The most common group of acquired and potentially treatable skeletal muscle disorders. Five major forms: dermatomyositis (DM), inclusion body myositis (IBM), polymyositis (PM), immune-mediated necrotizing myopathy, and antisynthetase syndrome. Usually present as progressive and symmetric muscle weakness; extraocular muscles spared but pharyngeal weakness (dysphagia) and head drop from neck muscle weakness are common. Respiratory muscles may be affected in advanced cases. IBM is characterized by early involvement of quadriceps (leading to falls) and distal muscles; IBM may have an asymmetric pattern. Progression is over weeks or months in PM and DM, but typically over years in IBM. Skin involvement in DM may consist of a heliotrope rash (blue-purple discoloration) on the upper eyelids with edema, a flat red rash on the face and upper trunk, or erythema over knuckles (Gottron's sign). A variety of cancers are associated with DM. Features of each disorder are summarized in Table 198-1 Inflammatory Myopathies: Clinical and Laboratory Features.