Inclusion body myositis. An electrophysiologic study.

Inclusion body myositis is a rare and slowly progressive myositis associated with cytoplasmic inclusions and fibrillar nuclear material. These histopathologic findings are of unknown significance. The clinical presentation of IBM has marked similarities to that of chronic polymyositis with proximal greater than distal weakness and muscle wasting more pronounced in the lower than upper extremities. In contrast to polymyositis, however, relatively few individuals report neck flexor weakness or dysphagia. Corticosteroid treatment is usually ineffective. The clinical, histopathologic and electrophysiologic findings in a patient with IBM are presented. Of particular interest in this report is the detailed motor unit recruitment frequency data. A number of previous IBM reports fail to mention specific electrophysiologic data or present evidence suggestive of a possible combined neuropathic and myopathic disease. Recruitment intervals of 150 ms or greater in combination with decreased motor unit duration and amplitudes in the involved muscles imply a myopathic pathophysiology. These findings are discussed in relation to electrophysiologic data from previously reported cases.