Erythema Multiforme

Related Subjects: | Nikolsky's sign | Koebner phenomenon | Erythema Multiforme | Pyoderma gangrenosum | Erythema Nodosum | Dermatitis Herpetiformis | Lichen Planus | Acanthosis Nigricans | Acne Rosacea | Acne Vulgaris | Alopecia | Vitiligo | Urticaria | Basal Cell Carcinoma | Malignant Melanoma | Squamous Cell Carcinoma | Mycosis Fungoides (Sezary Syndrome) | Xeroderma Pigmentosum | Bullous Pemphigoid | Pemphigus Vulgaris | Seborrheic Dermatitis | Pityriasis/Tinea Versicolor Infections | Pityriasis Rosea | Scabies | Dermatomyositis | Toxic Epidermal Necrolysis | Stevens-Johnson Syndrome | Atopic Eczema/Atopic Dermatitis | Psoriasis

🎯 Erythema multiforme (EM) is an acute, immune-mediated skin reaction usually triggered by infections (esp. HSV) or drugs. It is typically mild and self-limiting, but in rare cases, it can progress to Stevens–Johnson syndrome (SJS). 🚨

ℹ️ About

• Polymorphic eruption with characteristic target lesions.
• Classically associated with Herpes simplex virus infection.
• Represents a type IV hypersensitivity reaction.

🩺 Clinical Features

• Target lesions – concentric rings: 🔴 red rim, ⚪ pale center, 🔴 central dot.
• Symmetrical distribution, esp. acral (hands, feet, extensor surfaces).
• Systemic: fever, malaise, arthralgia.
• Mucosal involvement: mild/absent in EM minor; significant in EM major/SJS.
• Often follows HSV (1–3 weeks prior) or Mycoplasma pneumoniae infection (respiratory symptoms).
• Red eyes 👁️, photophobia, and sore joints may occur.
• Tip: Marking lesions helps monitor spread/resolution.

🎯 Typical Target Lesion

• Tender erythematous ring → pale halo → central dusky spot/vesicle.

🦠 Causes

• Viruses: HSV-1/2 🦠, Hepatitis B, EBV, Enteroviruses.
• Bacteria: Group A Strep, Mycoplasma pneumoniae, TB.
• Fungi: Histoplasma, Coccidioides.
• Drugs 💊: Sulphonamides, NSAIDs, Penicillin, Isoniazid, Tetracyclines.
• Other: CTDs (SLE), pregnancy, radiation, malignancy (leukaemia, lymphoma).

🔍 Differentials

• Urticaria (wheals, transient, itchy)
• SJS/TEN (more mucosal/systemic involvement, higher severity)
• Drug eruptions, viral exanthems

🧾 Investigations

• Bloods: FBC, U&E, LFTs, CRP/ESR.
• Pregnancy test (if relevant before certain drugs).
• CXR if pneumonia/TB suspected.
• Skin biopsy 🔬: dermal oedema, lymphocytic infiltrate, keratinocyte necrosis at dermo-epidermal junction.

💊 Management

• General: Discontinue offending drug, treat trigger infection.
• Mild EM:
  • Supportive care: antihistamines for itch, rest.
  • Aciclovir PO if HSV suspected.
  • Topical corticosteroids for skin lesions.
• Severe EM / EM major:
  • Systemic steroids (short course) or immunosuppressants (e.g. azathioprine in refractory cases).
  • IV Aciclovir if HSV suspected and unable to tolerate PO.
  • IV fluids + nutritional support if mucosal involvement limits intake.
  • Ophthalmology 👁️ and dermatology 👨‍⚕️ reviews essential.

🚨 Key warning: Always distinguish EM from SJS/TEN — both can follow drugs, but SJS/TEN has extensive mucosal involvement and systemic toxicity, requiring ICU/burns unit care.

🩺 Case 1 — Herpes-Associated Erythema Multiforme

A 21-year-old student presents with target-like lesions on the hands and forearms following a recurrent cold sore. He is otherwise well, with no mucosal involvement. Diagnosis: erythema multiforme minor triggered by HSV. Management: 💊 Symptomatic care with antihistamines, topical steroids for itching, and oral aciclovir if recurrent HSV-associated episodes. Avoid: ❌ Confusing with urticaria (EM lesions are fixed, not fleeting); avoid unnecessary systemic steroids in mild HSV-related EM.

🩺 Case 2 — Drug-Induced Erythema Multiforme Major

A 34-year-old woman develops widespread target lesions with central blistering and painful oral ulcers one week after starting carbamazepine. She has fever and malaise. Diagnosis: erythema multiforme major with mucosal involvement. Management: 🚑 Immediate withdrawal of the offending drug; supportive care (hydration, analgesia, wound care). Hospital admission if severe mucosal disease or systemic compromise. Avoid: ❌ Re-exposure to the causative drug; avoid mislabelling as Stevens–Johnson syndrome unless extensive epidermal detachment is present (distinction matters for prognosis).