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Juvenile dermatomyositis (JDM), a systemic autoimmune disease with onset in childhood, is characterized by chronic skeletal muscle and cutaneous inflammation of unknown cause. JDM is relatively responsive to immunosuppressive therapy, and rapid diagnosis and adequate therapy improve outcomes.

JDM is the most common clinical subset of a larger family of disorders known as the idiopathic inflammatory myopathies (IIMs) (Table 202-1). Juvenile polymyositis (JPM), which constitutes 4% to 8% of childhood myositis cases, has similar features without the characteristic cutaneous manifestations, but patients with JPM often have more severe and distal weakness. Overlap myositis, which constitutes 6% to 12% of childhood IIM, occurs when JDM or JPM is associated with another autoimmune disease, such as systemic lupus erythematosus, scleroderma, juvenile idiopathic arthritis, systemic vasculitis, or type 1 diabetes mellitus. JDM and JPM also have been reported in combination with primary immunodeficiencies, such as Wiskott-Aldrich syndrome and common variable immunodeficiency, without apparent infectious triggers. Other clinical forms of IIMs rarely have been described in children and include clinically amyopathic dermatomyositis (DM), as well as focal, orbital, cancer-associated, eosinophilic, granulomatous myositis and immune-mediated necrotizing myopathy (see Table 202-1).


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