A 51-Year-Old Woman with Long-Standing Exercise Intolerance

McArdle disease is a glycogen storage myopathy which is caused by mutations in the PYGM gene with resultant myophosphorylase deficiency. Patients with McArdle disease typically present with exercise intolerance and recurrent rhabdomyolysis. These patients may also manifest fixed proximal limb weakness and persistent CK elevation. Muscle biopsy is useful to differentiate McArdle disease from other metabolic myopathies, such as other glycogen storage myopathies, lipid storage myopathies, and mitochondrial myopathies, to direct genetic testing. Here we present a patient with McArdle disease who was initially misdiagnosed with inflammatory myopathy and was unnecessarily exposed to immunosuppressive therapies. A correct diagnosis was achieved by a muscle biopsy, which led to appropriate management.