Excerpt from Polymyositis

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Background

Polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM) are the major members of a group of skeletal muscle diseases called the idiopathic inflammatory myopathies. Clinical features, characteristic muscle biopsy findings, immune markers, and histopathologic findings differentiate these illnesses. No strictly defined diagnostic criteria for PM or DM exist; however, Bohan and Peter proposed the criteria most widely cited. These criteria include the typical rash of DM, findings at history and physical examination that reveal symmetric proximal muscular weakness, elevated serum muscle enzyme levels, electromyographic evidence of myopathic abnormalities, and characteristic findings at muscle biopsy.

PM and DM have many shared clinical features. Both are inflammatory myopathies that present as symmetric muscle weakness that develops over weeks to months. Initial treatment with corticosteroids usually produces a response; however, nonresponders require further treatment. Both conditions may be associated with malignancies. Despite these similarities, muscle biopsy findings and characteristic skin findings of DM reveal each as a distinct clinical entity.

Although classified as an inflammatory myopathy, IBM shows minimal evidence of inflammation. This is the most common inflammatory myopathy in patients older than 50 years. It presents as an asymmetric, distal weakness and also has distinct biopsy findings. Studies so far have not yielded significant response to treatment. IBM will not be discussed further in this article.

Pathophysiology

In both PM and DM, immune-mediated muscle inflammation and vascular damage occur. In PM, the immune system is primed to act against previously unrecognized muscle antigens. In DM, complement-mediated damage to endomysial vessels and microvasculature of the dermis occurs.

Frequency

United States

Overall, the annual incidence of inflammatory myopathy is 1 case per 100,000 persons per year.

Mortality/Morbidity

• The 5-year mortality rate is 20%.
• Mortality is most often related to associated malignancy or pulmonary complications; however, elderly patients with cardiac involvement or dysphagia also have a higher mortality rate.
• Outcome of treatment varies widely and is related to clinical and histologic features.

Race

PM and DM are more common among blacks. Estimated black-to-white incidence for PM is 5:1 and for DM is 3:1.

Sex

Both PM and DM are more common in females, with an approximate 2:1 ratio.

Age

• PM is a disease of adults, and it is rare in persons younger than 20 years.
• Although DM is primarily a disease of adults, it also is observed in children, usually those aged 5-14 years.

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