What is Polymyositis?

Polymyositis (PM) is an uncommon autoimmune disease which causes inflammation leading to destruction of muscle and increasing muscular weakness. As the disease progresses, muscle tissue may be replaced by functionless scar tissue.

The cause of the disease has not been found. It begins when an unknown trigger stimulates the immune system to spontaneously produce autoantibodies against normal healthy tissue. Some researchers speculate that there may be an infective element involved such as bacteria, parasites or viruses. However there is no hard evidence to date. it is also not known to be an infectious disease, hence it is not passed on from person to person.

The muscles affected are typically those closest to the trunk or torso, or what is known as the proximal muscles. The muscles in the hips, thighs, shoulders, upper arms and neck are usually affected. This results in weakness that is symmetrical, affecting both the left and right sides of the body. This muscle weakness can be moderate to severe.

The signs and symptoms of PM can occur gradually, so it may be difficult to pinpoint exactly when it may have started. It can occur over days, weeks or months, and the symptoms may fluctuate from time to time. The loss of strength in the proximal muscles can be noticed as difficulty getting up from chairs, climbing stairs or lifting above the shoulders. Trouble with swallowing and lifting the head from the pillow can occur. Occasionally, the muscles ache and are tender to the touch.There may be a general feeling of discomfort or fatigue. weight loss and/or a low-grade fever may also be present.

Polymyositis is more common in females than males, the ratio being about 2:1. It can affect all age and racial groups, but it is usually rare in people below age 20. About 60% of cases occur between 30-60 years of age.

PM is a chronic illness with periods of increased symptoms, called flares or relapses, and periods of minimal or no symptoms, known as remissions.

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 such as systemic lupus erythematosus (SLE), scleroderma and rheumatoid arthritis. Hence it can be a difficult disease to diagnose as the symptoms may mimic other connective tissue diseases.