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.

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.

Hypermobile Ehlers-Danlos syndrome (hEDS) is generally considered the least severe type of EDS, although significant complications, primarily musculoskeletal, can and do occur. The skin is often soft and may be mildly hyperextensible. Subluxations and dislocations are common; they may occur spontaneously or with minimal trauma and can be acutely painful. Degenerative joint disease is common. Chronic pain, distinct from that associated with acute dislocations, is a serious complication of the condition and can be both physically and psychologically disabling. Easy bruising, functional bowel disorders, and cardiovascular autonomic dysfunction are common. Aortic root dilation, when present, is typically of a mild degree with no increased risk of dissection in the absence of significant dilation. Psychological dysfunction, psychosocial impairment, and emotional problems are common.

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.

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.

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.

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).

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.

Congenital myopathy-1B (CMYP1B) is an autosomal recessive disorder of skeletal muscle characterized by severe hypotonia and generalized muscle weakness apparent soon after birth or in early childhood with delayed motor development, generalized muscle weakness and atrophy, and difficulty walking or running. Affected individuals show proximal muscle weakness with axial and shoulder girdle involvement, external ophthalmoplegia, and bulbar weakness, often resulting in feeding difficulties and respiratory insufficiency. Orthopedic complications such as joint laxity, distal contractures, hip dislocation, cleft palate, and scoliosis are commonly observed. Serum creatine kinase is normal. The phenotype is variable in severity (Jungbluth et al., 2005; Bharucha-Goebel et al., 2013). Some patients show symptoms in utero, including reduced fetal movements, polyhydramnios, and intrauterine growth restriction. The most severely affected patients present in utero with fetal akinesia, arthrogryposis, and lung hypoplasia resulting in fetal or perinatal death (McKie et al., 2014). Skeletal muscle biopsy of patients with recessive RYR1 mutations can show variable features, including multiminicores (Ferreiro and Fardeau, 2002), central cores (Jungbluth et al., 2002), congenital fiber-type disproportion (CFTD) (Monnier et al., 2009), and centronuclear myopathy (Wilmshurst et al., 2010). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (117000).

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

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.

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.

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).

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).