Summary
Glycogen storage type V disease (McArdle's disease) is due to muscle phosphorylase deficiency. Patients present with a syndrome of muscular exercise intolerance with myalgia, cramps, fatigue, and muscle weakness. Massive elevation of creatine-kinase and rhabdomyolysis with myoglobinuria (dark urine) after exercise is noted in half of all patients, potentially leading to acute kidney failure. A 'second wind' phenomenon is observed in many patients. The clinical aspect is usually very classical, but some patients have very moderate forms, while in others, onset is very early in life with hypotonia, generalized muscle weakness and progressive respiratory failure. Biologically, lack of lactate elevation in blood during ischaemic forearm test, excess glycogen and deficient phosphorylase in the muscle biopsy are diagnostic. The condition is autosomal recessive. The gene has been located on chromosome 11 (11q13) and cloned, and mutations have been identified. Mutation R49X may account for over 40% of the alleles in Caucasian populations. Treatment is based on controlled physical training in order to develop mitochondrial oxidation capacities in muscles, and programmed glucose intake according to exercising periods. Diets with high protein intake have yielded variable results. *Authors: Dr I. Maire, Dr R. Froissart (February 2005)*.
