What is McArdle disease?

• McArdle disease [1] (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 [2], 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.

[1] Myopathy due to a defect in muscle glycogen breakdown. McArdle B (1951) Clin. Sci. 10: 13-33.

[2] 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.