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Erythema multiforme
Erythema multiforme (EM) is an acute immune reaction, usually to herpes simplex infection, presenting with typical three-ring target lesions on the limbs; it is distinct from SJS/TEN, which are mainly caused by drugs.
Why it matters in the exam
The stem shows a young adult with round target lesions on the palms and forearms a week after a "cold sore" on the lip, and asks about the cause and treatment. The key decision is EM versus SJS/TEN: in EM the target lesions are typical and raised, mainly on the limbs, and detached skin is under 10%; in SJS the lesions are flat and atypical, start on the trunk and are usually caused by a drug. Also asked are recurrent EM — treated with long-term aciclovir prophylaxis — and mycoplasma-related EM in children, where mucosal involvement is prominent. Remember that the disease is self-limiting over two to four weeks, is usually not life-threatening, and that the right distinction avoids both needless admission and a missed severe drug reaction. In the exam, look in the stem for the clue of a preceding infection — a cold sore, cough or fever — and the absence of a new drug, and check whether the lesions are raised and whether they appear on the palms, because these are the signs that separate the diseases and point to the right management, including the decision to stop a drug, to admit and to start long-term prevention.
Key points
- Erythema multiforme is an acute immune reaction, mostly to HSV, with typical three-ring target lesions.
- Lesions appear mainly on the limbs and palms, and in the major form the mucosa is also involved.
- SJS/TEN are a separate entity, usually drug-induced, with flat lesions, onset on the trunk and wider skin detachment.
- EM is self-limiting within weeks and treated symptomatically; recurrent EM is treated with preventive antivirals.
- In children, mycoplasma causes a picture with prominent mucosal involvement that needs macrolide treatment.
Summary table
| Feature | EM minor | EM major | SJS/TEN (for comparison) |
|---|---|---|---|
| Main cause | HSV | HSV, mycoplasma | Drugs (allopurinol, anticonvulsants, sulfonamides, nevirapine) |
| Lesions | Raised typical targets, three rings | Typical targets with mucosal involvement | Flat atypical targets, dusky macules, blisters |
| Distribution | Limbs, palms, extensor surfaces | Limbs and face, mucosa | Trunk and face first, spreading |
| Detached skin area | None | Under 10% | SJS under 10%, TEN over 30% |
| Treatment | Symptomatic; preventive aciclovir in recurrent EM | Supportive care, treat the cause, steroids in selected cases | Stop the suspected drug immediately, burns unit, SCORTEN score |
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Sources
- British Association of Dermatologists guideline on erythema multiforme, 2022 — Classification and management
- CDC Sexually Transmitted Infections Treatment Guidelines, 2021 — Genital herpes — suppressive therapy
- Fitzpatrick's Dermatology, 9th edition (2019) — Erythema multiforme
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.