Erythema Multiforme
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Direct answer
Erythema multiforme is an acute, self-limiting hypersensitivity reaction that produces target or iris lesions, most often after herpes simplex infection. The minor form involves the skin or a single mucosal site, while the major form involves two or more mucosal sites with systemic upset. Unlike Stevens-Johnson syndrome, classic erythema multiforme shows no sheet-like epidermal detachment, which is the key distinguishing feature.
What you must remember
- Target lesion: a dusky centre surrounded by pale oedema and an outer erythematous ring, giving the iris appearance.
- Triggers: herpes simplex, especially recurrent HSV-associated disease, and Mycoplasma pneumoniae, with drugs such as sulphonamides and anticonvulsants.
- Minor form: skin lesions alone or a single mucosal site, without significant systemic features.
- Major form: at least two mucosal sites with systemic symptoms such as fever and malaise.
- Recurrent disease: HSV-associated episodes are prevented with suppressive acyclovir.
- Histology: interface dermatitis with keratinocyte apoptosis and basal cell damage, which overlaps with SJS.
- Severity markers: epidermal detachment is absent in EM, under 10 per cent of body surface area in SJS, and over 30 per cent in TEN.
- Treatment: supportive care and treatment of the trigger, with systemic corticosteroids reserved for severe forms where benefit is debated.
Where candidates lose marks
Points of confusion here usually involve calling every target lesion erythema multiforme.
- Trap: calling every target lesion erythema multiforme Correct: atypical targets, mucosal erosions and epidermal detachment indicate SJS, a drug reaction needing urgent review.
- Trap: assuming HSV-triggered erythema multiforme needs no prevention Correct: recurrent HSV-associated erythema multiforme is prevented with suppressive antiviral therapy.
- Trap: reading erythema multiforme major as the same as SJS Correct: major erythema multiforme has multiple mucosal sites but no sheet-like epidermal loss, which separates it from SJS.
Frequently asked questions
Which infection most often triggers erythema multiforme?
Herpes simplex virus is the commonest infectious trigger of erythema multiforme, and recurrent herpes-associated episodes are frequent. Mycoplasma pneumoniae is another important trigger, particularly in younger patients.
How does minor erythema multiforme differ from the major form?
Minor erythema multiforme involves the skin or a single mucosal site without significant systemic upset, whereas the major form involves at least two mucosal sites with systemic symptoms. Major disease may need hospital care and close monitoring.
What distinguishes erythema multiforme from Stevens-Johnson syndrome?
Erythema multiforme shows no sheet-like epidermal detachment, while Stevens-Johnson syndrome shows detachment of under 10 per cent of body surface area. Atypical targets, flat dusky macules and prominent mucosal erosions favour Stevens-Johnson syndrome.
What histological pattern suggests erythema multiforme?
Erythema multiforme shows an interface dermatitis with keratinocyte apoptosis and basal cell damage, but this pattern overlaps with Stevens-Johnson syndrome. Clinical correlation is therefore essential before histology is used to label the reaction.
How is recurrent herpes-associated erythema multiforme prevented?
Recurrent herpes-associated erythema multiforme is prevented with suppressive antiviral therapy, most often acyclovir. The regimen is individualised to how often episodes recur, and it reduces the chance of further target lesions.