AKT · Dermatology/Reactive & systemic signs

Erythema multiforme

Immune-mediated reaction (usually HSV / Mycoplasma)

Overview

An acute, usually self-limiting, immune-mediated reaction characterised by TARGET lesions, most often triggered by herpes simplex virus (and Mycoplasma in children) rather than drugs. The key job is to separate this generally benign condition from SJS/TEN.

Recognise

Typical TARGET (iris) lesions — three zones: a dusky/blistered centre, a pale ring, and an erythematous outer ring
Acral predominance — symmetrical on the hands/feet, extensor limbs, palms and soles
EM minor (little/no mucosal involvement) vs EM major (significant mucosal involvement but far less skin detachment than SJS/TEN)

Red flags

Extensive mucosal involvement, skin pain, sheet-like detachment or systemic toxicity → reconsider SJS/TEN (a different, more dangerous entity)

Differentials & how to tell them apart

Stevens-Johnson syndrome / TENdrug trigger, atypical flat targets, extensive mucosal involvement and skin DETACHMENT — EM has typical raised acral targets, is usually post-infective and detaches little

Urticariafleeting weals (each <24h) without the fixed 3-zone target

Urticarial vasculitisweals lasting >24h, bruising

Viral exanthemmorbilliform, no true target lesions

Clinical image

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Erythema multiforme — clinical imagesDermNet

Investigations

Clinical. Look for the trigger — HSV (recent cold sore), Mycoplasma (especially children, with respiratory symptoms). Biopsy only if uncertain.

Management

Treat the trigger (e.g. HSV) + symptomatic care; it is usually self-limiting

1Identify and treat the trigger (aciclovir for HSV, antibiotics for Mycoplasma); symptomatic care for the skin. EM minor self-resolves over ~2 weeks.Gate: Distinguish EM from SJS/TEN — EM has typical raised three-zone TARGET lesions with an acral distribution and is usually POST-INFECTIVE (HSV/Mycoplasma) with minimal detachment, whereas SJS/TEN is drug-induced with extensive mucosal disease and sheet-like skin loss
2Recurrent HSV-associated EM → continuous aciclovir prophylaxis; EM major with mucosal disease → supportive care and specialist input.

Treat/suppress the trigger — aciclovir for HSV-associated disease; antibiotics for Mycoplasma

Symptomatic care (antihistamine, topical steroid) — EM minor is self-limiting

Aciclovir prophylaxis — for recurrent HSV-driven erythema multiforme

Key points

EM is usually HSV-driven and benign; do not over-call it as SJS. Recurrent EM is a clue to recurrent herpes — prophylactic aciclovir helps. The 3-zone target is the recognition sign.

Monitor & prognosis

Resolution; recurrence pattern.

EM minor resolves without scarring; recurrences occur with recurrent HSV.

Source: NICE CKS; DermNet