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Chapter 202: Juvenile Dermatomyositis

Lisa G. Rider

INTRODUCTION

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).

TABLE 202-1CLINICAL CLASSIFICATION OF THE JUVENILE IDIOPATHIC INFLAMMATORY MYOPATHIES (IIMS)
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TABLE 202-1CLINICAL CLASSIFICATION OF THE JUVENILE IDIOPATHIC INFLAMMATORY MYOPATHIES (IIMS)
Subgroup Important Features
Juvenile dermatomyositis (JDM) Characteristic skin rashes of Gottron papules on extensor surfaces or heliotrope rash over eyelids, with chronic, progressive, proximal, and axial weakness. CD4+ T cells and plasmacytoid dendritic cells in muscle in perivascular distribution with resultant type I interferon signature. Calcinosis is present in 25–40%, cutaneous or gastrointestinal ulceration in up to 25%, and lipodystrophy in 10% of patients; may have systemic manifestations. Accounts for 85% of juvenile IIM.
Myositis associated with another autoimmune disease (overlap myositis) Patients meet criteria for myositis and another autoimmune disease. Overlap with scleroderma, systemic lupus erythematosus, or juvenile idiopathic arthritis is most common in children. Raynaud phenomenon, interstitial lung disease, arthritis, and calcinosis are more frequent in this subgroup. Higher mortality, related to lung disease. Seen in 6–12% of juvenile IIM.
Polymyositis Characteristic skin rashes are absent. May have severe weakness, including distal weakness, falling episodes, higher serum creatine kinase level, myalgia, more frequent cardiac involvement. Pathogenesis involves CD8+ muscle endomysial inflammation. Seen in 4–8% of juvenile IIM.
Clinically amyopathic dermatomyositis Typical JDM skin rashes without muscle involvement for at least 2 years, or with subclinical muscle involvement detected by additional testing (elevated serum muscle enzymes, abnormal electromyogram, magnetic resonance imaging, or muscle biopsy). Calcinosis, cutaneous ulcerations, and interstitial lung disease are lower in frequency, and associated malignancy has not been reported, in contrast to adults where these features are more frequent. Seen in 1% of juvenile IIM.
Focal myositis Most often presents as an enlarged mass within the affected muscle, which is usually painful or tender to palpation. The most common sites of involvement ...

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