Fabry disease is the most common of the lysosomal storage disorders and results from deficient activity of the enzyme alpha-galactosidase A (a-Gal A), leading to progressive lysosomal deposition of globotriaosylceramide and its derivatives in cells throughout the body. The classic form, occurring in males with less than 1% a-Gal A enzyme activity, usually has its onset in childhood or adolescence with periodic crises of severe pain in the extremities (acroparesthesia), the appearance of vascular cutaneous lesions (angiokeratomas), sweating abnormalities (anhidrosis, hypohidrosis, and rarely hyperhidrosis), characteristic corneal and lenticular opacities, and proteinuria. Gradual deterioration of renal function to end-stage renal disease (ESRD) usually occurs in men in the third to fifth decade. In middle age, most males successfully treated for ESRD develop cardiac and/or cerebrovascular disease, a major cause of morbidity and mortality. Heterozygous females typically have milder symptoms at a later age of onset than males. Rarely, females may be relatively asymptomatic throughout a normal life span or may have symptoms as severe as those observed in males with the classic phenotype. In contrast, late-onset forms occur in males with greater than 1% a-Gal A activity. Clinical manifestations include cardiac disease, which usually presents in the sixth to eighth decade with left ventricular hypertrophy, cardiomyopathy, arrhythmia, and proteinuria; renal failure, associated with ESRD but without the skin lesions or pain; or cerebrovascular disease presenting as stroke or transient ischemic attack.

Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease (MJD), is characterized by progressive cerebellar ataxia and variable findings including pyramidal signs, a dystonic-rigid extrapyramidal syndrome, significant peripheral amyotrophy and generalized areflexia, progressive external ophthalmoplegia, action-induced facial and lingual fasciculations, and bulging eyes. Neurologic findings tend to evolve as the disorder progresses.

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.

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.

Gitelman syndrome (GTLMNS) is an autosomal recessive renal tubular salt-wasting disorder characterized by hypokalemic metabolic alkalosis with hypomagnesemia and hypocalciuria. It is the most common renal tubular disorder among Caucasians (prevalence of 1 in 40,000). Most patients have onset of symptoms as adults, but some present in childhood. Clinical features include transient periods of muscle weakness and tetany, abdominal pains, and chondrocalcinosis (summary by Glaudemans et al., 2012). Gitelman syndrome is sometimes referred to as a mild variant of classic Bartter syndrome (607364). For a discussion of genetic heterogeneity of Bartter syndrome, see 607364.

Autosomal dominant hypocalcemia-1 is associated with low or normal serum parathyroid hormone concentrations (PTH). Approximately 50% of patients have mild or asymptomatic hypocalcemia; about 50% have paresthesias, carpopedal spasm, and seizures; about 10% have hypercalciuria with nephrocalcinosis or kidney stones; and more than 35% have ectopic and basal ganglia calcifications (summary by Nesbit et al., 2013). Thakker (2001) noted that patients with gain-of-function mutations in the CASR gene, resulting in generally asymptomatic hypocalcemia with hypercalciuria, have low-normal serum PTH concentrations and have often been diagnosed with hypoparathyroidism because of the insensitivity of earlier PTH assays. Because treatment with vitamin D to correct the hypocalcemia in these patients causes hypercalciuria, nephrocalcinosis, and renal impairment, these patients need to be distinguished from those with other forms of hypoparathyroidism (see 146200). Thakker (2001) suggested the designation 'autosomal dominant hypocalcemic hypercalciuria' for this CASR-related disorder. Genetic Heterogeneity of Autosomal Dominant Hypocalcemia Autosomal dominant hypocalcemia-2 (HYPOC2; 615361) is caused by mutation in the GNA11 gene (139313) on chromosome 19p13.

Amyotrophic lateral sclerosis-parkinsonism/dementia complex of Guam is a neurodegenerative disorder with unusually high incidence among the Chamorro people of Guam. Both ALS and parkinsonism-dementia are chronic, progressive, and uniformly fatal disorders in this population. Both diseases are known to occur in the same kindred, the same sibship, and even the same individual. See PARK7 (606324) for discussion of a similar phenotype caused by mutation in the DJ1 gene (602533).

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.

Satoyoshi syndrome is a rare disorder characterized by progressive, painful, intermittent muscle spasms, diarrhea or unusual malabsorption, endocrinopathy with amenorrhea, and secondary skeletal abnormalities. The disorder is also called komuragaeri disease by the Japanese; in Japanese 'komura' means calf and 'gaeri' means 'turnover' or spasm. All cases have apparently been sporadic, even when occurring in large families (Ehlayel and Lacassie, 1995).

