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.

  1. Muscle of the shoulder, arms, pelvis, and thighs are the most frequent targets, but the reasons are unknown.
  2. As the disease spreads, other muscles may become involved. For example, the diaphragm and chest wall muscles, which are needed for breathing, may be attacked.
  3. The pharyngeal muscles, which are needed for swallowing, may be weakened to the point that swallowing solid foods becomes impossible. Heart muscle is another potential target.
  4. When dermatomyositis attacks the skin, there is a characteristic purplish rash, found mostly on the face and upper chest, although it may occur anywhere on the body.
  5. Associated with this rash, patients will often have swelling around the eyes, especially upon rising in the morning.
  6. There may also be scaly, reddened eruptions over the small joints of the fingers, sometimes over the eyelids, and occasionally on the shoulders or upper back.
  7. These eruptions, which may be somewhat itchy, are so specific that they almost indicate the diagnosis.