Patients with polymyositis and dermatomyositis who undergo plasmapheresis therapy. Pathologic findings.

Muscle biopsy specimens were studied in 20 of 26 patients who had polymyositis or dermatomyositis prior to treatment with plasmapheresis and immunosuppressive therapy. Morphologic findings from biopsy specimens were similar among patients with polymyositis and dermatomyositis, except that perifascicular atrophy was a prominent feature in patients with dermatomyositis and inconspicuous or absent in patients with polymyositis; immunofluorescent staining of muscle for immunoglobulins and/or complement was noted more frequently for patients with dermatomyositis than those with polymyositis. Patients judged clinically to have highly active disease had the most severe changes, which consisted of muscle necrosis, regeneration, phagocytosis, and inflammation; those with mild and moderately active disease were indistinguishable pathologically. No correlation between duration of disease and degree of pathologic alteration was found. Six of seven patients with both pretreatment and posttreatment biopsies showed marked improvement in the extent of pathologic alteration, and a statistically significant reduction in the degree of muscle atrophy following treatment.