An infant presents with 1 day of an itchy red rash with low-grade fever and mild swelling of the hands and feet. She had nasal congestion and a cough last week, but otherwise has been well. Exam is notable for edema in the distal extremities, blanching erythematous annular plaques, and dermatographism. There is no mucosal involvement. (Image source: PMID 25843790)
What is this rash? What causes it? How do we treat it?
Urticaria multiforme is a self-limited urticarial hypersensitivity eruption that primarily occurs in infants and young children. Because of the annular nature of the rash, association with a preceding infection, and often dramatic presentation, the condition is commonly misdiagnosed as erythema multiforme or other more serious cutaneous conditions. The rash typically self-resolves over days and only requires supportive care.