congenital atonic pseudoparalysis observed especially in infants and characterized by absence of muscular tone only in muscles innervated by the spinal nerves; also known as Oppenheim''s disease or syndrome or congenital atonic pseudoparalysis.

Camurati-Engelmann disease (CED) is characterized by hyperostosis of the long bones and the skull, proximal muscle weakness, limb pain, a wide-based, waddling gait, and joint contractures. Facial features such as macrocephaly, frontal bossing, enlargement of the mandible, proptosis, and cranial nerve impingement resulting in facial palsy are seen in severely affected individuals later in life.

The clinical manifestations of glycogen storage disease type IV (GSD IV) discussed in this entry span a continuum of different subtypes with variable ages of onset, severity, and clinical features. Clinical findings vary extensively both within and between families. The fatal perinatal neuromuscular subtype presents in utero with fetal akinesia deformation sequence, including decreased fetal movements, polyhydramnios, and fetal hydrops. Death usually occurs in the neonatal period. The congenital neuromuscular subtype presents in the newborn period with profound hypotonia, respiratory distress, and dilated cardiomyopathy. Death usually occurs in early infancy. Infants with the classic (progressive) hepatic subtype may appear normal at birth, but rapidly develop failure to thrive; hepatomegaly, liver dysfunction, and progressive liver cirrhosis; hypotonia; and cardiomyopathy. Without liver transplantation, death from liver failure usually occurs by age five years. Children with the non-progressive hepatic subtype tend to present with hepatomegaly, liver dysfunction, myopathy, and hypotonia; however, they are likely to survive without progression of the liver disease and may not show cardiac, skeletal muscle, or neurologic involvement. The childhood neuromuscular subtype is rare and the course is variable, ranging from onset in the second decade with a mild disease course to a more severe, progressive course resulting in death in the third decade.

Multiple symmetric lipomatosis (MSL) is an autosomal recessive metabolic disorder characterized by the growth of unencapsulated masses of adipose tissue with predilection for the cervical and thoracic regions. The lipoma growth is striking and disfiguring, and growth around the neck may cause difficulty swallowing or breathing. The age at onset ranges from childhood to young adulthood. Most, but not all, patients develop axonal peripheral neuropathy, which can appear at any age and varies in severity. Laboratory studies in MSL show low leptin (164160), low adiponectin (605441), variably increased lactate, and increased FGF21 (609436). Some patients may have insulin resistance. The disorder is exclusively associated with a particular MFN2 mutation (R707W; 608507.0013), usually in the homozygous state, but sometimes in the compound heterozygous state (Rocha et al., 2017; Capel et al., 2018).

Marinesco-Sjögren syndrome (MSS) is characterized by cerebellar ataxia with cerebellar atrophy, dysarthria, nystagmus, early-onset (not necessarily congenital) cataracts, myopathy, muscle weakness, and hypotonia. Additional features may include psychomotor delay, hypergonadotropic hypogonadism, short stature, and various skeletal abnormalities. Children with MSS usually present with muscular hypotonia in early infancy; distal and proximal muscular weakness is noticed during the first decade of life. Later, cerebellar findings of truncal ataxia, dysdiadochokinesia, nystagmus, and dysarthria become apparent. Motor function worsens progressively for some years, then stabilizes at an unpredictable age and degree of severity. Cataracts can develop rapidly and typically require lens extraction in the first decade of life. Although many adults have severe disabilities, life span in MSS appears to be near normal.

The spectrum of MECP2-related phenotypes in females ranges from classic Rett syndrome to variant Rett syndrome with a broader clinical phenotype (either milder or more severe than classic Rett syndrome) to mild learning disabilities; the spectrum in males ranges from severe neonatal encephalopathy to pyramidal signs, parkinsonism, and macroorchidism (PPM-X) syndrome to severe syndromic/nonsyndromic intellectual disability. Females: Classic Rett syndrome, a progressive neurodevelopmental disorder primarily affecting girls, is characterized by apparently normal psychomotor development during the first six to 18 months of life, followed by a short period of developmental stagnation, then rapid regression in language and motor skills, followed by long-term stability. During the phase of rapid regression, repetitive, stereotypic hand movements replace purposeful hand use. Additional findings include fits of screaming and inconsolable crying, autistic features, panic-like attacks, bruxism, episodic apnea and/or hyperpnea, gait ataxia and apraxia, tremors, seizures, and acquired microcephaly. Males: Severe neonatal-onset encephalopathy, the most common phenotype in affected males, is characterized by a relentless clinical course that follows a metabolic-degenerative type of pattern, abnormal tone, involuntary movements, severe seizures, and breathing abnormalities. Death often occurs before age two years.

Sandhoff disease comprises a phenotypic continuum encompassing acute infantile, subacute juvenile, and late-onset disease. Although classification into these phenotypes is somewhat arbitrary, it is helpful in understanding the variation observed in the timing of disease onset, presenting manifestations, rate of progression, and life span. Acute infantile Sandhoff disease (onset age <6 months). Infants are generally normal at birth followed by progressive weakness and slowing of developmental progress, then developmental regression and severe neurologic impairment. Seizures are common. Death usually occurs between ages two and three years. Subacute juvenile Sandhoff disease (onset age 2-5 years). After attaining normal developmental milestones, developmental progress slows, followed by developmental regression and neurologic impairment (abnormal gait, dysarthria, and cognitive decline). Death (usually from aspiration) typically occurs in the early to late teens. Late-onset Sandhoff disease (onset older teen years or young adulthood). Nearly normal psychomotor development is followed by a range of neurologic findings (e.g., weakness, spasticity, dysarthria, and deficits in cerebellar function) and psychiatric findings (e.g., deficits in executive function and memory). Life expectancy is not necessarily decreased.

Adrenocorticotropic hormone (ACTH) hypersecretion by corticotroph adenomas of the pituitary result in excess cortisol secretion, or Cushing disease. The clinical features of Cushing disease include central obesity, moon facies, 'buffalo hump,' diabetes, hypertension, fatigue, easy bruising, depression, and reproductive disorders. Cushing disease is associated with increased morbidity and mortality, mainly due to cardiovascular or cerebrovascular disease and infections (summary by Perez-Rivas et al., 2015). Mutations in the USP8 gene, leading to an upregulated epidermal growth factor receptor (EGFR; 131550) pathway, have been identified in about 36 to 62% of corticotroph adenomas (summary by Mete and Lopes, 2017).

INSR-related severe syndromic insulin resistance comprises a phenotypic spectrum that is a continuum from the severe phenotype Donohue syndrome (DS) (also known as leprechaunism) to the milder phenotype Rabson-Mendenhall syndrome (RMS). DS at the severe end of the spectrum is characterized by severe insulin resistance (hyperinsulinemia with associated fasting hypoglycemia and postprandial hyperglycemia), severe prenatal growth restriction and postnatal growth failure, hypotonia and developmental delay, characteristic facies, and organomegaly involving heart, kidneys, liver, spleen, and ovaries. Death usually occurs before age one year. RMS at the milder end of the spectrum is characterized by severe insulin resistance that, although not as severe as that of DS, is nonetheless accompanied by fluctuations in blood glucose levels, diabetic ketoacidosis, and – in the second decade – microvascular complications. Findings can range from severe growth delay and intellectual disability to normal growth and development. Facial features can be milder than those of DS. Complications of longstanding hyperglycemia are the most common cause of death. While death usually occurs in the second decade, some affected individuals live longer.

Adenylosuccinase deficiency is an autosomal recessive inborn error of metabolism caused by an enzymatic defect in de novo purine synthesis (DNPS) pathway. ADSL deficiency leads to the accumulation of toxic intermediates, including succinyladenosine (S-Ado) and succinylaminoimidazole carboxamide riboside (SAICAr) in body fluids. There are 3 major phenotypic forms of the disorder that correlate with different values of the S-Ado and SAICAr concentration ratios (S-Ado/SAICAr) in the cerebrospinal fluid. These include the most severe fatal neonatal encephalopathy (S-Ado/SAICAr ratio less than 1); childhood form (type I) with severe psychomotor retardation (S-Ado/SAICAr ratio close to 1), and a milder form (type II) with psychomotor retardation or hypotonia (S-Ado/SAICAr ratio greater than 2) (summary by Baresova et al., 2012).

The phenotype of acid sphingomyelinase deficiency (ASMD) occurs along a continuum. Individuals with the severe early-onset form, infantile neurovisceral ASMD, were historically diagnosed with Niemann-Pick disease type A (NPD-A). The later-onset, chronic visceral form of ASMD is also referred to as Niemann-Pick disease type B (NPD-B). A phenotype with intermediate severity is also known as chronic neurovisceral ASMD (NPD-A/B). The most common presenting symptom in NPD-A is hepatosplenomegaly, usually detectable by age three months; over time the liver and spleen become massive in size. Psychomotor development progresses no further than the 12-month level, after which neurologic deterioration is relentless. Failure to thrive typically becomes evident by the second year of life. A classic cherry-red spot of the macula of the retina, which may not be present in the first few months, is eventually present in all affected children. Interstitial lung disease caused by storage of sphingomyelin in pulmonary macrophages results in frequent respiratory infections and often respiratory failure. Most children succumb before the third year of life. NPD-B generally presents later than NPD-A, and the manifestations are less severe. NPD-B is characterized by progressive hepatosplenomegaly, gradual deterioration in liver and pulmonary function, osteopenia, and atherogenic lipid profile. No central nervous system (CNS) manifestations occur. Individuals with NPD-A/B have symptoms that are intermediate between NPD-A and NPD-B. The presentation in individuals with NPD-A/B varies greatly, although all are characterized by the presence of some CNS manifestations. Survival to adulthood can occur in individuals with NPD-B and NPD-A/B.

GLB1-related disorders comprise two phenotypically distinct lysosomal storage disorders: GM1 gangliosidosis and mucopolysaccharidosis type IVB (MPS IVB). The phenotype of GM1 gangliosidosis constitutes a spectrum ranging from severe (infantile) to intermediate (late-infantile and juvenile) to mild (chronic/adult). Type I (infantile) GM1 gangliosidosis begins before age 12 months. Prenatal manifestations may include nonimmune hydrops fetalis, intrauterine growth restriction, and placental vacuolization; congenital dermal melanocytosis (Mongolian spots) may be observed. Macular cherry-red spot is detected on eye exam. Progressive central nervous system dysfunction leads to spasticity and rapid regression; blindness, deafness, decerebrate rigidity, seizures, feeding difficulties, and oral secretions are observed. Life expectancy is two to three years. Type II can be subdivided into the late-infantile (onset age 1-3 years) and juvenile (onset age 3-10 years) phenotypes. Central nervous system dysfunction manifests as progressive cognitive, motor, and speech decline as measured by psychometric testing. There may be mild corneal clouding, hepatosplenomegaly, and/or cardiomyopathy; the typical course is characterized by progressive neurologic decline, progressive skeletal disease in some individuals (including kyphosis and avascular necrosis of the femoral heads), and progressive feeding difficulties leading to aspiration risk. Type III begins in late childhood to the third decade with generalized dystonia leading to unsteady gait and speech disturbance followed by extrapyramidal signs including akinetic-rigid parkinsonism. Cardiomyopathy develops in some and skeletal involvement occurs in most. Intellectual impairment is common late in the disease with prognosis directly related to the degree of neurologic impairment. MPS IVB is characterized by skeletal dysplasia with specific findings of axial and appendicular dysostosis multiplex, short stature (below 15th centile in adults), kyphoscoliosis, coxa/genu valga, joint laxity, platyspondyly, and odontoid hypoplasia. First signs and symptoms may be apparent at birth. Bony involvement is progressive, with more than 84% of adults requiring ambulation aids; life span does not appear to be limited. Corneal clouding is detected in some individuals and cardiac valvular disease may develop.

Lysinuric protein intolerance (LPI) typically presents after an infant is weaned from breast milk or formula; variable findings include recurrent vomiting and episodes of diarrhea, episodes of stupor and coma after a protein-rich meal, poor feeding, aversion to protein-rich food, failure to thrive, hepatosplenomegaly, and muscular hypotonia. Over time, findings include: poor growth, osteoporosis, involvement of the lungs (progressive interstitial changes, pulmonary alveolar proteinosis) and of the kidneys (progressive glomerular and proximal tubular disease), hematologic abnormalities (normochromic or hypochromic anemia, leukopenia, thrombocytopenia, erythroblastophagocytosis in the bone marrow aspirate), and a clinical presentation resembling the hemophagocytic lymphohistiocytosis/macrophagic activation syndrome. Hypercholesterolemia, hypertriglyceridemia, and acute pancreatitis can also be seen.

A rare hereditary ataxia with characteristics of neurogenic muscular atrophy associated with signs of cerebellar ataxia, degeneration of the retina and diabetes mellitus. Onset of the disease is in adolescence and the course is slowly progressive.

Spinal muscular atrophy (SMA) is characterized by muscle weakness and atrophy resulting from progressive degeneration and irreversible loss of the anterior horn cells in the spinal cord (i.e., lower motor neurons) and the brain stem nuclei. The onset of weakness ranges from before birth to adulthood. The weakness is symmetric, proximal > distal, and progressive. Before the genetic basis of SMA was understood, it was classified into clinical subtypes based on maximum motor function achieved; however, it is now apparent that the phenotype of SMN1-associated SMA spans a continuum without clear delineation of subtypes. With supportive care only, poor weight gain with growth failure, restrictive lung disease, scoliosis, and joint contractures are common complications; however, newly available targeted treatment options are changing the natural history of this disease.

Chorea-acanthocytosis (ChAc) is characterized by a progressive movement disorder, cognitive and behavior changes, a myopathy that can be subclinical, and chronic hyperCKemia in serum. Although the disorder is named for acanthocytosis of the red blood cells, this feature is variable. The movement disorder is mostly limb chorea, but some individuals present with parkinsonism. Dystonia is common and affects the oral region and especially the tongue, causing dysarthria and serious dysphagia with resultant weight loss. Habitual tongue and lip biting are characteristic, as well as tongue protrusion dystonia. Progressive cognitive and behavioral changes resemble those in a frontal lobe syndrome. Seizures are observed in almost half of affected individuals and can be the initial manifestation. Myopathy results in progressive distal muscle wasting and weakness. Mean age of onset in ChAc is about 30 years, although ChAc can develop as early as the first decade or as late as the seventh decade. It runs a chronic progressive course and may lead to major disability within a few years. Life expectancy is reduced, with age of death ranging from 28 to 61 years.

A subtype of autosomal recessive limb-girdle muscular dystrophy characterized by a childhood onset of progressive shoulder and pelvic girdle muscle weakness and atrophy frequently associated with calf hypertrophy, diaphragmatic weakness, and/or variable cardiac abnormalities. Mild to moderate elevated serum creatine kinase levels and positive Gowers sign are reported.

Fukuyama congenital muscular dystrophy (FCMD) is characterized by hypotonia, symmetric generalized muscle weakness, and CNS migration disturbances that result in changes consistent with cobblestone lissencephaly with cerebral and cerebellar cortical dysplasia. Mild, typical, and severe phenotypes are recognized. Onset typically occurs in early infancy with poor suck, weak cry, and floppiness. Affected individuals have contractures of the hips, knees, and interphalangeal joints. Later features include myopathic facial appearance, pseudohypertrophy of the calves and forearms, motor and speech delays, intellectual disability, seizures, ophthalmologic abnormalities including visual impairment and retinal dysplasia, and progressive cardiac involvement after age ten years. Swallowing disturbance occurs in individuals with severe FCMD and in individuals older than age ten years, leading to recurrent aspiration pneumonia and death.

Spinocerebellar ataxia type 1 (SCA1) is characterized by progressive cerebellar ataxia, dysarthria, and eventual deterioration of bulbar functions. Early in the disease, affected individuals may have gait disturbance, slurred speech, difficulty with balance, brisk deep tendon reflexes, hypermetric saccades, nystagmus, and mild dysphagia. Later signs include slowing of saccadic velocity, development of up-gaze palsy, dysmetria, dysdiadochokinesia, and hypotonia. In advanced stages, muscle atrophy, decreased deep tendon reflexes, loss of proprioception, cognitive impairment (e.g., frontal executive dysfunction, impaired verbal memory), chorea, dystonia, and bulbar dysfunction are seen. Onset is typically in the third or fourth decade, although childhood onset and late-adult onset have been reported. Those with onset after age 60 years may manifest a pure cerebellar phenotype. Interval from onset to death varies from ten to 30 years; individuals with juvenile onset show more rapid progression and more severe disease. Anticipation is observed. An axonal sensory neuropathy detected by electrophysiologic testing is common; brain imaging typically shows cerebellar and brain stem atrophy.

