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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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