(BMJ)—A 3-yo girl presented w/ a 3-day hx of rash and swelling of the extremities and a 2-day hx of fever. Two weeks before presentation, she began experiencing cough, intermittent fever, and malaise. Initially, her hands were itchy and swollen, then maculopapular lesions developed. The rash evolved into target lesions 7 days after onset. Exam: papules/vesicles on legs, buttocks, ankles, and lips. CBC, erythrocyte sedimentation rate, CRP, LFTs normal. What’s the dx?
Varicella zoster
Mycoplasma pneumoniae
Stevens-Johnson syndrome
Gianotti-Crosti syndrome
Hand-foot-and-mouth dz
You are correct. Tests were positive for IgM to Mycoplasma pneumoniae and negative for HSV and varicella zoster virus. This pt had classic erythema multiforme (EM) induced by M pneumoniae. Abx (eg, PCNs, cephalosporins), HSV, and M pneumoniae are the most common causes of EM in children.

Dx is made on the basis of distinctive target lesions, early cough, fever, no hx of meds, and a positive test for M pneumoniae IgM. CXR is unnecessary in cases of uncomplicated pneumonia, and bx isn’t required.

M pneumoniae-induced EM can present as a polymorphic rash (flat or maculopapular eruptions, or both, and vesicles) w/ typical target lesions, w/ or w/o mucosal involvement.

This pt was treated w/ topical steroids, PO azithromycin, and then PO prednisone due to increased rash, late swelling of extremities, and persistent fever, despite topical steroids. Two weeks later, the fever, malaise, and rash had resolved.

BMJ 2020;371:m4349