A subjective feeling of tiredness characterized by a lack of energy and motivation.

Pain in muscle.

A disorder of muscle unrelated to impairment of innervation or neuromuscular junction.

The term dystrophy means abnormal growth. However, muscular dystrophy is used to describe primary myopathies with a genetic basis and a progressive course characterized by progressive skeletal muscle weakness and wasting, defects in muscle proteins, and histological features of muscle fiber degeneration (necrosis) and regeneration. If possible, it is preferred to use other HPO terms to describe the precise phenotypic abnormalities.

Sudden and involuntary contractions of one or more muscles.

A type of myopathy associated with mitochondrial disease and characterized by findings on biopsy such as ragged red muscle fibers.

Sudden and involuntary contractions of one or more muscles brought on by physical exertion.

Any abnormality of the skeletal muscle cell. Muscle fibers are subdivided into two types. Type I fibers are fatigue-resistant and rich in oxidative enzymes (they stain light with the myosin ATPase reaction), and type II fibers are fast-contracting, fatigue-prone, and rich in glycolytic enzymes (these fibers stain darkly). Normal muscle tissue has a random distribution of type I and type II fibers.

The presence of abnormal electromyographic patterns indicative of myopathy, such as small-short polyphasic motor unit potentials.

An abnormal, increased fatiguability of the musculature.

Chronic muscle inflammation accompanied by muscle weakness.

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.

The dystrophinopathies cover a spectrum of X-linked muscle disease ranging from mild to severe that includes Duchenne muscular dystrophy, Becker muscular dystrophy, and DMD-associated dilated cardiomyopathy (DCM). The mild end of the spectrum includes the phenotypes of asymptomatic increase in serum concentration of creatine phosphokinase (CK) and muscle cramps with myoglobinuria. The severe end of the spectrum includes progressive muscle diseases that are classified as Duchenne/Becker muscular dystrophy when skeletal muscle is primarily affected and as DMD-associated DCM when the heart is primarily affected. Duchenne muscular dystrophy (DMD) usually presents in early childhood with delayed motor milestones including delays in walking independently and standing up from a supine position. Proximal weakness causes a waddling gait and difficulty climbing stairs, running, jumping, and standing up from a squatting position. DMD is rapidly progressive, with affected children being wheelchair dependent by age 12 years. Cardiomyopathy occurs in almost all individuals with DMD after age 18 years. Few survive beyond the third decade, with respiratory complications and progressive cardiomyopathy being common causes of death. Becker muscular dystrophy (BMD) is characterized by later-onset skeletal muscle weakness. With improved diagnostic techniques, it has been recognized that the mild end of the spectrum includes men with onset of symptoms after age 30 years who remain ambulatory even into their 60s. Despite the milder skeletal muscle involvement, heart failure from DCM is a common cause of morbidity and the most common cause of death in BMD. Mean age of death is in the mid-40s. DMD-associated DCM is characterized by left ventricular dilation and congestive heart failure. Females heterozygous for a DMD pathogenic variant are at increased risk for DCM.

Pompe disease is classified by age of onset, organ involvement, severity, and rate of progression. Infantile-onset Pompe disease (IOPD; individuals with onset before age 12 months with cardiomyopathy) may be apparent in utero but more typically onset is at the median age of four months with hypotonia, generalized muscle weakness, feeding difficulties, failure to thrive, respiratory distress, and hypertrophic cardiomyopathy. Without treatment by enzyme replacement therapy (ERT), IOPD commonly results in death by age two years from progressive left ventricular outflow obstruction and respiratory insufficiency. Late-onset Pompe disease (LOPD; including: (a) individuals with onset before age 12 months without cardiomyopathy; and (b) all individuals with onset after age 12 months) is characterized by proximal muscle weakness and respiratory insufficiency; clinically significant cardiac involvement is uncommon.

Glycogen storage disease type III (GSD III) is characterized by variable liver, cardiac muscle, and skeletal muscle involvement. GSD IIIa is the most common subtype, present in about 85% of affected individuals; it manifests with liver and muscle involvement. GSD IIIb, with liver involvement only, comprises about 15% of all affected individuals. In infancy and early childhood, liver involvement presents as hepatomegaly and failure to thrive, with fasting ketotic hypoglycemia, hyperlipidemia, and elevated hepatic transaminases. In adolescence and adulthood, liver disease becomes less prominent. Most individuals develop cardiac involvement with cardiac hypertrophy and/or cardiomyopathy. Skeletal myopathy manifesting as weakness may be evident in childhood and slowly progresses, typically becoming prominent in the third to fourth decade. The overall prognosis is favorable but cannot be predicted on an individual basis. Long-term complications such as muscular and cardiac symptoms as well as liver fibrosis/cirrhosis and hepatocellular carcinoma may have a severe impact on prognosis and quality of life. To date, it is unknown if long-term complications can be alleviated and/or avoided by dietary interventions.

Glycogen storage disease type V (GSDV, McArdle disease) is a metabolic myopathy characterized by exercise intolerance manifested by rapid fatigue, myalgia, and cramps in exercising muscles. Symptoms are usually precipitated by isometric exercise or sustained aerobic exercise. Most individuals improve their exercise tolerance by exploiting the "second-wind" phenomenon with relief of myalgia and fatigue after a few minutes of rest. Age of onset is frequently in the first decade of life but can vary; however, diagnosis is typically delayed as myalgia and fatigability are dismissed/overlooked. Fixed muscle weakness occurs in approximately 25% of affected individuals, is more likely to involve proximal muscles, and is more common in individuals of advanced age. Approximately 50% of affected individuals have recurrent episodes of myoglobinuria that can – on occasion – eventually result in acute renal failure.

Glycogen storage disease VII is an autosomal recessive metabolic disorder characterized clinically by exercise intolerance, muscle cramping, exertional myopathy, and compensated hemolysis. Myoglobinuria may also occur. The deficiency of the muscle isoform of PFK results in a total and partial loss of muscle and red cell PFK activity, respectively. Raben and Sherman (1995) noted that not all patients with GSD VII seek medical care because in some cases it is a relatively mild disorder.

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.

Lowe syndrome (oculocerebrorenal syndrome) is characterized by involvement of the eyes, central nervous system, and kidneys. Dense congenital cataracts are found in all affected boys and infantile glaucoma in approximately 50%. All boys have impaired vision; corrected acuity is rarely better than 20/100. Generalized hypotonia is noted at birth and is of central (brain) origin. Deep tendon reflexes are usually absent. Hypotonia may slowly improve with age, but normal motor tone and strength are never achieved. Motor milestones are delayed. Almost all affected males have some degree of intellectual disability; 10%-25% function in the low-normal or borderline range, approximately 25% in the mild-to-moderate range, and 50%-65% in the severe-to-profound range of intellectual disability. Affected males have varying degrees of proximal renal tubular dysfunction of the Fanconi type, including low molecular-weight (LMW) proteinuria, aminoaciduria, bicarbonate wasting and renal tubular acidosis, phosphaturia with hypophosphatemia and renal rickets, hypercalciuria, sodium and potassium wasting, and polyuria. The features of symptomatic Fanconi syndrome do not usually become manifest until after the first few months of life, except for LMW proteinuria. Glomerulosclerosis associated with chronic tubular injury usually results in slowly progressive chronic renal failure and end-stage renal disease between the second and fourth decades of life.

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.

Phosphoglycerate mutase deficiency is a disorder that primarily affects muscles used for movement (skeletal muscles). Beginning in childhood or adolescence, affected individuals experience muscle aches or cramping following strenuous physical activity. Some people with this condition also have recurrent episodes of myoglobinuria. Myoglobinuria occurs when muscle tissue breaks down abnormally and releases a protein called myoglobin, which is processed by the kidneys and released in the urine. If untreated, myoglobinuria can lead to kidney failure.\n\nIn some cases of phosphoglycerate mutase deficiency, microscopic tube-shaped structures called tubular aggregates are seen in muscle fibers. It is unclear how tubular aggregates are associated with the signs and symptoms of the disorder.

The spectrum of isolated sulfite oxidase deficiency ranges from classic early-onset (severe) disease to late-onset (mild) disease. Classic ISOD is characterized in the first few hours to days of life by intractable seizures, feeding difficulties, and rapidly progressive encephalopathy manifest as abnormal tone (especially opisthotonus, spastic quadriplegia, and pyramidal signs) followed by progressive microcephaly and profound intellectual disability. Lens subluxation or dislocation, another characteristic finding, may be evident after the newborn period. Children usually die during the first few months of life. Late-onset ISOD manifests between ages six and 18 months and is characterized by ectopia lentis (variably present), developmental delay/regression, movement disorder characterized by dystonia and choreoathetosis, ataxia, and (rarely) acute hemiplegia as a result of metabolic stroke. The clinical course may be progressive or episodic. In the episodic form encephalopathy, dystonia, choreoathetosis, and/or ataxia are intermittent.

Oculopharyngeal muscular dystrophy (OPMD) is characterized by ptosis and dysphagia due to selective involvement of the muscles of the eyelids and pharynx, respectively. For the vast majority of individuals with typical OPMD, the mean age of onset of ptosis is usually 48 years and of dysphagia 50 years; in 5%-10% of individuals with severe OPMD, onset of ptosis and dysphagia occur before age 45 years and is associated with lower limb girdle weakness starting around age 60 years. Swallowing difficulties, which determine prognosis, increase the risk for potentially life-threatening aspiration pneumonia and poor nutrition. Other manifestations as the disease progresses can include limitation of upward gaze, tongue atrophy and weakness, chewing difficulties, wet voice, facial muscle weakness, axial muscle weakness, and proximal limb girdle weakness predominantly in lower limbs. Some individuals with severe involvement will eventually need a wheelchair. Neuropsychological tests have shown altered scores in executive functions in some.

A mild subtype of autosomal recessive limb girdle muscular dystrophy characterized by slowly progressive proximal muscle weakness and wasting of the pelvic and shoulder girdles with onset that usually occurs during the second or third decade of life. Clinical presentation is variable and can include calf psuedohypertrophy, joint contractures, scapular winging, muscle cramping and/or facial and respiratory muscle involvement.

Aldolase A deficiency is an autosomal recessive disorder associated with hereditary hemolytic anemia (Kishi et al., 1987).

Systemic primary carnitine deficiency (CDSP) is a disorder of the carnitine cycle that results in defective fatty acid oxidation. It encompasses a broad clinical spectrum including the following: Metabolic decompensation in infancy typically presenting between age three months and two years with episodes of hypoketotic hypoglycemia, poor feeding, irritability, lethargy, hepatomegaly, elevated liver transaminases, and hyperammonemia triggered by fasting or common illnesses such as upper respiratory tract infection or gastroenteritis. Childhood myopathy involving heart and skeletal muscle with onset between age two and four years. Pregnancy-related decreased stamina or exacerbation of cardiac arrhythmia. Fatigability in adulthood. Absence of symptoms. The latter two categories often include mothers diagnosed with CDSP after newborn screening has identified low carnitine levels in their infants.

Carnitine-acylcarnitine translocase (CACT) is a critical component of the carnitine shuttle, which facilitates the transfer of long-chain fatty acylcarnitines across the inner mitochondrial membrane. CACT deficiency causes a defect in mitochondrial long-chain fatty acid ß-oxidation, with variable clinical severity. Severe neonatal-onset disease is most common, with symptoms evident within two days after birth; attenuated cases may present in the first months of life. Hyperammonemia and cardiac arrhythmia are prominent in early-onset disease, with high rates of cardiac arrest. Other clinical features are typical for disorders of long-chain fatty acid oxidation: poor feeding, lethargy, hypoketotic hypoglycemia, hypotonia, transaminitis, liver dysfunction with hepatomegaly, and rhabdomyolysis. Univentricular or biventricular hypertrophic cardiomyopathy, ranging from mild to severe, may respond to appropriate dietary and medical therapies. Hyperammonemia is difficult to treat and is an important determinant of long-term neurocognitive outcome. Affected individuals with early-onset disease typically experience brain injury at presentation, and have recurrent hyperammonemia leading to developmental delay / intellectual disability. Affected individuals with later-onset disease have milder symptoms and are less likely to experience recurrent hyperammonemia, allowing a better developmental outcome. Prompt treatment of the presenting episode to prevent hypoglycemic, hypoxic, or hyperammonemic brain injury may allow normal growth and development.

Dihydropyrimidinase deficiency (DPYSD) is an autosomal recessive disease characterized by the presence of dihydropyrimidinuria. The clinical phenotype is highly variable, ranging from early infantile onset of severe neurologic involvement, dysmorphic features, and feeding problems to late onset of mild intellectual disability and even asymptomatic individuals. Patients with a complete or partial deficiency have an increased risk of developing severe toxicity after administration of the anticancer drug 5-fluorouracil (5-FU) (summary by Nakajima et al., 2017). See also dihydropyrimidine dehydrogenase deficiency (274270), a similar disorder.

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.

McLeod neuroacanthocytosis syndrome (designated as MLS throughout this review) is a multisystem disorder with central nervous system (CNS), neuromuscular, cardiovascular, and hematologic manifestations in males: CNS manifestations are a neurodegenerative basal ganglia disease including movement disorders, cognitive alterations, and psychiatric symptoms. Neuromuscular manifestations include a (mostly subclinical) sensorimotor axonopathy and muscle weakness or atrophy of different degrees. Cardiac manifestations include dilated cardiomyopathy, atrial fibrillation, and tachyarrhythmia. Hematologically, MLS is defined as a specific blood group phenotype (named after the first proband, Hugh McLeod) that results from absent expression of the Kx erythrocyte antigen and weakened expression of Kell blood group antigens. The hematologic manifestations are red blood cell acanthocytosis and compensated hemolysis. Alloantibodies in the Kell and Kx blood group system can cause strong reactions to transfusions of incompatible blood and severe anemia in affected male newborns of Kell-negative mothers. Females heterozygous for XK pathogenic variants have mosaicism for the Kell and Kx blood group antigens. Although they usually lack CNS and neuromuscular manifestations, some heterozygous females may develop clinical manifestations including chorea or late-onset cognitive decline.

