RAPIDLY EVOLVING MYOPATHY WITH MYOSIN-DEFICIENT MUSCLE-FIBERS

Five patients with rapidly evolving, severe weakness had an unusual my opathy with virtually complete loss of myosin in 5 to 40% of muscle fi bers. Three of the 5 patients began to develop weakness 1 to 2 weeks a fter lung transplantation. The fourth became weak after a febrile illn ess. The fifth presented with diabetic ketoacidosis and weakness. All patients had received corticosteroid therapy. In all cases the weaknes s was progressive and led to severe disability, with respiratory failu re in 4 patients. Initial diagnostic testing did not localize an under lying cause for the weakness. Creatine kinase was normal or minimally elevated. Electromyography generally showed mildly myopathic or nondia gnostic changes. However, muscle biopsy revealed numerous small angula r 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 fr om previously described syndromes involving rapidly progressive weakne ss. Slow recovery over a period of months is the most common outcome.