Carnitine palmitoyltransferase II (CPT II) deficiency is a disorder of long-chain fatty-acid oxidation. The three clinical presentations are lethal neonatal form, severe infantile hepatocardiomuscular form, and myopathic form (which is usually mild and can manifest from infancy to adulthood). While the former two are severe multisystemic diseases characterized by liver failure with hypoketotic hypoglycemia, cardiomyopathy, seizures, and early death, the latter is characterized by exercise-induced muscle pain and weakness, sometimes associated with myoglobinuria. The myopathic form of CPT II deficiency is the most common disorder of lipid metabolism affecting skeletal muscle and the most frequent cause of hereditary myoglobinuria. Males are more likely to be affected than females.

'Familial pseudohyperkalemia' (PSHK) is a term that was coined to describe conditions in which a patient presents with pseudohyperkalemia as a result of a temperature-based abnormality in the transport of potassium (K) and sodium (Na) across the red cell membrane, in association with essentially normal hematology. PSHK can be considered to be the clinically benign, nonhemolytic cousin of hereditary stomatocytic leaky-cell, congenital hemolytic anemias (see 194380) (summary by Gore et al., 2002). For a discussion of clinical and genetic heterogeneity of the hereditary stomatocytoses, see 194380.

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 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.

Nivelon-Nivelon-Mabille syndrome (NNMS) is characterized by progressive microcephaly, vermis hypoplasia, and skeletal dysplasia. Variable features include infantile-onset seizures, dwarfism, generalized chondrodysplasia, and micromelia (Abdel-Salam et al., 2019).

Spinal and bulbar muscular atrophy (SBMA) is a gradually progressive neuromuscular disorder in which degeneration of lower motor neurons results in muscle weakness, muscle atrophy, and fasciculations. SBMA occurs only in males. Affected individuals often show gynecomastia, testicular atrophy, and reduced fertility as a result of mild androgen insensitivity.

MDGDC5 is an autosomal recessive muscular dystrophy characterized by variable age at onset, normal cognition, and no structural brain changes (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 muscular dystrophy-dystroglycanopathy type C, see MDDGC1 (609308).

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).

A form of axonal Charcot-Marie-Tooth disease, a peripheral sensorimotor neuropathy with symmetric weakness primarily occurring in the lower limbs and reaching the arms only after 5 to 10 years, occasional and predominantly distal sensory loss and reduced tendon reflexes. Presents with gait anomaly between the first and sixth decade and early onset is generally associated to a more severe phenotype that may include foot drop.

Most individuals with classic GBE1 adult polyglucosan body disease (GBE1-APBD) present after age 40 years with unexplained progressive neurogenic bladder, gait difficulties (i.e., spasticity and weakness) from mixed upper and lower motor neuron involvement, sensory loss predominantly in the distal lower extremities, autonomic dysfunction (associated with orthostatic hypotension and constipation), and mild cognitive difficulties (often executive dysfunction). Some affected individuals without classic GBE1-APBD have atypical phenotypes including Alzheimer disease-like dementia and axonal neuropathy, stroke-like episodes, and diaphragmatic failure; others may have a history of infantile liver disease.

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).

Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction (NMJ). Patients present clinically with onset of variable muscle weakness between infancy and adulthood. These disorders have been classified according to the location of the defect: presynaptic, synaptic, and postsynaptic. CMS10 is an autosomal recessive CMS resulting from a postsynaptic defect affecting endplate maintenance of the NMJ. Patients present with limb-girdle weakness in the first decade. Treatment with ephedrine or salbutamol may be beneficial; cholinesterase inhibitors should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Spinal muscular atrophy is characterized by degeneration of the anterior horn cells in the spinal cord, leading to symmetric muscle weakness and wasting. See also autosomal recessive adult-onset proximal spinal muscular atrophy (SMA4; 271150), caused by defect in the SMN1 gene (600354), and autosomal dominant childhood-onset proximal SMA (158600).

