Rapidly evolving myopathy with myosin‐deficient muscle fibers

Five patients with rapidly evolving, severe weakness had an unusual myopathy with virtually complete loss of myosin in 5 to 40% of muscle fibers. Three of the 5 patients began to develop weakness 1 to 2 weeks after lung transplantation. The fourth became weak after a febrile illness. The fifth presented with diabetic ketoacidosis and weakness. All patients had received corticosteroid therapy. In all cases the weakness was progressive and led to severe disability, with respiratory failure in 4 patients. Initial diagnostic testing did not localize an underlying cause for the weakness. Creatine kinase was normal or minimally elevated. Electromyography generally showed mildly myopathic or nondiagnostic changes. However, muscle biopsy revealed numerous small angular fibers with no myosin ATPase staining at any pH. Immunocytochemical staining and ultrastructural studies confirmed a severe loss of myosin in many fibers. This rapidly evolving myopathy with myosin‐deficient muscle fibers appears to be different clinically and pathologically from previously described syndromes involving rapidly progressive weakness. Slow recovery over a period of months is the most common outcome.