Polymyositis and Dermatomyositis

J. Robert Polk and Lawrence J. Kagen

Disease.. classification Pathogenesis Clinical..presentafipn Laboratory.. studies Differential.. diagnosis Treatment


Polymyositis (PM) is an inflammatory disease of striated skeletal muscle. In some patients, a characteristic skin rash is present, thus the term dermatomyositis. Dermatomyositis (DM) was described by Unverricht in 1887. PM occurs at any age, but most cases occur between the fourth and sixth decades of life, with a mild female preponderance. A childhood form of DM has been recognized. Estimates of the prevalence of PM range from 0.2 to 0.6 cases per 100,000 population. DM may be associated with malignancy.

I. Disease classification. Various classification systems have been proposed. It should be remembered that not all patients who are weak have PM. The following is a useful classification system:

A. Dermatomyositis

1. Childhood.

2. Adulthood.

B. Polymyositis.

C. Myositis associated with other connective tissue and inflammatory disorders (e.g., systemic lupus erythematosus, rheumatoid arthritis, scleroderma, sarcoidosis).

D. Myositis associated with malignancy.

E. Inclusion body myositis.

F. Myositis secondary to infectious agents.

G. Drug- and toxin-induced myositis.

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