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.

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.

X-linked recessive Charcot-Marie-Tooth disease-4 with or without cerebellar ataxia (CMTX4) is a mitochondrial disorder manifest as progressive neurologic dysfunction with highly variable features. The age at onset ranges from infancy to young adulthood, and patients can present with different features, including hearing loss, delayed motor development, or difficulty walking due to peripheral neuropathy and/or cerebellar ataxia. Most patients develop all features, including a progressive sensorimotor axonal neuropathy and deafness due to auditory neuropathy. Additional more variable features can include cognitive impairment, cerebellar atrophy on brain imaging, cerebellar signs, such as dysarthria, abnormal extraocular movements, tremor, and dysmetria, as well as spasticity. There is significant intrafamilial variability: the variable features are consistent with mitochondrial dysfunction. Prolonged treatment with riboflavin may result in some mild improvement in the ataxia (summary by Rinaldi et al., 2012, Heimer et al., 2018, Bogdanova-Mihaylova et al., 2019).

Danon disease is a multisystem condition with predominant involvement of the heart, skeletal muscles, and retina, with overlying cognitive dysfunction. Males are typically more severely affected than females. Males usually present with childhood onset concentric hypertrophic cardiomyopathy that is progressive and often requires heart transplantation. Rarely, hypertrophic cardiomyopathy can evolve to resemble dilated cardiomyopathy. Most affected males also have cardiac conduction abnormalities. Skeletal muscle weakness may lead to delayed acquisition of motor milestones. Learning disability and intellectual disability, most often in the mild range, are common. Additionally, affected males can develop retinopathy with subsequent visual impairment. The clinical features in females are broader and more variable. Females are more likely to have dilated cardiomyopathy, with a smaller proportion requiring heart transplantation compared to affected males. Cardiac conduction abnormalities, skeletal muscle weakness, mild cognitive impairment, and pigmentary retinopathy are variably seen in affected females.

The dystrophinopathies cover a spectrum of X-linked muscle disease ranging from mild to severe that includes Duchenne muscular dystrophy, Becker muscular dystrophy, and DMD-associated dilated cardiomyopathy (DCM). The mild end of the spectrum includes the phenotypes of asymptomatic increase in serum concentration of creatine phosphokinase (CK) and muscle cramps with myoglobinuria. The severe end of the spectrum includes progressive muscle diseases that are classified as Duchenne/Becker muscular dystrophy when skeletal muscle is primarily affected and as DMD-associated DCM when the heart is primarily affected. Duchenne muscular dystrophy (DMD) usually presents in early childhood with delayed motor milestones including delays in walking independently and standing up from a supine position. Proximal weakness causes a waddling gait and difficulty climbing stairs, running, jumping, and standing up from a squatting position. DMD is rapidly progressive, with affected children being wheelchair dependent by age 12 years. Cardiomyopathy occurs in almost all individuals with DMD after age 18 years. Few survive beyond the third decade, with respiratory complications and progressive cardiomyopathy being common causes of death. Becker muscular dystrophy (BMD) is characterized by later-onset skeletal muscle weakness. With improved diagnostic techniques, it has been recognized that the mild end of the spectrum includes men with onset of symptoms after age 30 years who remain ambulatory even into their 60s. Despite the milder skeletal muscle involvement, heart failure from DCM is a common cause of morbidity and the most common cause of death in BMD. Mean age of death is in the mid-40s. DMD-associated DCM is characterized by left ventricular dilation and congestive heart failure. Females heterozygous for a DMD pathogenic variant are at increased risk for DCM.

Merosin-deficient congenital muscular dystrophy is an autosomal recessive form of muscular dystrophy characterized by muscle weakness apparent at birth or in the first 6 months of life. Patients show hypotonia, poor suck and cry, and delayed motor development; most never achieve independent ambulation. Most patients also have periventricular white matter abnormalities on brain imaging, but mental retardation and/or seizures occur only rarely (summary by Xiong et al., 2015).

Episodic ataxia type 1 (EA1) is a potassium channelopathy characterized by constant myokymia and dramatic episodes of spastic contractions of the skeletal muscles of the head, arms, and legs with loss of both motor coordination and balance. During attacks individuals may experience a number of variable symptoms including vertigo, blurred vision, diplopia, nausea, headache, diaphoresis, clumsiness, stiffening of the body, dysarthric speech, and difficulty in breathing, among others. EA1 may be associated with epilepsy. Other possible associations include delayed motor development, cognitive disability, choreoathetosis, and carpal spasm. Usually, onset is in childhood or early adolescence.

Carnitine palmitoyltransferase 1A (CPT1A) deficiency is a disorder of long-chain fatty acid oxidation. Clinical manifestations usually occur in an individual with a concurrent febrile or gastrointestinal illness when energy demands are increased; onset of symptoms is usually rapid. The recognized phenotypes are: acute fatty liver of pregnancy, in which the fetus has biallelic pathogenic variants in CPT1A that causes CPT1A deficiency; and hepatic encephalopathy, in which individuals (typically children) present with hypoketotic hypoglycemia and sudden onset of liver failure. Individuals with hepatic encephalopathy typically present with hypoglycemia, absent or low levels of ketones, and elevated serum concentrations of liver transaminases, ammonia, and total carnitine. Between episodes of hepatic encephalopathy, individuals appear developmentally and cognitively normal unless previous metabolic decompensation has resulted in neurologic damage.

Autosomal recessive limb-girdle muscular dystrophy-6 (LGMDR6) is a very rare and severe neuromuscular disorder with onset in most patients in the first decade of life. Generalized muscle weakness affecting predominantly proximal and distal muscles of the limbs is progressive, and patients require walking aids or become wheelchair-bound. Some patients have cardiomyopathy or heart rhythm abnormalities, or require ventilatory support (Alonso-Perez et al., 2022). For a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy, see LGMDR1 (253600).

Hereditary rippling muscle disease is an autosomal dominant disorder characterized by mechanically triggered contractions of skeletal muscle. In rippling muscle disease, mechanical stimulation leads to electrically silent muscle contractions that spread to neighboring fibers that cause visible ripples to move over the muscle. RMD is usually inherited as an autosomal dominant trait, but autosomal recessive inheritance has also been reported (Kubisch et al., 2005). Genetic Heterogeneity of Rippling Muscle Disease Another locus for RMD, designated RMD1 (600332), maps to chromosome 1q41.

Autosomal dominant multiple epiphyseal dysplasia (MED) presents in early childhood, usually with pain in the hips and/or knees after exercise. Affected children complain of fatigue with long-distance walking. Waddling gait may be present. Adult height is either in the lower range of normal or mildly shortened. The limbs are relatively short in comparison to the trunk. Pain and joint deformity progress, resulting in early-onset osteoarthritis, particularly of the large weight-bearing joints.

A rare genetic neuromuscular disease with characteristics of late onset of mild, progressive proximal muscle weakness, severe myalgia during and after exercise, and susceptibility to rhabdomyolysis. Intellectual disability is mild or absent. There are no abnormalities of the skin. Muscle biopsy shows focal depletion of mitochondria especially at the centre of muscle fibres, surrounded by enlarged mitochondria at the periphery.

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.

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.

A rare metabolic myopathy with characteristics of episodic myalgia with myoglobinuria which is induced by fever, viral or bacterial infection, prolonged exercise or alcohol abuse, and could, on occasion, lead to acute renal failure. Between episodes, patients may be asymptomatic or could present elevated creatine kinase levels and mild muscle weakness. There have been no further descriptions in the literature since 1997.

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. Weakness usually begins in mid-childhood or adolescence, but progression is slow and ambulation is 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.

Other signs and symptoms of myofibrillar myopathy can include a weakened heart muscle (cardiomyopathy), muscle pain (myalgia), loss of sensation and weakness in the limbs (peripheral neuropathy), and respiratory failure. Individuals with this condition may have skeletal problems including joint stiffness (contractures) and abnormal side-to-side curvature of the spine (scoliosis). Rarely, people with this condition develop clouding of the lens of the eyes (cataracts).\n\nThe signs and symptoms of myofibrillar myopathy vary widely among affected individuals, typically depending on the condition's genetic cause. Most people with this disorder begin to develop muscle weakness (myopathy) in mid-adulthood. However, features of this condition can appear anytime between infancy and late adulthood. Muscle weakness most often begins in the hands and feet (distal muscles), but some people first experience weakness in the muscles near the center of the body (proximal muscles). Other affected individuals develop muscle weakness throughout their body. Facial muscle weakness can cause swallowing and speech difficulties. Muscle weakness worsens over time.\n\nMyofibrillar myopathy is part of a group of disorders called muscular dystrophies that affect muscle function and cause weakness. Myofibrillar myopathy primarily affects skeletal muscles, which are muscles that the body uses for movement. In some cases, the heart (cardiac) muscle is also affected.

Limb-girdle muscular dystrophies resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239) represent the mildest end of the phenotypic spectrum of muscular dystrophies collectively known as dystroglycanopathies. The limb-girdle phenotype is characterized by onset of muscular weakness apparent after ambulation is achieved; mental retardation and mild brain anomalies are variable (Balci et al., 2005; review by Godfrey et al., 2007). The most severe end of the phenotypic spectrum of dystroglycanopathies is represented by congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A; see MDDGA1, 236670), previously designated Walker-Warburg syndrome (WWS) or muscle-eye-brain disease (MEB), and the intermediate range of the spectrum is represented by congenital muscular dystrophy-dystroglycanopathy with or without mental retardation (type B; see MDDGB1, 613155). Genetic Heterogeneity of Limb-Girdle Muscular Dystrophy-Dystroglycanopathy (Type C) Limb-girdle muscular dystrophy due to defective glycosylation of DAG1 is genetically heterogeneous. See also MDDGC2 (613158), caused by mutation in the POMT2 gene (607439); MDDGC3 (613157), caused by mutation in the POMGNT1 gene (606822); MDDGC4 (611588), caused by mutation in the FKTN gene (607440); MDDGC5 (607155), caused by mutation in the FKRP gene (606596); MDDGC7 (616052), caused by mutation in the ISPD gene (CRPPA; 614631); MDDGC8 (618135), caused by mutation in the POMGNT2 gene (614828); MDDGC9 (613818) caused by mutation in the DAG1 gene (128239); MDDGC12 (616094), caused by mutation in the POMK gene (615247); MDDGC14 (615352) caused by mutation in the GMPPB gene (615320); and MDDGC15 (612937), caused by mutation in the DPM3 gene (605951).

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).

Autosomal dominant limb-girdle muscular dystrophy-3 (LGMDD3) is characterized by slowly progressive proximal muscle weakness affecting the upper and lower limbs. Onset is usually in adulthood, but can occur during the teenage years. Affected individuals may also develop cataracts before age 50 (summary by Vieira et al., 2014). For a phenotypic description and a discussion of genetic heterogeneity of autosomal dominant limb-girdle muscular dystrophy, see LGMDD1 (603511).

MDDGB6 is an autosomal recessive congenital muscular dystrophy with impaired intellectual development and structural brain abnormalities (Longman et al., 2003). 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., 2009). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

Alpha-B crystallin-related myofibrillar myopathy is an autosomal dominant muscular disorder characterized by adult onset of progressive muscle weakness affecting both the proximal and distal muscles and associated with respiratory insufficiency, cardiomyopathy, and cataracts. There is phenotypic variability both within and between families (Fardeau et al., 1978; Selcen and Engel, 2003). A homozygous founder mutation in the CRYAB gene has been identified in Canadian aboriginal infants of Cree origin who have a severe fatal infantile hypertonic form of myofibrillar myopathy; see 613869. For a phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

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).

Congenital disorders of glycosylation (CDGs) are metabolic deficiencies in glycoprotein biosynthesis that usually cause severe mental and psychomotor retardation. Different forms of CDGs can be recognized by altered isoelectric focusing (IEF) patterns of serum transferrin. For a general discussion of CDGs, see CDG Ia (212065) and CDG Ib (602579).

The autosomal dominant TRPV4 disorders (previously considered to be clinically distinct phenotypes before their molecular basis was discovered) are now grouped into neuromuscular disorders and skeletal dysplasias; however, the overlap within each group is considerable. Affected individuals typically have either neuromuscular or skeletal manifestations alone, and in only rare instances an overlap syndrome has been reported. The three autosomal dominant neuromuscular disorders (mildest to most severe) are: Charcot-Marie-Tooth disease type 2C. Scapuloperoneal spinal muscular atrophy. Congenital distal spinal muscular atrophy. The autosomal dominant neuromuscular disorders are characterized by a congenital-onset, static, or later-onset progressive peripheral neuropathy with variable combinations of laryngeal dysfunction (i.e., vocal fold paresis), respiratory dysfunction, and joint contractures. The six autosomal dominant skeletal dysplasias (mildest to most severe) are: Familial digital arthropathy-brachydactyly. Autosomal dominant brachyolmia. Spondylometaphyseal dysplasia, Kozlowski type. Spondyloepiphyseal dysplasia, Maroteaux type. Parastremmatic dysplasia. Metatropic dysplasia. The skeletal dysplasia is characterized by brachydactyly (in all 6); the five that are more severe have short stature that varies from mild to severe with progressive spinal deformity and involvement of the long bones and pelvis. In the mildest of the autosomal dominant TRPV4 disorders life span is normal; in the most severe it is shortened. Bilateral progressive sensorineural hearing loss (SNHL) can occur with both autosomal dominant neuromuscular disorders and skeletal dysplasias.

