A type of idiopathic inflammatory myopathy characterized by evocative skin lesions and symmetrical proximal muscle weakness.

The cardinal symptom is severely pruritic skin lesions. Macular, papular, papulovesicular, urticarial, multiforme- and plaque-like variants are differentiated morphologically, hence the name polymorphous. Usually one morphology dominates in a single individual (monomorphous). The skin lesions develop a few hours to several days after sun exposure. Initially, patchy erythema develops, accompanied by pruritus. Distinct lesions then develop. The upper chest, upper arms, backs of the hands, thighs, and the sides of the face are the primary localizations. The skin lesions resolve spontaneously within several days of ceasing sun exposure and do not leave behind any traces.