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Polymyositis and dermatomyositis 

Polymyositis and dermatomyositis

Polymyositis and dermatomyositis

John H. Stone


July 30, 2015: This chapter has been re-evaluated and remains up-to-date. No changes have been necessary.

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date: 29 April 2017

Polymyositis and dermatomyositis are two types of idiopathic inflammatory myopathy. The pathological findings in polymyositis suggest an HLA class I-restricted immune response mediated by cytotoxic T cells; dermatomyositis appears to be associated with humorally mediated destruction of muscle-associated microvasculature.

Clinical features—polymyositis is characterized by symmetrical painless proximal muscle weakness that develops slowly, usually over weeks to months, and typically associated with significant elevation of serum creatine kinase and other muscle enzymes. The pattern of muscle involvement in dermatomyositis is clinically indistinguishable from that of polymyositis, but with cutaneous manifestations including Gottron’s sign, heliotrope rash, erythema, ‘mechanic’s hands’, periungual abnormalities, and calcinosis cutis. Extra-muscular features include interstitial lung disease (30% of cases), aspiration pneumonia, and associated malignancy (polymyositis 9%, dermatomyositis 15%).

Investigation and diagnosis—disease is associated with ‘myositis-specific’ autoantibodies (30% of cases), of which there are three main types—antisynthetases, antisignal recognition particle (anti-SRP) antibodies, and anti-Mi-2 antibodies—and ‘myositis-associated’ autoantibodies. Particular autoantibodies are associated with particular disease phenotypes. The definitive test for establishing the diagnosis of inflammatory myopathy and excluding other causes of muscle weakness is muscle biopsy. Magnetic resonance imaging can demonstrate muscle involvement, be repeated as a method of evaluating response to therapy, and be useful in selecting a muscle group for biopsy.

Management and prognosis—treatment is with glucocorticoids, usually beginning with 1 mg/kg/day of prednisone. Second-line agents, usually azathioprine or methotrexate, are given to patients with aggressive disease. Biological agents are likely to be increasingly used in the future. The 5-year survival rate of patients with polymyositis or dermatomyositis is greater than 80%, but morbidity from both the diseases themselves and their treatments is high.

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