Bartter syndrome refers to a group of disorders that are unified by autosomal recessive transmission of impaired salt reabsorption in the thick ascending loop of Henle with pronounced salt wasting, hypokalemic metabolic alkalosis, and hypercalciuria. Clinical disease results from defective renal reabsorption of sodium chloride in the thick ascending limb (TAL) of the Henle loop, where 30% of filtered salt is normally reabsorbed (Simon et al., 1997). Patients with antenatal forms of Bartter syndrome typically present with premature birth associated with polyhydramnios and low birth weight and may develop life-threatening dehydration in the neonatal period. Patients with classic Bartter syndrome (see BARTS3, 607364) present later in life and may be sporadically asymptomatic or mildly symptomatic (summary by Simon et al., 1996 and Fremont and Chan, 2012). For a discussion of genetic heterogeneity of Bartter syndrome, see 607364.

Okinawa-type hereditary motor and sensory neuropathy (HMSNO) is an autosomal dominant neurodegenerative disorder characterized by young adult onset of proximal or distal muscle weakness and atrophy, muscle cramps, and fasciculations, with later onset of distal sensory impairment. The disorder is slowly progressive and clinically resembles amyotrophic lateral sclerosis (ALS; see 105400) (summary by Ishiura et al., 2012).

This syndrome is characterised by the association of acanthosis nigricans, insulin resistance, severe muscle cramps and acral hypertrophy.

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.

Familial hypomagnesemia with secondary hypocalcemia (HOMG1) is a rare autosomal recessive disorder characterized by very low serum magnesium levels. Hypocalcemia is a secondary consequence of parathyroid failure and parathyroid hormone resistance as a result of severe magnesium deficiency. The disease typically manifests during the first months of life with generalized convulsions or signs of increased neuromuscular excitability, such as muscle spasms or tetany. Untreated, the disease may be fatal or lead to severe neurologic damage. Treatment includes immediate administration of magnesium, usually intravenously, followed by life-long high-dose oral magnesium (review by Knoers, 2009). Genetic Heterogeneity of Hypomagnesemia A form of hypomagnesemia due to kidney defects and high urinary magnesium excretion associated with hypocalciuria (HOMG2; 154020) is caused by mutation in the FXYD2 gene (601814). Renal hypomagnesemia-3 (HOMG3; 248250), associated with hypercalciuria and nephrocalcinosis, is caused by mutation in the CLDN16 gene (603959). Renal hypomagnesemia-4 (HOMG4; 611718), which is normocalciuric, is caused by mutation in the EGF gene (131530). Renal hypomagnesemia-5 (HOMG5; 248190), associated with hypercalciuria, nephrocalcinosis, and severe ocular involvement, is caused by mutation in the CLDN19 gene (610036). Renal hypomagnesemia-6 (HOMG6; 613882) is caused by mutation in the CNNM2 gene (607803). Renal hypomagnesemia-7 with or without dilated cardiomyopathy (HOMG7; 620152) is caused by mutation in the RRAGD gene (608268). Patients with Gitelman syndrome (263800) and Bartter syndrome (see 241200) also show hypomagnesemia, and steatorrhea and severe chronic diarrhea states, such as Crohn disease (see 226600) and Whipple disease, that can result in severe hypomagnesemia.

Bartter syndrome refers to a group of disorders that are unified by autosomal recessive transmission of impaired salt reabsorption in the thick ascending loop of Henle with pronounced salt wasting, hypokalemic metabolic alkalosis, and hypercalciuria. Clinical disease results from defective renal reabsorption of sodium chloride in the thick ascending limb (TAL) of the Henle loop, where 30% of filtered salt is normally reabsorbed (Simon et al., 1997). Patients with antenatal forms of Bartter syndrome typically present with premature birth associated with polyhydramnios and low birth weight and may develop life-threatening dehydration in the neonatal period. Patients with classic Bartter syndrome (see BARTS3, 607364) present later in life and may be sporadically asymptomatic or mildly symptomatic (summary by Simon et al., 1996 and Fremont and Chan, 2012). For a discussion of genetic heterogeneity of Bartter syndrome, see 607364.

Long-chain hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency and trifunctional protein (TFP) deficiency are caused by impairment of mitochondrial TFP. TFP has three enzymatic activities – long-chain enoyl-CoA hydratase, long-chain 3-hydroxyacyl-CoA dehydrogenase, and long-chain 3-ketoacyl-CoA thiolase. In individuals with LCHAD deficiency, there is isolated deficiency of long-chain 3-hydroxyacyl-CoA dehydrogenase, while deficiency of all three enzymes occurs in individuals with TFP deficiency. Individuals with TFP deficiency can present with a severe-to-mild phenotype, while individuals with LCHAD deficiency typically present with a severe-to-intermediate phenotype. Neonates with the severe phenotype present within a few days of birth with hypoglycemia, hepatomegaly, encephalopathy, and often cardiomyopathy. The intermediate phenotype is characterized by hypoketotic hypoglycemia precipitated by infection or fasting in infancy. The mild (late-onset) phenotype is characterized by myopathy and/or neuropathy. Long-term complications include peripheral neuropathy and retinopathy.

