Erythema Multiforme or Urticaria?

In the February 2007 issue (Photo Quiz, page 88), there is a case of a child with a rash diagnosed as erythema multiforme (EM) rather than urticaria. I assume that these were fixed lesions, which distinguished them from urticaria lesions? Also, if this rash had been caused by the antibiotic the patient had been taking for an acute otitis media infection, why did it first manifest after 4 days of treatment?

---- Valerie Welch, MD Monticello Clinic Monticello, Minn

Thanks very much for your probing questions. Rashes can be such an enigma. The majority of the lesions in this patient were fixed. Some did appear to be enhanced after the patient showered, however, and other lesions had snakelike borders that resembled those of urticaria.

EM has a broad spectrum of clinical manifestations. (Its name implies the breadth of possible features.) Because most clinicians do not obtain a biopsy when EM is the presumed diagnosis, the exact cause of the rash is usually not known.

In this child's case, antibiotics were presumed to be the cause. The child's mother reported that he was able to tolerate the medicine for 3 to 4 days.

I based my diagnosis of EM on the distribution and character of the rash. However, after reviewing some other pictures of this condition--and after consulting with some of my dermatologist colleagues--it could well be that this child had urticaria.

---- John W. Harrington, MD Chief of General Pediatrics Children's Hospital of The King's Daughters Norfolk, Va