Hereditary motor and sensory neuropathy with agenesis of the corpus callosum (HMSN/ACC), a neurodevelopmental and neurodegenerative disorder, is characterized by severe progressive sensorimotor neuropathy with resulting hypotonia, areflexia, and amyotrophy, and by variable degrees of dysgenesis of the corpus callosum. Mild-to-severe intellectual disability and "psychotic episodes" during adolescence are observed. Sensory modalities are moderately to severely affected beginning in infancy. The average age of onset of walking is 3.8 years; the average age of loss of walking is 13.8 years; the average age of death is 33 years.

Primrose syndrome is characterized by macrocephaly, hypotonia, developmental delay, intellectual disability with expressive speech delay, behavioral issues, a recognizable facial phenotype, radiographic features, and altered glucose metabolism. Additional features seen in adults: sparse body hair, distal muscle wasting, and contractures. Characteristic craniofacial features include brachycephaly, high anterior hairline, deeply set eyes, ptosis, downslanted palpebral fissures, high palate with torus palatinus, broad jaw, and large ears with small or absent lobes. Radiographic features include calcification of the external ear cartilage, multiple Wormian bones, platybasia, bathrocephaly, slender bones with exaggerated metaphyseal flaring, mild epiphyseal dysplasia, and spondylar dysplasia. Additional features include hearing impairment, ocular anomalies, cryptorchidism, and nonspecific findings on brain MRI.

Brown-Vialetto-Van Laere syndrome is a rare autosomal recessive neurologic disorder characterized by sensorineural hearing loss and a variety of cranial nerve palsies, usually involving the motor components of the seventh and ninth to twelfth (more rarely the third, fifth, and sixth) cranial nerves. Spinal motor nerves and, less commonly, upper motor neurons are sometimes affected, giving a picture resembling amyotrophic lateral sclerosis (ALS; 105400). The onset of the disease is usually in the second decade, but earlier and later onset have been reported. Hearing loss tends to precede the onset of neurologic signs, mostly progressive muscle weakness causing respiratory compromise. However, patients with very early onset may present with bulbar palsy and may not develop hearing loss until later. The symptoms, severity, and disease duration are variable (summary by Green et al., 2010). Genetic Heterogeneity of Brown-Vialetto-Van Laere Syndrome See also BVVLS2 (614707), caused by mutation in the SLC52A2 gene (607882) on chromosome 8q.

Cayman cerebellar ataxia (ATCAY) is an autosomal recessive neurologic disorder characterized by hypotonia from birth, variable psychomotor retardation, and cerebellar dysfunction, including nystagmus, intention tremor, dysarthria, ataxic gait, and truncal ataxia. Although the disorder was initially believed to be restricted to an isolated region of Grand Cayman Island (summary by Nystuen et al., 1996; Bomar et al., 2003), one Pakistani family with the disorder and an ATCAY mutation has been reported, thus expanding the ethnic distribution (Manzoor et al., 2018).

Hereditary neuralgic amyotrophy (HNA) is an autosomal dominant form of recurrent focal neuropathy characterized clinically by acute, recurrent episodes of brachial plexus neuropathy with muscle weakness and atrophy preceded by severe pain in the affected arm.

The spectrum of ASAH1-related disorders ranges from Farber disease (FD) to spinal muscular atrophy with progressive myoclonic epilepsy (SMA-PME). Classic FD is characterized by onset in the first weeks of life of painful, progressive deformity of the major joints; palpable subcutaneous nodules of joints and mechanical pressure points; and a hoarse cry resulting from granulomas of the larynx and epiglottis. Life expectancy is usually less than two years. In the other less common types of FD, onset, severity, and primary manifestations vary. SMA-PME is characterized by early-childhood-onset progressive lower motor neuron disease manifest typically between ages three and seven years as proximal lower-extremity weakness, followed by progressive myoclonic and atonic seizures, tremulousness/tremor, and sensorineural hearing loss. Myoclonic epilepsy typically begins in late childhood after the onset of weakness and can include jerking of the upper limbs, action myoclonus, myoclonic status, and eyelid myoclonus. Other findings include generalized tremor, and cognitive decline. The time from disease onset to death from respiratory complications is usually five to 15 years.

Bethlem myopathy-1 (BTHLM1) is a congenital muscular dystrophy characterized by distal joint laxity and a combination of distal and proximal joint contractures. The age at onset is highly variable, ranging from infancy to adulthood. Disease progression is slow and ambulation is usually retained into adulthood (summary by Butterfield et al., 2013). Genetic Heterogeneity of Bethlem Myopathy See Bethlem myopathy-1B (BTHLM1B; 620725), caused by mutation in the COL6A2 gene (120240) on chromosome 21q22; Bethlem myopathy-1C (620726), caused by mutation the COL6A3 gene (120250) on chromosome 2q37; and Bethlem myopathy-2 (BTHLM2; 616471), caused by mutation in the COL12A1 gene (120320) on chromosome 6q13-q14.

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

Nemaline myopathy is divided into six types. In order of decreasing severity, the types are: severe congenital, Amish, intermediate congenital, typical congenital, childhood-onset, and adult-onset. The types are distinguished by the age when symptoms first appear and the severity of symptoms; however, there is overlap among the various types. The severe congenital type is the most life-threatening. Most individuals with this type do not survive past early childhood due to respiratory failure. The Amish type solely affects the Old Order Amish population of Pennsylvania and is typically fatal in early childhood. The most common type of nemaline myopathy is the typical congenital type, which is characterized by muscle weakness and feeding problems beginning in infancy. Most of these individuals do not have severe breathing problems and can walk unassisted. People with the childhood-onset type usually develop muscle weakness in adolescence. The adult-onset type is the mildest of all the various types. People with this type usually develop muscle weakness between ages 20 and 50.\n\nNemaline myopathy is a disorder that primarily affects skeletal muscles, which are muscles that the body uses for movement. People with nemaline myopathy have muscle weakness (myopathy) throughout the body, but it is typically most severe in the muscles of the face; neck; trunk; and other muscles close to the center of the body (proximal muscles), such as those of the upper arms and legs. This weakness can worsen over time. Affected individuals may have feeding and swallowing difficulties, foot deformities, abnormal curvature of the spine (scoliosis), and joint deformities (contractures). Most people with nemaline myopathy are able to walk, although some affected children may begin walking later than usual. As the condition progresses, some people may require wheelchair assistance. In severe cases, the muscles used for breathing are affected and life-threatening breathing difficulties can occur.

Nemaline myopathy-6 is an autosomal dominant skeletal muscle disorder characterized by childhood onset of slowly progressive proximal muscle weakness, exercise intolerance, and slow movements with stiff muscles. Patients are unable to run or correct themselves from falling over. Histopathologic changes seen on skeletal muscle biopsy include nemaline rods, cores devoid of oxidative enzyme activity, and predominance of hypertrophic type 1 fibers. There is no cardiac or respiratory involvement (summary by Sambuughin et al., 2010).

Pierson syndrome (PIERS) is an autosomal recessive disorder comprising congenital nephrotic syndrome with diffuse mesangial sclerosis and distinct ocular abnormalities, including microcoria and hypoplasia of the ciliary and pupillary muscles, as well as other anomalies. Many patients die early, and those who survive tend to show neurodevelopmental delay and visual loss (summary by Zenker et al., 2004). Mutations in the LAMB2 gene also cause nephrotic syndrome type 5 with or without mild ocular anomalies (NPHS5; 614199).

Posterior column ataxia with retinitis pigmentosa (AXPC1) is an autosomal recessive neurologic disorder characterized by childhood-onset retinitis pigmentosa and later onset of gait ataxia due to sensory loss (summary by Ishiura et al., 2011).

Congenital myasthenic syndrome associated with AChR deficiency is a disorder of the postsynaptic neuromuscular junction (NMJ) clinically characterized by early-onset muscle weakness with variable severity. Electrophysiologic studies show low amplitude of the miniature endplate potential (MEPP) and current (MEPC) resulting from deficiency of AChR at the endplate. Patients with mutations in the CHRNE gene may have compensatory increased expression of the fetal subunit CHRNG (100730) and may respond to treatment with cholinergic agents, pyridostigmine, or amifampridine (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

A form of limb-girdle muscular dystrophy that usually has a childhood onset (but can range from the first to third decade of life) of severe progressive proximal weakness, eventually involving the distal muscles. Some patients may remain ambulatory but most are wheelchair dependant 20 years after onset. Caused by homozygous mutation in the titin gene (TTN).

A neurodegenerative disease with characteristics of progressive muscular paralysis reflecting degeneration of motor neurons in the primary motor cortex, corticospinal tracts, brainstem and spinal cord. Caused by heterozygous mutation in the VAPB gene on chromosome 20q13.

Isolated complex I deficiency is a rare inborn error of metabolism due to mutations in nuclear or mitochondrial genes encoding subunits or assembly factors of the human mitochondrial complex I (NADH: ubiquinone oxidoreductase) and is characterized by a wide range of manifestations including marked and often fatal lactic acidosis, cardiomyopathy, leukoencephalopathy, pure myopathy and hepatopathy with tubulopathy. Among the numerous clinical phenotypes observed are Leigh syndrome, Leber hereditary optic neuropathy and MELAS syndrome (see these terms).

PLP1 disorders of central nervous system myelin formation include a range of phenotypes from Pelizaeus-Merzbacher disease (PMD) to spastic paraplegia 2 (SPG2). PMD typically manifests in infancy or early childhood with nystagmus, hypotonia, and cognitive impairment; the findings progress to severe spasticity and ataxia. Life span is shortened. SPG2 manifests as spastic paraparesis with or without CNS involvement and usually normal life span. Intrafamilial variation of phenotypes can be observed, but the signs are usually fairly consistent within families. Heterozygous females may manifest mild-to-moderate signs of the disease.

X-linked myopathy with excessive autophagy (XMEA) is an X-linked recessive skeletal muscle disorder characterized by childhood onset of progressive muscle weakness and atrophy primarily affecting the proximal muscles. While onset is usually in childhood, it can range from infancy to adulthood. Many patients lose ambulation and become wheelchair-bound. Other organ systems, including the heart, are clinically unaffected. Muscle biopsy shows intracytoplasmic autophagic vacuoles with sarcolemmal features and a multilayered basal membrane (summary by Ramachandran et al., 2013; Kurashige et al., 2013, and Ruggieri et al., 2015). Danon disease (300257), caused by mutation in the LAMP2 gene (309060) on chromosome Xq24, is a distinct disorder with similar pathologic features.

Lethal congenital contracture syndrome-2 (LCCS2) is an autosomal recessive disorder characterized by severe multiple congenital contractures with muscle wasting and atrophy. Micrognathia and other craniofacial anomalies, including cleft palate, as well as cardiac defects and enlarged urinary bladder at birth have also been reported. Hydrops fetalis and multiple pterygia are absent. Most patients have died in the neonatal period, although 2 survived to early adolescence (Landau et al., 2003). For a general phenotypic description and a discussion of genetic heterogeneity of LCCS, see LCCS1 (253310).

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

Disease with characteristics of sensory neuropathy and cerebellar ataxia. Prevalence is unknown. Only 26 cases in a 5-generation American family of Irish ancestry have been reported to date. Onset is in the second and third decades of life with symptomatic onset ranging from 13 to 27 years. Patients initially present with axonal sensory neuropathy, while cerebellar ataxia and motor neuron dysfunction develop later. Linked to chromosome 7q22-q23 but the responsible gene mutation has not yet been identified.

X-linked deafness-5 is a neurologic disorder characterized by childhood onset of auditory neuropathy and later onset of distal sensory impairment affecting the peripheral nervous system (summary by Zong et al., 2015).

Phosphorylase kinase (PhK) deficiency causing glycogen storage disease type IX (GSD IX) results from deficiency of the enzyme phosphorylase b kinase, which has a major regulatory role in the breakdown of glycogen. The two types of PhK deficiency are liver PhK deficiency (characterized by early childhood onset of hepatomegaly and growth restriction, and often, but not always, fasting ketosis and hypoglycemia) and muscle PhK deficiency, which is considerably rarer (characterized by any of the following: exercise intolerance, myalgia, muscle cramps, myoglobinuria, and progressive muscle weakness). While symptoms and biochemical abnormalities of liver PhK deficiency were thought to improve with age, it is becoming evident that affected individuals need to be monitored for long-term complications such as liver fibrosis and cirrhosis.

X-linked spondyloepimetaphyseal dysplasia with hypomyelinating leukodystrophy (SEMDHL) is an X-linked recessive developmental disorder characterized by slowly progressive skeletal and neurologic abnormalities, including short stature, large and deformed joints, significant motor impairment, visual defects, and sometimes cognitive deficits. Affected individuals typically have normal early development in the first year or so of life, followed by development regression and the development of symptoms. Brain imaging shows white matter abnormalities consistent with hypomyelinating leukodystrophy (summary by Miyake et al., 2017).

MDDGB5 is an autosomal recessive congenital muscular dystrophy with impaired intellectual development and structural brain abnormalities (Brockington et al., 2001). It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (Mercuri et al., 2006). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

Charcot-Marie-Tooth disease, dominant intermediate-A (CMTDIA) is an autosomal dominant peripheral neuropathy characterized by onset of symptoms in the first or second decades of life. Affected individuals have difficulty walking with muscle cramps of the lower limbs; the motor symptoms may be worsened by cold. The disorder is slowly progressive, eventually involving all 4 limbs, but patients remain ambulatory. After age 40, patients develop more severe features, including distal muscle weakness and atrophy, pes cavus, areflexia, and distal sensory loss. Electrophysiologic studies yield nerve conduction velocities with 'intermediate' values between demyelinating and axonal neuropathy (see below). One such family has been reported (Rossi et al., 1985).

A rare syndromic renal disorder characterized by renal, neurologic and thyroid disease, associated with thrombocytopenia. There have been no further descriptions in the literature since 1978.

MPV17-related mitochondrial DNA (mtDNA) maintenance defect presents in the vast majority of affected individuals as an early-onset encephalohepatopathic (hepatocerebral) disease that is typically associated with mtDNA depletion, particularly in the liver. A later-onset neuromyopathic disease characterized by myopathy and neuropathy, and associated with multiple mtDNA deletions in muscle, has also rarely been described. MPV17-related mtDNA maintenance defect, encephalohepatopathic form is characterized by: Hepatic manifestations (liver dysfunction that typically progresses to liver failure, cholestasis, hepatomegaly, and steatosis); Neurologic involvement (developmental delay, hypotonia, microcephaly, and motor and sensory peripheral neuropathy); Gastrointestinal manifestations (gastrointestinal dysmotility, feeding difficulties, and failure to thrive); and Metabolic derangements (lactic acidosis and hypoglycemia). Less frequent manifestations include renal tubulopathy, nephrocalcinosis, and hypoparathyroidism. Progressive liver disease often leads to death in infancy or early childhood. Hepatocellular carcinoma has been reported.

Nemaline myopathy-2 (NEM2) is an autosomal recessive skeletal muscle disorder with a wide range of severity. The most common clinical presentation is early-onset (in infancy or childhood) muscle weakness predominantly affecting proximal limb muscles. Muscle biopsy shows accumulation of Z-disc and thin filament proteins into aggregates named 'nemaline bodies' or 'nemaline rods,' usually accompanied by disorganization of the muscle Z discs. The clinical and histologic spectrum of entities caused by variants in the NEB gene is a continuum, ranging in severity. The distribution of weakness can vary from generalized muscle weakness, more pronounced in proximal limb muscles, to distal-only involvement, although neck flexor weakness appears to be rather consistent. Histologic patterns range from a severe usually nondystrophic disturbance of the myofibrillar pattern to an almost normal pattern, with or without nemaline bodies, sometimes combined with cores (summary by Lehtokari et al., 2014). Genetic Heterogeneity of Nemaline Myopathy See also NEM1 (255310), caused by mutation in the tropomyosin-3 gene (TPM3; 191030) on chromosome 1q22; NEM3 (161800), caused by mutation in the alpha-actin-1 gene (ACTA1; 102610) on chromosome 1q42; NEM4 (609285), caused by mutation in the beta-tropomyosin gene (TPM2; 190990) on chromosome 9p13; NEM5A (605355), also known as Amish nemaline myopathy, NEM5B (620386), and NEM5C (620389), all caused by mutation in the troponin T1 gene (TNNT1; 191041) on chromosome 19q13; NEM6 (609273), caused by mutation in the KBTBD13 gene (613727) on chromosome 15q22; NEM7 (610687), caused by mutation in the cofilin-2 gene (CFL2; 601443) on chromosome 14q13; NEM8 (615348), caused by mutation in the KLHL40 gene (615340), on chromosome 3p22; NEM9 (615731), caused by mutation in the KLHL41 gene (607701) on chromosome 2q31; NEM10 (616165), caused by mutation in the LMOD3 gene (616112) on chromosome 3p14; and NEM11 (617336), caused by mutation in the MYPN gene (608517) on chromosome 10q21. Several of the genes encode components of skeletal muscle sarcomeric thin filaments (Sanoudou and Beggs, 2001). Mutations in the NEB gene are the most common cause of nemaline myopathy (Lehtokari et al., 2006).

