Late Presentation of McArdle's Disease Mimicking Polymyalgia Rheumatica: A Case Report and Review of the Literature.

McArdle disease or glycogen storage disease Type V is a genetic condition caused by PYGM gene mutations leading to exercise intolerance and fatigability. The condition most commonly presents in childhood. In rare cases, patients have presented with late-onset McArdle disease. We present a case of a 64-year-old male presenting with myalgia who was initially presented with polymyalgia rheumatica-type symptoms of proximal muscle pain and a response to steroids. At review, his background musculoskeletal symptoms were evaluated in detail. Following a muscle biopsy, skeletal muscle enzymatic assay, and genetic testing, he was diagnosed with late-onset McArdle's disease (homozygous PYGM genotype). The importance of recognition and early diagnosis is highlighted to enable the accurate diagnosis and conservative lifestyle advice, with the avoidance of other medical therapies for other disease mimics.