Dermatomyositis

Dermatomyositis (DM) is a systemic autoimmune inflammatory myopathy characterized by the combination of progressive proximal skeletal muscle weakness and distinctive cutaneous manifestations. It belongs to the group of idiopathic inflammatory myopathies (IIM) that also includes polymyositis, inclusion body myositis, and immune-mediated necrotizing myopathy. DM has a bimodal age distribution, affecting children (juvenile DM, peak onset 5-14 years) and adults (peak onset 40-60 years), with a female-to-male predominance of approximately 2:1. The annual incidence is approximately 1 per 100,000 population. The pathogenesis of DM involves a complement-mediated microangiopathy targeting the intramuscular blood vessels, distinguishing it from polymyositis, which involves direct T-cell-mediated attack on muscle fibers. In DM, autoantibodies and complement components (C5b-9 membrane attack complex/MAC) deposit on the endothelium of endomysial capillaries and small arterioles within skeletal muscle. This complement-mediated endothelial injury causes capillary necrosis and dropout (loss of capillaries), ischemic damage to muscle fibers, and perifascicular atrophy -- the histological hallmark of DM. Perifascicular atrophy refers to atrophy and degeneration of muscle fibers at the periphery of muscle fascicles (near the perimysium), reflecting their greater vulnerability to ischemic injury from...