Definition - one of a group of muscle diseases that interfere with the processing of carbohydrates for energy production

Cause - a genetic defect in the phosphorylase (also known as myophosphorylase) enzyme, which affects the breakdown of glycogen, the stored form of glucose (sugar)

Onset - usually before age 15

Symptoms - exercise intolerance; cramps, muscle pain and weakness shortly after beginning exercise; "second wind" after resting

Progression - generally not progressive, although weakness between episodes of exercise sometimes develops

Inheritance - autosomal recessive, meaning caused by the contribution of a defective gene from each parent

Facts About Metabolic Disease of Muscle