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Juvenile dermatomyositis (JDM) is a systemic autoimmune disease characterized by chronic skeletal muscle and cutaneous inflammation of unknown cause.1,2 Symptoms generally begin before age 18. JDM is relatively responsive to immunosuppressive therapy, and rapid diagnosis, and institution of adequate therapy improves outcomes.

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JDM is the most common clinical subset of a larger family of disorders known as the idiopathic inflammatory myopathies (IIM) (Table 205-1).1,3,4 Juvenile polymyositis (JPM), which constitutes 2% to 8% of childhood myositis cases, has similar features without the characteristic cutaneous manifestations, but may have more severe and distal weakness. Overlap myositis, constituting 3% to 10% 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 I 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 IIM have been described in children, including dermatomyositis sine myositis, and focal, orbital, cancer-associated, eosinophilic, inclusion-body, and granulomatous myositis, but these subsets occur in fewer than 1 in 1 million children and are not discussed here.

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Table 205-1. Clinical Classification of the Juvenile Idiopathic Inflammatory Myopathies (IIM)

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