Myopathy and Exercise Intolerance Associated with McArdle's Disease (Myophosphorylase Deficiency)

 

Onset is usually in the pediatric age group, but a late onset in adulthood may also occur.

 

Clinical features:

(1) exercise intolerance

(2) myalgias and muscle cramps

(3) asymmetric proximal muscle weakness

(4) recurrent episodes of rhabdomyolysis

(5) repeated episodes of acute renal failure

 

Laboratory findings:

(1) elevated serum creatine kinase and myoglobinuria during rhabdomyolysis

(2) absence of a rise in serum lactate after ischemic exercise testing of the forearm.

 

A skeletal muscle biopsy will show a vacuolar myopathy. Enzymatic staining shows an absence of myophosphorylase activity.