Polymyositis is an autoimmune disease seen in adults. It presents with bilateral proximal muscle weakness. Microscopic exam demonstrates endomysial lympho-cytic inflammation (mostly cytotoxic T8) and skeletal muscle fiber degeneration and regeneration. Patients respond to immunosuppression.
Dermatomyositis is a connective tissue disorder involving inflammation of skeletal muscle and skin. It can affect both children and adults. It presents with bilateral proximal muscle weakness, skin rash of the upper eyelids, and periorbital edema.
Microscopic exam demonstrates perimysial and vascular lymphocytic inflamma-tion, perifascicular fiber atrophy, and skeletal muscle fiber degeneration and regen-eration. Adult patients are at increased risk of lung, colon, breast, and gynecologic cancers.
Inclusion body myositis affects adults age >50, causing slowly progressive, asym-metrical, distal muscle weakness. Light microscopy demonstrates autophagic vacu-oles and inclusion bodies in addition to inflammation and necrosis. The disease is refractory to immunosuppressive therapy.