Could a patient with target-like lower extremity lesions and respiratory symptoms have erythema multiforme?

Erythema Multiforme in the Context of Target-Like Lower Extremity Lesions and Respiratory Symptoms

Yes, erythema multiforme (EM) is a strong diagnostic consideration for this patient, particularly given the target-like lesions on the lower extremities, and the respiratory symptoms suggest a possible Mycoplasma pneumoniae trigger, which is a well-established cause of EM, especially in children. 1

Key Diagnostic Features Supporting EM

The characteristic presentation of EM includes:

• Target or "iris" lesions consisting of a dark red center surrounded by a pink ring with well-defined circular appearance 1, 2
• Three-zone target configuration with central dark papule/vesicle, pale zone, and erythematous halo 1
• Predominant acral localization on extremities (especially extensor surfaces), spreading centripetally 3
• Fixed lesions that remain for a minimum of 7 days, distinguishing EM from urticaria where individual lesions resolve within 24 hours 1, 3

Respiratory Symptoms and Mycoplasma Connection

The combination of target-like skin lesions with respiratory symptoms is particularly suggestive of Mycoplasma pneumoniae-associated EM:

• Mycoplasma pneumoniae is a significant infectious trigger for EM, particularly common in children, and is associated with better prognosis 1
• This organism can cause both respiratory illness and EM simultaneously 4, 3
• Mycoplasma-induced EM may present with predominantly mucous membrane involvement with little or no cutaneous lesions (termed "Mycoplasma pneumoniae-associated mucositis") 1
• The mechanism involves a hypersensitivity reaction with cytotoxic T lymphocytes inducing keratinocyte apoptosis, typically appearing 10 days after viral infection 1

Critical Differential Diagnoses to Exclude

Stevens-Johnson Syndrome (SJS)/Toxic Epidermal Necrolysis (TEN) must be differentiated from EM:

• SJS/TEN features flat atypical targets or purpuric macules with epidermal detachment and positive Nikolsky sign 1
• EM typically presents with raised atypical targets, predominantly on limbs and extremities 1
• EM is mostly related to HSV or Mycoplasma infection, while SJS/TEN is usually triggered by drugs 1
• EM has lower mortality and better prognosis compared to SJS/TEN 1

Other conditions in the differential include:

• Rocky Mountain Spotted Fever (RMSF): Rash on palms and soles can occur with RMSF, but this typically begins as blanching pink macules evolving to maculopapules, not true target lesions 5
• Coccidioidomycosis: Can present with erythema multiforme or erythema nodosum in endemic areas, but requires endemic exposure history 5
• Tinea corporis: Presents with well-demarcated, sharply circumscribed patches with raised leading edge and satellite lesions, not true target morphology 2

Diagnostic Approach

Document lesion morphology precisely:

• Photograph and mark borders to track progression 1
• Look for the characteristic three-zone target configuration 1
• Assess for symmetrical distribution on bilateral extremities 3

Histopathological confirmation when clinical diagnosis is uncertain:

• EM shows variable epidermal damage ranging from individual cell apoptosis to confluent epidermal necrosis 1
• Histopathology helps exclude other blistering dermatoses that may mimic EM 1

Investigate potential triggers:

• Test for Mycoplasma pneumoniae given respiratory symptoms 1, 3
• Consider HSV testing, as it is the most common cause of EM 1, 4
• Review medication history for drugs that can precipitate EM (sulfonamides, penicillins, phenytoin, allopurinol) 4, 6

Management Considerations

For acute EM:

• Symptomatic treatment with topical steroids or antihistamines 7, 3
• Treat underlying infection if identified (Mycoplasma requires appropriate antibiotics) 3

For recurrent HSV-associated EM:

• Prophylactic antiviral therapy is first-line treatment 7, 3
• Note that topical acyclovir prophylaxis does not prevent further episodes; systemic therapy is required 4

For severe mucosal involvement:

• May require hospitalization for intravenous fluids and electrolyte repletion 3
• Antiseptic or anesthetic solutions for mucosal lesions 7

Common Pitfalls to Avoid

• Do not confuse EM with urticaria; EM lesions remain fixed for at least 7 days versus urticarial lesions resolving within 24 hours 1, 3
• Do not overlook Mycoplasma pneumoniae as a trigger when respiratory symptoms are present 1, 3
• Do not misdiagnose as SJS/TEN based solely on the presence of target lesions; assess for epidermal detachment and Nikolsky sign 1
• Do not dismiss the possibility of drug-induced EM; carefully review all medications 4, 6