Definition of McArdle disease

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McArdle disease: Glycogen storage disease type V and the most common type of glycogen storage disease. McArdle disease is an autosomal recessive disorder caused by mutations in the gene that encodes myophosphorylase, an enzyme that is essential for glycogenolysis. Exercise intolerance usually develops during childhood, along with pain, cramps, and fatigue in exercised muscle. These symptoms are more likely to be induced by brief, intense activities (such as weight lifting or sprinting) but can also occur after prolonged, low-intensity exercises (such as swimming or jogging). Severe muscle damage can lead to myoglobinuria (the release of myoglobin from muscle into the bloodstream) and renal failure. There is variation among patients with McArdle disease. Some patients become fatigued easily but have no clinically significant muscle cramps or pain, whereas others have with progressive atrophy and weakness of upper arm, and thigh muscles in late adulthood. In rare cases, McArdle disease is so severe it causes congenital weakness and progressive respiratory failure.