McArdle's disease resembling an inflammatory myopathy

Previous reports have shown that McArdle's disease may occasionally present in older patients (i.e. greater than 40 years of age) as a progressive myopathy that is clinically and sometimes electrophysiologically indistinguishable from idiopathic polymyositis. We report two such patients who in addition had muscle biopsies showing inflammatory infiltrates compatible with polymyositis. However, subsequent enzyme histochemistry demonstrated complete myophosphorylase deficiency and, in the absence of an alternative explanation, may be the reason for the inflammatory changes seen. These cases highlight the importance of a thorough evaluation of patients with an 'inflammatory myopathy' and the occasional diagnostic confusion that may arise in differentiating metabolic myopathies from idiopathic polymyositis. Diagnostic clarification is important to avoid the use of incorrect and potentially toxic treatment.