Pathologic changes of polymyositis and dermatomyositis

Pathologic changes of polymyositis and dermatomyositis

What are the major pathologic changes on light microscopy in the muscle biopsies of patients with polymyositis (PM) and dermatomyositis (DM)? 

Both polymyositis and dermatomyositis have the following:

• Variation and rounding of muscle fibers; occasional angular and atrophic fibers

• Fiber necrosis, phagocytosis

• Inflammatory infiltrate:

• In dermatomyositis, perivascular and perimysial inflammation composed of macrophages, B cells, and CD4 dendritic cells

• In polymyositis, nonnecrotic endomysial inflammation composed of CD8 T cells and macrophages

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