The word polymyositis (PM) means "many muscles inflamed".
Polymyositis is an inflammatory disease of muscle. The cause of the disease is not known. It begins when white blood cells, the immune cells of inflammation, spontaneously invade muscles. The muscles affected are mostly those closest to the trunk or torso. This results in weakness which can be severe. Patients can also feel fatigue, a general feeling of discomfort and have weight loss and/or low-grade fever. It is a chronic illness with periods of increased symptoms, called flares or relapses, and decreased symptoms, known as remissions.
Polymyositis is slightly more common in females. It affects all age groups, although its onset is most common in middle childhood and in the twenties. Polymyositis occurs throughout the world. Polymyositis can be associated with skin rash and is then referred to as "dermatomyositis." It also can affect other areas of the body and is, therefore, a systemic illness. Occasionally, it is associated with cancer, or with other diseases of connective tissue.
To date, no cause of polymyositis has been isolated by scientific researchers. There are indicators of heredity (genetic) susceptibility that can be found in some patients. There is indirect evidence of infection by a virus that has yet to be identified in a form of polymyositis that is particularly resistant to treatment, called inclusion body myositis.
Diagnosis of PM involves physical examination of muscle strength, blood tests for muscle enzymes, electrical tests of muscle and nerves, and confirmed by muscle biopsy.
Treatment of PM includes cortisone-related medications, immune suppressants, and physical therapy.