Definition
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Erythema Multiforme (EM): an acute, immune-mediated skin reaction characterized by target lesions.
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Often self-limiting but can be recurrent.
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Two main types:
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EM Minor: limited skin involvement, no mucosal lesions
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EM Major: mucosal involvement ± systemic symptoms; overlaps with Stevens-Johnson Syndrome
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Epidemiology & Risk Factors
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Peak incidence: young adults (20–40 years)
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Slight male predominance
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Common triggers:
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Infections (most common): HSV (herpes simplex virus), Mycoplasma pneumoniae
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Medications (less common): sulfonamides, NSAIDs, anticonvulsants
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Vaccinations or idiopathic
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Clinical Features
| Feature | Description | High-Yield Points |
|---|---|---|
| Skin lesions | Target (iris) lesions: concentric rings with central dusky area, raised border, erythematous halo | Classic lesion = targetoid papule or plaque |
| Distribution | Symmetrical, acral areas: hands, feet, extensor surfaces | May spread to trunk |
| Mucosal involvement | Minor: absent; Major: oral, ocular, or genital involvement | Oral erosions common in EM major |
| Systemic symptoms | Fever, malaise – usually mild in EM minor | Severe in EM major or overlap with SJS |
Course:
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Acute onset over 1–3 days
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Resolves in 1–2 weeks for EM minor
💡 PLAB Tip: Target lesions on acral surfaces are classic; differentiate from urticaria or drug rash.
Differential Diagnosis
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Urticaria: transient, blanching wheals; no target lesions
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Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: widespread blistering, mucosal involvement, systemic toxicity
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Fixed drug eruption: round, violaceous plaques that recur at same site
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Viral exanthems: diffuse rash, less defined target lesions
Investigations
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Clinical diagnosis usually sufficient
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Bloods: only if systemic involvement suspected (CBC, LFTs, renal function)
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HSV PCR or serology if recurrent EM suspected
Management
General Measures
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Identify and treat underlying trigger (HSV, bacterial infection, medication)
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Symptomatic relief: antihistamines, analgesics
EM Minor
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Usually self-limiting
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Topical corticosteroids for pruritus
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Supportive care
EM Major
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Hospital admission if extensive mucosal involvement or systemic symptoms
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Systemic corticosteroids sometimes used (controversial)
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Treat underlying cause (e.g., antiviral therapy for HSV)
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Supportive care: hydration, pain control, mouth care
Recurrent EM
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Suppressive antiviral therapy (acyclovir) if HSV-associated
Complications
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Painful mucosal erosions
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Secondary infection of erosions
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Rare scarring
PLAB-Style Questions
Q1: A 25-year-old patient presents with sudden onset of symmetrical, target-shaped lesions on the hands and feet after a recent cold sore. No mucosal involvement. Most likely diagnosis?
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A. Urticaria
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B. Erythema Multiforme Minor
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C. Stevens-Johnson Syndrome
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D. Fixed drug eruption
✅ Answer: B. Erythema Multiforme Minor
(Target lesions on acral surfaces, mild course, post-HSV)
Q2: Which feature differentiates EM Minor from Stevens-Johnson Syndrome?
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A. Symmetrical lesions
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B. Mucosal involvement
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C. Pruritus
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D. Triggered by infection
✅ Answer: B. Mucosal involvement
(SJS has significant mucosal and systemic involvement)
High-Yield Pearls
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Target lesions on acral sites = hallmark
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Most common trigger: herpes simplex virus
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EM Minor = self-limiting, EM Major may require hospitalization
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Symmetry and rapid onset are key diagnostic clues
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Recurrent EM often HSV-related → consider suppressive antivirals