The spectrum of COL4A1-related disorders includes: small-vessel brain disease of varying severity including porencephaly, variably associated with eye defects (retinal arterial tortuosity, Axenfeld-Rieger anomaly, cataract) and systemic findings (kidney involvement, muscle cramps, cerebral aneurysms, Raynaud phenomenon, cardiac arrhythmia, and hemolytic anemia). On imaging studies, small-vessel brain disease is manifest as diffuse periventricular leukoencephalopathy, lacunar infarcts, microhemorrhage, dilated perivascular spaces, and deep intracerebral hemorrhages. Clinically, small-vessel brain disease manifests as infantile hemiparesis, seizures, single or recurrent hemorrhagic stroke, ischemic stroke, and isolated migraine with aura. Porencephaly (fluid-filled cavities in the brain detected by CT or MRI) is typically manifest as infantile hemiparesis, seizures, and intellectual disability; however, on occasion it can be an incidental finding. HANAC (hereditary angiopathy with nephropathy, aneurysms, and muscle cramps) syndrome usually associates asymptomatic small-vessel brain disease, cerebral large vessel involvement (i.e., aneurysms), and systemic findings involving the kidney, muscle, and small vessels of the eye. Two additional phenotypes include isolated retinal artery tortuosity and nonsyndromic autosomal dominant congenital cataract.

Lactate dehydrogenase deficiency is a condition that affects how the body breaks down sugar to use as energy in cells, primarily muscle cells.\n\nThere are two types of this condition: lactate dehydrogenase-A deficiency (sometimes called glycogen storage disease XI) and lactate dehydrogenase-B deficiency.\n\nPeople with lactate dehydrogenase-A deficiency experience fatigue, muscle pain, and cramps during exercise (exercise intolerance). In some people with lactate dehydrogenase-A deficiency, high-intensity exercise or other strenuous activity leads to the breakdown of muscle tissue (rhabdomyolysis). The destruction of muscle tissue releases a protein called myoglobin, which is processed by the kidneys and released in the urine (myoglobinuria). Myoglobin causes the urine to be red or brown. This protein can also damage the kidneys, in some cases leading to life-threatening kidney failure. Some people with lactate dehydrogenase-A deficiency develop skin rashes. The severity of the signs and symptoms among individuals with lactate dehydrogenase-A deficiency varies greatly.\n\nPeople with lactate dehydrogenase-B deficiency typically do not have any signs or symptoms of the condition. They do not have difficulty with physical activity or any specific physical features related to the condition. Affected individuals are usually discovered only when routine blood tests reveal reduced lactate dehydrogenase activity.

Congenital myasthenic syndrome-12 is an autosomal recessive neuromuscular disorder characterized by onset of proximal muscle weakness in the first decade. EMG classically shows a decremental response to repeated nerve stimulation. Affected individuals show a favorable response to acetylcholinesterase (AChE) inhibitors (summary by Senderek et al., 2011). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

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.

PEOA6 is characterized by muscle weakness, mainly affecting the lower limbs, external ophthalmoplegia, exercise intolerance, and mitochondrial DNA (mtDNA) deletions on muscle biopsy. Symptoms may appear in childhood or adulthood and show slow progression (summary by Ronchi et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant progressive external ophthalmoplegia, see PEOA1 (157640).

An autosomal dominant hypocalcemia disease that has material basis in heterozygous mutation in the GNA11 gene on chromosome 19p13.

Myopathy with extrapyramidal signs is an autosomal recessive disorder characterized by early childhood onset of proximal muscle weakness and learning disabilities. While the muscle weakness is static, most patients develop progressive extrapyramidal signs that may become disabling (summary by Logan et al., 2014). Brain MRI in 1 patient showed congenital malformations, including polymicrogyria and cerebellar dysplasia (Wilton et al., 2020).

