Overview
Clinical Meaning
Dermatomyositis (DM) and polymyositis (PM) are idiopathic inflammatory myopathies (IIM) with distinct pathogenic mechanisms, clinical presentations, and management consideration...
Dermatomyositis (DM) and polymyositis (PM) are idiopathic inflammatory myopathies (IIM) with distinct pathogenic mechanisms, clinical presentations, and management considerations that the NP must differentiate. Dermatomyositis involves complement-mediated microangiopathy: autoantibodies activate complement (C5b-9 membrane attack complex) on endomysial capillary endothelium, causing capillary necrosis and dropout. The resulting ischemia produces perifascicular atrophy — the histological hallmark — with inflammatory infiltrate that is perivascular and perimysial (B cells, CD4+ T-helper cells, plasmacytoid dendritic cells). DM has pathognomonic cutaneous features: heliotrope rash (violaceous periorbital discoloration with edema), Gottron papules (violaceous papules over MCP/PIP/DIP joints), V-sign (chest), shawl sign (upper back), and mechanic's hands. Polymyositis involves direct CD8+ cytotoxic T-cell attack on muscle fibers. Autoreactive T cells recognize aberrantly expressed MHC class I molecules on muscle fiber surfaces and directly invade non-necrotic fibers (endomysial inflammatory infiltrate). There are NO cutaneous manifestations in PM. The diagnosis of PM requires exclusion of other causes of myopathy including inclusion body myositis (IBM), immune-mediated necrotizing myopathy (IMNM), drug-induced myopathy (statins), and endocrine myopathy (hypothyroidism, Cushing). Critical clinical distinctions: DM has a 15-30% association with underlying malignancy in adults (paraneoplastic...