Isomorphic phenomenon in erythema multiforme

The cutaneous lesions of erythema multiforme have a propensity to localize at previously inflamed or traumatized skin sites. Evidence of the isomorphic phenomenon may be demonstrated in most cases of erythema multiforme by a careful history and physical examination. We present three cases of erythema multiforme which illustrate localization of skin lesions around scratches, recent surgical scars, lacerations, traumatized nail folds and folliculitis. The isomorphic phenomenon may serve as an important clue in understanding the clinical features and possibly, the pathogenesis of erythema multiforme.