Autosomal dominant distal hereditary motor neuronopathy-6 (HMND6) is a neurologic disorder characterized by onset of slowly progressive distal lower limb weakness and atrophy between the second and fourth decades of life. Weakness usually begins in the calf muscles and later involves more proximal muscles. The severity is variable, and some patients have difficulty walking or running. Most also have upper limb involvement, particularly of the triceps and intrinsic hand muscles. Some patients may lose independent ambulation later in the disease course. Sensory impairment is typically not present, and cognition and bulbar function are normal (summary by Sumner et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant distal HMN (dHMN), see HMND1 (182960).

Tubular aggregates in muscle, first described by Engel (1964), are structures of variable appearance consisting of an outer tubule containing either one or more microtubule-like structures or amorphous material. They are a nonspecific pathologic finding that may occur in a variety of circumstances, including alcohol- and drug-induced myopathies, exercise-induced cramps or muscle weakness, and inherited myopathies. Tubular aggregates are derived from the sarcoplasmic reticulum (Salviati et al., 1985) and are believed to represent an adaptive mechanism aimed at regulating an increased intracellular level of calcium in order to prevent the muscle fibers from hypercontraction and necrosis (Martin et al., 1997; Muller et al., 2001). Genetic Heterogeneity of Tubular Aggregate Myopathy See also TAM2 (615883), caused by mutation in the ORAI1 gene (610277) on chromosome 12q24.

Vacuolar myopathy with CASQ1 aggregates is an autosomal dominant mild muscle disorder characterized by adult onset of muscle cramping and weakness as well as increased levels of serum creatine kinase (CK). The disorder is not progressive, and some patients may be asymptomatic (summary by Rossi et al., 2014).

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).

Autosomal recessive limb-girdle muscular dystrophy-18 (LGMD18) is characterized by childhood-onset of proximal muscle weakness resulting in gait abnormalities and scapular winging. Serum creatine kinase is increased. A subset of patients may show a hyperkinetic movement disorder with chorea, ataxia, or dystonia and global developmental delay (summary by Bogershausen et al., 2013). Additional more variable features include alacrima, achalasia, cataracts, or hepatic steatosis (Liang et al., 2015; Koehler et al., 2017). For a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy, see LGMDR1 (253600).

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).

Charcot-Marie-Tooth disease type 2DD is an autosomal dominant peripheral sensorimotor neuropathy mainly affecting the lower limbs. Affected individuals have gait impairment due to distal muscle weakness and atrophy. Some patients may also have involvement of the distal upper limbs, resulting in atrophy of the intrinsic hand muscles. The age at onset and severity of the disorder is highly variable, even within families, and those with earlier onset in late childhood or the teenage years tend to have a more severe disease course. Patients remain ambulatory even late in the disease, although some may require orthotic devices (summary by Lassuthova et al., 2018). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT type 2, see CMT2A (118210).

Immunodeficiency-58 is an autosomal recessive primary immunologic disorder characterized by early-onset skin lesions, including eczematous dermatitis, infectious abscesses, and warts, recurrent respiratory infections or allergies, and chronic persistent infections with candida, Molluscum contagiosum, mycobacteria, EBV, bacteria, and viruses. Some patients may have gastrointestinal involvement, including inflammatory bowel disease, EBV+ smooth muscle tumors, and esophagitis. Immunologic analysis shows defective T-cell function with decreased Treg cells and deficient CD3/CD28 costimulation responses in both CD4+ and CD8+ T cells. B-cell function may also be impaired (summary by Wang et al., 2016 and Alazami et al., 2018).

Episodic mitochondrial myopathy with or without optic atrophy and reversible leukoencephalopathy (MEOAL) is an autosomal recessive neuromuscular disorder characterized mainly by childhood onset of progressive muscle weakness and exercise intolerance. Patients have episodic exacerbation, which may be associated with increased serum creatine kinase or lactic acid. Additional more variable features may include optic atrophy, reversible leukoencephalopathy, and later onset of a sensorimotor polyneuropathy. The disorder results from impaired formation of Fe-S clusters, which are essential cofactors for proper mitochondrial function (summary by Gurgel-Giannetti et al., 2018)

