Glycogen storage disease type V

Glycogen storage disease type V, or McArdle disease, is an inherited disorder caused by deficiency of muscle glycogen phosphorylase. It impairs muscle glycogen breakdown, causing exercise intolerance and episodes of muscle injury.

Muscle glycogen phosphorylase: An enzyme that releases glucose-1-phosphate from glycogen in skeletal muscle. Its deficiency prevents working muscle from mobilizing its glycogen stores.
Glycogen: A branched polymer of glucose that animals store chiefly in liver and skeletal muscle. Muscle glycogen is present but cannot be effectively broken down in this disorder.
Forearm exercise test: A test that measures metabolites in blood before and after brief forearm exercise. Failure of lactate to rise after exercise can support the diagnosis.
Brian McArdle: A British physician who described a patient with exercise-induced muscle cramps and impaired lactate production in 1951. His report gave the disorder its common name.
Glycogenolysis: The enzymatic breakdown of glycogen into glucose-derived molecules. McArdle disease blocks its first step in skeletal muscle.
Glucose-1-phosphate: A phosphorylated glucose molecule produced during glycogen breakdown and converted to glucose-6-phosphate. Muscle glycogen phosphorylase normally releases it from glycogen.
Creatine kinase: An enzyme abundant in muscle that enters the blood after muscle injury. Blood levels are often elevated in McArdle disease, even between major episodes.
GSD nomenclature: A group of inherited disorders caused by defects in glycogen synthesis, breakdown, or regulation. Type V identifies the muscle phosphorylase deficiency among these glycogen disorders.
Second-wind phenomenon: An improvement in exercise tolerance after several minutes of activity, as muscles increase their use of circulating fuels. In McArdle disease, increased blood flow supplies glucose and fatty acids that partly bypass the blocked pathway.
Glycolysis: A metabolic pathway that converts glucose into pyruvate while producing ATP. Blocked access to glycogen limits one fuel source for glycolysis during exercise.

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Glycogen storage disease type IICompared with: It also causes exercise-related muscle symptoms, but affects cytosolic glycogen use.