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).

A rare, genetic, mitochondrial DNA-related mitochondrial myopathy disorder characterized by slowly progressive muscular weakness (proximal greater than distal), predominantly involving the facial muscles and scapular girdle, associated with insulin-dependent diabetes mellitus. Neurological involvement and congenital myopathy may be variably observed.

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.

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.

King-Denborough syndrome (KDS) is an autosomal dominant disorder characterized by the triad of congenital myopathy, dysmorphic features, and susceptibility to malignant hyperthermia (summary by Dowling et al., 2011).

X-linked infantile spinal muscular atrophy (XL-SMA) is characterized by congenital hypotonia, areflexia, and evidence of degeneration and loss of anterior horn cells (i.e., lower motor neurons) in the spinal cord and brain stem. Often congenital contractures and/or fractures are present. Intellect is normal. Life span is significantly shortened because of progressive ventilatory insufficiency resulting from chest muscle involvement.

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.

Dent disease, an X-linked disorder of proximal renal tubular dysfunction, is characterized by low molecular weight (LMW) proteinuria, hypercalciuria, and at least one additional finding including nephrocalcinosis, nephrolithiasis, hematuria, hypophosphatemia, chronic kidney disease (CKD), and evidence of X-linked inheritance. Males younger than age ten years may manifest only LMW proteinuria and/or hypercalciuria, which are usually asymptomatic. Thirty to 80% of affected males develop end-stage renal disease (ESRD) between ages 30 and 50 years; in some instances ESRD does not develop until the sixth decade of life or later. The disease may also be accompanied by rickets or osteomalacia, growth restriction, and short stature. Disease severity can vary within the same family. Males with Dent disease 2 (caused by pathogenic variants in OCRL) may also have mild intellectual disability, cataracts, and/or elevated muscle enzymes. Due to random X-chromosome inactivation, some female carriers may manifest hypercalciuria and, rarely, renal calculi and moderate LMW proteinuria. Females rarely develop CKD.

Uruguay faciocardiomusculoskeletal syndrome (FCMSU) is an X-linked disorder in which affected males have a distinctive facial appearance, muscular hypertrophy, and cardiac ventricular hypertrophy leading to premature death. Additional features include large, broad, and deformed hands and feet, congenital hip dislocation, and scoliosis (summary by Xue et al., 2016).

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).

A rare genetic neuromuscular disease with characteristics of a progressive muscle weakness starting in the anterior tibial muscles, later involving lower and upper limb muscles, associated with an increased serum creatine kinase levels and absence of dysferlin on muscle biopsy. There is evidence the disease is caused by homozygous mutation in the gene encoding dysferlin (DYSF) on chromosome 2p13. Patients become wheelchair dependent.

Dilated cardiomyopathy, a disorder characterized by cardiac dilation and reduced systolic function, represents an outcome of a heterogeneous group of inherited and acquired disorders. For background and phenotypic information on dilated cardiomyopathy, see CMD1A (115200).

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).

Recurrent myoglobinuria is characterized by recurrent attacks of rhabdomyolysis associated with muscle pain and weakness and followed by excretion of myoglobin in the urine. Renal failure may occasionally occur. Onset is usually in early childhood under the age of 5 years. Unlike the exercise-induced rhabdomyolyses such as McArdle syndrome (232600), carnitine palmitoyltransferase deficiency (see 255110), and the Creteil variety of phosphoglycerate kinase deficiency (311800), the attacks in recurrent myoglobinuria no relation to exercise, but are triggered by intercurrent illnesses, commonly upper respiratory tract infections (Ramesh and Gardner-Medwin, 1992). See 160010 for discussion of a possible autosomal dominant form of myoglobinuria. Severe rhabdomyolysis is a major clinical feature of anesthetic-induced malignant hyperthermia (145600), an autosomal dominant disorder.

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.

Autosomal recessive myosin storage congenital myopathy-7B (CMYP7B) is a skeletal muscle disorder characterized by the onset of scapuloperoneal muscle weakness in early childhood or young adulthood. Affected individuals have difficulty walking, steppage gait, and scapular winging due to shoulder girdle involvement. The severity and progression of the disorder is highly variable, even within families. Most patients develop respiratory insufficiency, nocturnal hypoventilation, and restrictive lung disease; some develop hypertrophic cardiomyopathy. Additional features include myopathic facies, high-arched palate, scoliosis, and muscle wasting with thin body habitus. Serum creatine kinase may be normal or elevated. Skeletal muscle biopsy shows variable findings, including myosin storage disease, type 1 fiber predominance, centralized nuclei, and multiminicore disease (Onengut et al., 2004; Tajsharghi et al., 2007; Beecroft et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (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.

Neutral lipid storage disease with myopathy (NLSDM) is an autosomal recessive muscle disorder characterized by adult onset of slowly progressive proximal muscle weakness affecting the upper and lower limbs and associated with increased serum creatine kinase; distal muscle weakness may also occur. About half of patients develop cardiomyopathy later in the disease course. Other variable features include diabetes mellitus, hepatic steatosis, hypertriglyceridemia, and possibly sensorineural hearing loss. Leukocytes and muscle cells show cytoplasmic accumulation of triglycerides (summary by Reilich et al., 2011). Neutral lipid storage disease with myopathy belongs to a group of disorders termed neutral lipid storage disorders (NLSDs). These disorders are characterized by the presence of triglyceride-containing cytoplasmic droplets in leukocytes and in other tissues, including bone marrow, skin, and muscle. Chanarin-Dorfman syndrome (CDS; 275630) is defined as NLSD with ichthyosis (NLSDI). Patients with NLSDM present with myopathy but without ichthyosis (summary by Fischer et al., 2007).

Ataxia with oculomotor apraxia type 2 (AOA2) is characterized by onset of ataxia between age three and 30 years after initial normal development, axonal sensorimotor neuropathy, oculomotor apraxia, cerebellar atrophy, and elevated serum concentration of alpha-fetoprotein (AFP).

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).

GNE myopathy is a slowly progressive muscle disease that typically presents between age 20 and 40 years with bilateral foot drop caused by anterior tibialis weakness. Lower-extremity muscle involvement progresses from the anterior to the posterior compartment of the lower leg, followed by hamstrings, then hip girdle muscles, with relative sparing of the quadriceps. A wheelchair may be needed about ten to 20 years after the onset of manifestations. The upper extremities, which may be affected within five to ten years of disease onset, do not necessarily follow a distal-to-proximal progression. In advanced stages, neck and core muscles can become affected.

A rare metabolic myopathy with characteristics of muscle cramping and/or stiffness after exercise (especially during heat exposure), post-exertional rhabdomyolysis and myoglobinuria and elevation of serum creatine kinase. Caused by mutation in the SLC16A1 gene.

With the current widespread use of multigene panels and comprehensive genomic testing, it has become apparent that the phenotypic spectrum of EPG5-related disorder represents a continuum. At the most severe end of the spectrum is classic Vici syndrome (defined as a neurodevelopmental disorder with multisystem involvement characterized by the combination of agenesis of the corpus callosum, cataracts, hypopigmentation, cardiomyopathy, combined immunodeficiency, microcephaly, and failure to thrive); at the milder end of the spectrum are attenuated neurodevelopmental phenotypes with variable multisystem involvement. Median survival in classic Vici syndrome appears to be 24 months, with only 10% of children surviving longer than age five years; the most common causes of death are respiratory infections as a result of primary immunodeficiency and/or cardiac insufficiency resulting from progressive cardiac failure. No data are available on life span in individuals at the milder end of the spectrum.

A rare genetic neuromuscular disorder characterized by proximal and symmetrical muscle weakness (particularly of neck, sternomastoid, facial and diaphragm muscles), spinal rigidity, joint contractures (Achilles tendon, elbows, hands), generalized muscle hypertrophy and early respiratory failure (usually in the first decade of life). Patients typically present delayed motor milestones and grossly elevated serum creatine kinase levels, and with disease progression, forced expiratory abdominal squeeze and nocturnal hypoventilation.

Any familial isolated dilated cardiomyopathy in which the cause of the disease is a mutation in the DES gene.

Limb-girdle muscular dystrophies are characterized clinically by predominantly proximal muscle weakness of variable severity and dystrophic changes on muscle biopsy. LGMDR4 is in general a severe form of the disorder, with some patients developing symptoms before 8 years of age and losing the ability to ambulate in their second decade. Some patients have a milder course, with weakness evident in the teenage years and loss of walking ability in their fourth decade (summary by Lim et al., 1995 and Bonnemann et al., 1996). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy, see LGMDR1 (253600).

Poikiloderma with neutropenia (PN) is characterized by an inflammatory eczematous rash (ages 6-12 months) followed by post-inflammatory poikiloderma (age >2 years) and chronic noncyclic neutropenia typically associated with recurrent sinopulmonary infections in the first two years of life and (often) bronchiectasis. There is increased risk for myelodysplastic syndrome and, rarely, acute myelogenous leukemia. Other ectodermal findings include nail dystrophy and palmar/plantar hyperkeratosis. Most affected individuals also have reactive airway disease and some have short stature, hypogonadotropic hypogonadism, midfacial retrusion, calcinosis cutis, and non-healing skin ulcers.

A rare syndrome with characteristics of camptodactyly, muscle hypoplasia and weakness, skeletal anomalies, facial dysmorphism and abnormal dermatoglyphics. Dysmorphic features include facial asymmetry, hypertelorism, broad nasal bridge, long philtrum and a small mouth. Winging scapulae, scoliosis, syndactyly and clinodactyly are commonly observed. The affected patients usually have normal mental development. The molecular basis of the syndrome has not yet been elucidated.

Ataxia with oculomotor apraxia type 1 (AOA1) is characterized by childhood onset of slowly progressive cerebellar ataxia, followed by oculomotor apraxia and a severe primary motor peripheral axonal motor neuropathy. The first manifestation is progressive gait imbalance (mean age of onset: 4.3 years; range: 2-10 years), followed by dysarthria, then upper-limb dysmetria with mild intention tremor. Oculomotor apraxia, usually noticed a few years after the onset of ataxia, progresses to external ophthalmoplegia. All affected individuals have generalized areflexia followed by a peripheral neuropathy and quadriplegia with loss of ambulation about seven to ten years after onset. Hands and feet are short and atrophic. Chorea and upper-limb dystonia are common. Intellect remains normal in some individuals; in others, different degrees of cognitive impairment have been observed.

A rare genetic distal arthrogryposis syndrome with characteristics of plantar flexion contractures typically presenting with toe-walking in infancy, variably associated with milder contractures of the hip, elbow, wrist and finger joints. No ocular or neurological abnormalities are associated and serum creatine phosphokinase levels are normal.

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).

Hereditary myopathy with early respiratory failure (HMERF) is a slowly progressive myopathy that typically begins in the third to fifth decades of life. The usual presenting findings are gait disturbance relating to distal leg weakness or nocturnal respiratory symptoms due to respiratory muscle weakness. Weakness eventually generalizes and affects both proximal and distal muscles. Most affected individuals require walking aids within a few years of onset; some progress to wheelchair dependence and require nocturnal noninvasive ventilatory support about ten years after onset. The phenotype varies even among individuals within the same family: some remain ambulant until their 70s whereas others may require ventilator support in their 40s.

Progressive external ophthalmoplegia-4 is an autosomal dominant form of mitochondrial disease that variably affects skeletal muscle, the nervous system, the liver, and the gastrointestinal tract. Age at onset ranges from infancy to adulthood. The phenotype ranges from relatively mild, with adult-onset skeletal muscle weakness and weakness of the external eye muscles, to severe, with a multisystem disorder characterized by delayed psychomotor development, lactic acidosis, constipation, and liver involvement (summary by Young et al., 2011). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant progressive external ophthalmoplegia, see PEOA1 (157640).

Combined oxidative phosphorylation deficiency type 3 is an extremely rare clinically heterogenous disorder described in about 5 patients to date. Clinical signs included hypotonia, lactic acidosis, and hepatic insufficiency, with progressive encephalomyopathy or hypertrophic cardiomyopathy.

Megaconial-type congenital muscular dystrophy (MDCMC) is an autosomal recessive disorder characterized by early-onset muscle wasting and impaired intellectual development. Some patients develop fatal cardiomyopathy. Muscle biopsy shows peculiar enlarged mitochondria that are prevalent toward the periphery of the fibers but are sparse in the center (summary by Mitsuhashi et al., 2011).

A mild form of limb-girdle muscular dystrophy with characteristics of muscle weakness in the four limbs, mild scapular winging, severe atrophy of the quadriceps and anterior tibialis muscles, calf hypertrophy and lack of respiratory and cardiac involvement.

Autosomal dominant myopathy with rimmed ubiquitin-positive autophagic vacuolation (MRUPAV) is characterized by adult onset of slowly progressive skeletal muscle weakness variably affecting the distal or proximal lower limbs. Some patients may also have upper limb involvement or neck muscle weakness, but respiratory and bulbar involvement only rarely occurs. EMG studies show a myopathic process, and myotonia may also be observed. Skeletal muscle biopsy shows myopathic features, rimmed vacuoles, and abnormal subsarcolemmal protein aggregation with activation of the autophagy pathway (Ruggieri et al., 2020).

