Inclusion body myositis. A corticosteroid-resistant idiopathic inflammatory myopathy.

In seven patients with slowly progressive muscle weakness, inclusion body myositis (IBM) was diagnosed on biopsy. None had stigmata of collagen-vascular disease or malignancy. Serum creatine kinase levels were mildly or moderately increased. The six patients treated with prednisone did not improve. Needle electromyography showed a "myopathic" pattern in all patients, but four also had diffuse neurogenic changes with normal nerve conductions. Histologic study of muscle showed a mixture of small rounded fibers varying in size, atrophic angulated fibers forming small groups, and hypertrophic fibers. Variable amounts of inflammation, necrosis, and regeneration were seen in all specimens. All showed numerous intracytoplasmic vacuoles lined with purple-blue granules. Electron microscopy showed membranous whorls and masses of abnormal filaments measuring 14 to 18 nm in diameter. Although IBM seems to be a distinct type of inflammatory myopathy, its etiology and pathogenesis are not clear.