Polymyositis and dermatomyositis are connective tissue diseases in which there is muscle weakness and tenderness. The two disorders are very similar, except dermatomyositis involves the skin as well as the muscles, while polymyositis involves only the muscles.

The major manifestation of polymyositis is inflammation leading to destruction of muscle and increasing muscular weakness. As the disease progresses, the muscle tissue is replaced by functionless scar tissue. Both have features in common...