Calpainopathy is characterized by symmetric and progressive weakness of proximal limb-girdle muscles. The age at onset of muscle weakness ranges from two to 40 years. The phenotype shows intra- and interfamilial variability ranging from severe to mild. Three autosomal recessive calpainopathy phenotypes have been identified based on the distribution of muscle weakness and age at onset: Pelvifemoral limb-girdle muscular dystrophy (LGMD) (Leyden-Möbius LGMD) phenotype, the most frequently observed calpainopathy phenotype, in which muscle weakness is first evident in the pelvic girdle and later in the shoulder girdle, with onset that may occur as early as before age 12 years or as late as after age 30 years. Scapulohumeral LGMD (Erb LGMD) phenotype, usually a milder phenotype with infrequent early onset, in which muscle weakness is first evident in the shoulder girdle and later in the pelvic girdle. HyperCKemia, usually observed in children or young individuals, in which individuals are asymptomatic and have high serum creatine kinase (CK) concentrations. The autosomal dominant form of calpainopathy shows a variability of clinical phenotype, ranging from almost asymptomatic to wheelchair dependence after age 60 years in few cases with a generally milder phenotype than the recessive form. Clinical findings of calpainopathy include the tendency to walk on tiptoe, difficulty in running, scapular winging, waddling gait, and slight hyperlordosis. Other findings include symmetric weakness of proximal more than distal muscles in the limbs, trunk, and periscapular area; laxity of the abdominal muscles; Achilles tendon shortening; scoliosis; and joint contractures. Affected individuals typically do not have cardiac involvement or intellectual disability.

Mevalonic aciduria (MEVA), the first recognized defect in the biosynthesis of cholesterol and isoprenoids, is a consequence of a deficiency of mevalonate kinase (ATP:mevalonate 5-phosphotransferase; EC 2.7.1.36). Mevalonic acid accumulates because of failure of conversion to 5-phosphomevalonic acid, which is catalyzed by mevalonate kinase. Mevalonic acid is synthesized from 3-hydroxy-3-methylglutaryl-CoA, a reaction catalyzed by HMG-CoA reductase (142910). Mevalonic aciduria is characterized by dysmorphology, psychomotor retardation, progressive cerebellar ataxia, and recurrent febrile crises, usually manifesting in early infancy, accompanied by hepatosplenomegaly, lymphadenopathy, arthralgia, and skin rash. The febrile crises are similar to those observed in hyperimmunoglobulinemia D and to periodic fever syndrome (HIDS; 260920), which is also caused by mutation in the MVK gene (summary by Prietsch et al., 2003).

Any familial isolated dilated cardiomyopathy in which the cause of the disease is a mutation in the FKTN gene.

MDDGC4 is an autosomal recessive muscular dystrophy with onset in infancy or early childhood. Cognition and brain structure are usually normal (Godfrey et al., 2006). 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., 2009).

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 ANO5 muscle disease is a continuum that ranges from asymptomatic hyperCKemia and exercise-induced myalgia to proximal and/or distal muscle weakness. The most typical presentation is limb-girdle muscular dystrophy type 2L (LGMD2L) with late-onset proximal lower-limb weakness in the fourth or fifth decade (range 15-70 years). Less common is Miyoshi-like disease (Miyoshi muscular dystrophy 3) with early-adult-onset calf distal myopathy (around age 20 years). Incidental hyperCKemia may be present even earlier. Initial symptoms are walking difficulties, reduced sports performance, and difficulties in standing on toes as well as nonspecific exercise myalgia and/or burning sensation in the calf muscles. Muscle weakness and atrophy are frequently asymmetric. Cardiac findings can include cardiomyopathy and arrhythmias and/or left ventricular dysfunction. Bulbar or respiratory symptoms have not been reported. Females have milder disease manifestations than males. Disease progression is slow in both the LGMD and distal forms; ambulation is preserved until very late in the disease course. Life span is normal.

Syndrome with characteristics of severe psychomotor retardation, failure to thrive and intolerance to wheat and dairy products. So far, only two cases have been described. The disease is caused by mutations in the COG8 gene, which encodes a subunit of the COG complex. This complex is involved vesicle transport in the Golgi apparatus.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the NSUN2 gene.

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.

Salih myopathy is characterized by muscle weakness (manifest during the neonatal period or in early infancy) and delayed motor development; children acquire independent walking between ages 20 months and four years. In the first decade of life, global motor performance is stable or tends to improve. Moderate joint and neck contractures and spinal rigidity may manifest in the first decade but become more obvious in the second decade. Scoliosis develops after age 11 years. Cardiac dysfunction manifests between ages five and 16 years, progresses rapidly, and leads to death between ages eight and 20 years, usually from heart rhythm disturbances.

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.

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.

Emery-Dreifuss muscular dystrophy is characterized classically by the triad of weakness of the shoulder and pelvic girdle muscles, contractures of the elbows, neck, and Achilles tendon, and cardiac involvement, most commonly arrhythmias (summary by Jimenez-Escrig et al., 2012). For a discussion of genetic heterogeneity of EDMD, see 310300.

Congenital generalized lipodystrophy type 4 (CGL4) combines the phenotype of classic Berardinelli-Seip lipodystrophy (608594) with muscular dystrophy and cardiac conduction anomalies (Hayashi et al., 2009). For a general description and a discussion of genetic heterogeneity of congenital generalized lipodystrophy, see CGL1 (608594).

The spectrum of ANO5 muscle disease is a continuum that ranges from asymptomatic hyperCKemia and exercise-induced myalgia to proximal and/or distal muscle weakness. The most typical presentation is limb-girdle muscular dystrophy type 2L (LGMD2L) with late-onset proximal lower-limb weakness in the fourth or fifth decade (range 15-70 years). Less common is Miyoshi-like disease (Miyoshi muscular dystrophy 3) with early-adult-onset calf distal myopathy (around age 20 years). Incidental hyperCKemia may be present even earlier. Initial symptoms are walking difficulties, reduced sports performance, and difficulties in standing on toes as well as nonspecific exercise myalgia and/or burning sensation in the calf muscles. Muscle weakness and atrophy are frequently asymmetric. Cardiac findings can include cardiomyopathy and arrhythmias and/or left ventricular dysfunction. Bulbar or respiratory symptoms have not been reported. Females have milder disease manifestations than males. Disease progression is slow in both the LGMD and distal forms; ambulation is preserved until very late in the disease course. Life span is normal.

LMNA-related congenital muscular dystrophy (L-CMD) is a condition that primarily affects muscles used for movement (skeletal muscles). It is part of a group of genetic conditions called congenital muscular dystrophies, which cause weak muscle tone (hypotonia) and muscle wasting (atrophy) beginning very early in life.\n\nIn people with L-CMD, muscle weakness becomes apparent in infancy or early childhood and can worsen quickly. The most severely affected infants develop few motor skills, and they are never able to hold up their heads, roll over, or sit. Less severely affected children may learn to sit, stand, and walk before muscle weakness becomes apparent. First the neck muscles weaken, causing the head to fall forward (dropped-head syndrome). As other skeletal muscles become weaker, these children may ultimately lose the ability to sit, stand, and walk unassisted.\n\nOther features of L-CMD often include spinal rigidity and abnormal curvature of the spine (scoliosis and lordosis); joint deformities (contractures) that restrict movement, particularly in the hips and legs; and an inward-turning foot. People with L-CMD also have an increased risk of heart rhythm abnormalities (arrhythmias).\n\nOver time, muscle weakness causes most infants and children with L-CMD to have trouble eating and breathing. The breathing problems result from restrictive respiratory insufficiency, which occurs when muscles in the chest are weakened and the ribcage becomes increasingly rigid. This problem can be life-threatening, and many affected children require support with a machine to help them breathe (mechanical ventilation).

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.

MDDGB4 is a rare autosomal recessive congenital muscular dystrophy that is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies.' In contrast to most dystroglycanopathies, impaired intellectual development is not a feature of MDDGB4 (Godfrey et al., 2007). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

Any autosomal dominant Emery-Dreifuss muscular dystrophy in which the cause of the disease is a mutation in the SYNE2 gene.

Any autosomal dominant Emery-Dreifuss muscular dystrophy in which the cause of the disease is a mutation in the SYNE1 gene.

Myofibrillar myopathy-6 is an autosomal dominant severe neuromuscular disorder characterized by onset in the first decade of rapidly progressive generalized and proximal muscle weakness, respiratory insufficiency, cardiomyopathy, and skeletal deformities related to muscle weakness. Muscle biopsy shows fiber-type grouping, disruption of the Z lines, and filamentous inclusions, and sural nerve biopsy shows a neuropathy, often with giant axonal neurons. Most patients are severely affected by the second decade and need cardiac transplant, ventilation, and/or a wheelchair (summary by Jaffer et al., 2012). For a phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy (MFM), see MFM1 (601419).

Generally, Parkinson's disease that begins after age 50 is called late-onset disease. The condition is described as early-onset disease if signs and symptoms begin before age 50. Early-onset cases that begin before age 20 are sometimes referred to as juvenile-onset Parkinson's disease.\n\nParkinson's disease can also affect emotions and thinking ability (cognition). Some affected individuals develop psychiatric conditions such as depression and visual hallucinations. People with Parkinson's disease also have an increased risk of developing dementia, which is a decline in intellectual functions including judgment and memory.\n\nOften the first symptom of Parkinson's disease is trembling or shaking (tremor) of a limb, especially when the body is at rest. Typically, the tremor begins on one side of the body, usually in one hand. Tremors can also affect the arms, legs, feet, and face. Other characteristic symptoms of Parkinson's disease include rigidity or stiffness of the limbs and torso, slow movement (bradykinesia) or an inability to move (akinesia), and impaired balance and coordination (postural instability). These symptoms worsen slowly over time.\n\nParkinson's disease is a progressive disorder of the nervous system. The disorder affects several regions of the brain, especially an area called the substantia nigra that controls balance and movement.

Limb-girdle muscular dystrophy-dystroglycanopathy type C15 (MDDGC15) is an autosomal recessive disorder characterized by progressive proximal muscle weakness, manifest initially as unsteady gait, but later including more distal muscles, and dilated cardiomyopathy. The age at onset varies widely from the first decade to adulthood; those with earlier onset may have delayed motor development. Laboratory studies show increased serum creatine kinase and muscle biopsy shows dystrophic features with decreased alpha-dystroglycan (DAG1; 128239). Biochemical studies often show evidence of abnormal N-glycosylation of serum proteins, consistent with a congenital disorder of glycosylation (CDG) (summary by Svahn et al., 2019). For a discussion of genetic heterogeneity of muscular dystrophy- dystroglycanopathy type C, see MDDGC1 (609308). For a discussion of the classification of CDGs, see CDG1A (212065).

Congenital disorder of glycosylation type It (CDG1T) is an autosomal recessive disorder characterized by a wide range of clinical manifestations and severity. The most common features include cleft lip and bifid uvula, apparent at birth, followed by hepatopathy, intermittent hypoglycemia, short stature, and exercise intolerance, often accompanied by increased serum creatine kinase. Less common features include rhabdomyolysis, dilated cardiomyopathy, and hypogonadotropic hypogonadism (summary by Tegtmeyer et al., 2014). For a discussion of the classification of CDGs, see CDG1A (212065).

Muscle beta-enolase deficiency is a glycolysis disorder reported in one patient to date and characterized clinically by exercise intolerance and myalgia due to severe enolase deficiency in muscle.

Malignant hyperthermia susceptibility (MHS) is a pharmacogenetic disorder of skeletal muscle calcium regulation associated with uncontrolled skeletal muscle hypermetabolism. Manifestations of malignant hyperthermia (MH) are precipitated by certain volatile anesthetics (i.e., halothane, isoflurane, sevoflurane, desflurane, enflurane), either alone or in conjunction with a depolarizing muscle relaxant (specifically, succinylcholine). The triggering substances cause uncontrolled release of calcium from the sarcoplasmic reticulum and may promote entry of extracellular calcium into the myoplasm, causing contracture of skeletal muscles, glycogenolysis, and increased cellular metabolism, resulting in production of heat and excess lactate. Affected individuals experience acidosis, hypercapnia, tachycardia, hyperthermia, muscle rigidity, compartment syndrome, rhabdomyolysis with subsequent increase in serum creatine kinase (CK) concentration, hyperkalemia with a risk for cardiac arrhythmia or even cardiac arrest, and myoglobinuria with a risk for renal failure. In nearly all cases, the first manifestations of MH (tachycardia and tachypnea) occur in the operating room; however, MH may also occur in the early postoperative period. There is mounting evidence that some individuals with MHS will also develop MH with exercise and/or on exposure to hot environments. Without proper and prompt treatment with dantrolene sodium, mortality is extremely high.

Malignant hyperthermia susceptibility (MHS) is a pharmacogenetic disorder of skeletal muscle calcium regulation associated with uncontrolled skeletal muscle hypermetabolism. Manifestations of malignant hyperthermia (MH) are precipitated by certain volatile anesthetics (i.e., halothane, isoflurane, sevoflurane, desflurane, enflurane), either alone or in conjunction with a depolarizing muscle relaxant (specifically, succinylcholine). The triggering substances cause uncontrolled release of calcium from the sarcoplasmic reticulum and may promote entry of extracellular calcium into the myoplasm, causing contracture of skeletal muscles, glycogenolysis, and increased cellular metabolism, resulting in production of heat and excess lactate. Affected individuals experience acidosis, hypercapnia, tachycardia, hyperthermia, muscle rigidity, compartment syndrome, rhabdomyolysis with subsequent increase in serum creatine kinase (CK) concentration, hyperkalemia with a risk for cardiac arrhythmia or even cardiac arrest, and myoglobinuria with a risk for renal failure. In nearly all cases, the first manifestations of MH (tachycardia and tachypnea) occur in the operating room; however, MH may also occur in the early postoperative period. There is mounting evidence that some individuals with MHS will also develop MH with exercise and/or on exposure to hot environments. Without proper and prompt treatment with dantrolene sodium, mortality is extremely high.

