Polymyositis is a rare autoimmune disease that affects the muscles. It causes inflammation, weakness and sometimes damage to the muscles, making it difficult for people to move. Some synonyms that can be used for polymyositis include idiopathic inflammatory myopathy, myositis, and dermatomyositis. Idiopathic inflammatory myopathy refers to a group of rare autoimmune diseases that attack muscle fibers, resulting in inflammation and weakness. Myositis is a broader term used to describe muscle inflammation and can include polymyositis and dermatomyositis. Dermatomyositis, on the other hand, is a type of myositis that also affects the skin, causing redness, swelling and sometimes rashes.