What causes an erythema, asymptomatic rash with coin-shaped lesions in a pediatric patient?

Differential Diagnosis of Asymptomatic Coin-Shaped Erythematous Rash in Pediatric Patients

The most likely diagnosis is nummular (coin-shaped) eczema or atopic dermatitis presenting in a nummular pattern, though erythema multiforme, tinea corporis, and pityriasis rosea must be considered based on specific clinical features.

Primary Diagnostic Considerations

Nummular Eczema/Atopic Dermatitis

• Coin-shaped (nummular) lesions can represent a variant of atopic dermatitis, particularly when diffusely distributed across the body 1
• The absence of pruritus is atypical but does not exclude the diagnosis, as some children may not report itching or may have minimal symptoms 1
• Lesions typically appear as round, erythematous plaques with dry, scaly surfaces unless treated with topical corticosteroids 1
• Distribution patterns vary by age: infants show involvement of cheeks, trunk, and extensor surfaces; older children demonstrate more localized chronic lesions 1

Erythema Multiforme

• Target or "coin-shaped" lesions with central clearing are characteristic, typically appearing symmetrically on extremities (especially extensor surfaces) and spreading centripetally 2, 3, 4
• The asymptomatic presentation is unusual, as most cases involve some degree of discomfort 3
• Individual lesions remain fixed for minimum 7 days, distinguishing them from urticaria which resolves within 24 hours 3
• Common triggers include herpes simplex virus, Mycoplasma pneumoniae infection, medications, or recent vaccinations 3, 5, 4
• Typical target lesions have three zones: dusky center, pale edematous middle ring, and erythematous outer ring 3, 4

Tinea Corporis (Ringworm)

• Fungal infection presenting as annular, erythematous plaques with raised, scaly borders and central clearing 6
• Lesions are typically pruritic, though mild cases may be minimally symptomatic 6
• Can appear as multiple coin-shaped lesions when widespread 6

Pityriasis Rosea

• Characterized by a herald patch followed by bilateral, symmetric distribution in a "Christmas tree" pattern on the trunk 6
• Individual lesions are oval with collarette scaling 6
• Mild pruritus may be present but is not universal 6

Critical Diagnostic Algorithm

Step 1: Assess Lesion Morphology

• True target lesions with three distinct zones suggest erythema multiforme 3, 4
• Homogeneous erythematous plaques with scaling favor eczematous processes 1
• Annular lesions with raised borders and central clearing indicate tinea corporis 6
• Oval lesions with collarette scaling in Christmas tree pattern indicate pityriasis rosea 6

Step 2: Evaluate Distribution Pattern

• Symmetric involvement of extremities (especially extensor surfaces) strongly suggests erythema multiforme 2, 3
• Flexural involvement favors atopic dermatitis in older children 1
• Trunk and extremities with sparing of face suggests nummular eczema 1
• Bilateral trunk distribution in Christmas tree pattern confirms pityriasis rosea 6

Step 3: Obtain Targeted History

• Recent viral illness (especially herpes simplex or upper respiratory infection) or new medications within 2-3 weeks suggests erythema multiforme 3, 4
• Recent vaccination (particularly BCG) may trigger erythema multiforme 5
• Personal or family history of atopy supports eczematous diagnosis 1
• Contact with animals or other children with ringworm suggests tinea 6

Step 4: Assess for Systemic Symptoms

• Fever preceding rash onset by several days, then resolving before rash appears, suggests roseola 6
• Respiratory symptoms with rash suggest possible Mycoplasma pneumoniae-associated erythema multiforme 3, 4
• Absence of systemic symptoms is consistent with nummular eczema or tinea 1, 6

Diagnostic Testing

When to Perform Testing

• KOH preparation and fungal culture if annular lesions with raised borders and central clearing are present 6
• Skin biopsy is rarely needed but can differentiate between eczematous and interface dermatitis patterns 1
• Mycoplasma pneumoniae serology and PCR if erythema multiforme is suspected with respiratory symptoms 4
• Herpes simplex virus testing if history suggests recurrent episodes 3, 4

Management Approach

For Nummular Eczema/Atopic Dermatitis

• Liberal application of fragrance-free emollients is first-line therapy regardless of severity 1
• Apply emollients immediately after 10-15 minute lukewarm bath 1
• Topical corticosteroids appropriate for age, site, and disease extent 1
• Avoid irritants and maintain cool environmental temperature 1

For Erythema Multiforme

• Symptomatic treatment with topical corticosteroids or antihistamines 3
• Immediate Mycoplasma pneumoniae testing and empiric antibiotics (azithromycin) if respiratory symptoms present 1, 4
• Supportive care with monitoring for progression 5
• Prophylactic antiviral therapy if herpes simplex virus-associated and recurrent 3, 4

For Tinea Corporis

• Topical antifungal therapy for localized disease 6
• Systemic antifungals for extensive involvement 6

Critical Pitfalls to Avoid

• Do not assume asymptomatic presentation excludes serious conditions—erythema multiforme can present without significant symptoms initially 5
• Do not confuse erythema multiforme with Stevens-Johnson syndrome—SJS presents with widespread purpuric macules, blisters, and mucosal involvement, not isolated target lesions 1, 3
• Do not delay Mycoplasma testing if erythema multiforme is suspected—this requires specific antibiotic therapy 1, 4
• Do not restrict diet in children with suspected atopic dermatitis without professional supervision—dietary manipulation is rarely beneficial and can cause nutritional deficiencies 1
• Mark lesion borders with ink and observe for 24-48 hours if diagnostic uncertainty exists—expanding lesions favor infectious or inflammatory processes 1