Malignant hyperthermia susceptibility (MHS) is a pharmacogenetic disorder of skeletal muscle calcium regulation associated with uncontrolled skeletal muscle hypermetabolism. Manifestations of malignant hyperthermia (MH) are precipitated by certain volatile anesthetics (i.e., halothane, isoflurane, sevoflurane, desflurane, enflurane), either alone or in conjunction with a depolarizing muscle relaxant (specifically, succinylcholine). The triggering substances cause uncontrolled release of calcium from the sarcoplasmic reticulum and may promote entry of extracellular calcium into the myoplasm, causing contracture of skeletal muscles, glycogenolysis, and increased cellular metabolism, resulting in production of heat and excess lactate. Affected individuals experience acidosis, hypercapnia, tachycardia, hyperthermia, muscle rigidity, compartment syndrome, rhabdomyolysis with subsequent increase in serum creatine kinase (CK) concentration, hyperkalemia with a risk for cardiac arrhythmia or even cardiac arrest, and myoglobinuria with a risk for renal failure. In nearly all cases, the first manifestations of MH (tachycardia and tachypnea) occur in the operating room; however, MH may also occur in the early postoperative period. There is mounting evidence that some individuals with MHS will also develop MH with exercise and/or on exposure to hot environments. Without proper and prompt treatment with dantrolene sodium, mortality is extremely high.

Congenital disorders of glycosylation (CDG) are a group of hereditary multisystem disorders that are commonly associated with severe psychomotor and mental retardation. The characteristic biochemical abnormality of CDGs is the hypoglycosylation of glycoproteins, which is routinely determined by isoelectric focusing (IEF) of serum transferrin. Type I CDG comprises those disorders in which there is a defect in the assembly of lipid-linked oligosaccharides or their transfer onto nascent glycoproteins, whereas type II CDG comprises defects of trimming, elongation, and processing of protein-bound glycans (summary by Hansske et al., 2002). For a general discussion of CDGs, see CDG1A (212065).

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).

Infantile-onset autophagic vacuolar myopathy is characterized by increased cardiac and skeletal muscle glycogen with normal acid maltase (GAA; 606800). Skeletal muscle biopsy shows characteristic intracytoplasmic vacuoles that stain for sarcolemmal proteins and complement proteins. Similar pathologic findings are seen in Danon disease (300257), caused by mutation in the LAMP2 gene (309060) on chromosome Xq24, and X-linked myopathy with excessive autophagy (XMEA; 310440), which has been mapped to Xq28.

Myotonic dystrophy type 2 (DM2) is characterized by myotonia and muscle dysfunction (proximal and axial weakness, myalgia, and stiffness), and less commonly by posterior subcapsular cataracts, cardiac conduction defects, insulin-insensitive type 2 diabetes mellitus, and other endocrine abnormalities. While myotonia (involuntary muscle contraction with delayed relaxation) has been reported during the first decade, onset is typically in the third to fourth decade, most commonly with fluctuating or episodic muscle pain that can be debilitating and proximal and axial weakness of the neck flexors and the hip flexors. Subsequently, weakness occurs in the elbow extensors and finger flexors. Facial weakness and weakness of the ankle dorsiflexors are less common. Myotonia rarely causes severe symptoms. In a subset of individuals, calf hypertrophy in combination with brisk reflexes is notable.

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.

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.

Autosomal recessive limb-girdle muscular dystrophy-3 (LGMDR3) affects mainly the proximal muscles and results in difficulty walking. Most individuals have onset in childhood; the disorder is progressive. Other features may include scapular winging, calf pseudohypertrophy, and contractures. Cardiomyopathy has rarely been reported (summary by Babameto-Laku et al., 2011). For a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy, see LGMDR1 (253600).

TK2-related mitochondrial DNA (mtDNA) maintenance defect is a phenotypic continuum that ranges from severe to mild. To date, approximately 107 individuals with a molecularly confirmed diagnosis have been reported. Three main subtypes of presentation have been described: Infantile-onset myopathy with neurologic involvement and rapid progression to early death. Affected individuals experience progressive muscle weakness leading to respiratory failure. Some individuals develop dysarthria, dysphagia, and/or hearing loss. Cognitive function is typically spared. Juvenile/childhood onset with generalized proximal weakness and survival to at least 13 years. Late-/adult-onset myopathy with facial and limb weakness and mtDNA deletions. Some affected individuals develop respiratory insufficiency, chronic progressive external ophthalmoplegia, dysphagia, and dysarthria.

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. It represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (van Reeuwijk et al., 2005). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

MDDGB3 is an autosomal recessive congenital muscular dystrophy with impaired intellectual development and mild brain abnormalities (Clement et al., 2008). 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., 2009). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. It represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (Beltran-Valero de Bernabe et al., 2004). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. It represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (Godfrey et al., 2007). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

MDDGB2 is an autosomal recessive congenital muscular dystrophy associated with impaired intellectual development and mild structural brain abnormalities (Yanagisawa et al., 2007). It is part of a group of similar disorders, collectively known as 'dystroglycanopathies,' resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239) (Godfrey et al., 2007). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

MDDGC3 is a rare form of autosomal recessive limb-girdle muscular dystrophy with normal cognition (Clement et al., 2008). It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (Godfrey et al., 2007). For a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type C, see MDDGC1 (609308).

MDDGC2 is an autosomal recessive muscular dystrophy with onset after ambulation is achieved. Cognition is normal (Biancheri et al., 2007). It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (Godfrey et al., 2007). For a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type C, see MDDGC1 (609308).

Limb-girdle muscular dystrophy type 1H (LGMD1H) is an autosomal dominant disorder characterized by adult onset of progressive proximal muscle weakness affecting both the upper and lower limbs (Bisceglia et al., 2010). For a phenotypic description and a discussion of genetic heterogeneity of autosomal dominant limb-girdle muscular dystrophy, see LGMDD1 (603511).

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).

An autosomal recessive muscular dystrophy caused by mutations in the POMGNT1 gene. It is associated with characteristic brain and eye malformations, profound mental retardation, and death usually in the first years of life.

Infantile mitochondrial myopathy due to reversible COX deficiency is a rare mitochondrial disorder characterized by onset in infancy of severe hypotonia and generalized muscle weakness associated with lactic acidosis, but is distinguished from other mitochondrial disorders in that affected individuals recover spontaneously after 1 year of age (summary by Mimaki et al., 2010). See also transient infantile liver failure (LFIT; 613070), which is a similar disorder.

Mitochondrial membrane protein-associated neurodegeneration (MPAN) is characterized initially by gait changes followed by progressive spastic paresis, progressive dystonia (which may be limited to the hands and feet or more generalized), neuropsychiatric abnormalities (emotional lability, depression, anxiety, impulsivity, compulsions, hallucinations, perseveration, inattention, and hyperactivity), and cognitive decline. Additional early findings can include dysphagia, dysarthria, optic atrophy, axonal neuropathy, parkinsonism, and bowel/bladder incontinence. Survival is usually well into adulthood. End-stage disease is characterized by severe dementia, spasticity, dystonia, and parkinsonism.

CAV3-related distal myopathy is one form of distal myopathy, a group of disorders characterized by weakness and loss of function affecting the muscles farthest from the center of the body (distal muscles), such as those of the hands and feet. People with CAV3-related distal myopathy experience wasting (atrophy) and weakness of the small muscles in the hands and feet that generally become noticeable in adulthood. A bump or other sudden impact on the muscles, especially those in the forearms, may cause them to exhibit repetitive tensing (percussion-induced rapid contraction). The rapid contractions can continue for up to 30 seconds and may be painful. Overgrowth (hypertrophy) of the calf muscles can also occur in CAV3-related distal myopathy. The muscles closer to the center of the body (proximal muscles) such as the thighs and upper arms are normal in this condition.

Congenital myopathy-10A (CMYP10A) is a severe autosomal recessive skeletal muscle disorder characterized by generalized hypotonia, respiratory insufficiency, and poor feeding apparent from birth. Decreased fetal movements may be observed. More variable features include high-arched palate, distal joint contractures, foot deformities, scoliosis, areflexia, and dysphagia. Many patients show eventration of the diaphragm. Affected individuals become ventilator-dependent in the first months or years of life and never achieve walking; many die in childhood (Logan et al., 2011). Patients with more damaging mutations in the MEGF10 gene, including nonsense or frameshift null mutations, show the more severe phenotype (CMYP10A), whereas those with missense mutations affecting conserved cysteine residues in the EGF-like domain show the less severe phenotype with later onset of respiratory failure and minicores on muscle biopsy (CMYP10B) (Croci et al., 2022). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (117000).

Congenital myopathy-10B (CMYP10B) is an autosomal recessive skeletal muscle disorder characterized by infantile- or childhood-onset myopathy, areflexia, dysphagia, and respiratory distress that usually requires nocturnal ventilation. Other common features include facial and neck muscle weakness, feeding difficulties, contractures, scoliosis, high-arched palate, hyporeflexia, and difficulties walking. The disorder is slowly progressive and most patients follow a chronic course. Muscle biopsy shows variable findings, including type 1 fiber predominance, minicore lesions, and myofibrillar disorganization (Boyden et al., 2012; Harris et al., 2018). Patients with missense mutations affecting conserved cysteine residues in the EGF-like domain show the mild variant phenotype (CMYP10B) with later onset of respiratory failure and minicores on muscle biopsy, whereas patients with more damaging mutations, including nonsense or frameshift null mutations, show the severe variant phenotype (CMYP10A) (Croci et al., 2022). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (117000).

Primary coenzyme Q10 (CoQ10) deficiency is usually associated with multisystem involvement, including neurologic manifestations such as fatal neonatal encephalopathy with hypotonia; a late-onset slowly progressive multiple-system atrophy-like phenotype (neurodegeneration with autonomic failure and various combinations of parkinsonism and cerebellar ataxia, and pyramidal dysfunction); and dystonia, spasticity, seizures, and intellectual disability. Steroid-resistant nephrotic syndrome (SRNS), the hallmark renal manifestation, is often the initial manifestation either as isolated renal involvement that progresses to end-stage renal disease (ESRD), or associated with encephalopathy (seizures, stroke-like episodes, severe neurologic impairment) resulting in early death. Hypertrophic cardiomyopathy (HCM), retinopathy or optic atrophy, and sensorineural hearing loss can also be seen.

CDG2L is an autosomal recessive multisystem disorder apparent from birth or early infancy. It is characterized by poor growth, gastrointestinal and liver abnormalities, delayed psychomotor development, hypotonia, recurrent infections, hematologic abnormalities, increased bleeding tendency, and hyperhidrosis or hyperkeratosis. More variable features include nonspecific dysmorphic facial features and cardiac septal defects. The disorder often results in death in infancy or the first years of life (summary by Rymen et al., 2015). For a general discussion of CDGs, see CDG1A (212065) and CDG2A (212066).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. It represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (summary by Roscioli et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

CDG2K is an autosomal recessive disorder with a variable phenotype. Affected individuals show psychomotor retardation and growth retardation, and most have short stature. Other features include dysmorphism, hypotonia, eye abnormalities, acquired microcephaly, hepatomegaly, and skeletal dysplasia. Serum transferrin analysis shows a CDG type II pattern (summary by Foulquier et al., 2012). For a general discussion of CDGs, see CDG1A (212065) and CDG2A (212066).

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 DNA depletion syndrome-11 is an autosomal recessive mitochondrial disorder characterized by onset in childhood or adulthood of progressive external ophthalmoplegia (PEO), muscle weakness and atrophy, exercise intolerance, and respiratory insufficiency due to muscle weakness. More variable features include spinal deformity, emaciation, and cardiac abnormalities. Skeletal muscle biopsies show deletion and depletion of mitochondrial DNA (mtDNA) with variable defects in respiratory chain enzyme activities (summary by Kornblum et al., 2013). For a discussion of genetic heterogeneity of autosomal recessive mtDNA depletion syndromes, see MTDPS1 (603041).

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).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is an autosomal recessive disorder with congenital muscular dystrophy resulting in muscle weakness early in life and brain and eye anomalies. It is usually associated with delayed psychomotor development and shortened life expectancy. The phenotype includes the alternative clinical designations Walker-Warburg syndrome (WWS) and muscle-eye-brain disease (MEB). The disorder represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (summary by Stevens et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

Any familial isolated dilated cardiomyopathy in which the cause of the disease is a mutation in the CRYAB gene.

Myofibrillar myopathy refers to a genetically heterogeneous group of muscular disorders characterized by a pathologic morphologic pattern of myofibrillar degradation and abnormal accumulation of proteins involved with the sarcomeric Z disc (summary by Foroud et al., 2005). For a general phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

Amyotrophic lateral sclerosis-21 (ALS21) is an autosomal dominant neurodegenerative disorder affecting upper and lower motor neurons, resulting in muscle weakness and respiratory failure. Some patients may develop myopathic features or dementia (summary by Johnson et al., 2014). For a discussion of genetic heterogeneity of amyotrophic lateral sclerosis, see ALS1 (105400).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is an autosomal recessive disorder with congenital muscular dystrophy resulting in muscle weakness early in life and brain and eye anomalies. It is usually associated with delayed psychomotor development and shortened life expectancy. The phenotype includes the alternative clinical designations Walker-Warburg syndrome (WWS) and muscle-eye-brain disease (MEB). The disorder represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as dystroglycanopathies (summary by Stevens et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is a autosomal recessive disorder associated with severe neurologic defects and resulting in early infantile death. The phenotype includes the alternative clinical designations Walker-Warburg syndrome (WWS) and muscle-eye-brain disease (MEB). The disorder represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as dystroglycanopathies (summary by Buysse et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

MDDGB14 is an autosomal recessive congenital muscular dystrophy characterized by severe muscle weakness apparent in infancy and impaired intellectual development. Some patients may have additional features, such as microcephaly, cardiac dysfunction, seizures, or cerebellar hypoplasia. It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (summary by Carss et al., 2013). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

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).

