As of 2015, doctors haven't discovered the cause of polymyositis, according to the Muscular Dystrophy Association. Similar to an autoimmune disease, polymyositis causes inflammation in muscle tissue when the immune system attacks healthy muscle fibers. Muscles in the neck, shoulders, hips, upper arms and thighs are commonly affected.
Polymyositis causes the affected muscles to feel weakened, tender or painful, and harmful inflammation can spread to vital organs such as the heart and lungs, states the MDA. Symptoms usually start after age 20.
Arthritis, difficulty swallowing and heart arrhythmias are also symptoms of polymyositis, notes the National Institute of Neurological Disorders and Strokes. Polymyositis has no cure, but doctors use medications and therapeutic techniques, such as orthotics and heat therapy, to relieve the symptoms. Immunosuppressant drugs help doctors manage the inflammation.