McArdle disease is a glycogen storage disease caused by mutations in the PYGM gene encoding myophosphorylase. It manifests classically with childhood-onset exercise-induced pain.
Here, the authors report the characteristics of 2 unrelated patients with a new homozygous mutation of the PYGM gene. In both patients, symptoms had developed progressively in the 2 preceding years, and there was no history of exercise intolerance. Both patients demonstrated myogenic abnormalities on EMG, multiple glycogen-containing vacuoles and undetectable muscle myophosphorylase activity on muscle biopsy, and a novel homozygous frameshift p.Lys42Profs*48 PYGM mutation.
This report expands the phenotype and genotype of McArdle disease, and suggests that PYGM mutations should be looked for in patients with very late-onset myopathy with no previous history of exercise intolerance.
