Muscle phosphorylase deficiency in childhood.

Myophosphorylase deficiency (McArdle's syndrome) is an uncommon condition characterized by exercise intolerance, muscle cramps and myoglobinuria. The present report describes the clinical, histochemical, electronmicroscopic and biochemical findings in a 12-year-old boy with myophosphorylase deficiency. The diagnosis should have been suspected when the boy was 6 years old. Most index cases have not been diagnosed until adult life, but this syndrome has to be considered in the differential diagnosis of exercise intolerance in childhood.