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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: 18 October 2021

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%)....

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