Oculopharyngodistal myopathy-1 (OPDM1) is an autosomal dominant disorder characterized by adult-onset ptosis, external ophthalmoplegia, facial muscle weakness, distal limb muscle weakness and atrophy, and pharyngeal involvement, resulting in dysphagia and dysarthria. Skeletal muscle biopsy shows myopathic changes with rimmed vacuoles. There are variable manifestations of the disorder regarding muscle involvement and severity (summary by Ishiura et al., 2019). Genetic Heterogeneity of Oculopharyngodistal Myopathy See also OPDM2 (618940), caused by trinucleotide repeat expansion in the GIPC1 gene (605072) on chromosome 19p13; OPDM3 (619473), caused by trinucleotide repeat expansion in the NOTCH2NLC gene (618025) on chromosome 1q21; and OPDM4 (619790), caused by trinucleotide repeat expansion in the RILPL1 gene (614092) on chromosome 12q24. Oculopharyngeal muscular dystrophy (OPMD; 164300) is a similar disorder with overlapping features. It is caused by a similar heterozygous trinucleotide repeat expansion in the PABPN1 gene (602279) (summary by Durmus et al., 2011).

Autosomal recessive spastic paraplegia-83 (SPG83) is a neurologic disorder characterized by progressive lower limb spasticity resulting in gait instability. Patients develop symptoms in the second decade, consistent with juvenile onset. Some patients may have myalgia or mild dysarthria, but the phenotype is considered to be a pure type of SPG with no additional neurologic abnormalities (summary by Husain et al., 2020). For a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see SPG5A (270800).

Myofibrillar myopathy-10 (MFM10) is an autosomal recessive structural muscle disorder characterized by onset of muscle pain, cramping, and exercise fatigue in the first or second decades of life. Some patients have mild contractures of the large joints apparent in early childhood. Affected individuals have a characteristic appearance of a thick neck and prominent shoulder girdle with anteverted shoulders and a tendency toward kyphosis. There is no apparent muscle weakness, but some affected individuals show progressive muscle rigidity leading to limited mobility. There is variable cardiac involvement, ranging from chest pain with left ventricular hypertrophy to subclinical signs such as abnormal EKG or elevated cardiac enzymes. Skeletal muscle biopsy shows structural abnormalities with myofibrillar disorganization and accumulation of autophagocytic vacuoles (summary by Hedberg-Oldfors et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

Charcot-Marie-Tooth disease type 2GG (CMT2GG) is an autosomal dominant axonal peripheral neuropathy characterized by slowly progressive distal muscle weakness and atrophy primarily affecting the lower limbs and causing difficulty walking. The onset is usually in adulthood, although rare patients may have mild symptoms from childhood. Some individuals may also have involvement of the hands. Although most patients have hypo- or areflexia at the ankles, distal sensory impairment is not always present, indicating a spectrum of disease encompassing both distal hereditary neuropathy and axonal CMT. Electrophysiologic studies are consistent with a axonal process (summary by Mendoza-Ferreira et al., 2020). For a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

Charcot-Marie-Tooth disease type 2FF (CMT2FF) is an autosomal dominant progressive axonal sensorimotor peripheral neuropathy characterized by early-childhood onset of difficulties walking or running due to atrophy and weakness of the lower limbs. Most patients have foot and ankle deformities, requiring surgery or walking aids. Some patients lose independent ambulation. There is also prominent involvement of the upper limbs, with weakness and atrophy of the forearm, wrist, and intrinsic hand muscles. Proximal muscle function is preserved. Affected individuals have variable distal sensory impairment. Most patients have hyporeflexia, although brisk reflexes, suggesting upper motor involvement, have been described in 1 family. Sural nerve biopsy showed abnormal myelination (Rebelo et al., 2021). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT type 2, see CMT2A (118210).

Autosomal dominant congenital disorder of glycosylation type Iw (CDG1WAD) is characterized by variable skeletal anomalies, short stature, macrocephaly, and dysmorphic features; about half of patients have impaired intellectual development. Additional features include increased muscle tone and muscle cramps (Wilson et al., 2021).

Charcot-Marie-Tooth disease type 2Z (CMT2Z) is an autosomal dominant axonal peripheral neuropathy characterized by onset, usually in the first decade, of distal lower limb muscle weakness and sensory impairment. The disorder is progressive, and affected individuals tend to develop upper limb and proximal muscle involvement in an asymmetric pattern, resulting in severe disability late in adulthood. Rare occurrence of global developmental delay with impaired intellectual development or learning difficulties has been observed. In some instances, the same mutation may result in different phenotypic manifestations (CMT2Z or DIGFAN), which highlights the clinical spectrum associated with MORC2 mutations and may render the classification of patients into one or the other disorder challenging (summary by Sevilla et al., 2016, Ando et al., 2017, Guillen Sacoto et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

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).