What is McArdle disease?
McArdle disease  (Glycogen Storage Disease Type V, GSD5) is an ultra rare muscle myopathy with an incidence of approx. 1 in 100,000. (Muscle GSDs are very different to liver GSDs.)
Deficiency of the myophosphorylase enzyme , caused by mutations in the PYGM gene, results in an inability to utilize glycogen in skeletal muscle.
Maximal isometric or anaerobic activity for more than about 6 seconds will risk fixed contracture.
A serious shortage of energy occurs in the first 10 minutes of activity. This leads to premature fatigue, exaggerated heart rate, pain and muscle spasm.
If activity is continued in the presence of pain, a fixed contracture can occur with risks of rhabdomyolysis, acute renal failure and/or compartment syndrome (page 9).
Patients must be able to recognize the signs that urgent hospital attendance is required (page 9).
A series of CK tests (page 10c) can help patients understand and avoid future serious episodes.
In many cases diagnosis is only achieved approx. 20 years after presentation. Muscle damage or atrophy may have occurred by then, causing disability.
Diagnosis in childhood, provision of specialist advice and learning to manage the condition well, can lead to being less severely affected.
Patients must learn to attain ‘second wind’ (page 12f). Regular exercise enhances aerobic metabolism and reduces risk of muscle damage.
 Myopathy due to a defect in muscle glycogen breakdown. McArdle B (1951) Clin. Sci. 10: 13-33.
 A functional disorder of muscle associated with the absence of phosphorylase. Mommaerts WF, Illingworth B, Pearson CM, Guillory RJ, Seraydarian K (1956) Proc Natl Acad Sci U S A 45: 791-797.
McArdle Disease Medical Overview