What is the diagnosis and treatment for a young adult with no significant underlying medical conditions presenting with Erythema multiforme?

Erythema Multiforme: Diagnosis and Management

Immediate Diagnostic Confirmation

For a young adult with suspected erythema multiforme, confirm the diagnosis by identifying characteristic target or "iris" lesions with three distinct concentric zones (dark red center, pale ring, erythematous halo), distributed symmetrically on the extremities, particularly extensor surfaces, with lesions remaining fixed for at least 7 days. 1, 2, 3

Key Diagnostic Features to Document

• Lesion morphology: True targetoid configuration with dusky red and white concentric rings, not flat atypical targets (which suggest Stevens-Johnson syndrome) 1, 2
• Distribution pattern: Symmetric involvement of distal extremities, especially extensor surfaces, spreading centripetally 3, 4
• Lesion duration: Fixed lesions persisting minimum 7 days (unlike urticaria which resolves within 24 hours) 1, 3
• Mucosal involvement: Minimal to absent in erythema multiforme minor; if extensive mucosal erosions with hemorrhagic crusting are present, consider Stevens-Johnson syndrome instead 5, 2

Critical Differentiation from Stevens-Johnson Syndrome

Erythema multiforme and Stevens-Johnson syndrome are separate disorders despite historical confusion 6, 3:

• EM: Typical raised target lesions, predominantly on extremities, HSV-triggered, better prognosis, lower mortality 1, 2
• SJS/TEN: Flat atypical targets or purpuric macules, positive Nikolsky sign, epidermal detachment, drug-induced, prodrome of fever/malaise, 10-30% mortality 5, 2

Identify and Address the Underlying Trigger

Most Common Causes in Young Adults

Herpes simplex virus (HSV) is the most frequent trigger, followed by Mycoplasma pneumoniae and medications 1, 2, 6:

• HSV-associated EM: Lesions typically appear 10 days after viral infection due to hypersensitivity reaction with cytotoxic T lymphocytes inducing keratinocyte apoptosis 1
• Mycoplasma pneumoniae: Particularly common in children/young adults, may present with predominantly mucosal involvement with minimal cutaneous lesions 1
• Medications: Less common than in SJS/TEN, but document all drugs taken in preceding 2 months 2

Essential Investigations

• HSV PCR or serology to identify viral trigger 2
• Mycoplasma pneumoniae testing if respiratory symptoms present 1
• Skin biopsy from lesional skin if diagnosis uncertain (shows variable epidermal damage from individual cell apoptosis to confluent necrosis) 1, 2
• Direct immunofluorescence from perilesional biopsy to exclude immunobullous disorders (pemphigus, pemphigoid) 2

Treatment Algorithm

Acute Erythema Multiforme (First Episode)

Stop any suspected causal medication immediately and treat identified infections before initiating symptomatic therapy 6, 3:

For Mild Cases (Limited Skin Involvement, No Mucosal Disease)

• Topical high-potency corticosteroids to affected skin areas 6, 3
• Oral antihistamines for pruritus 6, 3
• Supportive care: Lesions typically resolve without sequelae in 3-5 weeks 3, 7

For Moderate Cases (Mucosal Involvement Present)

• Topical antiseptic or anesthetic solutions (lidocaine viscous, diphenhydramine/aluminum hydroxide) for oral lesions 6, 3
• Soft bland diet to minimize mucosal trauma 8
• Adequate hydration monitoring 3, 4

For Severe Mucosal Disease

• Hospitalization for intravenous fluids and electrolyte repletion 3
• Systemic corticosteroids (prednisone 0.5-1 mg/kg for 7 days) only for severe cases per FDA indication for "severe erythema multiforme (Stevens-Johnson syndrome)" 9, 6

Recurrent Erythema Multiforme (HSV-Associated)

For patients with recurrent episodes triggered by HSV, initiate continuous antiviral prophylaxis as first-line therapy 6, 3, 7:

First-Line Antiviral Prophylaxis

• Acyclovir 400 mg twice daily continuously 7
• Valacyclovir 500 mg daily (if no response to acyclovir; greater bioavailability, more convenient dosing) 7
• Famciclovir 250 mg twice daily (alternative if valacyclovir ineffective) 7

Second-Line Therapies (If Antiviral Prophylaxis Fails)

Refer to dermatology for consideration of 6, 7:

• Immunosuppressive agents (azathioprine, dapsone)
• Antimalarials (hydroxychloroquine)
• Other systemic therapies tailored to individual response

Common Pitfalls to Avoid

• Do not confuse with urticaria: EM lesions remain fixed for ≥7 days; urticarial lesions resolve within 24 hours 1, 3
• Do not miss Stevens-Johnson syndrome: Presence of epidermal detachment, positive Nikolsky sign, or flat atypical targets requires immediate hospitalization and different management 5, 2
• Do not use systemic corticosteroids routinely: Reserve for severe mucosal disease; evidence for benefit in mild-moderate EM is limited 9, 6
• Do not delay antiviral prophylaxis in recurrent cases: Continuous suppressive therapy is highly effective for HSV-triggered recurrent EM 6, 3, 7