STAC3 disorder is characterized by congenital myopathy, musculoskeletal involvement of the trunk and extremities, feeding difficulties, and delayed motor milestones. Most affected individuals have weakness with myopathic facies, scoliosis, kyphosis or kyphoscoliosis, and contractures. Other common findings are ptosis, abnormalities of the palate (including cleft palate), and short stature. Risk for malignant hyperthermia susceptibility and restrictive lung disease are increased. Intellect is typically normal. Originally described in individuals from the Lumbee Native American tribe (an admixture of Cheraw Indian, English, and African American ancestry) in the state of North Carolina and reported as Native American myopathy, STAC3 disorder has now been identified in numerous other populations worldwide.

Nathalie syndrome has characteristics of deafness, cataract, muscular atrophy, skeletal abnormalities, growth retardation, underdeveloped secondary sexual characteristics and electrocardiographic abnormalities. It has been described in a Dutch family: in three sisters (one named Nathalie) and their brother.

Collagen VI-related dystrophies (COL6-RDs) represent a continuum of overlapping clinical phenotypes with Bethlem muscular dystrophy at the milder end, Ullrich congenital muscular dystrophy (UCMD) at the more severe end, and a phenotype in between UCMD and Bethlem muscular dystrophy, referred to as intermediate COL6-RD. Bethlem muscular dystrophy is characterized by a combination of proximal muscle weakness and joint contractures. Hypotonia and delayed motor milestones occur in early childhood; mild hypotonia and weakness may be present congenitally. By adulthood, there is evidence of proximal weakness and contractures of the elbows, Achilles tendons, and long finger flexors. The progression of weakness is slow, and more than two thirds of affected individuals older than age 50 years remain independently ambulatory indoors, while relying on supportive means for mobility outdoors. Respiratory involvement is not a consistent feature. UCMD is characterized by congenital weakness, hypotonia, proximal joint contractures, and striking hyperlaxity of distal joints. Decreased fetal movements are frequently reported. Some affected children acquire the ability to walk independently; however, progression of the disease results in a loss of ambulation by age ten to eleven years. Early and severe respiratory insufficiency occurs in all individuals, resulting in the need for nocturnal noninvasive ventilation (NIV) in the form of bilevel positive airway pressure (BiPAP) by age 11 years. Intermediate COL6-RD is characterized by independent ambulation past age 11 years and respiratory insufficiency that is later in onset than in UCMD and results in the need for NIV in the form of BiPAP by the late teens to early 20s. In contrast to individuals with Bethlem muscular dystrophy, those with intermediate COL6-RD typically do not achieve the ability to run, jump, or climb stairs without use of a railing.

Congenital myopathy-1B (CMYO1B) is an autosomal recessive disorder of skeletal muscle characterized by severe hypotonia and generalized muscle weakness apparent soon after birth or in early childhood with delayed motor development, generalized muscle weakness and atrophy, and difficulty walking or running. Affected individuals show proximal muscle weakness with axial and shoulder girdle involvement, external ophthalmoplegia, and bulbar weakness, often resulting in feeding difficulties and respiratory insufficiency. Orthopedic complications such as joint laxity, distal contractures, hip dislocation, cleft palate, and scoliosis are commonly observed. Serum creatine kinase is normal. The phenotype is variable in severity (Jungbluth et al., 2005; Bharucha-Goebel et al., 2013). Some patients show symptoms in utero, including reduced fetal movements, polyhydramnios, and intrauterine growth restriction. The most severely affected patients present in utero with fetal akinesia, arthrogryposis, and lung hypoplasia resulting in fetal or perinatal death (McKie et al., 2014). Skeletal muscle biopsy of patients with recessive RYR1 mutations can show variable features, including multiminicores (Ferreiro and Fardeau, 2002), central cores (Jungbluth et al., 2002), congenital fiber-type disproportion (CFTD) (Monnier et al., 2009), and centronuclear myopathy (Wilmshurst et al., 2010). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Hereditary myopathy with lactic acidosis (HML) is an autosomal recessive muscular disorder characterized by childhood onset of exercise intolerance with muscle tenderness, cramping, dyspnea, and palpitations. Biochemical features include lactic acidosis and, rarely, rhabdomyolysis. It is a chronic disorder with remission and exacerbation of the muscle phenotype (summary by Sanaker et al., 2010).

Dysferlinopathy includes a spectrum of muscle disease characterized by two major phenotypes: Miyoshi muscular dystrophy (MMD) and limb-girdle muscular dystrophy type 2B (LGMD2B); and two minor phenotypes: asymptomatic hyperCKemia and distal myopathy with anterior tibial onset (DMAT). MMD (median age of onset 19 years) is characterized by muscle weakness and atrophy, most marked in the distal parts of the legs, especially the gastrocnemius and soleus muscles. Over a period of years, the weakness and atrophy spread to the thighs and gluteal muscles. The forearms may become mildly atrophic with decrease in grip strength; the small muscles of the hands are spared. LGMD2B is characterized by early weakness and atrophy of the pelvic and shoulder girdle muscles in adolescence or young adulthood, with slow progression. Other phenotypes in this spectrum are scapuloperoneal syndrome and congenital muscular dystrophy. Asymptomatic hyperCKemia is characterized by marked elevation of serum CK concentration only. DMAT is characterized by early and predominant distal muscle weakness, particularly of the muscles of the anterior compartment of the legs.

Spastic paraplegia-31 (SPG31) is an autosomal dominant neurologic disorder characterized primarily by spasticity of the lower limbs, resulting in gait abnormalities and muscle weakness. There is a bimodal age at onset with a peak in the second and fourth decades of life. Most affected individuals have a 'pure' form of the disorder with gait difficulties and hyperreflexia of the lower limbs. However, there is phenotypic heterogeneity, and some patients have a 'complex' form of the disorder with additional signs and symptoms, such as peripheral neuropathy, cerebellar ataxia, postural tremor, or urinary symptoms. The disorder is slowly progressive, and there is intrafamilial variability and incomplete penetrance (summary by Goizet et al., 2011 and Toft et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant spastic paraplegia, see SPG3A (182600).

Autosomal recessive lethal congenital contracture syndrome (LCCS) is the most severe, neonatally lethal, form of arthrogryposis (see 108120), a disorder characterized by congenital nonprogressive joint contractures. The contractures can involve the upper or lower limbs and/or the vertebral column, leading to various degrees of flexion or extension limitations evident at birth (summary by Markus et al., 2012). Genetic Heterogeneity of Lethal Congenital Contracture Syndrome See also lethal congenital contracture syndrome-2 (LCCS2; 607598), caused by mutation in the ERBB3 gene (190151); LCCS3 (611369), caused by mutation in the PIP5K1C gene (606102); LCCS4 (614915), caused by mutation in the MYBPC1 gene (160794); LCCS5 (615368), caused by mutation in the DNM2 gene (602378); LCCS6 (616248), caused by mutation in the ZBTB42 gene (613915); LCCS7 (616286), caused by mutation in the CNTNAP1 gene (602346); LCCS8 (616287), caused by mutation in the ADCY6 gene (600294); LCCS9 (616503), caused by mutation in the ADGRG6 gene (612243); LCCS10 (617022), caused by mutation in the NEK9 gene (609798); and LCCS11 (617194), caused by mutation in the GLDN gene (608603).

Deafness-vitiligo-achalasia syndrome is characterized by the association of deafness, short stature, vitiligo, muscle wasting, and achalasia.

ACTH-independent macronodular adrenal hyperplasia (AIMAH) is an endogenous form of adrenal Cushing syndrome characterized by multiple bilateral adrenocortical nodules that cause a striking enlargement of the adrenal glands. Although some familial cases have been reported, the vast majority of AIMAH cases are sporadic. Patients typically present in the fifth or sixth decade of life, approximately 10 years later than most patients with other causes of Cushing syndrome (Swain et al., 1998; Christopoulos et al., 2005). Approximately 10 to 15% of adrenal Cushing syndrome is due to primary bilateral ACTH-independent adrenocortical pathology. The 2 main subtypes are AIMAH and primary pigmented nodular adrenocortical disease (PPNAD; see 610489), which is often a component of the Carney complex (160980) and associated with mutations in the PRKAR1A gene (188830). AIMAH is rare, representing less than 1% of endogenous causes of Cushing syndrome (Swain et al., 1998; Christopoulos et al., 2005). See also ACTH-independent Cushing syndrome (615830) due to somatic mutation in the PRKACA gene (601639). Cushing 'disease' (219090) is an ACTH-dependent disorder caused in most cases by pituitary adenomas that secrete excessive ACTH. Genetic Heterogeneity of ACTH-Independent Macronodular Adrenal Hyperplasia AIMAH2 (615954) is caused by germline mutation on 1 allele of the ARMC5 gene (615549) coupled with a somatic mutation in the other allele. AIMAH3 (620990) is caused by germline mutation on 1 allele of the KDM1A gene (609132) coupled with a somatic mutation in the other allele.

Microhydranencephaly (MHAC) is a severe neurodevelopmental defect characterized by extreme microcephaly, profound motor and mental retardation, spasticity, and incomplete cerebral formation. Radiologic studies show gross dilation of the ventricles resulting from the absence of cerebral hemispheres or severe delay in their development, as well as hypoplasia of the corpus callosum, cerebellum, and brainstem (summary by Guven et al., 2012).

Spastic paraplegia 11 (SPG11) is characterized by progressive spasticity and weakness of the lower limbs frequently associated with the following: mild intellectual disability with learning difficulties in childhood and/or progressive cognitive decline; peripheral neuropathy; pseudobulbar involvement; and increased reflexes in the upper limbs. Less frequent findings include: cerebellar signs (ataxia, nystagmus, saccadic pursuit); retinal degeneration; pes cavus; scoliosis; and parkinsonism with characteristic brain MRI features that include thinning of the corpus callosum. Onset occurs mainly during infancy or adolescence (range: age 1-31 years) and in rare cases as late as age 60 years. Most affected individuals become wheelchair bound one or two decades after disease onset.

3-Methylcrotonylglycinuria is an autosomal recessive disorder of leucine catabolism. The clinical phenotype is highly variable, ranging from neonatal onset with severe neurologic involvement to asymptomatic adults. There is a characteristic organic aciduria with massive excretion of 3-hydroxyisovaleric acid and 3-methylcrotonylglycine, usually in combination with a severe secondary carnitine deficiency. MCC activity in extracts of cultured fibroblasts of patients is usually less than 2% of control (summary by Baumgartner et al., 2001). Also see 3-methylcrotonylglycinuria I (MCC1D; 210200), caused by mutation in the alpha subunit of 3-methylcrotonyl-CoA carboxylase (MCCC1; 609010).

ALS2-related disorder involves retrograde degeneration of the upper motor neurons of the pyramidal tracts and comprises a clinical continuum of the following three phenotypes: Infantile ascending hereditary spastic paraplegia (IAHSP), characterized by onset of spasticity with increased reflexes and sustained clonus of the lower limbs within the first two years of life, progressive weakness and spasticity of the upper limbs by age seven to eight years, and wheelchair dependence in the second decade with progression toward severe spastic tetraparesis and a pseudobulbar syndrome caused by progressive cranial nerve involvement. Juvenile primary lateral sclerosis (JPLS), characterized by upper motor neuron findings of pseudobulbar palsy and spastic quadriplegia without dementia or cerebellar, extrapyramidal, or sensory signs. Juvenile amyotrophic lateral sclerosis (JALS or ALS2), characterized by onset between ages three and 20 years. All affected individuals show a spastic pseudobulbar syndrome (spasticity of speech and swallowing) together with spastic paraplegia. Some individuals are bedridden by age 12 to 50 years.

Triosephosphate isomerase deficiency (TPID) is an autosomal recessive multisystem disorder characterized by congenital hemolytic anemia, and progressive neuromuscular dysfunction beginning in early childhood. Many patients die from respiratory failure in childhood. The neurologic syndrome is variable, but usually includes lower motor neuron dysfunction with hypotonia, muscle weakness and atrophy, and hyporeflexia. Some patients may show additional signs such as dystonic posturing and/or spasticity. Laboratory studies show intracellular accumulation of dihydroxyacetone phosphate (DHAP), particularly in red blood cells (summary by Fermo et al., 2010).

Diaphyseal medullary stenosis with malignant fibrous histiocytoma is an autosomal dominant bone dysplasia characterized by pathologic fractures due to abnormal cortical growth and diaphyseal medullary stenosis. The fractures heal poorly, and there is progressive bowing of the lower extremities. In 2 families, affected individuals also showed a limb-girdle myopathy, with muscle weakness and atrophy. Approximately 35% of affected individuals develop an aggressive form of bone sarcoma consistent with malignant fibrous histiocytoma or osteosarcoma. Thus, the disorder may be considered a tumor predisposition syndrome (summary by Camacho-Vanegas et al., 2012).

Amyotrophic lateral sclerosis is a neurodegenerative disorder characterized by the death of motor neurons in the brain, brainstem, and spinal cord, resulting in fatal paralysis. ALS usually begins with asymmetric involvement of the muscles in middle adult life. Approximately 10% of ALS cases are familial (Siddique and Deng, 1996). ALS is sometimes referred to as 'Lou Gehrig disease' after the famous American baseball player who was diagnosed with the disorder. Rowland and Shneider (2001) and Kunst (2004) provided extensive reviews of ALS. Some forms of ALS occur with frontotemporal dementia (FTD); see 105500. Ranganathan et al. (2020) provided a detailed review of the genes involved in different forms of ALS with FTD, noting that common disease pathways involve disturbances in RNA processing, autophagy, the ubiquitin proteasome system, the unfolded protein response, and intracellular trafficking. The current understanding of ALS and FTD is that some forms of these disorders represent a spectrum of disease with converging mechanisms of neurodegeneration. Familial ALS is distinct from a form of ALS with dementia reported in cases on Guam (105500) (Espinosa et al., 1962; Husquinet and Franck, 1980), in which the histology is different and dementia and parkinsonism complicate the clinical picture. Genetic Heterogeneity of Amyotrophic Lateral Sclerosis ALS is a genetically heterogeneous disorder, with several causative genes and mapped loci. ALS6 (608030) is caused by mutation in the FUS gene (137070) on chromosome 16p11; ALS8 (608627) is caused by mutation in the VAPB gene (605704) on chromosome 13; ALS9 (611895) is caused by mutation in the ANG gene (105850) on chromosome 14q11; ALS10 (612069) is caused by mutation in the TARDBP gene (605078) on 1p36; ALS11 (612577) is caused by mutation in the FIG4 gene (609390) on chromosome 6q21; ALS12 (613435) is caused by mutation in the OPTN gene (602432) on chromosome 10p13; ALS15 (300857) is caused by mutation in the UBQLN2 gene (300264) on chromosome Xp11; ALS18 (614808) is caused by mutation in the PFN1 gene (176610) on chromosome 17p13; ALS19 (615515) is caused by mutation in the ERBB4 gene (600543) on chromosome 2q34; ALS20 (615426) is caused by mutation in the HNRNPA1 gene (164017) on chromosome 12q13; ALS21 (606070) is caused by mutation in the MATR3 gene (164015) on chromosome 5q31; ALS22 (616208) is caused by mutation in the TUBA4A gene (191110) on chromosome 2q35; ALS23 (617839) is caused by mutation in the ANXA11 gene (602572) on chromosome 10q23; ALS26 (619133) is caused by mutation in the TIA1 gene (603518) on chromosome 2p13; ALS27 (620285) is caused by mutation in the SPTLC1 gene (605712) on chromosome 9q22; and ALS28 (620452) is caused by mutation in the LRP12 gene (618299) on chromosome 8q22. Loci associated with ALS have been found on chromosomes 18q21 (ALS3; 606640) and 20p13 (ALS7; 608031). Intermediate-length polyglutamine repeat expansions in the ATXN2 gene (601517) contribute to susceptibility to ALS (ALS13; 183090). Susceptibility to ALS24 (617892) is conferred by mutation in the NEK1 gene (604588) on chromosome 4q33, and susceptibility to ALS25 (617921) is conferred by mutation in the KIF5A gene (602821) on chromosome 12q13. Susceptibility to ALS has been associated with mutations in other genes, including deletions or insertions in the gene encoding the heavy neurofilament subunit (NEFH; 162230); deletions in the gene encoding peripherin (PRPH; 170710); and mutations in the dynactin gene (DCTN1; 601143). Some forms of ALS show juvenile onset. See juvenile-onset ALS2 (205100), caused by mutation in the alsin (606352) gene on 2q33; ALS4 (602433), caused by mutation in the senataxin gene (SETX; 608465) on 9q34; ALS5 (602099), caused by mutation in the SPG11 gene (610844) on 15q21; and ALS16 (614373), caused by mutation in the SIGMAR1 gene (601978) on 9p13.

