How do the appearances of erythema multiforme and tinea corporis differ?

Distinguishing Erythema Multiforme from Tinea Corporis

Erythema multiforme (EM) and tinea corporis have distinctly different clinical appearances that can be reliably differentiated through careful examination of morphology, distribution pattern, and associated symptoms.

Key Distinguishing Features

Erythema Multiforme

• Characteristic lesions: Target or "iris" lesions with concentric zones of color change (dark center, pale ring, and erythematous outer ring) 1
• Distribution: Symmetrical, predominantly on extremities (especially extensor surfaces) and face; tends to start acral and progress proximally 2
• Evolution: Acute onset with rapid development over 24-48 hours
• Associated features: Often preceded by HSV infection (most common trigger), mycoplasma infection, or drug reactions 1
• Mucosal involvement: May have oral, ocular, or genital mucosal erosions, especially in EM major 3

Tinea Corporis

• Characteristic lesions: Annular or ring-shaped lesions with central clearing and active, scaly, erythematous borders 4
• Distribution: Asymmetrical, can appear anywhere on the body but commonly on exposed areas
• Evolution: Gradual enlargement over days to weeks
• Associated features: Pruritus is common; may have vesicles or pustules at the active border
• Border characteristics: Well-defined, raised, scaly, advancing border that is more erythematous than the center 4

Diagnostic Approach

Clinical Examination

1. Inspect lesion morphology:

   • EM: Look for true target lesions with three zones of color change
   • Tinea: Look for annular lesions with scaly, raised borders and central clearing

2. Evaluate distribution:

   • EM: Symmetrical, predominant on extremities
   • Tinea: Random, asymmetrical distribution

3. Check for associated symptoms:

   • EM: May have systemic symptoms (fever, malaise), especially in severe cases
   • Tinea: Usually localized pruritus without systemic symptoms

Confirmatory Testing

• For suspected tinea corporis: KOH preparation of skin scrapings from the active border to identify fungal hyphae 3
• For suspected EM: Skin biopsy may be necessary in atypical presentations 3

Important Distinctions

Erythema Multiforme

• Often follows a preceding trigger (HSV infection in 70-90% of recurrent cases) 2
• May have constitutional symptoms in severe cases
• Target lesions are pathognomonic when present
• May have mucosal involvement (oral, ocular, genital) 3
• Self-limited course (2-3 weeks) without treatment 5

Tinea Corporis

• Caused by dermatophyte fungi
• Typically has a raised, scaly border with central clearing
• Pruritus is a prominent feature
• Responds to topical antifungal therapy
• Gradually enlarges if untreated 4

Potential Pitfalls and Caveats

• Early tinea corporis may not show the classic central clearing and can be mistaken for other conditions
• EM without typical target lesions may be confused with urticaria or drug eruptions
• Atypical presentations of either condition may require biopsy for definitive diagnosis 3
• The presence of scale strongly suggests tinea corporis rather than EM
• Marking the borders of an ambiguous lesion to monitor for expansion can help differentiate between the two conditions 4

Management Implications

The correct diagnosis is crucial as management differs significantly:

• Tinea corporis: Requires antifungal therapy (topical for limited disease, systemic for extensive disease)
• Erythema multiforme: Requires identification and treatment of underlying cause (antiviral therapy for HSV-associated EM, discontinuation of offending medication) 5

By carefully assessing the morphology, distribution, and evolution of skin lesions, clinicians can accurately differentiate between erythema multiforme and tinea corporis, leading to appropriate management and improved outcomes.