An elevation of the level of the enzyme creatine kinase (also known as creatine phosphokinase (CK; EC 2.7.3.2) in the blood. CK levels can be elevated in a number of clinical disorders such as myocardial infarction, rhabdomyolysis, and muscular dystrophy.

Corticosteroid-binding globulin deficiency is a condition with subtle signs and symptoms, the most frequent being extreme tiredness (fatigue), especially after physical exertion. Many people with this condition have unusually low blood pressure (hypotension). Some affected individuals have a fatty liver or experience chronic pain, particularly in their muscles. These features vary among affected individuals, even those within the same family.\n\nMany people with corticosteroid-binding globulin deficiency have only one or two of these features; others have no signs and symptoms of the disorder and are only diagnosed after a relative is found to be affected.\n\nSome people with corticosteroid-binding globulin deficiency also have a condition called myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). The features of ME/CFS are prolonged fatigue that interferes with daily activities, as well as general symptoms, such as sore throat or headaches.

Dimethylglycine dehydrogenase deficiency (DMGDHD) is an inborn error of metabolism characterized by a fish-like odor, chronic fatigue, and increased level of the muscle form of creatine kinase in serum (Moolenaar et al., 1999).

Stormorken syndrome is an autosomal dominant disorder characterized by mild bleeding tendency due to platelet dysfunction, thrombocytopenia, anemia, asplenia, tubular aggregate myopathy, congenital miosis, and ichthyosis. Additional features may include headache or recurrent stroke-like episodes (summary by Misceo et al., 2014).

Four phenotypes comprise the RRM2B mitochondrial DNA maintenance defects (RRM2B-MDMDs): RRM2B encephalomyopathic MDMD, the most severe phenotype, usually manifesting shortly after birth as hypotonia, poor feeding, and faltering growth requiring hospitalization. Subsequent assessments are likely to reveal multisystem involvement including sensorineural hearing loss, renal tubulopathy, and respiratory failure. Autosomal dominant progressive external ophthalmoplegia (adPEO), typically adult onset; other manifestations can include ptosis, bulbar dysfunction, fatigue, and muscle weakness. RRM2B autosomal recessive progressive external ophthalmoplegia (arPEO), a typically childhood-onset predominantly myopathic phenotype of PEO, ptosis, proximal muscle weakness, and bulbar dysfunction. RRM2B mitochondrial neurogastrointestinal encephalopathy (MNGIE)-like, characterized by progressive ptosis, ophthalmoplegia, gastrointestinal dysmotility, cachexia, and peripheral neuropathy. To date, 78 individuals from 52 families with a molecularly confirmed RRM2B-MDMD have been reported.

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).

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. Treatment with acetylcholinesterase inhibitors or amifampridine may be helpful (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Mitochondrial complex I deficiency nuclear type 29 (MC1DN29) is an autosomal recessive metabolic disorder that usually presents in childhood, adolescence, or adulthood with exercise intolerance and easy fatigue with myalgias and muscle weakness. However, a severe multisystem presentation with chronic renal failure and cardiomyopathy in infancy has been reported (Sanchez-Caballero et al., 2016; Alston et al., 2016). For a discussion of genetic heterogeneity of mitochondrial complex I deficiency, see 252010.