McArdle Disease

McArdle disease, also known as glycogen storage disease type V, is an inherited metabolic disorder in which the muscles lack the enzyme myophosphorylase, which is needed to break down stored glycogen for energy during exercise. Without this enzyme, muscle cells cannot mobilize their glycogen stores, so people with the condition experience exercise intolerance, painful cramps and fatigue early into physical activity, sometimes followed by a second wind if they slow down and allow other fuel sources to take over. Repeated episodes of severe muscle breakdown, called rhabdomyolysis, can release myoglobin into the bloodstream and injure the kidneys. The disease is named for the British physician Brian McArdle, who first described it in 1951, and it is managed mainly by pacing physical activity and avoiding sudden intense exertion.