Symptoms of McArdle disease
clinical presentation seen in myophosphorylase deficiency (McArdle disease; glycogenosis type V)
McArdle disease is one of the two most common GSD. It is autosomal recessive due to mutations in the myophosphorylase gene. It is a common cause of recurrent rhabdomyolysis and myoglobinuria, second only to CPT deficiency. Myophosphorylase degrades glycogen to glucose-1-phosphate and hence is important in calling up stored energy for muscle use. The cardinal manifestation of this deficiency is exercise intolerance associated with pain, fatigue, cramps, and/or weakness. The degree of intolerance varies among affected individuals. Symptoms generally follow activities of high intensity and short duration. Symptoms should resolve with rest. In fact, at rest, affected individuals function well and adjust their activities to a level below their threshold for symptoms. Some individuals experience the “second wind” phenomenon, which is a marked improvement in exercise tolerance about 10 minutes into aerobic exercise involving large muscle masses (jogging or cycling). For unknown reason, severe cramps and myoglobinuria are rare before adolescence. Elevated CK at rest is a common finding. The forearm exercise test (FET) is usually diagnostic showing a flat venous lactate curve. Definitive diagnosis requires histochemical and biochemical testing showing the enzyme deficiency in muscle or genetic testing. Low-dose creatine monohydrate and pre-exercise sucrose or glucose may improve exercise performance and tolerance in patients with this condition.