[Muscle histopathology and responsiveness to steroid therapy in polymyositis and dermatomyositis].

We carried out muscle biopsies on 22 patients with polymyositis and dermatomyositis, and evaluated the relationship between muscle histopathological findings and steroid responsiveness. We classified the histopathological findings of the biopsied muscles into 4 types based on the distribution and extent of inflammatory cellular infiltration. The first "perivascular infiltration" type was seen in 7 patients who had inflammatory cells mainly around the small vessels, the second "endomysial infiltration" type in 5 patients who had inflammatory cells mainly around the muscle fibers, the third "diffuse infiltration" type in 4 patients who had scattered inflammatory cells around the small vessels and the muscle fibers, and the fourth "scant infiltration" type in 6 patients who had no prominent inflammatory cells. Most of the patients with the first or the second type showed a good response to the initial steroid therapy, while 3 patients with the diffuse infiltration and 4 with the scant infiltration types poorly responded. Histopathological typing of biopsied muscles is one of the useful markers to predict the responsiveness to the initial steroid therapy.