[Histopathologic aspects of polymyositis and dermatomyositis. Correlation with the clinical course. Study of 57 cases].

Muscle biopsies from 57 patients with dermatomyositis or polymyositis were histologically evaluated and compared with the disease's clinical course. Perifascicular atrophy, perivascular infiltrates and tubular inclusions in endothelial cells were significantly more frequent in young patients with dermatomyositis. On the other hand, in adult polymyositis, which evolves more slowly, necrosis with slight muscular atrophy and perinecrotic infiltrates was observed. This division into two groups was clear when the clinical evolution and histological patterns were compared. The mean age of each group was different, but there was a large overlap. Two different pathogenetic mechanisms can be envisaged: primary involvement of muscle capillaries with muscle ischemia in young patients with dermatomyositis and primary involvement of muscle fibers in adults afflicted with polymyositis.