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.

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.

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).

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.

Deficiency of very long-chain acyl-coenzyme A dehydrogenase (VLCAD), which catalyzes the initial step of mitochondrial beta-oxidation of long-chain fatty acids with a chain length of 14 to 20 carbons, is associated with three phenotypes. The severe early-onset cardiac and multiorgan failure form typically presents in the first months of life with hypertrophic or dilated cardiomyopathy, pericardial effusion, and arrhythmias, as well as hypotonia, hepatomegaly, and intermittent hypoglycemia. The hepatic or hypoketotic hypoglycemic form typically presents during early childhood with hypoketotic hypoglycemia and hepatomegaly, but without cardiomyopathy. The later-onset episodic myopathic form presents with intermittent rhabdomyolysis provoked by exercise, muscle cramps and/or pain, and/or exercise intolerance. Hypoglycemia typically is not present at the time of symptoms.

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.

Any tubular aggregate myopathy in which the cause of the disease is a mutation in the ORAI1 gene.

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.

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.

A form of limb-girdle muscular dystrophy presenting in infancy with muscle weakness and delayed motor development (eventually learning to walk at 18 months of age) followed by progressive proximal weakness, pseudohypertrophy of calf muscles, mild facial weakness and borderline intelligence.

Perrault syndrome is characterized by sensorineural hearing loss (SNHL) in males and females and ovarian dysfunction in females. SNHL is bilateral and ranges from profound with prelingual (congenital) onset to moderate with early-childhood onset. When onset is in early childhood, hearing loss can be progressive. Ovarian dysfunction ranges from gonadal dysgenesis (absent or streak gonads) manifesting as primary amenorrhea to primary ovarian insufficiency (POI) defined as cessation of menses before age 40 years. Fertility in affected males is reported as normal (although the number of reported males is limited). Neurologic features described in some individuals with Perrault syndrome include learning difficulties and developmental delay, cerebellar ataxia, and motor and sensory peripheral neuropathy.

Polyglucosan body myopathy-2 is an autosomal recessive disorder characterized by proximal muscle weakness of the lower limbs resulting in gait disturbances. Some patients also have involvement of the upper limbs and/or distal muscle weakness. The age at onset is highly variable, and the disorder is slowly progressive. Muscle biopsy shows accumulation of polyglucosan, which contains abnormally long and poorly branched glucosyl chains and is variably resistant to digestion by alpha-amylase (summary by Malfatti et al., 2014). For a discussion of genetic heterogeneity of PGBM, see PGBM1 (615895).

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.

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).

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+").

Reducing-body myopathy (RBM) is a rare myopathy characterized pathologically by the presence of intracytoplasmic inclusion bodies strongly stained by menadione-linked alpha-glycerophosphate dehydrogenase (MAG) in the absence of substrate, alpha-glycerophosphate. The term 'reducing body' refers to the reducing activity of the inclusions to nitroblue tetrazolium (NBT) in the absence of substrate. This condition is also commonly associated with rimmed vacuoles and cytoplasmic bodies. The clinical features of RBM are variable; a severe form has onset in infancy or early childhood and results in severe disability or early death (RBMX1A; 300717), and a less severe form has onset in late childhood or adulthood (RBMX1B) (summary by Liewluck et al., 2007 and Shalaby et al., 2009).

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).

Hyperphosphatasia with impaired intellectual development syndrome-6 (HPMRS6) is an autosomal recessive multisystem disorder characterized by global developmental delay, dysmorphic features, seizures, and congenital cataracts. Severity is variable, and the disorder may show a range of phenotypic and biochemical abnormalities, including increased serum alkaline phosphatase levels (summary by Ilkovski et al., 2015). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of HPMRS, see HPMRS1 (239300). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, and congenital muscular dystrophy. The phenotype includes the alternative clinical designation Walker-Warburg syndrome (WWS), which is associated with death in infancy. The disorder represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1), collectively known as 'dystroglycanopathies' (summary by Geis et al., 2013 and Riemersma et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

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).

Bethlem myopathy-2 (BTHLM2) is characterized by congenital hypotonia and myopathy. Motor development is delayed, but muscle strength improves with age, and patients are able to achieve ambulation. Proximal joint contractures that improve over time, as well as joint hyperlaxity, are also present. Muscle biopsy shows mild variability in fiber diameter, without degeneration or regeneration (Zou et al., 2014; Hicks et al., 2014). For a discussion of genetic heterogeneity of Bethlem myopathy, see BTHLM1 (158810).

Reducing-body myopathy (RBM) is a rare myopathy characterized pathologically by the presence of intracytoplasmic inclusion bodies strongly stained by menadione-linked alpha-glycerophosphate dehydrogenase (MAG) in the absence of substrate, alpha-glycerophosphate. The term 'reducing body' refers to the reducing activity of the inclusions to nitroblue tetrazolium (NBT) in the absence of substrate. This condition is also commonly associated with rimmed vacuoles and cytoplasmic bodies. The clinical features of RBM are variable; a severe form has onset in infancy or early childhood and results in severe disability or early death, and a less severe form has onset in late childhood or adulthood (RBMX1B; 300718) (summary by Liewluck et al., 2007 and Shalaby et al., 2009).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is a genetically heterogeneous autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and early death. The phenotype commonly includes cobblestone (type II) lissencephaly, cerebellar malformations, and retinal malformations. More variable features include macrocephaly or microcephaly, hypoplasia of midline brain structures, ventricular dilatation, microphthalmia, cleft lip/palate, and congenital contractures (Dobyns et al., 1989). Those with a more severe phenotype characterized as Walker-Warburg syndrome often die within the first year of life, whereas those characterized as having muscle-eye-brain disease may rarely acquire the ability to walk and to speak a few words. These are part of a group of disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (Godfrey et al., 2007). Genetic Heterogeneity of Congenital Muscular Dystrophy-Dystroglycanopathy with Brain and Eye Anomalies (Type A) Muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is genetically heterogeneous and can be caused by mutation in other genes involved in DAG1 glycosylation: see MDDGA2 (613150), caused by mutation in the POMT2 gene (607439); MDDGA3 (253280), caused by mutation in the POMGNT1 gene (606822); MDDGA4 (253800), caused by mutation in the FKTN gene (607440); MDDGA5 (613153), caused by mutation in the FKRP gene (606596); MDDGA6 (613154), caused by mutation in the LARGE gene (603590); MDDGA7 (614643), caused by mutation in the ISPD gene (CRPPA; 614631); MDDGA8 (614830) caused by mutation in the GTDC2 gene (POMGNT2; 614828); MDDGA9 (616538), caused by mutation in the DAG1 gene (128239); MDDGA10 (615041), caused by mutation in the TMEM5 gene (RXYLT1; 605862); MDDGA11 (615181), caused by mutation in the B3GALNT2 gene (610194); MDDGA12 (615249), caused by mutation in the SGK196 gene (POMK; 615247); MDDGA13 (615287), caused by mutation in the B3GNT1 gene (B4GAT1; 605517); and MDDGA14 (615350), caused by mutation in the GMPPB gene (615320).

Autosomal dominant intermediate Charcot-Marie-Tooth disease E with focal segmental glomerulonephritis is characterized by the neurologic features of CMT, including distal muscle weakness and atrophy and distal sensory loss, and the features of FSGS, including proteinuria, progression to end-stage renal disease, and a characteristic histologic pattern on renal biopsy (summary by Boyer et al., 2011). Isolated focal segmental glomerulosclerosis-5 (FSGS5; 613237) is also caused by heterozygous mutation in the INF2 gene. For a discussion of genetic heterogeneity of CMTDI, see 606482.

Myofibrillar myopathy-8 (MFM8) is an autosomal recessive myopathy characterized by slowly progressive proximal muscle weakness and atrophy affecting the upper and lower limbs, resulting in increased falls, gait problems, difficulty running or climbing stairs, and upper limb weakness or scapular winging. Some patients develop distal muscle weakness and atrophy. The phenotype may also be consistent with a clinical diagnosis of limb-girdle muscular dystrophy (LGMD). Age at symptom onset ranges from infancy to adulthood. Ambulation is generally preserved and cardiac involvement is rare, but respiratory compromise with decreased forced vital capacity often occurs. Muscle biopsy shows a mix of myopathic features, including myofibrillar inclusions and sarcomeric disorganization; some patients have been reported to have dystrophic changes on muscle biopsy (O'Grady et al., 2016; Daimaguler et al., 2021). There is significant phenotypic variation, even in patients with the same mutation, which must be taken into account when counseling affecting individuals (Woods et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

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).

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).

Distal arthrogryposis with impaired proprioception and touch is an autosomal recessive neurologic disorder characterized by loss of certain mechanosensation modalities resulting in ataxia, difficulty walking, dysmetria, muscle weakness and atrophy, and progressive skeletal contractures. Patients have onset of symptoms in early childhood (summary by Chesler et al., 2016 and Delle Vedove et al., 2016).

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.

A rare genetic neurodegenerative disease with characteristics of sudden onset of progressive motor deterioration and regression of developmental milestones. Manifestations include dystonia and muscle spasms, dysphagia, dysarthria, and eventually loss of speech and ambulation. Brain MRI shows predominantly striatal abnormalities. The disease is potentially associated with a fatal outcome.

SLC39A14 deficiency is characterized by evidence between ages six months and three years of delay or loss of motor developmental milestones (e.g., delayed walking, gait disturbance). Early in the disease course, children show axial hypotonia followed by dystonia, spasticity, dysarthria, bulbar dysfunction, and signs of parkinsonism including bradykinesia, hypomimia, and tremor. By the end of the first decade they develop severe, generalized, pharmaco-resistant dystonia, limb contractures, and scoliosis, and lose independent ambulation. Cognitive impairment appears to be less prominent than motor disability. Some affected children have succumbed in their first decade due to secondary complications such as respiratory infections.

Axonal Charcot-Marie-Tooth disease type 2CC is an autosomal dominant peripheral neuropathy that predominantly affects the lower limbs, resulting in muscle weakness and atrophy and gait impairment. Other features include distal sensory impairment and less severe involvement of the upper limbs. The age at onset and severity are variable (summary by Rebelo et al., 2016). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT type 2, see CMT2A (118210).

Lipid storage myopathy due to FLAD1 deficiency is an autosomal recessive inborn error of metabolism that includes variable mitochondrial dysfunction. The phenotype is extremely heterogeneous: some patients have a severe disorder with onset in infancy and cardiac and respiratory insufficiency resulting in early death, whereas others have a milder course with onset of muscle weakness in adulthood. Some patients show significant improvement with riboflavin treatment (summary by Olsen et al., 2016).

Developmental and epileptic encephalopathy-53 (DEE53) is a severe autosomal recessive neurodegenerative disorder characterized by onset of intractable seizures in infancy. Affected individuals show hypotonia and very poor or absent global development, resulting in severe intellectual disability and spastic quadriplegia. Some patients may die in childhood (summary by Hardies et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

MDCCAID is an autosomal recessive form of muscular dystrophy with onset of progressive muscle weakness in early childhood. Almost all patients also have early-onset cataracts, most have intellectual disability of varying severity, and some have seizures (summary by Wiessner et al., 2017 and Osborn et al., 2017).

Autosomal recessive myopathy with rigid spine and distal joint contractures (MRRSDC) is characterized by onset of slowly progressive muscle weakness in the first or second decades of life. There is initial involvement of the proximal lower limbs, followed by distal upper and lower limb muscle weakness and atrophy. Other features include joint contractures, rigid spine, and restricted pulmonary function; some patients may have mild cardiac involvement (summary by Kayman-Kurekci et al., 2014).

MDDGC9 is an autosomal recessive muscular dystrophy showing onset in early childhood. It is part of a group of similar disorders resulting from defective glycosylation of DAG1, collectively known as 'dystroglycanopathies' (summary by Hara et al., 2011). For a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type C, see MDDGC1 (609308).

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).

MDDGC14 is an autosomal recessive form of muscular dystrophy characterized by onset in early childhood of mild proximal muscle weakness. Some patients may have additional features, such as mild intellectual disability or seizures. It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (summary by Carss et al., 2013). Some patients with GMPPB mutations may show features consistent with a congenital myasthenic syndrome (see, e.g., CMS1A; 601462), such as fatigability and decremental compound muscle action potential response to repetitive nerve stimulation; these patients may show a positive therapeutic response to treatment with pyridostigmine (Belaya et al., 2015). For a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type C, see MDDGC1 (609308).

ISCA1-related multiple mitochondrial dysfunctions syndrome (ISCA1-MMDS) is a severe neurodegenerative condition typically characterized by either no attainment of developmental milestones or very early loss of achieved milestones, seizures in early infancy, development of spasticity with exaggerated deep tendon reflexes, nystagmus, and risk for sensorineural hearing loss. Affected individuals may also demonstrate elevated blood lactate levels with an elevated lipid-lactate peak on brain MR spectroscopy. Further brain MRI findings may include extensive cerebral and cerebellar deep white matter hyperintensities, marked dilatation of the cerebral ventricles, and pachygyria. Prognosis is poor and most individuals succumb to an intercurrent illness in early childhood.

