Polymyositis (pol-e-mi-o-SI-tis) is an uncommon connective tissue disease. It's a type of inflammatory myopathy, which is characterized by muscle inflammation and weakness. The most noticeable characteristic of polymyositis is weakness of the skeletal muscles, which control movement.
Polymyositis can occur at any age, but it mostly affects adults sometime between their 30s and 50s. It's more common in blacks than in whites, and women are affected more often than men are. Polymyositis signs and symptoms usually develop gradually, over weeks or months.
Periods of remission in polymyositis, during which symptoms improve spontaneously, rarely occur. However, treatment can improve your muscle strength and function. (http://www.mayoclinic.com/health/polymyositis/DS00334)
Polymyositis is slightly more common in females. It affects all age groups, although its onset is most common in middle childhood and in the 20s. 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, referred to as a systemic illness. Occasionally, it is associated with cancer or with other diseases of connective tissue (see systemic lupus erythematosus, scleroderma and rheumatoid arthritis).
What causes polymyositis?
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. This form of polymyositis is diagnosed by the pathologist, a physician specialist who interprets the microscope findings of muscle tissue. The muscle tissue in this form of polymyositis displays clear areas within the muscle cells (called vacuoles) when viewed under the magnification of a microscope.