Polymyositis & Dermatomyositis

Polymyositis is a disease of muscle featuring inflammation of the muscle fibers. 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 typically those closest to the trunk or torso. This results in weakness that can be severe. Polymyositis is a chronic illness with periods of increased symptoms, called flares or relapses, and minimal or no 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 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).

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.

Researchers from Sweden at the national meeting of the American College of Rheumatology in 2007 reported their findings that T-cells of the immune system in some polymyositis or dermatomyositis patients reacted against cytomegalovirus (CMV) and that detectable antibodies against CMV were present. Their conclusion was that there may be subsets of patients who develop their disease, in part, because of infection with this particular virus.

Aside from diseases with which polymyositis can be associated (as mentioned above), many other diseases and conditions can mimic polymyositis. These include nerve-muscle diseases (such as muscular dystrophies), drug toxins (such as alcohol, cocaine, steroids, colchicine, hydroxychloroquine, and cholesterol-lowering drugs, called statins), metabolic disorders (where muscle cells are unable to process chemicals normally), hormone disorders (such as abnormal thyroid), calcium and magnesium conditions, and infectious diseases (such as influenza virus, AIDS, streptococcus and Lyme bacteria, pork tapeworm and schistosomiasis).

Source: MedicineNet.com
http://www.medicinenet.com/polymyositis/article.htm