Unforeseen cardiac involvement in McArdle's disease.

McArdle's disease (glycogen storage disease type V) is a rare autosomal recessive metabolic myopathy due to myophosphorylase deficiency. It classically manifests by exercise intolerance, leg cramps, muscle pain and occasionally exercise induced myoglobinuria. The onset of exercise intolerance is typically in the second or third decades of life. It has a specific predilection to skeletal muscle involvement, yet cardiac muscle involvement is very rare. This report describes an unusual case of a 33 year-old man with known McArdle's disease who presented with an incidental finding of severe obstructive hypertrophic cardiomyopathy.