Sialuria is a rare inborn error of metabolism in which excessive free sialic acid is synthesized. Clinical features include hepatosplenomegaly, coarse facial features, and varying degrees of developmental delay (summary by Enns et al., 2001).

Snyder-Robinson syndrome (SRS) is an X-linked intellectual disability syndrome characterized by asthenic build, facial dysmorphism with a prominent lower lip, kyphoscoliosis, osteoporosis, speech abnormalities, and seizures. Developmental delay usually presents as failure to meet early developmental milestones and then evolves to moderate to profound intellectual disability (which appears to remain stable over time) and variable motor disability. Asthenic habitus and low muscle mass usually develop during the first year, even in males who are ambulatory. During the first decade, males with SRS develop osteoporosis, resulting in fractures in the absence of trauma.

Van den Ende-Gupta syndrome (VDEGS) is an autosomal recessive disorder characterized by severe contractual arachnodactyly from birth and distinctive facial dysmorphism, including triangular face, malar hypoplasia, narrow nose, everted lips, and blepharophimosis. Skeletal anomalies include slender ribs, hooked clavicles, and dislocated radial head. There is no neurologic involvement (summary by Patel et al., 2014).

Eiken syndrome (EKNS) is an autosomal recessive skeletal dysplasia characterized by delayed ossification of bones, epiphyseal dysplasia, and bone remodeling abnormalities. Type A1 brachydactyly (see 112500), supernumerary epiphyses of proximal phalanges and metacarpals, and failure of eruption of primary teeth have also been described. Defining radiologic features include delayed ossification of epiphyses and primary ossification centers of short tubular bones, modeling abnormalities of tubular bones, and angel-shaped phalanges (Jacob et al., 2019). See 603740 for a disorder with similar radiologic features.

Raine syndrome (RNS) is a neonatal osteosclerotic bone dysplasia of early and aggressive onset that usually results in death within the first few weeks of life, although there have been some reports of survival into childhood. Radiographic studies show a generalized increase in the density of all bones and a marked increase in the ossification of the skull. The increased ossification of the basal structures of the skull and facial bones underlies the characteristic facial features, which include narrow prominent forehead, proptosis, depressed nasal bridge, and midface hypoplasia. Periosteal bone formation is also characteristic of this disorder and differentiates it from osteopetrosis and other known lethal and nonlethal osteosclerotic bone dysplasias. The periosteal bone formation typically extends along the diaphysis of long bones adjacent to areas of cellular soft tissue (summary by Simpson et al., 2009). Some patients survive infancy (Simpson et al., 2009; Fradin et al., 2011).

Wiedemann-Steiner syndrome (WSS) is characterized by developmental delay, intellectual disability, and characteristic facial features, with or without additional congenital anomalies. The facial features include thick eyebrows with lateral flare, vertically narrow and downslanted palpebral fissures, widely spaced eyes, long eyelashes, wide nasal bridge, broad nasal tip, thin vermilion of the upper lip, and thick scalp hair. About 60% of affected individuals have hypertrichosis cubiti ("hairy elbows"), which was once thought to be pathognomic for the syndrome, with a majority having hypertrichosis of other body parts. Other clinical features include feeding difficulties, prenatal and postnatal growth restriction, epilepsy, ophthalmologic anomalies, congenital heart defects, hand anomalies (such as brachydactyly and clinodactyly), hypotonia, vertebral anomalies (especially fusion anomalies of the cervical spine), renal and uterine anomalies, immune dysfunction, brain malformations, and dental anomalies.

KAT6B disorders include genitopatellar syndrome (GPS) and Say-Barber-Biesecker-Young-Simpson variant of Ohdo syndrome (SBBYSS) which are part of a broad phenotypic spectrum with variable expressivity; individuals presenting with a phenotype intermediate between GPS and SBBYSS have been reported. Both phenotypes are characterized by some degree of global developmental delay / intellectual disability; hypotonia; genital abnormalities; and skeletal abnormalities including patellar hypoplasia/agenesis, flexion contractures of the knees and/or hips, and anomalies of the digits, spine, and/or ribs. Congenital heart defects, small bowel malrotation, feeding difficulties, slow growth, cleft palate, hearing loss, and dental anomalies have been observed in individuals with either phenotype.

The acromesomelic dysplasias are disorders in which there is disproportionate shortening of skeletal elements, predominantly affecting the middle segments (forearms and forelegs) and distal segments (hands and feet) of the appendicular skeleton. Acromesomelic dysplasia-1 (AMD1) is characterized by severe dwarfism (height below 120 cm) with shortening of the middle and distal segments of the limbs. This condition is usually diagnosed at birth and becomes more obvious in the first 2 years of life. X-rays show short broad fingers, square flat feet, and shortening of the long bones (particularly the forearms). The radius is bowed; the ulna is shorter than the radius, and its distal end is occasionally hypoplastic. The skull is dolichocephalic and a shortness of the trunk, with decreased vertebral height and narrowing of the lumbar interpedicular distances, is consistently observed. Facial appearance and intelligence are normal (summary by Faivre et al., 2000). Genetic Heterogeneity of Acromesomelic Dysplasia Additional autosomal recessive forms of acromesomelic dysplasia include acromesomelic dysplasia-2A (200700), -2B (228900), and -2C (201250), all caused by mutation in the GDF5 gene (601146) on chromosome 20q11; AMD3 (200700), caused by mutation in the BMPR1B gene (603248) on chromosome 4q22; and AMD4 (619636), caused by mutation in the PRKG2 gene (601591) on chromosome 4q21. An autosomal dominant form of acromesomelic dysplasia has also been reported (see 112910).

