What is the difference between erythema multiforme and nummular eczema in terms of diagnosis, treatment, and management?

Distinguishing Erythema Multiforme from Nummular Eczema

Erythema multiforme presents with fixed, three-zone target ("iris") lesions predominantly on the extremities that last at least 7 days and is triggered by infections (especially HSV) or drugs, while nummular eczema presents with coin-shaped, pruritic, scaly plaques that respond to topical corticosteroids and emollients.

Key Diagnostic Features

Erythema Multiforme

• Lesion morphology: Classic target lesions consist of three distinct concentric zones—a dark red center (often with vesicle or papule), surrounded by a pale pink ring, with an outer erythematous halo 1, 2
• Distribution: Predominantly acral, affecting extensor surfaces of extremities (hands, feet, elbows, knees) with centripetal spread 3
• Duration: Individual lesions remain fixed for a minimum of 7 days, distinguishing it from urticaria which resolves within 24 hours 2, 3
• Mucosal involvement: May affect oral, ocular, or genital mucosa, particularly in erythema multiforme major 4
• Onset: Acute, self-limited course typically appearing 10 days after viral infection 1

Nummular Eczema

While the provided evidence focuses primarily on seborrheic dermatitis rather than nummular eczema specifically, the general eczematous conditions share common features:

• Lesion morphology: Coin-shaped (discoid), scaly, erythematous plaques with crusting or weeping 5
• Distribution: Typically on trunk and extremities, but not specifically acral 5
• Duration: Chronic, relapsing course with lichenification in long-standing cases 5
• Pruritus: Intense itching is a hallmark feature 5

Etiologic Differences

Erythema Multiforme Triggers

• Primary infectious causes: Herpes simplex virus (most common), Mycoplasma pneumoniae (especially in children), hepatitis C, Coxsackie virus, Epstein-Barr virus 1, 6
• Medications: Allopurinol, phenobarbital, phenytoin, sulfonamides, penicillins, TNF-α inhibitors 6
• Mechanism: Hypersensitivity reaction with cytotoxic T lymphocytes inducing keratinocyte apoptosis 1

Nummular Eczema Triggers

• Aggravating factors: Hot water, harsh soaps/detergents that remove natural skin lipids, excessive heat 5
• Secondary infection: Staphylococcus aureus or herpes simplex superinfection may complicate the condition 5

Histopathologic Differentiation

Erythema Multiforme

• Variable epidermal damage ranging from individual keratinocyte apoptosis to confluent epidermal necrosis 1
• Interface dermatitis with lymphocytic infiltrate 1
• Direct immunofluorescence is negative, helping exclude immunobullous disorders 2

Eczematous Conditions

• Spongiosis (intercellular edema in epidermis) 5
• Acanthosis and parakeratosis in chronic lesions 5
• Perivascular lymphocytic infiltrate 5

Critical Differential Diagnosis Pitfalls

Avoid confusing erythema multiforme with Stevens-Johnson syndrome (SJS)/toxic epidermal necrolysis (TEN): SJS/TEN features flat atypical targets or purpuric macules (not true three-zone targets), demonstrates positive Nikolsky sign with epidermal detachment, is predominantly drug-induced, and carries significantly higher mortality 1, 2

Document lesion evolution: Photograph and mark lesion borders to track whether they remain fixed (EM) or migrate/resolve quickly (urticaria) 2

Management Approach

Erythema Multiforme Treatment

• Identify and treat underlying cause: Stop implicated medications; treat HSV or Mycoplasma pneumoniae infection 3, 7
• Acute symptomatic treatment: Topical corticosteroids or antihistamines for pruritus 3, 7
• Recurrent HSV-associated EM: Prophylactic antiviral therapy (acyclovir, valacyclovir, or famciclovir) is first-line; if resistant, consider switching antivirals or adding dapsone 6, 7
• Severe mucosal involvement: May require hospitalization for IV fluids and electrolyte repletion 3

Nummular Eczema Treatment

• Gentle skin care: Use mild, pH-neutral non-soap cleansers with tepid water; avoid hot water and harsh soaps 5
• Emollients: Apply fragrance-free moisturizers immediately after bathing to damp skin, reapply every 3-4 hours 5
• Topical corticosteroids: Low-potency (hydrocortisone 1%) for mild cases; use the least potent preparation needed to control symptoms 5
• Avoid overtreatment: Limit facial corticosteroid use to 2-4 weeks maximum due to risk of skin atrophy and telangiectasia 5
• Treat secondary infection: Oral flucloxacillin for Staphylococcus aureus; oral acyclovir for herpes simplex superinfection 5

When to Refer to Dermatology

• Diagnostic uncertainty or atypical presentation 5
• Failure to respond after 4 weeks of appropriate first-line therapy 5
• Suspected immunobullous disorder requiring direct immunofluorescence 2
• Need for skin biopsy and histopathologic confirmation 1
• Recurrent severe flares despite optimal therapy 5