chronic, progressive inflammation of skeletal muscles (muscle), particularly the muscles of the shoulders and pelvis.

Initially muscles may be swollen slightly, and the first symptoms to appear are usually muscle weakness and sometimes pain. A weakening of muscles close to the torso is common. Occasionally muscles of the esophagus and larynx are affected, which causes difficulty in swallowing and speaking. When muscles of the lungs (lung) are involved, difficulty in breathing may result. Muscles of the heart and the gastrointestinal tract also may be affected. As the disease progresses, the affected muscles atrophy and harden. Symptoms may occur in alternating cycles of remission and exacerbation.

The muscle cell damage that occurs in polymyositis is thought to result from an attack on muscle tissue by white blood cells called T lymphocytes (lymphocyte), which normally are produced by the immune system to fight infection. The factor that precipitates this autoimmune response is not known, but there is evidence that viral infections trigger some cases of polymyositis.

Polymyositis is more prevalent in women. It sometimes is associated with an increased incidence of several types of malignancies, including lung (lung cancer), colon (colorectal cancer), and breast cancers (breast cancer). The disorder dermatomyositis, which involves the skin as well as the muscles, shares many characteristics with polymyositis, but the two are believed to arise from different causes.

The medications most commonly used to treat polymyositis are corticosteroids, such as prednisone, which reduce inflammation. A number of immunosuppressive medications, such as methotrexate, also are used. Early diagnosis and treatment tends to reduce the severity of the disease.