What is the diagnosis and treatment for a target lesion with small red spots all over the trunk?

Erythema Multiforme: Diagnosis and Management

The clinical presentation of target lesions with small red spots distributed across the trunk is most consistent with erythema multiforme (EM), which requires identification and treatment of the underlying trigger (most commonly HSV or Mycoplasma pneumoniae), along with supportive care. 1, 2

Diagnostic Features

The hallmark of EM is the characteristic target or "iris" lesion consisting of a dark red center surrounded by a pink ring with well-defined circular appearance 1, 2. These lesions demonstrate a three-zone configuration: central dark papule/vesicle, pale zone, and erythematous halo 2.

Critical distinguishing feature: EM lesions remain fixed for a minimum of 7 days, unlike urticaria which resolves within 24 hours 1, 2. The lesions predominantly affect limbs and extremities, though trunk involvement occurs 1.

Primary Triggers to Identify

• Herpes simplex virus (HSV) is the most common infectious trigger, with lesions typically appearing 10 days after viral infection through a hypersensitivity reaction involving cytotoxic T lymphocytes inducing keratinocyte apoptosis 2
• Mycoplasma pneumoniae is another significant trigger, particularly common in children and associated with better prognosis 2
• Anticancer agents can cause toxic erythema resembling EM 2

Critical Differential Diagnoses to Exclude

Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis (SJS/TEN)

Must be excluded urgently as it carries significantly higher mortality. Key differentiating features 3:

• SJS/TEN presents with flat atypical targets or purpuric macules with epidermal detachment 3
• Positive Nikolsky sign (pathognomonic for SJS/TEN) 3
• Multiple mucosal surface involvement 3
• Systemic symptoms with neutropenia and significantly elevated liver enzymes 3
• EM is mostly HSV-related while SJS/TEN is usually drug-triggered 2

Other Differentials

• Tinea corporis: annular erythematous patches with well-defined scaly borders, but lacks the three-zone target configuration 1
• Urticaria: individual lesions resolve within 24 hours versus EM's fixed lesions for ≥7 days 2
• Viral exanthem: maculopapular distribution without targetoid morphology 2

Diagnostic Workup

Document lesion morphology precisely, looking for the three-zone target configuration 2. Key steps:

• Photograph and mark lesion borders to track progression 2
• Histopathological confirmation when clinical diagnosis is uncertain, showing variable epidermal damage ranging from individual cell apoptosis to confluent epidermal necrosis 2
• Assess for epidermal detachment or Nikolsky sign to exclude SJS/TEN 3
• Determine body surface area involvement: <10% suggests SJS if present 3

Treatment Approach

The primary management strategy focuses on identifying and treating the underlying trigger 1:

1. Supportive care with antihistamines and topical corticosteroids 1
2. Self-limited condition typically resolving in 2-4 weeks 1
3. If HSV-triggered, consider antiviral therapy to prevent recurrence 2
4. If Mycoplasma-induced, appropriate antibiotic therapy 2

Critical Pitfalls to Avoid

• Do not dismiss as simple drug eruption if targetoid lesions present with systemic involvement, as this suggests more severe reaction 3
• Do not use systemic corticosteroids without specialist consultation if SJS/TEN cannot be excluded, as their role is controversial and may increase infection risk 3
• Do not confuse with molluscum contagiosum (umbilicated papules) or scabies (burrows and excoriations), which lack target morphology 2