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PATHOGENESIS

Dermatomyositis is believed to result from an immune-mediated process triggered by “outside” factors (e.g. malignancy, drugs, infectious agents)

in genetically predisposed individuals. Table 42.3 illustrates some of the evidence that supports this theory. Serum anti-nuclear autoantibodies are often present, as are other myositis-specific or myositisassociated autoantibodies, as summarized in Table 42.4. The anti-synthetase antibodies are directed against cytoplasmic antigens; therefore the anti-nuclear antibody test may be negative. Patients with anti-synthetase antibodies often have overlap syndromes. The term anti-synthetase syndrome has been coined to refer to patients with these autoantibodies, fever, erosive polyarthritis, “mechanic’s hands”, Raynaud phenomenon, and interstitial lung disease.

Table 42.1 Revised classification system for the idiopathic inflammatory dermatomyopathies. This classification scheme recognizes, with equal weighting, the cutaneous and muscle manifestations of this group of disorders. Adapted from Sontheimer RD. Would a new name hasten the acceptance of amyopathic dermatomyositis (dermatomyositis sine myositis) as a distinctive subset within the idiopathic inflammatory dermatomyopathies spectrum of clinical illness? J Am Acad Dermatol 2002;46:626–36.

Table 42.2 Clinical subtypes of dermatomyositis – demographics and associated findings.

Table 42.3 Pathogenesis of dermatomyositis. Other environmental triggers such as ultraviolet radiation may play a role (e.g. correlation between rate of dermatomyositis with anti-Mi-2 antibodies and proximity to the equator). BCG, Bacillus Calmette–Guérin; BPH, benign prostatic hyperplasia; ICAM, intercellular adhesion molecule; MAP, mitogen-activated protein; TNF, tumor necrosis factor.

Table 42.4 Serum autoantibodies in adult and juvenile dermatomyositis (DM). The autoantigen CADM-140 was subsequently found to be identical to two previously identified gene products, interferon induced with helicase C domain protein 1 (IFIH1) and melanoma differentiation-associated gene 5 (MDA5). amyo, amyopathic; CADM-140, clinically amyopathic dermatomyositis antibody, 140 kD; CK, creatine kinase; NA, not applicable; NXP-2, nuclear matrix protein-2; SAE, small ubiquitin-like modifier activating enzyme; SRP, signal recognition particle; TIF1-γ (transcription intermediary factor 1-γ). Note: Autoantibody frequencies vary depending on the ethnicity/population in which the study was conducted. Adapted from references 29, 39–42.