Lethal congenital contracture syndrome-3 (LCCS3) is a severe autosomal recessive form of arthrogryposis characterized by multiple joint contractures with muscle wasting and atrophy (Narkis et al., 2007). For a general phenotypic description and a discussion of genetic heterogeneity of LCCS, see LCCS1 (253310).

Leukoencephalopathy with brain stem and spinal cord involvement and lactate elevation (LBSL) is characterized by slowly progressive cerebellar ataxia and spasticity with dorsal column dysfunction (decreased position and vibration sense) in most individuals. The neurologic dysfunction involves the legs more than the arms. The tendon reflexes are retained. Deterioration of motor skills usually starts in childhood or adolescence, but occasionally not until adulthood. Dysarthria develops over time. Occasional findings include epilepsy; learning problems; cognitive decline; and reduced consciousness, neurologic deterioration, and fever following minor head trauma. Individuals with neonatal or early-infantile onset have a severe disease course often associated with early death. Those with childhood onset have slow progression with wheelchair dependence in the teens or twenties. Adult onset is associated with slow progression and mild impairment.

An autosomal dominant form of amyotrophic lateral sclerosis caused by mutation(s) in the FIG4 gene, encoding polyphosphoinositide phosphatase.

A pure form of hereditary spastic paraplegia with characteristics of slowly progressive spastic paraplegia of lower extremities with an age of onset ranging from childhood to adulthood and patients presenting with spastic gait, increased tendon reflexes in lower limbs, extensor plantar response, weakness and atrophy of lower limb muscles and, in rare cases, pes cavus. No abnormalities are noted on magnetic resonance imaging.

CD59-mediated hemolytic anemia with immune-mediated polyneuropathy is an autosomal recessive disorder characterized by infantile onset of a relapsing-remitting polyneuropathy, often exacerbated by infection, and manifest as hypotonia, limb muscle weakness, and hyporeflexia. Immunosuppressive treatment may result in some clinical improvement (summary by Nevo et al., 2013).

Alopecia, neurologic defects, and endocrinopathy syndrome (ANES) is an autosomal recessive disorder characterized by alopecia with skin involvement including multiple facial nevi and flexural hyperpigmentation; moderately to severely impaired intellectual development; progressive motor decline; and endocrine deficiency (summary by Spiegel et al., 2010).

A neurodegenerative disease with characteristics of progressive muscular paralysis reflecting degeneration of motor neurons in the primary motor cortex, corticospinal tracts, brainstem and spinal cord. There is evidence this disease is caused by heterozygous mutation in the TARDBP gene that encodes the TDP43 protein on chromosome 1p36.

Turner-type X-linked syndromic intellectual developmental disorder (MRXST) is a neurodevelopmental disorder with a highly variable phenotype. Some affected families show X-linked recessive inheritance, with only males being affected and carrier females having no abnormal findings. In other affected families, males are severely affected, and female mutation carriers show milder cognitive abnormalities or dysmorphic features. In addition, there are female patients with de novo mutations who show the full phenotype, despite skewed X-chromosome inactivation. Affected individuals show global developmental delay from infancy, with variably impaired intellectual development and poor or absent speech, often with delayed walking. Dysmorphic features are common and can include macrocephaly, microcephaly, deep-set eyes, hypotelorism, small palpebral fissures, dysplastic, large, or low-set ears, long face, bitemporal narrowing, high-arched palate, thin upper lip, and scoliosis or mild distal skeletal anomalies, such as brachydactyly or tapered fingers. Males tend to have cryptorchidism. Other features, such as hypotonia, seizures, and delayed bone age, are more variable (summary by Moortgat et al., 2018).

Emery-Dreifuss muscular dystrophy (EDMD) is characterized by the clinical triad of: joint contractures that begin in early childhood; slowly progressive muscle weakness and wasting initially in a humero-peroneal distribution that later extends to the scapular and pelvic girdle muscles; and cardiac involvement that may manifest as palpitations, presyncope and syncope, poor exercise tolerance, and congestive heart failure along with variable cardiac rhythm disturbances. Age of onset, severity, and progression of muscle and cardiac involvement demonstrate both inter- and intrafamilial variability. Clinical variability ranges from early onset with severe presentation in childhood to late onset with slow progression in adulthood. In general, joint contractures appear during the first two decades, followed by muscle weakness and wasting. Cardiac involvement usually occurs after the second decade and respiratory function may be impaired in some individuals.

A rare, genetic, muscular dystrophy disease characterized by the co-occurrence of late onset scapular and peroneal muscle weakness, principally manifesting with distal lower limb and proximal upper limb weakness and scapular winging.

Christianson syndrome (referred to as CS in this GeneReview), an X-linked disorder, is characterized in males by cognitive dysfunction, behavioral disorder, and neurologic findings (e.g., seizures, ataxia, postnatal microcephaly, and eye movement abnormalities). Males with CS typically present with developmental delay, later meeting criteria for severe intellectual disability (ID). Behaviorally, autism spectrum disorder and hyperactivity are common, and may resemble the behaviors observed in Angelman syndrome. Hypotonia and oropharyngeal dysphagia in infancy may result in failure to thrive. Seizures, typically beginning before age three years, can include infantile spasms and tonic, tonic-clonic, myoclonic, and atonic seizures. Subsequently, regression (e.g., loss of ambulation and ability to feed independently) may occur. Manifestations in heterozygous females range from asymptomatic to mild ID and/or behavioral issues.

SUCLA2-related mitochondrial DNA (mtDNA) depletion syndrome, encephalomyopathic form with methylmalonic aciduria is characterized by onset of the following features in infancy or childhood (median age of onset 2 months; range of onset birth to 6 years): psychomotor retardation, hypotonia, dystonia, muscular atrophy, sensorineural hearing impairment, postnatal growth retardation, and feeding difficulties. Other less frequent features include distinctive facial features, contractures, kyphoscoliosis, gastroesophageal reflux, ptosis, choreoathetosis, ophthalmoplegia, and epilepsy (infantile spasms or generalized convulsions). The median survival is 20 years; approximately 30% of affected individuals succumb during childhood. Affected individuals may have hyperintensities in the basal ganglia, cerebral atrophy, and leukoencephalopathy on head MRI. Elevation of methylmalonic acid (MMA) in the urine and plasma is found in a vast majority of affected individuals, although at levels that are far below those typically seen in individuals with classic methylmalonic aciduria.

Spastic paraplegia-18B (SPG18B) is a severe autosomal recessive neurologic disorder characterized by onset in early childhood of progressive spastic paraplegia resulting in motor disability. Most affected individuals have severe psychomotor retardation. Some may develop significant joint contractures (summary by Alazami et al., 2011 and Yildirim et al., 2011).

A mild form of axonal Charcot-Marie-Tooth disease, a peripheral sensorimotor neuropathy, with characteristics of distal legs sensory loss and weakness that can be asymmetric. Tendon reflexes are reduced in the knees and absent in ankles. Progression is slow.

A rare genetic congenital muscular dystrophy due to extracellular matrix protein anomaly. The disease has characteristics of early motor development delay and muscle weakness with mild elevation of serum creatine kinase that may be followed by progressive disease course with predominantly proximal muscle weakness and atrophy, motor development regress, scoliosis and respiratory insufficiency. There is evidence this disease is caused by compound heterozygous mutation in the ITGA7 gene on chromosome 12q13.

CDG IIe is caused by a mutation that impairs the integrity of the conserved oligomeric Golgi (COG) complex and alters Golgi trafficking, resulting in the disruption of multiple glycosylation pathways. For a general discussion of CDGs, see CDG1A (212065).

Cystinosis comprises three allelic phenotypes: Nephropathic cystinosis in untreated children is characterized by renal Fanconi syndrome, poor growth, hypophosphatemic/calcipenic rickets, impaired glomerular function resulting in complete glomerular failure, and accumulation of cystine in almost all cells, leading to cellular dysfunction with tissue and organ impairment. The typical untreated child has short stature, rickets, and photophobia. Failure to thrive is generally noticed after approximately age six months; signs of renal tubular Fanconi syndrome (polyuria, polydipsia, dehydration, and acidosis) appear as early as age six months; corneal crystals can be present before age one year and are always present after age 16 months. Prior to the use of renal transplantation and cystine-depleting therapy, the life span in nephropathic cystinosis was no longer than ten years. With these interventions, affected individuals can survive at least into the mid-forties or fifties with satisfactory quality of life. Intermediate cystinosis is characterized by all the typical manifestations of nephropathic cystinosis, but onset is at a later age. Renal glomerular failure occurs in all untreated affected individuals, usually between ages 15 and 25 years. The non-nephropathic (ocular) form of cystinosis is characterized clinically only by photophobia resulting from corneal cystine crystal accumulation.

Camurati-Engelmann Disease not associated with TGFB1. This is an n-of-1 use case where only one patient or family has been described with this disorder.

In a report on the 37th ENMC Workshop, Rudel and Lehmann-Horn (1997) stated that the sodium channelopathies can be divided into 3 different forms: paramyotonia, potassium-aggravated myotonia, and periodic paralysis. Potassium-aggravated myotonia includes mild myotonia fluctuans, severe myotonia permanens, and acetazolamide-responsive myotonia.

Microcephaly, seizures, and developmental delay (MCSZ) is an autosomal recessive neurodevelopmental disorder with onset in infancy. There is a range of phenotypic severity: some patients develop refractory seizures in infancy, consistent with a developmental and epileptic encephalopathy (DEE), whereas others have more well-controlled seizures and a more protracted course associated with cerebellar atrophy and peripheral neuropathy (Shen et al., 2010 and Poulton et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

A rare mitochondrial disease due to a defect in mitochondrial protein synthesis with a variable phenotype that includes onset in infancy or early childhood of failure to thrive and psychomotor regression (after initial normal development), as well as ocular manifestations (such as ptosis, nystagmus, optic atrophy, ophthalmoplegia and reduced vision). Additional manifestations include bulbar paresis with facial weakness, hypotonia, difficulty chewing, dysphagia, mild dysarthria, ataxia, global muscle atrophy, and areflexia. It has a relatively slow disease progression with patients often living into the third decade of life.

Thiamine metabolism dysfunction syndrome-4 (THMD4) is an autosomal recessive metabolic disorder characterized by childhood onset of episodic encephalopathy, often associated with a febrile illness, and causing transient neurologic dysfunction. Most patients recover fully, but some may have mild residual weakness. Affected individuals also develop a slowly progressive axonal polyneuropathy beginning in childhood. Brain imaging during the acute episodes shows lesions consistent with bilateral striatal degeneration or necrosis (summary by Spiegel et al., 2009). For a discussion of genetic heterogeneity of disorders due to thiamine metabolism dysfunction, see THMD1 (249270).

Autosomal recessive limb-girdle muscular dystrophy-17 (LGMDR17) is characterized by early childhood onset of proximal muscle weakness and atrophy without skin involvement. One family has shown rapid progression of the disorder in adolescence (summary by Gundesli et al., 2010). For a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy, see LGMDR1 (253600).

SUCLG1-related mitochondrial DNA (mtDNA) depletion syndrome, encephalomyopathic form with methylmalonic aciduria is characterized in the majority of affected newborns by hypotonia, muscle atrophy, feeding difficulties, and lactic acidosis. Affected infants commonly manifest developmental delay / cognitive impairment, growth retardation / failure to thrive, hepatopathy, sensorineural hearing impairment, dystonia, and hypertonia. Notable findings in some affected individuals include hypertrophic cardiomyopathy, epilepsy, myoclonus, microcephaly, sleep disturbance, rhabdomyolysis, contractures, hypothermia, and/or hypoglycemia. Life span is shortened, with median survival of 20 months.

Combined oxidative phosphorylation deficiency-6 (COXPD6) is an X-linked recessive severe encephalomyopathic disorder with onset in utero or in infancy. Affected patients have hypotonia and severely impaired psychomotor development associated with variably decreased enzymatic activity of mitochondrial respiratory complexes in skeletal muscle or fibroblasts. More variable features may include sensorimotor neuropathy, seizures, severe muscle weakness, abnormal signals in the basal ganglia, hypertrophic cardiomyopathy, deafness, swallowing difficulties, and respiratory insufficiency. Death in childhood may occur (summary by Berger et al., 2011). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Multiple congenital anomalies-hypotonia-seizures syndrome-2 (MCAHS2) is an X-linked recessive neurodevelopmental disorder characterized by dysmorphic features, neonatal hypotonia, early-onset myoclonic seizures, and variable congenital anomalies involving the central nervous, cardiac, and urinary systems. Some affected individuals die in infancy (summary by Johnston et al., 2012). The phenotype shows clinical variability with regard to severity and extraneurologic features. However, most patients present in infancy with early-onset epileptic encephalopathy associated with developmental arrest and subsequent severe neurologic disability; these features are consistent with a form of developmental and epileptic encephalopathy (DEE) (summary by Belet et al., 2014, Kato et al., 2014). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of nomenclature and genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Williams distal myopathy is an autosomal dominant slowly progressive muscular disorder characterized by distal muscle weakness and atrophy affecting the upper and lower limbs. Onset occurs around the third to fourth decades of life, and patients remain ambulatory even after long disease duration. Muscle biopsy shows nonspecific changes with no evidence of rods, necrosis, or inflammation (summary by Duff et al., 2011). Mutation in the FLNC gene can also cause myofibrillar myopathy-5 (MFM5; 609524), which shows a different pattern of muscle involvement and different histologic changes.

IMD31C is a disorder of immunologic dysregulation with highly variable manifestations resulting from autosomal dominant gain-of-function mutations in STAT1 (600555). Most patients present in infancy or early childhood with chronic mucocutaneous candidiasis (CMC). Other highly variable features include recurrent bacterial, viral, fungal, and mycoplasmal infections, disseminated dimorphic fungal infections, enteropathy with villous atrophy, and autoimmune disorders, such as hypothyroidism or diabetes mellitus. A subset of patients show apparently nonimmunologic features, including osteopenia, delayed puberty, and intracranial aneurysms. Laboratory studies show increased activation of gamma-interferon (IFNG; 147570)-mediated inflammation (summary by Uzel et al., 2013 and Sampaio et al., 2013).

FKBP14 kyphoscoliotic Ehlers-Danlos syndrome (FKBP14-kEDS) is characterized by congenital muscle hypotonia and weakness (typically improving during childhood), progressive scoliosis, joint hypermobility, hyperelastic skin, gross motor developmental delay, myopathy, and hearing impairment. Most affected children achieve independent walking between ages two and four years. A decline of motor function in adulthood may be seen, but affected individuals are likely to be able to participate in activities of daily living in adulthood and maintain independent walking. Occasional features underlying systemic connective tissue involvement include aortic rupture and arterial dissection, subdural hygroma, insufficiency of cardiac valves, bluish sclerae, bladder diverticula, inguinal or umbilical herniae, and premature rupture of membranes during pregnancy. Rarer findings may include bifid uvula with submucous or frank cleft palate, speech/language delay without true cognitive impairment, and rectal prolapse.

Spinocerebellar ataxia-36 (SCA36) is a slowly progressive neurodegenerative disorder characterized by adult-onset gait ataxia, eye movement abnormalities, tongue fasciculations, and variable upper motor neuron signs. Some affected individuals may develop hearing loss (summary by Garcia-Murias et al., 2012). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Charcot-Marie-Tooth disease type 4F (CMT4F) is an autosomal recessive demyelinating neuropathy characterized by distal sensory impairment and distal muscle weakness and atrophy affecting the lower more than the upper limbs. Nerve conduction velocities are decreased and sural nerve biopsy shows loss of myelinated fibers. The age at onset is variable and can range from childhood to adult years. When the onset is in infancy, the phenotype is characterized as Dejerine-Sottas syndrome (DSS; 145900). For a phenotypic description and a discussion of genetic heterogeneity of autosomal recessive demyelinating Charcot-Marie-Tooth disease, see CMT4A (214400).

Emery-Dreifuss muscular dystrophy is a genetically heterogeneous muscular disease that presents with muscular dystrophy, joint contractures, and cardiomyopathy with conduction defects (summary by Liang et al., 2011). For a discussion of genetic heterogeneity of EDMD, see 310300.

EXOSC3 pontocerebellar hypoplasia (EXOSC3-PCH) is characterized by abnormalities in the posterior fossa and degeneration of the anterior horn cells. At birth, skeletal muscle weakness manifests as hypotonia (sometimes with congenital joint contractures) and poor feeding. In persons with prolonged survival, spasticity, dystonia, and seizures become evident. Within the first year of life respiratory insufficiency and swallowing difficulties are common. Intellectual disability is severe. Life expectancy ranges from a few weeks to adolescence. To date, 82 individuals (from 58 families) with EXOSC3-PCH have been described.

