Erythema multiforme — clinical SAQ

Stem

A 22-year-old student presents with a 3-day eruption on the palms, dorsal hands, forearms and knees. Two weeks earlier she noticed a cold sore. The lesions are symmetrical and raised, with a dusky centre, a pale oedematous ring and an erythematous outer ring. There are two shallow inner-lip erosions, but she drinks normally and has no eye, genital or respiratory symptoms. She takes no new medicines and is systemically well.^[1]

Questions

a) Give the diagnosis and the morphology that supports it. (2 marks)^[1]

b) Name four features that would make you urgently reconsider SJS/TEN. (2 marks)^[1]

c) What trigger assessment is appropriate in this patient, and when would you investigate for RIME/MIRM? (2 marks)^[1][2]

d) Outline acute management. (2 marks)^[1]

e) If she later develops frequent, burdensome recurrences, state the first-line preventive strategy and one evidence-supported regimen. (2 marks)^[4]

Model answer

a) Diagnosis and morphology — 2 marks

This is erythema multiforme with mild oral involvement (non-severe EM). A few lip erosions do not automatically make the episode “EM major.” The supporting lesion is a raised target with three zones: dusky or blistered centre, pale oedematous ring and outer erythematous ring, in a symmetrical acral distribution.^[1]

b) SJS/TEN red flags — 2 marks

Award 0.5 mark each for four: skin pain; purpuric macules or flat atypical targets that blister; widespread or confluent blistering; epidermal detachment; positive Nikolsky sign; trunk-predominant progression; systemic illness; extensive erosive mucositis; or a plausible recent drug exposure. A central blister within a raised three-zone target remains compatible with EM; morphology and detachment lead the urgent distinction.^[1]

c) Trigger assessment — 2 marks

Take a complete infection, recurrence and medicine timeline. The cold sore makes HSV a plausible trigger, but the timing does not prove causation. Do not order routine HSV serology; if an active suspicious herpetic lesion is available, PCR sampling is more informative. Typical mild EM otherwise needs no shotgun infection panel.^[1]

Prominent oral, ocular or urogenital mucositis after a respiratory illness, especially with sparse or variable skin lesions in a young person, should prompt RIME/MIRM assessment: lung examination, chest imaging when indicated, and respiratory M. pneumoniae PCR with context-appropriate serology. RIME/MIRM is not simply another name for EM major.^[2][1]

d) Acute management — 2 marks

She can be treated as an outpatient because intake is preserved and no eye, genital, respiratory or epidermal-necrolysis red flag is present. Explain the diagnosis; use bland emollient or petrolatum, consider a topical corticosteroid for inflamed skin, provide appropriate simple analgesia and gentle oral/local-anaesthetic care, and give strict return advice for skin pain, blistering that becomes widespread/confluent or develops on flat purpuric lesions, detachment, reduced intake or eye symptoms. Starting oral aciclovir after the EM eruption is established has not been shown to shorten this episode.^[1]

e) Prevention of recurrent EM — 2 marks

For frequent or burdensome recurrent EM, first line is continuous oral antiviral prophylaxis after diagnostic and renal/medicine review. An evidence-supported adult regimen is aciclovir 400 mg orally twice daily for at least 6 months; a small placebo-controlled trial showed fewer recurrences during continuous therapy. Valaciclovir 500 mg orally twice daily or famciclovir 250 mg orally twice daily are review-supported alternatives, but the overall treatment evidence is limited and dosing must be individualised.^[3][4][1]

Marking cautions

• Do not award “EM major” solely because two mild oral erosions are present.
• Do not accept “HSV serology proves recurrent HSV-associated EM.”
• Do not accept an acute aciclovir course as established treatment for the current EM eruption.
• Do not describe MIRM as synonymous with EM; it is the M. pneumoniae-associated form of RIME, and its relationship to EM remains debated.^[1][2]

References4ShowHide

1. [1]Kechichian E, Dupin N, Wetter DA, Ortonne N, Agbo-Godeau S, Chosidow O. Erythema multiforme EClinicalMedicine, 2024.PMID 39583748
2. [2]Canavan TN, Mathes EF, Frieden I, Shinkai K. Mycoplasma pneumoniae-induced rash and mucositis as a syndrome distinct from Stevens-Johnson syndrome and erythema multiforme: a systematic review J Am Acad Dermatol, 2015.PMID 25592340
3. [3]Tatnall FM, Schofield JK, Leigh IM. A double-blind, placebo-controlled trial of continuous acyclovir therapy in recurrent erythema multiforme Br J Dermatol, 1995.PMID 7888365
4. [4]de Risi-Pugliese T, Sbidian E, Ingen-Housz-Oro S, Le Cleach L. Interventions for erythema multiforme: a systematic review J Eur Acad Dermatol Venereol, 2019.PMID 30680804