Diagnosis: Urticaria multiforme
Based on the appearance of the plaques and the recent history of amoxicillin use, we diagnosed urticaria multiforme (UM) in this patient. UM is a benign, self-limited, cutaneous, histamine-dependent, hypersensitivity reaction that occurs predominantly among patients ages 4 months to 4 years.1
In UM, the eruption typically occurs one to 3 days after a viral infection or the administration of certain medications.2 A low-grade fever may or may not precede the eruption, and symptoms are usually limited to pruritus. Small urticarial papules and plaques initially appear and then coalesce to form blanching, erythematous, annular, polycyclic wheals.
Unlike “ordinary” acute urticaria, which is characterized by round or oval-shaped erythematous, edematous papules and plaques, UM plaques will have annular, gyrate, serpiginous, polycyclic, and/or target lesions with ecchymotic or dusky-appearing centers.1,3 Individual UM lesions typically last less than 24 hours, while the disease itself can persist for 2 to 12 days, until all lesions heal and the skin returns to normal.2
The diagnosis of UM is a clinical one. When a patient presents with urticarial lesions, ask about the timing of rash onset and the duration of individual lesions. Also ask whether the patient has had a fever, dermatographism, acral edema, or other symptoms, such as arthralgias and myalgias.2
Although a biopsy is typically not performed on this type of lesion, the histology will be the same as that seen in ordinary acute urticaria: dermal edema with some perivascular and interstitial infiltrates of eosinophils, neutrophils, and lymphocytes.3
Differential Dx includes serum sickness-like reaction, EM
Patients with serum sickness-like reaction (SSLR) will have a more severe clinical presentation than those with UM, characterized by a high-grade fever, lymphadenopathy, myalgia, and arthralgia.2 Also, the time between viral infection/medication administration and onset of eruption is greater in SSLR: 7 to 21 days, as opposed to one to 3 days in UM.1 In addition, the individual lesions of UM only last about a day, whereas the plaques of SSLR can last from a few days to a few weeks.