However, DM is not simply PM with the addition of a rash. Each disease has characteristic clinical differences, histopathologic findings, and immune markers. These include the typical rash of DM, findings at history and physical examination that reveal symmetric proximal muscular weakness, elevated serum muscle enzyme levels, electromyographic evidence of myopathic abnormalities, and characteristic findings at muscle biopsy. PM 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, DM has an insidious onset and involves the proximal muscles. However, this disease also involves a characteristic rash. In both diseases, immune-mediated muscle inflammation and vascular damage occur. In PM, the immune system is primed to act against previously unrecognized muscle antigens. In DM, complement-mediated damage to endomysial vessels and microvasculature of the dermis occurs.