Diagnosis: Urticaria multiforme
The patient’s lab work came back within normal range, except for an elevated white blood cell count (19,700/mm3; reference range, 4500-13,500/mm3). His mild systemic symptoms, skin lesions without blistering or necrosis, acral edema, and the absence of lymphadenopathy pointed to a diagnosis of urticaria multiforme.
Urticaria multiforme, also called acute annular urticaria or acute urticarial hypersensitivity syndrome, is a histamine-mediated hypersensitivity reaction characterized by transient annular, polycyclic, urticarial lesions with central ecchymosis. The incidence and prevalence are not known. Urticaria multiforme is considered common, but it is frequently misdiagnosed.1 It typically manifests in children ages 4 months to 4 years and begins with small erythematous macules, papules, and plaques that progress to large blanchable wheals with dusky blue centers.1-3 Lesions are usually located on the face, trunk, and extremities and are often pruritic (60%-94%).1-3 Individual lesions last less than 24 hours, but new ones may appear. The rash generally lasts 2 to 12 days.1,3
Patients often report a preceding viral illness, otitis media, recent use of antibiotics, or recent immunizations. Dermatographism due to mast cell–mediated cutaneous hypersensitivity at sites of minor skin trauma is common (44%).1 Patients often have associated facial or acral edema (72%).1 Children with significant edema of the feet may find walking difficult, which should not be confused with arthritis or arthralgias. Generally, patients are nontoxic in appearance, and systemic symptoms are limited to a low-grade fever.1-3
The diagnosis is made clinically and should not require a skin biopsy or extensive laboratory testing.When performed, laboratory studies, including CBC, erythrocyte sedimentation rate, C-reactive protein, and urinalysis are routinely normal.
Erythema multiforme and urticarial vasculitis are part of the differential
The differential diagnosis in this case includes erythema multiforme, Henoch-Schönlein purpura, serum sickness-like reaction, and urticarial vasculitis (TABLE1,2,4).
Continue to: Erythema multiforme