Cramps following exercise

A 13-year old boy presented with a 10-year history of severe muscle cramps experienced an hour after prolonged exercise. There was no history of exercise intolerance or myoglobinuria. A muscle biopsy showed a lipid myopathy and a deficiency of muscle carnitine palmityl transferase. He has responded to a high carbohydrate, low fat diet with added carbohydrate intake preceding extensive exercise. Diagnosis of this entity before an episode of rhabdomyolysis is unusual.