Any amyotrophic lateral sclerosis in which the cause of the disease is a mutation in the PFN1 gene.

Osteogenesis imperfecta (OI) is a connective tissue disorder characterized by bone fragility and low bone mass. Due to considerable phenotypic variability, Sillence et al. (1979) developed a classification of OI subtypes based on clinical features and disease severity: OI type I, with blue sclerae (166200); perinatal lethal OI type II, also known as congenital OI (166210); OI type III, a progressively deforming form with normal sclerae (259420); and OI type IV, with normal sclerae (166220). Most cases of OI are autosomal dominant with mutations in 1 of the 2 genes that code for type I collagen alpha chains, COL1A1 (120150) and COL1A2 (120160). Martinez-Glez et al. (2012) described osteogenesis imperfecta type XIII, an autosomal recessive form of the disorder characterized by normal teeth, faint blue sclerae, severe growth deficiency, borderline osteoporosis, and an average of 10 to 15 fractures a year affecting both upper and lower limbs and with severe bone deformity.

Lethal congenital contracture syndrome-4 (LCCS4) is a severe form of neuromuscular arthrogryposis characterized by contractures leading to various degrees of flexion or extension limitations evident at birth (Markus et al., 2012). For a general phenotypic description and discussion of genetic heterogeneity of LCCS, see LCCS1 (253310).

Pontocerebellar hypoplasia type 7 (PCH7) is a severe neurologic condition characterized by delayed psychomotor development, hypotonia, breathing abnormalities, and gonadal abnormalities (summary by Anderson et al., 2011). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

A rare subtype of autosomal dominant Charcot-Marie-Tooth disease type 2 with characteristics of adolescent to adulthood-onset of symmetrical, slowly progressive distal muscle weakness and atrophy (with a predominant weakness of the distal lower limbs) associated with reduced or absent deep tendon reflexes, pes cavus and mild to moderated deep sensory impairment. There is evidence this disease is caused by a heterozygous loss-of-function mutation in the DHTKD1 gene on chromosome 10p14.

CHCHD10-related disorders are characterized by a spectrum of adult-onset neurologic phenotypes that can include: Mitochondrial myopathy (may also be early onset): weakness, amyotrophy, exercise intolerance. Amyotrophic lateral sclerosis (ALS): progressive degeneration of upper motor neurons and lower motor neurons. Frontotemporal dementia (FTD): slowly progressive behavioral changes, language disturbances, cognitive decline, extrapyramidal signs. Late-onset spinal motor neuronopathy (SMA, Jokela type): weakness, cramps, and/or fasciculations; areflexia. Axonal Charcot-Marie-Tooth neuropathy: slowly progressive lower-leg muscle weakness and atrophy, small hand muscle weakness, loss of tendon reflexes, sensory abnormalities. Cerebellar ataxia: gait ataxia, kinetic ataxia (progressive loss of coordination of lower- and upper-limb movements), dysarthria/dysphagia, nystagmus, cerebellar oculomotor disorder. Because of the recent discovery of CHCHD10-related disorders and the limited number of affected individuals reported to date, the natural history of these disorders (except for SMAJ caused by the p.Gly66Val pathogenic variant) is largely unknown.

Mitochondrial complex III deficiency nuclear type 2 is an autosomal recessive severe neurodegenerative disorder that usually presents in childhood, but may show later onset, even in adulthood. Affected individuals have motor disability, with ataxia, apraxia, dystonia, and dysarthria, associated with necrotic lesions throughout the brain. Most patients also have cognitive impairment and axonal neuropathy and become severely disabled later in life (summary by Ghezzi et al., 2011). The disorder may present clinically as spinocerebellar ataxia or Leigh syndrome, or with psychiatric disturbances (Morino et al., 2014; Atwal, 2014; Nogueira et al., 2013). For a discussion of genetic heterogeneity of mitochondrial complex III deficiency, see MC3DN1 (124000).

A subtype of Charcot-Marie-Tooth type 4 with characteristics of childhood onset of slowly progressing, demyelinating sensorimotor neuropathy, focally folded myelin sheaths in nerve biopsy, reduced nerve conduction velocities and the typical Charcot-Marie-Tooth phenotype (i.e. distal muscle weakness and atrophy, and sensory loss). There is evidence this disease is caused by homozygous or compound heterozygous mutation in the SBF1 gene on chromosome 22q.

Multiple system atrophy (MSA) is a distinct clinicopathologic entity that manifests as a progressive adult-onset neurodegenerative disorder causing parkinsonism, cerebellar ataxia, and autonomic, urogenital, and pyramidal dysfunction in various combinations. Two main subtypes are recognized: 'subtype C,' characterized predominantly by cerebellar ataxia, and 'subtype P,' characterized predominantly by parkinsonism. MSA is characterized pathologically by the degeneration of striatonigral and olivopontocerebellar structures and glial cytoplasmic inclusions (GCIs) that consist of abnormally phosphorylated alpha-synuclein (SNCA; 163890) or tau (MAPT; 157140) (Gilman et al., 1998; Gilman et al., 2008; Scholz et al., 2009). 'Subtype C' of MSA has been reported to be more prevalent than 'subtype P' in the Japanese population (65-67% vs 33-35%), whereas 'subtype P' has been reported to be more prevalent than 'subtype C' in Europe (63% vs 34%) and North America (60% vs 13%, with 27% of cases unclassified) (summary by The Multiple-System Atrophy Research Collaboration, 2013). MSA is similar clinically and pathologically to Parkinson disease (PD; 168600) and Lewy body dementia (127750). See also PARK1 (168601), which is specifically caused by mutation in the SNCA gene. Pure autonomic failure manifests as orthostatic hypotension and other autonomic abnormalities without other neurologic involvement. Although there is some phenotypic overlap, the relationship of pure autonomic failure to MSA is unclear (Vanderhaeghen et al., 1970; Schatz, 1996).

Myoadenylate deaminase deficiency (MMDD) is an autosomal recessive condition that can manifest as exercise-induced muscle pain, occasionally associated with rhabdomyolysis and/or increased serum creatine kinase, or even infantile hypotonia. However, the finding of homozygous mutations among asymptomatic individuals have suggested to some (e.g., Verzijl et al., 1998) that AMPD1 deficiency may be a harmless entity (summary by Castro-Gago et al., 2011). Genetta et al. (2001) stated that AMPD1 deficiency is the most prevalent genetic disease in humans, the number of people heterozygous approaching 10% of Caucasians and individuals of African descent (Sabina et al., 1989). A small percentage of homozygous-deficient individuals, approximately 1.8% of the population, display symptoms of chronic fatigue and lost productivity as well as a predisposition to stress-related ailments, including heart disease and stroke, according to Genetta et al. (2001).

Mitochondrial DNA depletion syndrome-12B is an autosomal recessive mitochondrial disorder characterized by childhood onset of slowly progressive hypertrophic cardiomyopathy and generalized skeletal myopathy resulting in exercise intolerance, and, in some patients, muscle weakness and atrophy. Skeletal muscle biopsy shows ragged-red fibers, mtDNA depletion, and accumulation of abnormal mitochondria (summary by Echaniz-Laguna et al., 2012). For a discussion of genetic heterogeneity of mtDNA depletion syndromes, see MTDPS1 (603041).

Infantile hypotonia with psychomotor retardation and characteristic facies (IHPRF) is a severe autosomal recessive neurologic disorder with onset at birth or in early infancy. Affected individuals show very poor, if any, normal cognitive development. Some patients are never learn to sit or walk independently (summary by Al-Sayed et al., 2013). Genetic Heterogeneity of Infantile Hypotonia with Psychomotor Retardation and Characteristic Facies See also IHPRF2 (616801), caused by mutation in the UNC80 gene (612636) on chromosome 2q34; and IHPRF3 (616900), caused by mutation in the TBCK gene (616899) on chromosome 4q24.

Inclusion body myopathy associated with Paget disease of bone (PDB) and/or frontotemporal dementia (IBMPFD) is characterized by adult-onset proximal and distal muscle weakness (clinically resembling a limb-girdle muscular dystrophy syndrome), early-onset PDB, and premature frontotemporal dementia (FTD). Muscle weakness progresses to involve other limb and respiratory muscles. PDB involves focal areas of increased bone turnover that typically lead to spine and/or hip pain and localized enlargement and deformity of the long bones; pathologic fractures occur on occasion. Early stages of FTD are characterized by dysnomia, dyscalculia, comprehension deficits, and paraphasic errors, with minimal impairment of episodic memory; later stages are characterized by inability to speak, auditory comprehension deficits for even one-step commands, alexia, and agraphia. Mean age at diagnosis for muscle disease and PDB is 42 years; for FTD, 56 years. Dilated cardiomyopathy, amyotrophic lateral sclerosis, and Parkinson disease are now known to be part of the spectrum of findings associated with IBMPFD.

FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome is a multi-system disorder characterized primarily by congenital or early-onset lactic acidosis and growth failure, feeding difficulty, hypotonia, and developmental delay. Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes. All affected individuals alive at the time they were reported (median age: 3.5 years) demonstrated significant developmental delay. Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia). Survival varies; the median age of reported deaths was two years (range 2 days – 75 months), although surviving individuals as old as 36 years have been reported. To date FBXL4-related mtDNA depletion syndrome has been reported in 50 individuals.

Combined oxidative phosphorylation deficiency-18 (COXPD18) is an autosomal recessive disorder of mitochondrial function characterized by intrauterine growth retardation, hypotonia, visual impairment, speech delay, and lactic acidosis associated with decreased mitochondrial respiratory chain activity. Affected patients may also show hematologic abnormalities, mainly macrocytic anemia (summary by Hildick-Smith et al., 2013). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Spastic paraplegia-64 (SPG64) is a neurologic disorder characterized by childhood onset of progressive spastic paraplegia with impaired intellectual development, gait impairment, dysarthria, and white matter abnormalities on brain imaging. Some individuals show neurocognitive regression (Calame et al., 2022). For a discussion of genetic heterogeneity of autosomal recessive SPG, see SPG5A (270800).

An extremely rare and complex form of hereditary spastic paraplegia with characteristics of onset in infancy of spastic paraplegia (presenting with delayed walking and a scissors gait) associated with short stature and normal cognition. Periventricular deep white matter changes in the corpus callosum are noted on brain imaging. SPG63 is caused by a homozygous mutation in the AMPD2 gene (1p13.3) encoding AMP deaminase 2.

Hereditary fibrosing poikiloderma with tendon contractures, myopathy, and pulmonary fibrosis (POIKTMP) is characterized by the skin findings of poikiloderma (typically beginning in the first six months and mainly localized to the face), hypohidrosis with heat intolerance, mild lymphedema of the extremities, chronic erythematous and scaly skin lesions on the extremities, sclerosis of the digits, and mild palmoplantar keratoderma. Scalp hair, eyelashes, and/or eyebrows are typically sparse. Muscle contractures are usually seen in childhood and can be present as early as age two years. The majority of affected individuals develop progressive weakness of the proximal and distal muscles of all four limbs. Some adults develop progressive interstitial pulmonary fibrosis, which can be life threatening within three to four years after respiratory symptoms appear. Other features are exocrine pancreatic insufficiency, liver impairment, hematologic abnormalities, relative short stature, and cataract.

A rare pure or complex form of hereditary spastic paraplegia with characteristics of onset in infancy of progressive lower limb spasticity, abnormal gait, increased deep tendon reflexes and extensor plantar responses that may be associated with intellectual disability. Additional signs such as contractures in the lower limbs, amyotrophy, clubfoot and optic atrophy, have also been reported. Caused by homozygous mutation in the NT5C2 gene on chromosome 10q24.

MPPH (megalencephaly-postaxial polydactyly-polymicrogyria-hydrocephalus) syndrome is a developmental brain disorder characterized by megalencephaly (brain overgrowth) with the cortical malformation bilateral perisylvian polymicrogyria (BPP). At birth the occipital frontal circumference (OFC) ranges from normal to 6 standard deviations (SD) above the mean for age, sex, and gestational age; in older individuals the range is from 3 to 10 SD above the mean. A variable degree of ventriculomegaly is seen in almost all children with MPPH syndrome; nearly 50% of individuals have frank hydrocephalus. Neurologic problems associated with BPP include oromotor dysfunction (100%), epilepsy (50%), and mild-to-severe intellectual disability (100%). Postaxial hexadactyly occurs in 50% of individuals with MPPH syndrome.

Neurodevelopmental disorder with dysmorphic features, spasticity, and brain abnormalities (NEDDSBA) is an autosomal recessive neurodevelopmental disorder characterized by severely delayed global development, with hypotonia, impaired intellectual development, and poor or absent speech. Most patients have spasticity with limb hypertonia and brisk tendon reflexes. Additional features include nonspecific dysmorphic facial features, structural brain abnormalities, and cortical visual impairment (summary by Bosch et al., 2015). Novarino et al. (2014) labeled the disorder 'spastic paraplegia-67' (SPG67). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Polyglucosan body myopathy-1 (PGBM1) is an autosomal recessive disorder characterized by onset in childhood of progressive proximal muscle weakness, resulting in difficulties in ambulation. Most patients also develop progressive dilated cardiomyopathy, which may necessitate cardiac transplant in severe cases. A small subset of patients present with severe immunodeficiency and a hyperinflammatory state in very early childhood (summary by Boisson et al., 2012 and Nilsson et al., 2013). Genetic Heterogeneity of Polyglucosan Body Myopathy See also PGBM2 (616199), caused by mutation in the GYG1 gene (603942) on chromosome 3q24.

STING-associated vasculopathy with onset in infancy is an autoinflammatory vasculopathy causing severe skin lesions, particularly affecting the face, ears, nose, and digits, and resulting in ulceration, eschar formation, necrosis, and, in some cases, amputation. Many patients have interstitial lung disease. Tissue biopsy and laboratory findings show a hyperinflammatory state, with evidence of increased beta-interferon (IFNB1; 147640) signaling (summary by Liu et al., 2014).

Familial partial lipodystrophy type 6 (FPLD6) is characterized by abnormal subcutaneous fat distribution, with variable excess accumulation of fat in the face, neck, shoulders, axillae, back, abdomen, and pubic region, and reduction in subcutaneous fat of the lower extremities. Progressive adult-onset myopathy is seen in some patients, and there is variable association with diabetes, hypertriglyceridemia, low high-density lipoprotein (HDL) cholesterol, and hepatic steatosis (Zolotov et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of familial partial lipodystrophy (FPLD), see 151660.

Pontocerebellar hypoplasia type 1C is a severe autosomal recessive neurodegenerative disorder characterized by severe muscle weakness and failure to thrive apparent in the first months of life. Affected infants showed delayed psychomotor development, often with visual and hearing impairment, and may die of respiratory failure. Brain imaging typically shows cerebellar hypoplasia, hypoplasia of the corpus callosum, and immature myelination (summary by Boczonadi et al., 2014). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Nemaline myopathy-10 (NEM10) is an autosomal recessive severe congenital myopathy characterized by early-onset generalized muscle weakness and hypotonia with respiratory insufficiency and feeding difficulties. Many patients present antenatally with decreased fetal movements, and most die of respiratory failure in early infancy (summary by Yuen et al., 2014). Patients with a stable and much milder disease course have been described (Schatz et al., 2018). For a discussion of genetic heterogeneity of nemaline myopathy, see NEM3 (161800).

Ruijs et al. (2003) reported a Moroccan boy with a chromosomal breakage who died of hepatocellular carcinoma at age 17 years. The boy was noted to have growth retardation at age 3 years; at age 7 he was found to have thoracic kyphosis, frontal bossing, and a delayed bone age of approximately 3 years. He underwent surgery for severe bilateral posterior subcapsular cataracts at age 14. Examination at age 15 showed short stature and low weight, with premature graying of scalp hair, small frontotemporal diameter, small deep-set eyes, bulbous nose with high nasal bridge, small upper lip, and micrognathia. In addition, he had thoracic kyphoscoliosis, sloping shoulders, mild pectus excavatum, moderate bilateral contractures of both elbows, bilateral clinodactyly, and pes planus. At age 17, he developed abdominal pain, and ultrasonography revealed a liver mass; biopsy confirmed hepatocellular carcinoma. Because of the advanced stage, no treatment was possible, and he died 2 months later. Although his parents were not known to be consanguineous, they originated from the same small Moroccan village. Lessel et al. (2014) studied 2 brothers from a nonconsanguineous Australian family of European ancestry who exhibited low body weight, micrognathia, triangular face, muscular atrophy, lipodystrophy, bilateral simian creases, delayed bone age, and mild joint restrictions in the fingers and elbows. In addition, both brothers developed early-onset hepatocellular carcinoma, at ages 16 and 14 years, respectively. The older brother died at age 18 from complications of acute fulminant hepatic failure. Analysis of patient tumor biopsies showed strong focal accumulations of cancer biomarkers as well as a high proliferative index compared to healthy liver or to cells from idiopathic hepatocellular carcinoma.

