Polymyositis usually presents as symmetric pain and weakness of the proximal limb and neck flexor muscles. Less commonly, the distal musculature or muscles of glutition are involved.
Symptoms develop over several weeks to months.
Similarly, dermatomyositis has an insidious onset and involves proximal muscles. However, this disease also includes the presence of a characteristic rash. These include the typical rash of dermatomyositis, a history and physical examination which reveals symmetric, proximal muscular weakness, elevated serum muscle enzymes, electromyographic evidence of myopathic abnormalities, and characteristic muscle biopsy findings. Much of the literature groups PM and DM together as one disease. However, dermatomyositis is not simply polymyositis with the addition of a rash.