Mitochondrial myopathy and ataxia (MMYAT) is an autosomal recessive mtDNA depletion disorder characterized by cerebellar ataxia, congenital muscle involvement with histologic findings ranging from myopathic to dystrophic, and pigmentary retinopathy (summary by Donkervoort et al., 2019).

COXPD33 is an autosomal recessive multisystem disorder resulting from a defect in mitochondrial energy metabolism. The phenotype is highly variable, ranging from death in infancy to adult-onset progressive external ophthalmoplegia (PEO) and myopathy. A common finding is cardiomyopathy and increased serum lactate (summary by Feichtinger et al., 2017). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

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).

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.

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.

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.

Laing distal myopathy is characterized by early-onset weakness (usually before age 5 years) that initially involves the dorsiflexors of the ankles and great toes and then the finger extensors, especially those of the third and fourth fingers. Weakness of the neck flexors is seen in most affected individuals and mild facial weakness is often present. After distal weakness has been present for more than ten years, mild proximal weakness may be observed. Life expectancy is normal.

CMTDIG is an autosomal dominant neurologic disorder with a highly variable phenotype. Most affected individuals have onset in the first or second decades of slowly progressive distal motor weakness and atrophy, resulting in gait instability and distal upper limb impairment, as well as distal sensory impairment. More severely affected individuals may have pes cavus and claw hands and become wheelchair-bound, whereas other affected individuals have later onset with a milder disease course. Electrophysiologic studies tend to show median motor nerve conduction velocities (NCV) in the 'intermediate' range, between 25 and 45 m/s (summary by Berciano et al., 2017). In a review of intermediate CMT, Berciano et al. (2017) noted that advanced axonal degeneration may induce secondary demyelinating changes resulting in decreased NCV and attenuated compound muscle action potential (CMAP) in median nerve conduction studies. They thus suggested that testing the upper arm, axilla to elbow, may provide more accurate assessment of NCV and CMAP and reveal an intermediate phenotype (review by Berciano et al., 2017). For a discussion of genetic heterogeneity of CMTDI, see 606482.

A rare lethal congenital myopathy syndrome characterized by decreased fetal movements and polyhydramnios in utero and the presence of akinesia, severe hypotonia with respiratory insufficiency, absent reflexes, joint contractures, skeletal abnormalities with thin ribs and bones, intracranial and retinal hemorrhages and decreased birth weight in the neonate.

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).

Autosomal dominant limb-girdle muscular dystrophy is characterized by proximal and/or distal muscle weakness and atrophy. The age at onset is variable and can range from the first to the sixth decade, although later onset is less common. Most patients present with proximal muscle weakness that progresses to distal involvement, but some can present with distal impairment. The severity is variable: patients with a more severe phenotype can lose ambulation after several decades and have facial weakness with bulbar and respiratory involvement. Muscle biopsy shows dystrophic changes with protein aggregates, myofibrillar degeneration, and rimmed vacuoles (summary by Ruggieri et al., 2015). Genetic Heterogeneity of Autosomal Dominant Limb-Girdle Muscular Dystrophy Other forms of autosomal dominant LGMD include LGMDD2 (608423), previously LGMD1F, caused by mutation in the TNPO3 gene (610032) on chromosome 7q32; LGMDD3 (609115), previously LGMD1G, caused by mutation in the HNRNPDL gene (607137) on chromosome 4q21; and LGMDD4 (618129), previously LGMD1I, caused by mutation in the CAPN3 gene (114240) on chromosome 15q15. For a discussion of autosomal recessive LGMD, see 253600.

Myofibrillar myopathy-4 (MFM4) is an autosomal dominant disorder characterized by adult-onset distal muscle weakness primarily affecting the lower limbs at onset. Affected individuals usually present with gait difficulties in their forties, followed by slow progression with eventual involvement of the hands and proximal muscles of the lower limbs. Rare patients may develop cardiomyopathy. Skeletal muscle biopsy shows myopathic changes with myofibrillar changes (Selcen and Engel, 2005; Griggs et al., 2007). For a phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

SMALED2A is an autosomal dominant form of spinal muscular atrophy characterized by early childhood onset of muscle weakness and atrophy predominantly affecting the proximal and distal muscles of the lower extremity, although some patients may show upper extremity involvement. The disorder results in delayed walking, waddling gait, difficulty walking, and loss of distal reflexes. Some patients may have foot deformities or hyperlordosis, and some show mild upper motor signs, such as spasticity. Sensation, bulbar function, and cognitive function are preserved. The disorder shows very slow progression throughout life (summary by Oates et al., 2013). For discussion of genetic heterogeneity of lower extremity-predominant spinal muscular atrophy, see SMALED1 (158600).

Autosomal dominant limb-girdle muscular dystrophy-4 (LGMDD4) is characterized by onset of proximal muscle weakness in young adulthood. Affected individuals often have gait difficulties; some may have upper limb involvement. Other features include variably increased serum creatine kinase, myalgia, and back pain. The severity and expressivity of the disorder is highly variable, even within families (summary by Vissing et al., 2016). For a discussion of genetic heterogeneity of autosomal dominant limb-girdle muscular dystrophy, see 603511.

MDDGC8 is an autosomal recessive muscular dystrophy with onset in childhood. The phenotype is highly variable: some patients may have gait difficulties and impaired intellectual development, whereas others may be clinically asymptomatic. Common features include calf hypertrophy and increased serum creatine kinase, and muscle biopsy often shows dystrophic features (summary by Endo et al., 2015). The disorder is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (Godfrey et al., 2007). For a discussion of genetic heterogeneity of muscular dystrophy- dystroglycanopathy type C, see MDDGC1 (609308).

The clinical manifestations of LAMA2 muscular dystrophy (LAMA2-MD) comprise a continuous spectrum ranging from severe congenital muscular dystrophy type 1A (MDC1A) to milder late-onset LAMA2-MD. MDC1A is typically characterized by neonatal profound hypotonia, poor spontaneous movements, and respiratory failure. Failure to thrive, gastroesophageal reflux, aspiration, and recurrent chest infections necessitating frequent hospitalizations are common. As disease progresses, facial muscle weakness, temporomandibular joint contractures, and macroglossia may further impair feeding and can affect speech. In late-onset LAMA2-MD onset of manifestations range from early childhood to adulthood. Affected individuals may show muscle hypertrophy and develop a rigid spine syndrome with joint contractures, usually most prominent in the elbows. Progressive respiratory insufficiency, scoliosis, and cardiomyopathy can occur.

A rare fatal inborn error of metabolism disorder with characteristics of respiratory distress and severe hypotonia at birth, severe global developmental delay, early-onset intractable seizures, myopathic facies with craniofacial dysmorphism (trigonocephaly/progressive microcephaly, low anterior hairline, arched eyebrows, hypotelorism, strabismus, small nose, prominent philtrum, thin upper lip, high-arched palate, micrognathia, malocclusion), severe, congenital flexion joint contractures and elevated serum creatine kinase levels. Scoliosis, optic atrophy, mild hepatomegaly, and hypoplastic genitalia may also be associated. There is evidence the disease is caused by homozygous or compound heterozygous mutation in the DPM2 gene on chromosome 9q34.

Fatal infantile hypertonic myofibrillar myopathy is a severe autosomal recessive muscular dystrophy with onset in the first weeks of life after a normal neonatal period. Affected infants show rapidly progressive muscular rigidity of the trunk and limbs associated with increasing respiratory difficulty resulting in death before age 3 years (summary by Del Bigio et al., 2011).

Progressive myoclonic epilepsy-6 (EPM6) is an autosomal recessive neurologic disorder characterized by onset of ataxia in the first years of life, followed by action myoclonus and seizures later in childhood, and loss of independent ambulation in the second decade. Cognition is not usually affected, although mild memory difficulties may occur in the third decade (summary by Corbett et al., 2011). For a discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800).

A rare subtype of autosomal recessive limb-girdle muscular dystrophy disorder with characteristics of infantile to childhood-onset of slowly progressive, principally proximal shoulder and/or pelvic-girdle muscular weakness that typically presents with positive Gowers'' sign and is associated with elevated creatine kinase levels, hyporeflexia, joint and achilles tendon contractures and muscle hypertrophy usually of the thighs, calves and/or tongue. Other highly variable features include cerebellar, cardiac and ocular abnormalities.

Spinocerebellar ataxia with axonal neuropathy-3 (SCAN3) is an autosomal recessive neuromuscular disorder characterized by onset in the first decade of slowly progressive distal muscle weakness and atrophy and distal sensory impairment due to an axonal peripheral neuropathy. Affected individuals have gait disturbances and sometimes manual dexterity difficulties, as well as cerebellar ataxia associated with cerebellar atrophy on brain imaging. Additional features usually include dysarthria, hyporeflexia, and increased serum creatine kinase. Some patients may have impaired intellectual development (summary by Higuchi et al., 2018). For a discussion of genetic heterogeneity of SCAN, see SCAN1 (607250).

Recurrent metabolic crises with variable encephalomyopathic features and neurologic regression (MECREN) is an autosomal recessive metabolic disorder with a highly variable phenotype. Most affected individuals present in the first years of life with episodic lactic acidosis associated with illness or stress, resulting in transient or permanent neurologic dysfunction. Some patients may recover, whereas others show subsequent variable developmental regression of motor and cognitive skills. Other features may include dystonia, hypotonia with inability to sit or walk, seizures, and abnormal signals in the basal ganglia. There is significant phenotypic heterogeneity, even among patients with the same mutation (summary by Almannai 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)

Autosomal dominant adult-onset distal myopathy-6 (MPD6) is a muscle disorder characterized by slowly progressive distal muscle weakness, primarily affecting the lower limbs and resulting in gait difficulties. Some patients develop involvement of proximal and upper limb muscles (summary by Savarese et al., 2019)

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).

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

Combined oxidative phosphorylation deficiency-40 (COXPD40) is an autosomal recessive mitochondrial disorder with onset in utero or soon after birth. Affected individuals have severe hypertrophic cardiomyopathy, poor growth, and sensorineural hearing loss. Laboratory studies show evidence of mitochondrial dysfunction, such as lactic acidosis. Patient-derived tissues and cells show variably decreased activities of mitochondrial respiratory complexes I, III, IV, and V. The disorder is lethal, with no reported patients surviving past infancy (summary by Friederich et al., 2018). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Combined oxidative phosphorylation deficiency-42 (COXPD42) is an autosomal recessive metabolic disorder characterized by onset of cardiomyopathy, respiratory insufficiency, lactic metabolic acidosis, and anemia in the first months of life. Patient tissue shows variable impairment of mitochondrial oxidative phosphorylation affecting mtDNA-encoded subunits I, III, and IV. All reported affected infants have died in the first year of life (summary by Friederich et al., 2018). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Oculopharyngodistal myopathy-2 (OPDM2) is an autosomal dominant muscle disorder characterized by onset of distal muscle weakness, mainly of the lower limbs, and/or ophthalmoplegia in the second or third decades of life. The disorder is slowly progressive, and patients develop facial weakness, bulbar weakness, and difficulty walking or climbing stairs. Some patients may have upper limb involvement and subclinical respiratory insufficiency. Laboratory studies show increased serum creatine kinase; skeletal muscle biopsy shows myopathic changes with abnormal cytoplasmic and intranuclear inclusions (summary by Deng et al., 2020). For a discussion of genetic heterogeneity of OPDM, see OPDM1 (164310).

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).

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).

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).

Mitochondrial complex IV deficiency nuclear type 11 (MC4DN11) is an autosomal recessive metabolic disorder characterized by a childhood-onset sensory neuronopathy and additional features which may include hypotonia, cerebellar ataxia, tremor, dystonia, choreoathetosis, and/or dysarthria. Patients may have variable motor delay, speech delay, or impaired intellectual development (summary by Doss et al., 2014; Otero et al., 2019; Xu et al., 2019; Dong et al., 2021). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Mitochondrial complex IV deficiency nuclear type 12 (MC4DN12) is an autosomal recessive metabolic disorder characterized by the onset of neurologic dysfunction in early infancy. Affected individuals demonstrate hypotonia with poor head control, profoundly delayed global development with inability to fix and follow, poor overall growth, abnormal spasms or myoclonus, and seizures. Most patients die in the first years of life; those that survive have spastic quadriplegia, feeding difficulties necessitating tube feeding, and profoundly impaired intellectual development with poor or absent communication. More variable features include cortical blindness, nystagmus, scoliosis, and hearing impairment. Brain imaging shows abnormalities consistent with Leigh syndrome (see 256000), as well as cystic cavitation. Laboratory studies show lactic acidosis, increased serum creatine kinase, and decreased levels and activity of mitochondrial respiratory complex IV (summary by Lim et al., 2014). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Congenital muscular dystrophies resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239) are characterized by early onset of muscle weakness, usually before ambulation is achieved; intellectual disability mild brain anomalies are variable (Balci et al., 2005; Godfrey et al., 2007). Congenital muscular dystrophy-dystroglycanopathies with or without impaired intellectual development (type B) represent the intermediate range of the spectrum of dystroglycanopathies. They are less severe than muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A; see MDDGA1, 236670), previously designated Walker-Warburg syndrome (WWS) or muscle-eye-brain disease (MEB), and more severe than limb-girdle muscular dystrophy-dystroglycanopathy (type C; see MDDGC1, 609308). Genetic Heterogeneity of Congenital Muscular Dystrophy-Dystroglycanopathy with or without Impaired Intellectual Development (Type B) Congenital muscular dystrophy with impaired intellectual development due to defective glycosylation of DAG1 is genetically heterogeneous. See also MDDGB2 (613156), caused by mutation in the POMT2 gene (607439); MDDGB3 (613151), caused by mutation in the POMGNT1 gene (606822); MDDGB4 (613152), caused by mutation in the FKTN gene (607440); MDDGB5 (616612), caused by mutation in the FKRP gene (606596); MDDGB6 (608840), caused by mutation in the LARGE gene (603590); MDDGB14 (615351), caused by mutation in the GMPPB gene (615320); and MDDGB15 (618992), caused by mutation in the DPM3 gene (605951).