Combined oxidative phosphorylation deficiency-24 (COXPD24) is an autosomal recessive mitochondrial disorder with wide phenotypic variability. Most patients present in infancy with delayed neurodevelopment, refractory seizures, hypotonia, and hearing impairment due to auditory neuropathy. Less common features may include cortical blindness, renal dysfunction, and/or liver involvement, suggestive of Alpers syndrome (MTDPS4A; 203700). Patients with the severe phenotype tend to have brain abnormalities on imaging, including cerebral atrophy and hyperintensities in the basal ganglia and brainstem, consistent with Leigh syndrome. Laboratory values may be normal or show increased lactate and evidence of mitochondrial respiratory chain defects, particularly in muscle. Some patients achieve little developmental milestones and may die in infancy or early childhood. However, some patients have a less severe phenotype manifest only by myopathy (summary by Sofou et al., 2015, Vanlander et al., 2015, and Mizuguchi et al., 2017). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

Any mitochondrial DNA depletion syndrome in which the cause of the disease is a mutation in the OPA1 gene.

Spinal muscular atrophy with congenital bone fractures is an autosomal recessive severe neuromuscular disorder characterized by onset of severe hypotonia with fetal hypokinesia in utero. This results in congenital contractures, consistent with arthrogryposis multiplex congenita, and increased incidence of prenatal fracture of the long bones. Affected infants have difficulty breathing and feeding and often die in the first days or months of life (summary by Knierim et al., 2016). For a discussion of genetic heterogeneity of spinal muscular atrophy with congenital bone fractures, see SMABF1 (616866).

Scapulohumeroperoneal myopathy is an autosomal dominant muscle disorder characterized by slowly progressive muscle weakness and atrophy affecting both proximal and distal muscles of the upper and lower limbs. Onset is usually in the first decade and can be as early as infancy, although some patients do not notice symptoms until young adulthood. There is marked variability in severity (summary by Zukosky et al., 2015).

Congenital disorder of glycosylation type IIo (CDG2O) is an autosomal recessive metabolic disorder characterized by infantile onset of progressive liver failure, hypotonia, and delayed psychomotor development. Laboratory abnormalities include elevated liver enzymes, coagulation factor deficiencies, hypercholesterolemia, and low ceruloplasmin. Serum isoelectric focusing of proteins shows a combined defect of N- and O-glycosylation, suggestive of a Golgi defect (summary by Jansen et al., 2016). For a general discussion of CDGs, see CDG1A (212065).

Autosomal recessive muscular dystrophy with cardiomyopathy and triangular tongue (MDRCMTT) is an autosomal recessive muscle disorder characterized by onset of severe and progressive muscle weakness and atrophy in childhood, resulting in loss of independent ambulation. Patients may also have dilated cardiomyopathy and have macroglossia with a small tip, resulting in a triangular appearance of the tongue (summary by Warman Chardon et al., 2015).

Rhizomelic chondrodysplasia punctata (RCDP) is a peroxisomal disorder characterized by disproportionately short stature primarily affecting the proximal parts of the extremities, a typical facial appearance including a broad nasal bridge, epicanthus, high-arched palate, dysplastic external ears, and micrognathia, congenital contractures, characteristic ocular involvement, dwarfism, and severe mental retardation with spasticity. Biochemically, plasmalogen synthesis and phytanic acid alpha-oxidation are defective. Most patients die in the first decade of life (summary by Wanders and Waterham, 2005). For a discussion of genetic heterogeneity of rhizomelic chondrodysplasia punctata, see 215100.

Charcot-Marie-Tooth disease type 4K (CMT4K) is an autosomal recessive demyelinating peripheral neuropathy characterized by onset in the first decade of distal muscle weakness and atrophy associated with impaired distal sensation. Both upper and lower limbs are affected. Affected individuals may also have nystagmus and late-onset cerebellar ataxia. Laboratory studies show increased serum lactate and isolated mitochondrial complex IV deficiency (summary by Echaniz-Laguna et al., 2013). For a phenotypic description and a discussion of genetic heterogeneity of autosomal recessive demyelinating Charcot-Marie-Tooth disease, see CMT4A (214400).

Autosomal recessive progressive external ophthalmoplegia with mitochondrial DNA deletions-2 (PEOB2) is a mitochondrial disorder characterized by adult onset of progressive external ophthalmoplegia, exercise intolerance, muscle weakness, and signs and symptoms of spinocerebellar ataxia, such as impaired gait and dysarthria. Some patients may have respiratory insufficiency. Laboratory studies are consistent with a defect in mtDNA replication (summary by Reyes et al., 2015). For a discussion of genetic heterogeneity of autosomal recessive PEO, see PEOB1 (258450).

Frontotemporal dementia and/or amyotrophic lateral sclerosis-4 is an autosomal dominant neurodegenerative disorder characterized by adult or late adult onset of cognitive impairment, behavioral abnormalities, and speech apraxia and/or upper and lower motor neuron signs. The phenotype is highly variable (summary by Freischmidt et al., 2015). For a discussion of genetic heterogeneity of FTDALS, see FTDALS1 (105550).

Frontotemporal dementia and/or amyotrophic lateral sclerosis-3 is an autosomal dominant neurodegenerative disorder characterized by adult or late adult onset of cognitive impairment, behavioral abnormalities, and speech apraxia and/or upper and lower motor neuron signs. Some patients may also develop Paget disease of bone. The phenotype is highly variable, even within families (summary by Rea et al., 2014). For a discussion of genetic heterogeneity of FTDALS, see FTDALS1 (105550).

Hypomyelinating leukodystrophy-10 (HLD10) is an autosomal recessive neurologic disorder characterized by postnatal progressive microcephaly, severely delayed psychomotor development, and hypomyelination on brain imaging (summary by Nakayama et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

Slow-channel congenital myasthenic syndrome (SCCMS) is a disorder of the postsynaptic neuromuscular junction (NMJ) characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the acetylcholine receptor channel, specifically from prolonged opening and activity of the channel, which causes prolonged synaptic currents resulting in a depolarization block. This is associated with calcium overload, which may contribute to subsequent degeneration of the endplate and postsynaptic membrane. Treatment with quinine, quinidine, or fluoxetine may be helpful; cholinesterase inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Lethal congenital contracture syndrome-7, an axoglial form of arthrogryposis multiplex congenita (AMC), is characterized by congenital distal joint contractures, polyhydramnios, reduced fetal movements, and severe motor paralysis leading to death early in the neonatal period (Laquerriere et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of lethal congenital contracture syndrome, see LCCS1 (253310).

The X-linked otopalatodigital (X-OPD) spectrum disorders, characterized primarily by skeletal dysplasia, include the following: Otopalatodigital syndrome type 1 (OPD1). Otopalatodigital syndrome type 2 (OPD2). Frontometaphyseal dysplasia type 1 (FMD1). Melnick-Needles syndrome (MNS). Terminal osseous dysplasia with pigmentary skin defects (TODPD). In OPD1, most manifestations are present at birth; females can present with severity similar to affected males, although some have only mild manifestations. In OPD2, females are less severely affected than related affected males. Most males with OPD2 die during the first year of life, usually from thoracic hypoplasia resulting in pulmonary insufficiency. Males who live beyond the first year of life are usually developmentally delayed and require respiratory support and assistance with feeding. In FMD1, females are less severely affected than related affected males. Males do not experience a progressive skeletal dysplasia but may have joint contractures and hand and foot malformations. Progressive scoliosis is observed in both affected males and females. In MNS, wide phenotypic variability is observed; some individuals are diagnosed in adulthood, while others require respiratory support and have reduced longevity. MNS in males results in perinatal lethality in all recorded cases. TODPD, seen only in females, is characterized by a skeletal dysplasia that is most prominent in the digits, pigmentary defects of the skin, and recurrent digital fibromata.

Sialidosis is an autosomal recessive disorder characterized by the progressive lysosomal storage of sialylated glycopeptides and oligosaccharides caused by a deficiency of the enzyme neuraminidase. Common to the sialidoses is the accumulation and/or excretion of sialic acid (N-acetylneuraminic acid) covalently linked ('bound') to a variety of oligosaccharides and/or glycoproteins (summary by Lowden and O'Brien, 1979). The sialidoses are distinct from the sialurias in which there is storage and excretion of 'free' sialic acid, rather than 'bound' sialic acid; neuraminidase activity in sialuria is normal or elevated. Salla disease (604369) is a form of 'free' sialic acid disease. Classification Lowden and O'Brien (1979) provided a logical nosology of neuraminidase deficiency into sialidosis type I and type II. Type I is the milder form, also known as the 'normosomatic' type or the cherry red spot-myoclonus syndrome. Sialidosis type II is the more severe form with an earlier onset, and is also known as the 'dysmorphic' type. Type II has been subdivided into juvenile and infantile forms. Other terms for sialidosis type II are mucolipidosis I and lipomucopolysaccharidosis.

A pure or complex form of hereditary spastic paraplegia with characteristics of onset in the first decade of life of spastic paraparesis (more prominent in lower than upper extremities) and unsteady gait, as well as increased deep tendon reflexes, amyotrophy, cerebellar ataxia and flexion contractures of the knees in some.

Lissencephaly-8 (LIS8) is an autosomal recessive neurologic disorder characterized by delayed psychomotor development, intellectual disability with poor or absent speech, early-onset refractory seizures, and hypotonia. Brain imaging shows variable features, including cortical gyral abnormalities and hypoplasia of the corpus callosum, brainstem, and cerebellum (Jerber et al., 2016). For a general description and a discussion of genetic heterogeneity lissencephaly, see LIS1 (607432).

Autosomal recessive limb-girdle muscular dystrophy-21 (LGMDR21) is characterized by progressive limb-girdle weakness with age of onset ranging from congenital to adult. Muscle imaging shows a specific and selective pattern of fatty muscle degeneration (summary by Servian-Morilla et al., 2020). For a discussion of genetic heterogeneity of autosomal recessive LGMD, see LGMDR1 (253600).

Congenital myasthenic syndrome-20 is an autosomal recessive neuromuscular disorder characterized by severe hypotonia associated with episodic apnea soon after birth. Patients have muscle weakness resulting in delayed walking, ptosis, poor sucking and swallowing, and generalized limb fatigability and weakness. EMG studies usually show a decremental response to repetitive nerve stimulation, and some patients may show a good response to AChE inhibitors (summary by Bauche et al., 2016). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Myofibrillar myopathy-7 (MFM7) is an autosomal recessive muscle disorder characterized by early childhood onset of slowly progressive muscle weakness that primarily affects the lower limbs and is associated with joint contractures (summary by Straussberg et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

Autosomal recessive progressive external ophthalmoplegia with mitochondrial DNA deletions-4 (PEOB4) is characterized by adult onset of eye muscle weakness and proximal limb muscle weakness associated with deletions of mtDNA on skeletal muscle biopsy, which results from defective mtDNA replication in post-mitotic muscle tissue. Additional features are more variable (summary by Ronchi et al., 2012). For a discussion of genetic heterogeneity of autosomal recessive PEO, see PEOB1 (258450).

Any autosomal recessive progressive external ophthalmoplegia in which the cause of the disease is a mutation in the TK2 gene.

Developmental and epileptic encephalopathy-51 (DEE51) is an autosomal recessive severe neurodevelopmental disorder characterized by onset of intractable seizures and hypotonia in the first days or weeks of life. Affected individuals have severely delayed psychomotor development and may show abnormal movements. Brain imaging shows nonspecific abnormalities, such as cerebral atrophy, cerebellar atrophy, and delayed myelination. Laboratory studies showed increased lactate, suggesting mitochondrial dysfunction (summary by Ait-El-Mkadem et al., 2017). For a discussion of genetic heterogeneity of DEE, see 308350.

NMIHBA is a severe autosomal recessive neurodevelopmental and neurodegenerative disorder characterized by global developmental delay apparent from infancy and profoundly impaired intellectual development. Affected individuals have microcephaly with accompanying dysmorphic features, truncal hypotonia, peripheral spasticity, and lack of independent ambulation or speech acquisition. Brain imaging shows variable abnormalities, including cortical atrophy, thin corpus callosum, cerebellar hypoplasia, and delayed myelination (summary by Zollo et al., 2017).

Pontocerebellar hypoplasia type 11 (PCH11) is an autosomal recessive neurodevelopmental disorder characterized by severely delayed psychomotor development with impaired intellectual development and poor speech, microcephaly, dysmorphic features, and pontocerebellar hypoplasia on brain imaging. Additional features are more variable (summary by Marin-Valencia et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

NEMMLAS is an autosomal recessive multisystemic disorder characterized by delayed psychomotor development, intellectual disability, and abnormal motor function, including hypotonia, dystonia, ataxia, and spasticity. Patient tissues may show deficiencies in one or more of the mitochondrial oxidative phosphorylation (OXPHOS) enzymes, but this is not a constant finding (summary by Wortmann et al., 2017).

Centronuclear myopathy-6 with fiber-type disproportion (CNM6) is an autosomal recessive, slowly progressive congenital myopathy with onset in infancy or early childhood. Patients may be hypotonic at birth, but all show delayed motor development and walking difficulties due to muscle weakness mainly affecting the proximal lower and upper limbs. Other features include scapular winging, scoliosis, and mildly decreased respiratory vital capacity. The phenotype and muscle biopsy abnormalities are variable, although centralized nuclei and fiber-type disproportion appear to be a common finding on muscle biopsy (summary by Vasli et al., 2017). For a discussion of genetic heterogeneity of centronuclear myopathy, see CNM1 (160150).

Schwartz-Jampel syndrome type 1 (SJS1) is a rare autosomal recessive disorder characterized by muscle stiffness (myotonia) and chondrodysplasia. Affected individuals usually present in childhood with permanent muscle stiffness or bone deformities. Common clinical features include mask-like facies (narrow palpebral fissures, blepharospasm, and pursed lips); permanent muscle stiffness with continuous skeletal muscle activity recorded on electromyography; dwarfism; pectus carinatum; kyphoscoliosis; bowing of long bones; and epiphyseal, metaphyseal, and hip dysplasia. The disorder is slowly progressive but does not appear to alter life span (summary by Stum et al., 2006).

Amyotrophic lateral sclerosis-24 (ALS24) is a fatal neurodegenerative disease characterized by adult-onset loss of motor neurons (Brenner et al., 2016).

Combined oxidative phosphorylation deficiency-13 (COXPD13) is an autosomal recessive multisystem disorder resulting from mitochondrial dysfunction. Affected individuals develop severe neurologic impairment in the first months of life, including hypotonia, abnormal dystonic movements, hearing loss, poor feeding, global developmental delay, and abnormal eye movements. Brain imaging shows signal abnormalities in putamen, basal ganglia, caudate nuclei, or corpus callosum, as well as delayed myelination. Analysis of patient tissues shows multiple defects in enzymatic activities of the mitochondrial respiratory chain, although some tissues may show normal values since tissue expression of the mitochondrial defect and metabolic needs of specific tissues are variable (summary by Vedrenne et al., 2012). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Hypermethioninemia due to adenosine kinase deficiency is an autosomal recessive inborn error of metabolism characterized by global developmental delay, early-onset seizures, mild dysmorphic features, and characteristic biochemical anomalies, including persistent hypermethioninemia with increased levels of S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy); homocysteine is typically normal (summary by Bjursell et al., 2011).

Congenital hypomyelinating neuropathy-2 is an autosomal dominant neurologic disorder characterized by early-onset hypotonia, severely delayed motor development, muscle weakness with areflexia, and severely decreased nerve conduction velocities (NCV) resulting from improper myelination of axons. The severity is variable: some patients may present at birth with contractures and respiratory insufficiency, whereas others may achieve walking (summary by Warner et al., 1996). CHN shows significant phenotypic overlap with Dejerine-Sottas syndrome (DSS; 145900), which is also a neuropathy with early onset. Some classify the disorders differently, noting that CHN is characterized by hypo- or amyelination resulting from a congenital defect in myelin formation, whereas DSS has features of continuous myelin breakdown, with demyelination and remyelination (summary by Smit et al., 2008). For a discussion of genetic heterogeneity of CHN, see CHN1 (605253).