Temple-Baraitser syndrome is a rare developmental disorder characterized by severe mental retardation and anomalies of the first ray of the upper and lower limbs with absence/hypoplasia of the nails. Most patients also have seizures; various dysmorphic facial features have been reported (summary by Jacquinet et al., 2010).

Spondylo-megaepiphyseal-metaphyseal dysplasia is a rare autosomal recessive skeletal dysplasia characterized by disproportionate short stature with a short and stiff neck and trunk; relatively long limbs that may show flexion contractures of the distal joints; delayed and impaired ossification of the vertebral bodies and the presence of large epiphyseal ossification centers and wide growth plates in the long tubular bones; and numerous pseudoepiphyses of the short tubular bones in hands and feet (summary by Hellemans et al., 2009).

Acrodysostosis-1 (ACRDYS1) is a form of skeletal dysplasia characterized by short stature, severe brachydactyly, facial dysostosis, and nasal hypoplasia. Affected individuals often have advanced bone age and obesity. Laboratory studies show resistance to multiple hormones, including parathyroid, thyrotropin, calcitonin, growth hormone-releasing hormone, and gonadotropin (summary by Linglart et al., 2011). However, not all patients show endocrine abnormalities (Lee et al., 2012). Genetic Heterogeneity of Acrodysostosis See also ACRDYS2 (614613), caused by mutation in the PDE4D gene (600129) on chromosome 5q12.

Miura-type epiphyseal chondrodysplasia (ECDM) is an overgrowth syndrome characterized by tall stature, arachnodactyly of the hands, macrodactyly of the great toes, scoliosis, coxa valga, and slipped capital femoral epiphysis (Miura et al., 2014). Multiple extra epiphyses are present in the hands (Boudin et al., 2018). Mutation in the NPR3 gene (108962) results in Boudin-Mortier syndrome (BOMOS; 619543), a similar phenotype of tall stature, arachnodactyly, elongated great toes, and multiple extra epiphyses.

Lamb-Shaffer syndrome is a neurodevelopmental disorder characterized by global developmental delay, intellectual disability, poor expressive speech, and mild dysmorphic facial features. Additional variable skeletal abnormalities may also be present (summary by Nesbitt et al., 2015).

Thauvin-Robinet-Faivre syndrome is an autosomal recessive disorder characterized by generalized overgrowth, mainly of height, and mildly delayed psychomotor development with mild or severe learning difficulties. More variable features may include congenital heart defects, kidney abnormalities, and skeletal defects. Patients may have an increased risk for Wilms tumor (summary by Akawi et al., 2016).

CAFDADD is a multisystemic developmental disorder with variable cardiac and digital anomalies and facial dysmorphism. Some patients may have seizures and ocular/aural abnormalities (Tokita et al., 2018).

Zimmermann-Laband syndrome-3 (ZLS3) is characterized by developmental delay, intellectual disability, coarse face, gingival hyperplasia, and nail hypoplasia/aplasia (Bauer et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of Zimmermann-Laband syndrome, see ZLS1 (135500).

SOFM is characterized by marked short stature, oligodontia, mild facial dysmorphism, and motor delay. Endosteal hyperostosis has also been observed, and patients may exhibit some features of progeria (Terhal et al., 2020; Beauregard-Lacroix et al., 2020).

Boudin-Mortier syndrome (BOMOS) is characterized by tall stature, arachnodactyly, disproportionately elongated great toes, and multiple extra epiphyses. Some patients also show joint hypermobility and dilation of the aortic root (Boudin et al., 2018). Mutation in the NPR2 gene (108961) results in a similar phenotype of increased stature and elongation of the digits, particularly of the great toes, with multiple extra epiphyses (epiphyseal chondrodysplasia, Miura type; 615923).

Neurodevelopmental disorder with hypotonia, impaired speech, and behavioral abnormalities (NEDHISB) is characterized by global developmental delay apparent since infancy or early childhood, hypotonia with delayed motor development, impaired intellectual development with significant speech delay or absent speech, and variable behavioral abnormalities, such as autism, repetitive actions, or aggression. About two-thirds of patients have early-onset seizures that range from intractable to self-limiting. More variable features include nonspecific dysmorphic facial features, distal skeletal anomalies, and brain imaging abnormalities. The phenotypic manifestations and severity are highly variable (Muir et al., 2021).

Neurodevelopmental disorder with poor growth, large ears, and dysmorphic facies (NEDGEF) is an autosomal recessive disorder characterized by these features as well as hypotonia and global developmental delay with impaired intellectual development. The severity is variable, even within families. Death in early childhood has been reported in 1 family (Alsaif et al., 2021).