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There is a great deal of confusion between three conditions: erythema multiforme, urticaria, and Stevens Johnson syndrome. What most pediatricians diagnose as erythema multiforme is instead urticaria, frequently giant urticaria (Table I).1 This is based on the mistaken belief that "multiforme" means many shapes of lesions at the time of examination. Erythema multiforme was described by Ferdinand von Hebra as an acute, self-limited condition characterized by the abrupt onset of fixed red papules, some of which evolved to develop a dusky central zone surrounded by a red outer zone that he called "target lesions."2 The red papules he described were "fixed," ie, the papules stayed at the same skin site for at least 7 days. In contrast, the urticaria mistakenly diagnosed as "erythema multiforme" has red annular skin lesions that move from skin site to skin site within hours. The red annular lesions will have normal skin in the center that gives the appearance of a red border with a "clear" center (Figure 1 ).' The red papules of true erythema multiforme would develop a dusky center, frequently demonstrating focal areas of necrosis by evolving into a central blister or crust.2,3 Thus, the center of von Hebra's target lesions were not normal unaffected skin, but damaged skin (Figure 2). Some of von Hebra's target lesions would evolve to demonstrate three zones of color change: an outer red zone, a middle dusky zone, and a central blister or whitish zone. Such lesions are called "iris lesions."2 It is now believed that target lesions and iris lesions are manifestations of the same type of cutaneous injury.3
The giant urticaria misdiagnosed as erythema multiforme frequently was associated with mild angioedema of the hands and feet, so "swelling" of the hands and feet mistakenly became a clinical feature of erythema multiforme (Figure 3).3 Von Hebra did not describe such swelling.2 New urticaria lesions will occur daily and continuously, while in erythema multiforme, all lesions appear abruptly within 72 hours (Table 1).
The list of "causes" of erythema multiforme in the literature is long and duplicates the "causes" of urticaria.3 In fact, erythema multiforme as described by von Hebra has a quite limited number of precipitating factors, with herpes simplex virus accounting for 80% of childhood cases.4 Other convincing causes...