Proteasome-associated autoinflammatory syndrome-1 (PRAAS1) is an autosomal recessive disorder characterized by early childhood onset of annular erythematous plaques on the face and extremities with subsequent development of partial lipodystrophy and laboratory evidence of immune dysregulation. More variable features include recurrent fever, severe joint contractures, muscle weakness and atrophy, hepatosplenomegaly, basal ganglia calcifications, and microcytic anemia (summary by Agarwal et al., 2010; Kitamura et al., 2011; Arima et al., 2011). This disorder encompasses Nakajo-Nishimura syndrome (NKJO); joint contractures, muscular atrophy, microcytic anemia, and panniculitis-induced lipodystrophy (JMP syndrome); and chronic atypical neutrophilic dermatosis with lipodystrophy and elevated temperature syndrome (CANDLE). Among Japanese patients, this disorder is best described as Nakajo-Nishimura syndrome, since both Nakajo (1939) and Nishimura et al. (1950) contributed to the original phenotypic descriptions. Genetic Heterogeneity of Proteasome-Associated Autoinflammatory Syndrome See also PRAAS2 (618048), caused by mutation in the POMP gene (613386) on chromosome 13q12; PRAAS3 (617591), caused by mutation in the PSMB4 gene (602177) on chromosome 1q21; PRAAS4 (619183), caused by mutation in the PSMG2 gene (609702) on chromosome 18p11; PRAAS5 (619175), caused by mutation in the PSMB10 gene (176847) on chromosome 16q22; and PRAAS6 (620796), caused by mutation in the PSMB9 gene (177045) on chromosome 6p21.

Mitochondrial complex I deficiency nuclear type 23 (MC1DN23) is an autosomal recessive nuclear-encoded mitochondrial disease with clinical presentations ranging from movement disorder phenotypes (dystonia and/or spasticity) to isolated optic atrophy. MRI findings may include basal ganglia abnormalities or optic atrophy (summary by Magrinelli et al., 2022).

Childhood-onset neurodegeneration with cerebellar atrophy (CONDCA) is a severe autosomal recessive neurodevelopmental disorder affecting the central and peripheral nervous system. Patients present in the first year of life with global developmental delay, impaired intellectual development, poor or absent speech, and motor abnormalities. Brain imaging shows cerebellar atrophy. The severity is variable, but death in childhood may occur (Shashi et al., 2018).

SMALED2B is a severe neuromuscular disorder with onset in utero. Affected individuals show decreased fetal movements and are usually born with congenital contractures consistent with arthrogryposis multiplex congenita (AMC). After birth, they have severe hypotonia and muscle atrophy as well as respiratory insufficiency due to muscle weakness. Some patients may have dysmorphic facial features and/or abnormalities on brain imaging. Many patients die in early childhood (summary by Storbeck et al., 2017) For discussion of genetic heterogeneity of lower extremity-predominant spinal muscular atrophy, see SMALED1 (158600).

A very rare complex subtype of hereditary spastic paraplegia that presents in infancy with delayed motor development (crawling, walking) and has characteristics of lower limb spasticity, increased deep tendon reflexes, extensor plantar responses, impaired vibratory sensation at ankles, amyotrophy and borderline intellectual disability. Additional signs may include gait disturbances, Achilles tendon contractures, and scoliosis and cerebellar abnormalities.

Fetal akinesia deformation sequence-4 (FADS4) is an autosomal recessive disorder characterized by decreased fetal movements due to impaired neuromuscular function, resulting in significant congenital contractures and death in utero or soon after birth (summary by Bonnin et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of FADS, see 208150.

Congenital arthrogryposis with anterior horn cell disease (CAAHD) is an autosomal recessive neuromuscular disorder with highly variable severity. Affected individuals are usually noted to have contractures in utero on prenatal ultrasound studies, and present at birth with generalized contractures manifest as arthrogryposis multiplex congenita (AMC). Patients have severe hypotonia with respiratory insufficiency, often resulting in death in infancy or early childhood. Some patients may survive into later childhood with supportive care, but may be unable to walk or sit independently due to a combination of muscle weakness and contractures. Cognition may be normal. The disorder also includes multiple congenital anomalies associated with AMC and hypotonia, including high-arched palate, myopathic facies, and bulbar weakness. Neuropathologic studies demonstrate severe loss of anterior horn cells in the spinal cord, as well as diffuse motor neuron axonopathy (summary by Smith et al., 2017 and Tan et al., 2017). Distinction from Lethal Congenital Contracture Syndrome 1 Biallelic mutation in the GLE1 gene can also cause LCCS1, which is lethal in utero. However, distinguishing between LCCS1 and CAAHD is controversial. Smith et al. (2017) suggested that differentiating between the 2 disorders has limited utility, and that they may represent a genotype/phenotype correlation rather than 2 different disease entities. In contrast, Said et al. (2017) concluded that LCCS1 represents a distinct clinical entity in which all affected individuals die prenatally and exhibit no fetal movements. Vuopala et al. (1995) differentiated CAAHD from LCCS1, noting that both are prevalent in Finland. LCCS1 is always fatal during the fetal period, presenting with severe hydrops and intrauterine growth retardation. In LCCS1, the spinal cord is macroscopically thinned because of an early reduction of the anterior horn and a paucity of anterior horn cells. The skeletal muscles are extremely hypoplastic, even difficult to locate. Infants with CAAHD survive longer than those with LCCS1, and when present, hydrops and intrauterine growth retardation are mild. The macroscopic findings of the central nervous system and skeletal muscles are closer to normal, although microscopic analysis also shows degeneration of anterior horn cells. In addition, birthplaces of ancestors of affected individuals do not show clustering in the northeast part of Finland, as is the case with LCCS1.

Congenital myopathy-19 (CMYO19) is an autosomal recessive skeletal muscle disorder characterized by infantile-onset of progressive muscle weakness and atrophy associated with scoliosis, variably impaired walking, and dysmorphic facial features (Feichtinger et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Neurodevelopmental disorder with hypotonia and variable intellectual and behavioral abnormalities (NEDHIB) is characterized by early-onset hypotonia, delayed walking, poor speech, and impaired intellectual development. Additional features may include feeding difficulties, dysmorphic features, and visual defects. Brain imaging tends to show delayed myelination, thin corpus callosum, and/or enlarged ventricles. The severity of the disorder is highly variable; initial evidence suggests that the severity may depend on the type of mutation (summary by Haijes et al., 2019).

SPTLC1-related hereditary sensory neuropathy (HSN) is an axonal form of hereditary motor and sensory neuropathy distinguished by prominent early sensory loss and later positive sensory phenomena including dysesthesia and characteristic "lightning" or "shooting" pains. Loss of sensation can lead to painless injuries, which, if unrecognized, result in slow wound healing and subsequent osteomyelitis requiring distal amputations. Motor involvement is present in all advanced cases and can be severe. After age 20 years, the distal wasting and weakness may involve proximal muscles, possibly leading to wheelchair dependency by the seventh or eighth decade. Sensorineural hearing loss is variable.

Emery-Dreifuss muscular dystrophy inherited in an X-linked recessive pattern and caused by mutations in the EMD gene, encoding emerin.

Neurodevelopmental disorder with hypotonia, microcephaly, and seizures (NEDHYMS) is an autosomal recessive disorder characterized by global developmental delay with axial hypotonia, inability to sit or walk, and severely impaired intellectual development with absent language. Most patients develop early-onset intractable seizures that prevent normal development. Additional features include feeding difficulties with poor overall growth and microcephaly. Some patients may have spastic quadriplegia, poor eye contact due to cortical blindness, variable dysmorphic features, and nonspecific abnormalities on brain imaging (summary by Tan et al., 2020).

Facioscapulohumeral muscular dystrophy (FSHD) typically presents with weakness of the facial muscles, the stabilizers of the scapula, or the dorsiflexors of the foot. Severity is highly variable. Weakness is slowly progressive and approximately 20% of affected individuals eventually require a wheelchair. Life expectancy is not shortened.

Distal myopathy with rimmed vacuoles (DMRV) is an autosomal dominant myopathic disorder characterized by adult onset of muscle weakness affecting the distal upper and lower limbs, which may result in walking difficulties, as well as proximal weakness of the shoulder girdle muscles. Muscle biopsy shows rimmed vacuoles (summary by Bucelli et al., 2015).

Spinal muscular atrophy (SMA) is characterized by muscle weakness and atrophy resulting from progressive degeneration and irreversible loss of the anterior horn cells in the spinal cord (i.e., lower motor neurons) and the brain stem nuclei. The onset of weakness ranges from before birth to adulthood. The weakness is symmetric, proximal > distal, and progressive. Before the genetic basis of SMA was understood, it was classified into clinical subtypes based on maximum motor function achieved; however, it is now apparent that the phenotype of SMN1-associated SMA spans a continuum without clear delineation of subtypes. With supportive care only, poor weight gain with growth failure, restrictive lung disease, scoliosis, and joint contractures are common complications; however, newly available targeted treatment options are changing the natural history of this disease.

Frontotemporal dementia and/or amyotrophic lateral sclerosis-6 (FTDALS6) is an autosomal dominant neurodegenerative disorder with highly variable manifestations. Some patients present in adulthood with progressive FTD, often classified as the 'behavioral variant,' which is characterized by reduced empathy, impulsive behavior, personality changes, and reduced verbal output. Other patients present with features of amyotrophic lateral sclerosis (ALS), which is a fatal neurodegenerative disease characterized by upper and lower motor neuron dysfunction resulting in rapidly progressive paralysis and death from respiratory failure. The pathologic hallmarks of this disease include pallor of the corticospinal tract due to loss of motor neurons (in ALS). In both ALS and FTD, there are ubiquitin-positive inclusions within surviving neurons as well as deposition of pathologic TDP43 (TARDBP; 605078) or p62 (SQSTM1; 601530) aggregates. Patients with a D395G mutation (601023.0014) have been shown to develop pathologic tau (MAPT; 157140) aggregates. Some patients with the disorder may have features of both diseases, and there is significant interfamilial and intrafamilial phenotypic variability (summary by Johnson et al., 2010; Wong et al., 2018; Al-Obeidi et al., 2018; Darwich et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of FTDALS, see FTDALS1 (105550).

Proteasome-associated autoinflammatory syndrome-4 (PRAAS4) is an autosomal recessive immunologic disorder characterized by onset of panniculitis and erythematous skin lesions in early infancy. Additional features include hepatosplenomegaly, lymphadenopathy, fever, generalized lipodystrophy, myositis, and joint contractures, as well as delayed motor and speech development. Autoimmune features, such as hemolytic anemia, may also occur. Laboratory studies show elevation of acute phase reactants and abnormal activation of the type I interferon response. Treatment with the JAK (see 147795) inhibitor ruxolitinib may result in clinical improvement (summary by de Jesus et al., 2019). For a discussion of genetic heterogeneity of PRAAS, see PRAAS1 (256040).

Pontocerebellar hypoplasia type 16 (PCH16) is an autosomal recessive severe neurodevelopmental disorder characterized by hypotonia and severe global developmental delay apparent from early infancy. Although the severity of the disorder is variable, most affected individuals achieve only a few, if any, developmental milestones. Most are unable to walk or speak, have eye abnormalities with poor visual contact, and develop early-onset epilepsy. Other features may include stereotypic movements, spasticity, and progressive microcephaly. Brain imaging shows pontocerebellar hypoplasia, often with thin corpus callosum, atrophy of the thalamus and basal ganglia, enlarged ventricles, and white matter abnormalities (summary by Ucuncu et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Autosomal recessive limb-girdle muscular dystrophy-27 (LGMDR27) is characterized by progressive muscle weakness primarily affecting the lower limbs and resulting in walking difficulty or loss of ambulation. The age at onset is highly variable, from infancy to young adulthood. Patients with infantile onset may have a more severe disease course with rapid progression. Upper limb involvement and distal muscle weakness may also occur. Additional more variable features include neck muscle weakness, scoliosis, and joint contractures. Less common features include impaired intellectual development or speech delay, cardiomyopathy, and cardiac arrhythmia. Muscle biopsy shows nonspecific dystrophic changes (Coppens et al., 2021). For a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy, see LGMDR1 (253600).

PEBAT is an autosomal recessive neurodevelopmental disorder characterized by severely delayed psychomotor development apparent soon after birth or in infancy, profound intellectual disability, poor or absent speech, and seizures. Most patients are never able to walk due to hypotonia or spasticity. Brain imaging shows cerebral and cerebellar atrophy, thin corpus callosum, and secondary hypomyelination. The disorder shows progressive features, including microcephaly, consistent with a neurodegenerative process (summary by Miyake et al., 2016; Flex et al., 2016).

Spastic paraplegia-76 is an autosomal recessive neurologic disorder characterized by young-adult onset of slowly progressive spasticity of the lower limbs resulting in gait difficulties. Most affected individuals have upper limb involvement and additional features such as foot deformities and dysarthria. Cognition is unaffected (summary by Gan-Or et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see SPG5A (270800).

Autosomal recessive limb-girdle muscular dystrophy-25 (LGMDR25) is characterized by slowly progressive onset of proximal lower limb weakness in adulthood. Affected individuals also develop cardiac arrhythmias resulting in syncopal episodes as young adults or later in life (summary by Schindler et al., 2016). For a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy (LGMD), see LGMDR1 (253600).

Autosomal recessive SPG9B is a neurologic disorder characterized by early-onset complex spastic paraplegia. Affected individuals had delayed psychomotor development, intellectual disability, and severe motor impairment. More variable features include dysmorphic facial features, tremor, and urinary incontinence (summary by Coutelier et al., 2015). For a discussion of genetic heterogeneity of autosomal recessive SPG, see SPG5A (270800).

A pure form of hereditary spastic paraplegia with characteristics of adult onset of crural spastic paraparesis, hyperreflexia, extensor plantar responses, proximal muscle weakness, mild muscle atrophy, decreased vibration sensation at ankles and mild urinary dysfunction. Foot deformities have been reported to eventually occur in some patients. No abnormalities are noted on brain magnetic resonance imaging and peripheral nerve conduction velocity studies.

Autosomal recessive spinocerebellar ataxia-21 (SCAR21) is a neurologic disorder characterized by onset of cerebellar ataxia associated with cerebellar atrophy in early childhood. Affected individuals also have recurrent episodes of liver failure in the first decade, resulting in chronic liver fibrosis, as well as later onset of a peripheral neuropathy. Mild learning disabilities may also occur (summary by Schmidt et al., 2015). The phenotype is highly variable: all patients appear to have episodic and severe liver dysfunction in early childhood that tends to resolve with age. Affected individuals also show mild developmental or language delay and/or later onset of variable neurologic features, such as motor dysfunction (summary by Lenz et al., 2018).

Carey-Fineman-Ziter syndrome-1 (CFZS1) is a multisystem congenital disorder characterized by hypotonia, Moebius sequence (bilateral congenital facial palsy with impairment of ocular abduction), Pierre Robin complex (micrognathia, glossoptosis, and high-arched or cleft palate), delayed motor milestones, and failure to thrive. More variable features include dysmorphic facial features, brain abnormalities, and intellectual disability. It has been postulated that many clinical features in CFZS1 may be secondary effects of muscle weakness during development or brainstem anomalies (summary by Pasetti et al., 2016). Di Gioia et al. (2017) determined that CFZS1 represents a slowly progressive congenital myopathy resulting from a defect in myoblast fusion. Genetic Heterogeneity of Carey-Fineman-Ziter Syndrome Carey-Fineman-Ziter syndrome-2 (CFZS2) is caused by mutation in the MYMX gene (619912) on chromosome 6p21.

Combined oxidative phosphorylation deficiency-55 (COXPD55) is characterized by global developmental delay, hypotonia, short stature, and impaired intellectual development with speech disabilities in childhood. Indolent progressive external ophthalmoplegia phenotype has been described in 1 patient (summary by Olahova et al., 2021). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Tessadori-Bicknell-van Haaften neurodevelopmental syndrome-2 (TEBIVANED2) is characterized by poor overall growth, profound global developmental delay with absent speech, and characteristic dysmorphic facial features, including hypertelorism, abnormal nose, and wide mouth (Tessadori et al., 2020). For a discussion of genetic heterogeneity of Tessadori-Bicknell-van Haaften neurodevelopmental syndrome, see TEBIVANED1 (619758).

NMAN is an autosomal recessive neurologic disorder characterized by onset in the first or second decade of a peripheral axonal neuropathy predominantly affecting motor more than sensory nerves. The axonal neuropathy is reminiscent of Charcot-Marie-Tooth disease type 2 (see, e.g., CMT2A1, 118210) and distal hereditary motor neuropathy (see, e.g., HMND1, 182960). Individuals with NMAN also have delayed muscle relaxation and action myotonia associated with neuromyotonic discharges on needle EMG resulting from hyperexcitability of the peripheral nerves (summary by Zimon et al., 2012).

Autosomal recessive distal hereditary motor neuronopathy-6 (HMNR6) is a neuromuscular disorder characterized by onset of distal muscle weakness in early infancy. Affected individuals often present at birth with distal joint contractures or foot deformities and show delayed motor development, often with inability to walk or frequent falls. Hypo- or hyperreflexia may be observed; limb muscle atrophy may also be present. Patients often show respiratory distress or diaphragmatic palsy. Electrophysiologic studies are consistent with a peripheral motor neuropathy without sensory involvement (Maroofian et al., 2019). For a discussion of genetic heterogeneity of autosomal recessive distal HMN, see HMNR1 (604320).

