Definition of Polymyositis - adult:

Polymyositis is a relatively uncommon inflammatory disease that leads to muscle weakness, swelling (inflammation), tenderness, and damage. It is part of a larger group of diseases called myositis.

Causes, incidence, and risk factors:

Polymyositis is a skeletal muscle disease that is also known as idiopathic inflammatory myopathy. The cause is unknown. An autoimmune reaction or infection of the skeletal muscle may cause the disease.

Polymyositis can affect people at any age, but it most commonly occurs between ages 50 and 70, or in children ages 5 to 15. It affects women twice as often as men. It is more common in African Americans than Caucasians. Overall, about 1 in 100,000 people are diagnosed with some form of inflammatory myopathy each year.

Other types of inflammatory muscle diseases exist, including inclusion body myositis.