Myositis refers to a group of diseases that involve inflammation of the muscles. The characteristics of those affected vary for each disease. Dermatomyositis (DM), polymyositis (PM), and inclusion body myositis (IBM) are the three major groups of inflammatory myopathy.

Polymyositis, which inflames and weakens muscles in many areas of the body, affects about six to eight people per 100,000. Approximately one person per 100,000 is newly diagnosed with this disease each year.

Dermatomyositis, which harms both muscle fibers and skin, affects about ten people per million. Estimates on the number of new cases diagnosed each year vary, ranging from about one to eight people per million. The exact number of people affected is hard to determine due to the rarity of the disease and the lack of consistent criteria used to diagnose it.

Inclusion body myositis, which involves a progressive weakening of muscles throughout the body, primarily affects adults over age 50. Many patients who have it are initially misdiagnosed with PM. IBM is the most common cause of muscle disease in adulthood. The true incidence is not known, but is much more common than either PM or DM.

Polymyositis most commonly begins between the ages of 40 and 60. The disease rarely affects people under the age of 20, although some cases have been reported in this age group.

Dermatomyositis most commonly begins in adults between the ages of 40 and 60. A juvenile form of the disease usually begins in children between the ages of five and fifteen. Inclusion body myositis usually begins after age 50 and is rare before age 50.

Polymyositis and dermatomyositis affect more women than men. Estimates range from 1.5 to 2 women affected for each man affected by either of these diseases. When PM/DM is associated with connective tissue diseases, the estimates of women to men affected are at the higher end of the range. In older patients with malignancies, the numbers of women and men affected are about equal.

Inclusion body myositis occurs more frequently in men than in women.

Larger numbers of Caucasian women are affected by dermatomyositis compared to African Americans. However, higher numbers of African Americans with other connective tissue diseases have DM or PM compared to Caucasians.

There have been reported cases of polymyositis, dermatomyositis, and inclusion body myositis in first-degree family members of patients with each of these diseases. First-degree family members include parents, children, and siblings. Inherited forms are rare, and PM, DM and IBM are primarily considered acquired diseases.

References

First published April 1, 2000
Last updated August 1, 2003
Copyright © 2000 Accordant Health Services, Inc. All Rights Reserved.

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