X-linked multiple congenital anomalies-neurodevelopmental syndrome (MCAND) is an X-linked recessive congenital multisystemic disorder characterized by poor growth, global developmental delay with impaired intellectual development, and variable abnormalities of the cardiac, skeletal, and genitourinary systems. Most affected individuals also have hypotonia and dysmorphic craniofacial features. Brain imaging typically shows enlarged ventricles and thin corpus callosum; some have microcephaly, whereas others have hydrocephalus. The severity of the disorder is highly variable, ranging from death in early infancy to survival into the second or third decade. Pathogenetically, the disorder results from disrupted gene expression and signaling during embryogenesis, thus affecting multiple systems (summary by Tripolszki et al., 2021 and Beck et al., 2021). Beck et al. (2021) referred to the disorder as LINKED syndrome (LINKage-specific deubiquitylation deficiency-induced Embryonic Defects).

Sulfide:quinone oxidoreductase-deficiency (SQORD) is characterized by a variable phenotype ranging from no clinical symptoms to episodes of encephalopathy and Leigh syndrome-like (see 256000) brain lesions, with acute symptoms triggered by infections and fasting. Other features may include lactic acidosis and decreased mitochondrial respiratory chain complex IV activity in tissues. Most affected individuals are asymptomatic. Patients with encephalopathy may recover or die in childhood (Friederich et al., 2020).

Mitochondrial complex IV deficiency nuclear type 22 (MC4DN22) is an autosomal recessive metabolic disorder characterized by neonatal hypertrophic cardiomyopathy, encephalopathy, and severe lactic acidosis with fatal outcome (Wintjes et al., 2021). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Combined oxidative phosphorylation deficiency-52 (COXPD52) is an autosomal recessive infantile mitochondrial complex II/III deficiency characterized by lactic acidemia, multiorgan system failure, and abnormal mitochondria. Intrafamilial variability has been reported (Farhan et al., 2014; Hershkovitz et al., 2021). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Autosomal recessive spinocerebellar ataxia-30 (SCAR30) is a progressive neurologic disorder characterized by childhood-onset global developmental delay with variably impaired intellectual development, motor dysfunction, and cerebellar ataxia. Affected individuals may also have psychiatric abnormalities, such as obsessive behavior, psychotic episodes, or hallucinations. Brain imaging usually shows cerebellar atrophy, although this may be an age-dependent feature (summary by Langer et al., 2018).

Infantile-onset myofibrillar myopathy-12 with cardiomyopathy (MFM12) is a severe autosomal recessive disorder affecting both skeletal and cardiac muscle tissue that is apparent in the first weeks of life. Affected infants show tremor or clonus at birth, followed by onset of rapidly progressive generalized muscle weakness and dilated cardiomyopathy and cardiac failure, usually resulting in death by 6 months of age. Skeletal and cardiac muscle tissues show hypotrophy of type I muscle fibers and evidence of myofibrillar disorganization (summary by Weterman et al., 2013). For a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

Oculopharyngodistal myopathy-3 (OPDM3) is a neuromyodegenerative disease characterized by progressive muscle weakness with ocular, facial, pharyngeal, and distal limb involvement, resulting in dysarthria and gait difficulties. The onset of the disorder is usually in adulthood, although childhood onset has rarely been reported. Additional features include hyporeflexia, proximal muscle weakness, neck muscle weakness, dysarthria, dysphagia, and ptosis. Some patients may develop pigmentary retinopathy, peripheral neuropathy, or hearing loss. Cognition is usually not affected, but there may be deficits or psychiatric manifestations. Brain imaging tends to show a leukoencephalopathy, often with a characteristic linear signal along the corticomedullary junction on brain imaging. Skin and muscle biopsy show intranuclear inclusions and rimmed vacuoles. Many of the clinical features are reminiscent of NIID, suggesting that these disorders likely fall within a broad phenotypic spectrum of diseases with neuromyodegenerative features associated with abnormal repeat expansions in this gene (summary by Ogasawara et al., 2020 and Yu et al., 2021). For a discussion of genetic heterogeneity of OPDM, see OPDM1 (164310).

Muscular dystrophy, congenital hearing loss, and ovarian insufficiency syndrome (MDHLO) is an autosomal recessive systemic disorder characterized by progressive muscle weakness, sensorineural hearing loss, and endocrine abnormalities, mainly primary amenorrhea due to ovarian insufficiency. Features of the disorder appear soon after birth, although endocrine anomalies are not noted until puberty. The severity of the phenotype is variable: some patients may lose ambulation and have significant respiratory insufficiency, whereas others retain the ability to walk (Foley et al., 2020).

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).

Individuals with TANGO2-related metabolic encephalopathy and arrhythmias can present in acute metabolic crisis (hypoglycemia, elevated lactate, mild hyperammonemia) or with developmental delay, regression, and/or seizures. The acute presentation varies from profound muscle weakness, ataxia, and/or disorientation to a comatose state. Individuals can present with intermittent acute episodes of rhabdomyolysis. The first episode of myoglobinuria has been known to occur as early as age five months. Acute renal tubular damage due to myoglobinuria can result in acute kidney injury and renal failure. During acute illness, transient electrocardiogram changes can be seen; the most common is QT prolongation. Life-threatening recurrent ventricular tachycardia or torsade de pointes occurs primarily during times of acute illness. Individuals who do not present in metabolic crises may present with gait incoordination, progressively unsteady gait, difficulty with speech, or clumsiness. Intellectual disability of variable severity is observed in almost all individuals. Seizures are observed outside the periods of crises in more than 75% of individuals. Hypothyroidism has been reported in more than one third of individuals.

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).

Charcot-Marie-Tooth disease type 2Y is an autosomal dominant peripheral neuropathy characterized by distal muscle weakness and atrophy associated with length-dependent sensory loss. Most patients have involvement of both the lower and upper limbs. The age at onset and the severity of the disorder are highly variable (summary by Gonzalez et al., 2014). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

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.

Inclusion body myopathy and brain white matter abnormalities (IBMWMA) is an autosomal dominant adult-onset disorder characterized predominantly by proximal limb girdle muscle weakness affecting the lower and upper limbs and resulting in gait difficulties and scapular winging. Additional features may include dysarthria, dysphagia, low back pain, and hyporeflexia. EMG is consistent with a myopathic process, although neuropathic findings have also been shown. Muscle biopsy shows fiber type variation, internal nuclei, rimmed vacuoles, and cytoplasmic protein aggregates or inclusions. Serum creatine kinase is usually elevated. Cognitive impairment or frontotemporal dementia occurs in some patients. The disorder is slowly progressive; some patients become wheelchair-bound after many years. Rare patients with this mutation develop ALS; some have both myopathy and ALS. Brain imaging shows white matter abnormalities using diffusion tensor imaging. The disorder is classified as multisystem proteinopathy-6 (MSP6) due to the characteristic disease mechanism of protein misfolding and abnormal tissue deposition (summary by Leoni et al., 2021).

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).

Oculopharyngodistal myopathy-4 (OPDM4) is an autosomal dominant neuromuscular disorder characterized by progressive ptosis, ophthalmoparesis, facial and masseter weakness, and muscle weakness of the distal limbs. Initial symptoms of the disorder, ptosis and limited eye movements, most commonly appear in the second or third decades. There is slow progression with development of dysarthria, dysphagia, and distal limb weakness and atrophy associated with absent deep tendon reflexes; sensation is normal. Serum creatine kinase is often increased, and skeletal muscle biopsy typically shows chronic myopathic changes with rimmed vacuoles and filamentous intranuclear inclusions (summary by Yu et al., 2022). For a discussion of genetic heterogeneity of OPDM, see OPDM1 (164310).

Dworschak-Punetha neurodevelopmental syndrome (DWOPNED) is an autosomal recessive disorder characterized mainly by global developmental delay and mildly impaired intellectual development (IQ range 77 to 85), often with behavioral abnormalities, including autism spectrum disorder and hyperactivity. Some affected individuals may have only speech delay or behavioral manifestations. More variable additional features include optic disc hypoplasia, ptosis, hypo- or hyperpigmented skin lesions, nonspecific dysmorphic facial features, and brain imaging abnormalities of the ventricles or corpus callosum. Of note, not all patients exhibit all features, and there is significant inter- and intrafamilial phenotypic variability (Dworschak et al., 2021).

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).

Myopathy with myalgia, increased serum creatine kinase, and with or without episodic rhabdomyolysis (MMCKR) is an autosomal recessive disorder of skeletal muscle 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).

Congenital muscular dystrophy with or without seizures (MYOS) is an autosomal recessive disorder characterized by severe muscle hypotonia apparent from birth, as well as developmental delay. Laboratory studies show increased serum creatine kinase and muscle biopsy shows nonspecific dystrophic features. Most patients develop seizures or have abnormal epileptiform findings on EEG studies; other variable findings may include feeding difficulties, nystagmus, myopathic facies, areflexia, and brain atrophy on MRI (summary by Larson et al., 2018 and Henige et al., 2021).

Susceptibility to rhabdomyolysis-1 (RHABDO1) is an autosomal recessive disorder characterized by recurrent episodes of rhabdomyolysis beginning in the teenage years. Some of the episodes may be triggered by exercise or heat; others occur spontaneously. Severe cases may result in acute renal failure or compartment syndrome. Affected individuals tend to have myalgia or muscle weakness in childhood and between episodes. Laboratory studies show increased serum creatine kinase and nonspecific myopathic features on skeletal muscle biopsy (Cabrera-Serrano et al., 2022).

Developmental delay with hypotonia, myopathy, and brain abnormalities (DEDHMB) is an autosomal recessive disorder characterized by global developmental delay and muscle weakness apparent in infancy. Affected individuals show severe motor delay and may not achieve independent walking due to central hypotonia and skeletal muscle myopathy. Some have poor overall growth with microcephaly, subtle dysmorphic features, and delayed language acquisition. Brain imaging shows cerebral atrophy, thinning of the corpus callosum, and delayed myelination (Shamseldin et al., 2016; Kotecha et al., 2021).

Congenital myopathy-18 (CMYP18) is a disorder of the skeletal muscle characterized by the onset of symptoms of muscle weakness in early childhood, including in utero and infancy. There is clinical heterogeneity in the manifestations and severity, ranging from fetal akinesia sequence causing early death to onset of symptoms in adulthood. Most affected individuals show delayed motor development with generalized hypotonia and progressive axial and limb muscle weakness beginning soon after birth or in infancy. Additional features may include swallowing difficulties, external ophthalmoplegia, ptosis, high-arched palate, and respiratory insufficiency, which can lead to death in severe cases. Muscle biopsy shows variable morphologic abnormalities, including alveolar changes in the intermyofibrillar network, fiber size variability, focal disorganization, internal nuclei, and dilated sarcoplasmic reticulum and T-tubules. The disorder results from a defect in excitation-contraction coupling in skeletal muscle (Schartner et al., 2017; Ravenscroft et al., 2021; Mauri et al., 2021; Yis et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (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).

The mitochondrial trifunctional protein, composed of 4 alpha and 4 beta subunits, catalyzes 3 steps in mitochondrial beta-oxidation of fatty acids: long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD), long-chain enoyl-CoA hydratase, and long-chain thiolase activities. Trifunctional protein deficiency is characterized by decreased activity of all 3 enzymes. Clinically, classic trifunctional protein deficiency can be classified into 3 main clinical phenotypes: neonatal onset of a severe, lethal condition resulting in sudden unexplained infant death (SIDS; 272120), infantile onset of a hepatic Reye-like syndrome, and late-adolescent onset of primarily a skeletal myopathy (summary by Spiekerkoetter et al., 2003). Some patients with MTP deficiency show a protracted progressive course associated with myopathy, recurrent rhabdomyolysis, and sensorimotor axonal neuropathy. These patients tend to survive into adolescence and adulthood (den Boer et al., 2003). See mitochondrial trifunctional protein deficiency-1 (609015), caused by mutation in the HADHA gene (600890), the alpha subunit of mitochondrial trifunctional protein.

Congenital myopathy-21 with early respiratory failure (CMYP21) is an autosomal recessive muscle disorder associated with diaphragmatic weakness and spinal rigidity. The age at symptom onset is highly variable, ranging from infancy to adulthood; the severity of the respiratory impairment, which can lead to death in the most severe cases, is also variable. Additional features, including developmental delay and hypertrophic cardiomyopathy, have been observed in one patient each (Weihl et al., 2023; Al-Kasbi et al., 2022). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (117000).

Autosomal recessive childhood-onset nemaline myopathy-5B (NEM5B) is a skeletal muscle disorder in which patients usually present with proximal muscle weakness of the lower and upper limbs in a limb-girdle distribution, resulting in gait abnormalities; however, most remain ambulatory even into late adulthood. Some affected individuals show delayed motor development. There is axial weakness and atrophy of the paraspinal muscles, along with kyphosis, scoliosis, and rigid spine, as well as variable limitations of the large joints. Most patients develop restrictive respiratory insufficiency with decreased forced vital capacity; some need noninvasive ventilation. Serum creatine kinase may be elevated. Muscle biopsy can show variable features, including nemaline rods, multiminicore lesions, endomysial fibrosis, and myofibrillar changes (Pellerin et al., 2020; Lee et al., 2022). For a discussion of genetic heterogeneity of nemaline myopathy, see NEM2 (256030).