McArdle's disease presenting as treatment resistant polymyositis.

The broad spectrum and often incomplete presentation of polymyositis frequently confound its distinction from other myopathies. In 18 months of screening for myophosphorylase in all muscle biopsy specimens at our institution, 2 cases of McArdle's disease were discovered in patients previously thought to have refractory PM. Given the important treatment implications of this distinction, all patients with "refractory PM" in whom the diagnosis has not been firmly established should be screened for myophosphorylase deficiency.