Childhood-onset neurodegeneration with multisystem involvement due to mitochondrial dysfunction (CONDMIM) is an autosomal recessive syndromic disorder characterized primarily by neurologic deficits. Patients show global developmental delay and variably impaired intellectual development with speech delay apparent from infancy. Affected individuals have hypotonia, poor feeding, poor overall growth, and respiratory distress early in life. Other features include visual impairment due to optic atrophy, sensorineural hearing loss, and neuromuscular abnormalities. The severity is highly variable. The disorder is progressive; about half of patients show developmental regression with loss of previous skills. Features suggestive of a mitochondrial disorder include cataracts, cardiomyopathy, diabetes mellitus, combined oxidative phosphorylation deficiency, and increased lactate. Some patients develop seizures, some have dysmorphic facial features, and some have nonspecific abnormalities on brain imaging. Death in childhood may occur (Kaiyrzhanov et al., 2022).

Myopathy with myalgia, increased serum creatine kinase, and with or without episodic rhabdomyolysis-1 (MMCKR1) is an autosomal recessive skeletal muscle disorder characterized by the onset of muscle cramping and stiffness on exertion in infancy or early childhood, although later (even adult) onset has also been reported. The features remit with rest, but some individuals develop mild proximal or distal muscle weakness. Rare affected individuals may demonstrate cardiac involvement, including left ventricular dysfunction or rhythm abnormalities. Laboratory studies show increased baseline serum creatine kinase levels with episodic spikes that may coincide with rhabdomyolysis. EMG shows myopathic changes, and muscle biopsy shows nonspecific myopathic or degenerative features (Lopes Abath Neto et al., 2021; Salzer-Sheelo et al., 2022). Genetic Heterogeneity of Myopathy with Myalgia, Increased Serum Creatine Kinase, and with or without Episodic Rhabdomyolysis MMCKR2 (620971) is caused by mutation in the DTNA gene (601239) on chromosome 18q12.

C9orf72 frontotemporal dementia and/or amyotrophic lateral sclerosis (C9orf72-FTD/ALS) is characterized most often by frontotemporal dementia (FTD) and upper and lower motor neuron disease (MND); however, atypical presentations also occur. Age at onset is usually between 50 and 64 years (range: 20-91 years) irrespective of the presenting manifestations, which may be pure FTD, pure amyotrophic lateral sclerosis (ALS), or a combination of the two phenotypes. The clinical presentation is highly heterogeneous and may differ between and within families, causing an unpredictable pattern and age of onset of clinical manifestations. The presence of MND correlates with an earlier age of onset and a worse overall prognosis.

Congenital myopathy-2C (CMYO2C) is an autosomal dominant disorder of the skeletal muscle characterized by severe congenital weakness usually resulting in death from respiratory failure in the first year or so of life. Patients present at birth with hypotonia, lack of antigravity movements, poor head control, and difficulties feeding or breathing, often requiring tube-feeding and mechanical ventilation. Decreased fetal movements may be observed in some cases. Of the patients with congenital myopathy caused by mutation in the ACTA1 gene, about 90% carry heterozygous mutations that are usually de novo and cause the severe infantile phenotype. Some patients with heterozygous mutations have a more typical and milder disease course with delayed motor development and proximal muscle weakness, but are able to achieve independent ambulation (CMYO2A; 161800). The severity of the disease most likely depends on the detrimental effect of the mutation, although there are probably additional modifying factors (Ryan et al., 2001; Laing et al., 2009; Sanoudou and Beggs, 2001; Agrawal et al., 2004; Nowak et al., 2013; Sewry et al., 2019; Laitila and Wallgren-Pettersson, 2021). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Sarcoplasmic body myopathy (MYOSB), also known as myoglobinopathy, is an autosomal dominant disorder characterized by adult-onset muscle weakness affecting the proximal and distal muscles. Affected individuals usually present with proximal and axial muscle weakness leading to gait disturbances, although some present with hand muscle weakness and atrophy. The disorder is slowly progressive, and patients may lose ambulation after a long disease course. Some individuals develop respiratory or cardiac symptoms, often needing nocturnal ventilation. Other more variable features may include neck muscle weakness and dysphagia; facial muscle weakness is uncommon (Olive et al., 2019; Hama et al., 2022).

Congenital myopathy-20 (CMYO20) is an autosomal recessive neuromuscular disorder that shows wide phenotypic variability. Some patients present in early childhood with proximal muscle weakness affecting the lower and upper limbs resulting in difficulties running and climbing, whereas others present soon after birth with congenital limb or distal contractures. Additional features may include dysmorphic facial features and global developmental delay. Skeletal muscle biopsy may show nemaline rods (Nilipour et al., 2018; Pehlivan et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

RECON progeroid syndrome (RECON) is a chromosomal instability disorder characterized by postnatal growth retardation, progeroid facial appearance, hypoplastic nose, prominent premaxilla, skin photosensitivity and xeroderma, muscle wasting with reduced subcutaneous fat, and slender elongated thumbs (Abu-Libdeh et al., 2022).

Autosomal dominant nemaline myopathy-5C (NEM5C) is a relatively mild skeletal muscle disorder with wide clinical variability, even within families. Affected individuals develop symptoms of muscle weakness in the first or second decades; those with earlier onset tend to have a more severe disease course. Features include difficulty walking on the heels, waddling gait, proximal muscle weakness affecting the upper and lower limbs, and Gowers sign. Additional features may include myopathic facies, high-arched palate, scoliosis or kyphosis, and ankle weakness. Patients remain ambulatory into late adulthood. Skeletal muscle biopsy shows hypotrophy of type 1 fibers, hypertrophy of type 2 fibers, fiber size variation, and myofibrillar disorganization. Nemaline rods in type 1 fibers are often observed, but are not always present (Konersman et al., 2017; Holling et al., 2022). For a discussion of genetic heterogeneity of nemaline myopathy, see NEM2 (256030).

Amyotrophic lateral sclerosis-28 (ALS28) is an autosomal dominant neurodegenerative disorder characterized by adult onset of slowly progressive limb muscle weakness and atrophy resulting in gait difficulties, loss of ambulation, and distal upper limb weakness. Facial involvement is rare, but some patients may have respiratory insufficiency. EMG and muscle biopsy show active and chronic denervation. Patient-derived motor neurons show accumulation of TDP43 (605078) and toxic intranuclear RNA accumulation (Kume et al., 2023). For discussion of genetic heterogeneity of amyotrophic lateral sclerosis, see ALS1 (105400).

Congenital myopathy-1A (CMYO1A) with susceptibility to malignant hyperthermia is an autosomal dominant disorder of skeletal muscle characterized by muscle weakness primarily affecting the proximal muscles of the lower limbs beginning in infancy or early childhood, although later onset of symptoms has been reported. There is significant phenotypic variability, even within families, and the wide clinical diversity most likely depends on the severity of the RYR1 mutation. The disorder is static or slowly progressive; affected individuals typically show delayed motor development and usually achieve independent walking, although many have difficulty running or climbing stairs. Additional features often include mild facial weakness, joint laxity, shoulder girdle weakness, and skeletal manifestations, such as dislocation of the hips, foot deformities, scoliosis, and Achilles tendon contractures. Some patients present with orthopedic deformities. Serum creatine kinase is usually not elevated. Respiratory involvement is rare and there is no central nervous system or cardiac involvement. Patients with dominant mutations in the RYR1 gene are at risk for malignant hyperthermia and both disorders may segregate in the same family. Historically, patients with congenital myopathy due to RYR1 mutations were diagnosed based on the finding of pathologic central cores (central core disease; CCD) on muscle biopsy, which represent areas that lack oxidative enzymes and mitochondrial activity in type 1 muscle fibers. However, additional pathologic findings may also be observed, including cores and rods, central nuclei, fiber type disproportion, multiminicores, and uniform type 1 fibers. These histopathologic features are not always specific to RYR1 myopathy and often change over time (Quinlivan et al., 2003; Jungbluth et al., 2007; Klein et al., 2012; Ogasawara and Nishino, 2021). Some patients with RYR1 mutations have pathologic findings on muscle biopsy, but are clinically asymptomatic (Shuaib et al., 1987; Quane et al., 1993). Rare patients with a more severe phenotype have been found to carry a heterozygous mutation in the RYR1 gene inherited from an unaffected parent. However, in these cases, there is a possibility of recessive inheritance (CMYO1B; 255320) with either a missed second RYR1 mutation in trans or a genomic rearrangement on the other allele that is undetectable on routine genomic sequencing, since the RYR1 gene is very large and genetic analysis may be difficult (Klein et al., 2012). Genetic Heterogeneity of Congenital Myopathy See also CMYO1B (255320), caused by mutation in the RYR1 gene (180901) on chromosome 19q13; CMYO2A (161800), CMYO2B (620265), and CMYO2C (620278), caused by mutation in the ACTA1 gene (102610) on chromosome 1q42; CMYO3 (602771), caused by mutation in the SELENON gene (606210) on chromosome 1p36; CMYO4A (255310) and CMYO4B (609284), caused by mutation in the TPM3 gene (191030) on chromosome 1q21; CMYO5 (611705), caused by mutation in the TTN gene (188840) on chromosome 2q31; CMYO6 (605637), caused by mutation in the MYH2 gene (160740) on chromosome 17p13; CMYO7A (608358) and CMYO7B (255160), caused by mutation in the MYH7 gene (160760) on chromosome 14q11; CMYO8 (618654), caused by mutation in the ACTN2 gene (102573) on chromosome 1q43; CMYO9A (618822) and CMYO9B (618823), caused by mutation in the FXR1 gene (600819) on chromosome 3q28; CMYO10A (614399) and CMYO10B (620249), caused by mutation in the MEGF10 gene (612453) on chromosome 5q23; CMYO11 (619967), caused by mutation in the HACD1 gene (610467) on chromosome 10p12; CMYO12 (612540), caused by mutation in the CNTN1 gene (600016) on chromosome 12q12; CMYO13 (255995), caused by mutation in the STAC3 gene (615521) on chromosome 12q13; CMYO14 (618414), caused by mutation in the MYL1 gene (160780) on chromosome 2q34; CMYO15 (620161), caused by mutation in the TNNC2 gene (191039) on chromosome 20q13; CMYO16 (618524), caused by mutation in the MYBPC1 gene (160794) on chromosome 12q23; CMYO17 (618975), caused by mutation in the MYOD1 gene (159970) on chromosome 11p15; CMYO18 (620246), caused by mutation in the CACNA1S gene (114208) on chromosome 1q32; CMYO19 (618578), caused by mutation in the PAX7 gene (167410) on chromosome 1p36; CMYO20 (620310), caused by mutation in the RYR3 gene (180903) on chromosome 15q13; CMYO21 (620326), caused by mutation in the DNAJB4 gene (611327) on chromosome 1p31; CMYO22A (620351) and CMYO22B (620369), both caused by mutation in the SCN4A gene (603967) on chromosome 17q23; CMYO23 (609285), caused by mutation in the TPM2 gene (190990) on chromosome 9p13; and CMYO24 (617336), caused by mutation in the MYPN gene (608517) on chromosome 10q21.

Autosomal dominant spastic paraplegia-91 with or without cerebellar ataxia (SPG91) is a highly variable neurologic disorder characterized by early-onset gait abnormalities due to spastic paraplegia of the lower limbs, sometimes with cerebellar ataxia. The age at onset is highly variable (congenital to young adult), although most patients have symptom onset in the first decade. Some patients present with a spastic paraplegia-predominant phenotype with significant pyramidal signs, whereas others present with an ataxic-predominant phenotype. In addition, although most patients have a more 'pure' phenotype restricted to gait abnormalities without additional features, others have a more 'complicated' phenotype with additional features such as sensory abnormalities, peripheral neuropathy, optic neuropathy, developmental delay, variably impaired intellectual development, and seizures. Many have normal brain imaging, but cerebellar atrophy may be observed in those with prominent cerebellar ataxia (Van de Vondel et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant spastic paraplegia, see SPG3A (182600).

Autosomal recessive distal hereditary motor neuronopathy-10 (HMNR10) is a slowly progressive disorder characterized by distal muscle weakness and atrophy predominantly affecting the lower limbs and resulting in gait abnormalities; upper limb involvement often occurs. Most individuals have juvenile or adult onset, but some may show earlier onset in infancy or childhood. Although most affected individuals have a pure distal motor neuropathy, some may also have signs of upper motor neuron disease, including pyramidal signs and hyperreflexia, and some may show mild sensory involvement or mild respiratory insufficiency. Foot deformities and calf atrophy are commonly observed. Intellectual development, cognitive function, and brain imaging are typically normal. Electrophysiologic studies are consistent with an axonal motor (sometimes sensorimotor) neuropathy. In general, patients with earlier onset have a more severe disorder with faster progression (summary by El-Bazzal et al., 2019; Demaegd et al., 2022). El-Bazzal et al. (2019) and Lazo and Morejon-Garcia (2023) noted that VRK1-related motor neuron disease is clinically heterogeneous and has been described by various clinical terms, including spinal muscular atrophy, distal spinal muscular atrophy, amyotrophic lateral sclerosis (ALS), juvenile-onset ALS, hereditary motor and sensory neuropathy, Charcot-Marie-Tooth disease, and pure distal motor neuropathy. VRK1 mutations result in functional insufficiency. For a discussion of genetic heterogeneity of autosomal recessive HMN, see HMNR1 (604320).

Congenital disorder of glycosylation type IIbb (CDG2BB) is an autosomal recessive disorder characterized by global developmental delay, severely impaired intellectual development, microcephaly, epilepsy, facial dysmorphism, and variable neurologic findings (Duan et al., 2023).

Immunodeficiency-115 with autoinflammation (IMD115) is an autosomal recessive disorder characterized by the onset of symptoms of immune dysregulation in early infancy. Affected individuals have immunodeficiency with recurrent bacterial, viral, and fungal infections, as well as autoinflammatory features, including arthritis and dermatitis. Some patients may have more systemic involvement, such as myopathy, gastrointestinal abnormalities, and anemia. Laboratory studies show variable B-cell and T-cell defects, sometimes with defective antibody responses and hypogammaglobulinemia (Boisson et al., 2015; Oda et al., 2019).

Childhood-onset neurodegeneration with cerebellar ataxia and cognitive decline (CONDCAC) is characterized by the onset of progressive gait and truncal ataxia in early childhood. Affected individuals have muscle weakness and atrophy and sensorimotor axonal neuropathy; some may lose ambulation. Additional features include cognitive decline or learning disabilities. Brain imaging shows cerebellar atrophy (Delle Vedove et al., 2022).

Autosomal recessive progressive external ophthalmoplegia-6 (PEOB6) is characterized by ptosis and ophthalmoplegia as well as other clinical manifestations and multiple mtDNA deletions in muscle (Shintaku et al., 2022). For a discussion of genetic heterogeneity of autosomal recessive PEO, see PEOB1 (258450).

Ullrich congenital muscular dystrophy-1 (UCMD1) is characterized by generalized muscle weakness and striking hypermobility of distal joints in conjunction with variable contractures of more proximal joints and normal intelligence. Additional findings may include kyphoscoliosis, protruded calcanei, and follicular hyperkeratosis. Some patients manifest at birth and never achieve independent ambulation, whereas others maintain ambulation into adulthood. Progressive scoliosis and deterioration of respiratory function is a typical feature (summary by Kirschner, 2013). For general phenotypic information and a discussion of genetic heterogeneity of Ullrich congenital muscular dystrophy, see UCMD1A (254090).

Neurodevelopmental disorder with hyperkinetic movements, seizures, and structural brain abnormalities (NEDMSB) is a severe autosomal recessive disorder characterized by failure to thrive in infancy, global developmental delay, hypotonia, motor abnormalities with inability to walk, involuntary movements, impaired intellectual development, absent speech, seizures, and structural brain abnormalities (Alkhater et al., 2018; Dafsari et al., 2022).

Thrombocytopenia-12 with or without myopathy (THC12) is an autosomal recessive disorder characterized by congenital thrombocytopenia apparent from infancy or early childhood. Most affected individuals have bleeding episodes, including petechiae, easy bruising, epistaxis, hematomas, menorrhagia, and increased bleeding after trauma or surgery, although rare patients may have thrombocytopenia without bleeding. Platelets are enlarged (macrothrombocytopenia), and there is an increase of circulating immature platelets, consistent with increased production. Patient platelets show hyposialylation due to GNE mutations, which causes increased removal of platelets from the circulation, shortened platelet lifespan, and resultant thrombocytopenia. In contrast to the thrombocytopenia, which is present since birth or early childhood, features of myopathy usually do not develop until the mid-twenties, similar to Nonaka myopathy (summary by Zhen et al., 2014, Izumi et al., 2014; Bottega et al., 2022). For a discussion of genetic heterogeneity of thrombocytopenia, see THC1 (313900).