Glycogen storage disease due to muscle glycogen phosphorylase deficiency

Glycogen storage disease due to muscle glycogen phosphorylase deficiency, also known as McArdle disease or Glycogen Storage Disease type V (GSD V), is a rare inherited metabolic disorder caused by mutations in the PYGM gene, which encodes the muscle-specific isoform of glycogen phosphorylase. This enzyme is essential for breaking down glycogen into glucose-1-phosphate in skeletal muscle, providing energy during exercise. Without functional muscle glycogen phosphorylase, individuals cannot effectively mobilize glycogen stores in their muscles, leading to an energy deficit during physical activity. The hallmark symptoms of McArdle disease include exercise intolerance, myalgia (muscle pain), early fatigue, and muscle cramps or contractures triggered by brief intense exercise (such as sprinting or lifting heavy objects) or sustained moderate activity. A characteristic feature is the 'second wind' phenomenon, in which patients experience relief of symptoms after a brief period of rest followed by resumption of exercise, as the body shifts to using alternative fuel sources such as free fatty acids and blood glucose. Episodes of rhabdomyolysis (acute muscle breakdown) can occur, leading to myoglobinuria (dark-colored urine) and, in severe cases, acute kidney failure. Serum creatine kinase (CK) levels are typically elevated, even at rest. McArdle disease primarily affects skeletal muscle, though cardiac muscle is not typically involved since it uses a different isoform of glycogen phosphorylase. Symptoms usually become apparent in childhood or adolescence, though diagnosis is often delayed until adulthood. Some patients may develop fixed proximal muscle weakness later in life. There is currently no cure for McArdle disease. Management focuses on lifestyle modifications, including aerobic conditioning programs, avoidance of strenuous exertion, and ingestion of simple sugars (such as sucrose) before planned exercise to provide an alternative energy source. A supervised, moderate aerobic exercise program has been shown to improve exercise capacity and quality of life. Vitamin B6 (pyridoxal phosphate) supplementation has been explored but has not demonstrated consistent clinical benefit.

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