Erythema Multiforme Causes and Clinical Differentiation

Primary Causes

• Mycoplasma pneumoniae is another significant infectious trigger, particularly common in children, and is associated with a better prognosis compared to other causes 1
• Anticancer agents can cause toxic erythema that may resemble EM, highlighting the need for careful diagnosis and management in patients undergoing cancer treatment 2
• Anticancer agents can also cause toxic erythema that may resemble EM, with similar clinical presentations 3

Clinical Differentiation from Similar Conditions

• The British Journal of Dermatology guidelines recommend distinguishing EM from Stevens-Johnson Syndrome (SJS)/Toxic Epidermal Necrolysis (TEN) based on clinical presentation, with EM typically presenting with typical target lesions or raised atypical targets, predominantly on limbs and extremities 4
• EM is mostly related to HSV infection, while SJS/TEN is usually triggered by drugs, and has lower mortality and better prognosis compared to SJS/TEN 4
• The British Journal of Dermatology guidelines also recommend histopathological confirmation when clinical diagnosis is uncertain, with EM showing variable epidermal damage ranging from individual cell apoptosis to confluent epidermal necrosis 4
• Histopathology helps exclude other blistering dermatoses that may mimic EM, and is an essential tool in the diagnostic approach 5

Special Presentations

• Mycoplasma-induced EM, especially in children, may present with predominantly mucous membrane involvement with little or no cutaneous lesions (termed "Mycoplasma pneumoniae-associated mucositis"), highlighting the need for careful diagnosis and management in this population 4

Erythema Multiforme Diagnostic Features

Lesion Characteristics

• The British Journal of Dermatology describes erythema multiforme as characterized by target or "iris" lesions, which consist of a dark red center surrounded by a pink ring, with a well-defined circular appearance 6, 7
• The lesions of erythema multiforme are distinguished from other conditions by their targetoid morphology with dusky red and white concentric rings, as described in the British Journal of Dermatology 6, 7

Differential Diagnosis

• Molluscum contagiosum presents as umbilicated papules, not circular lesions with dusky red and white centers, according to Clinical Infectious Diseases 8
• Scabies presents with burrows, papules, and excoriations in characteristic distributions, not target lesions, as noted in Clinical Infectious Diseases 8

Critical Differentiation from Stevens-Johnson Syndrome/TEN

• Stevens-Johnson Syndrome/TEN features flat atypical targets or purpuric macules, with epidermal detachment and positive Nikolsky sign, as described in the British Journal of Dermatology 6, 7

Differential Diagnoses for Erythema Multiforme Minor

Key Differential Diagnoses

• The American Academy of Pediatrics suggests that individual lesions in urticaria typically resolve within 24 hours, whereas EM lesions remain fixed for a minimum of 7 days, as noted in a patient with a previous episode of urticaria documented 2 weeks prior 9
• The Centers for Disease Control and Prevention recommends considering viral exanthem as a differential diagnosis, given the presence of undocumented fever with rash onset, maculopapular distribution on trunk and extremities, and age-appropriate presentation for common viral infections, such as roseola, enterovirus, or parvovirus B19 10

Critical Missing Diagnostic Information

• The Clinical Infectious Diseases journal suggests that documenting lesion evolution, including photographing and marking borders to track progression or migration, is crucial for differentiating between conditions, as noted in a case where the lesion evolution was monitored 9

Erythema Multiforme Diagnostic Approach

Primary Infectious Triggers

• The mechanism of erythema multiforme involves a hypersensitivity reaction with cytotoxic T lymphocytes inducing keratinocyte apoptosis, typically appearing 10 days after viral infection, as recommended by the Centers for Disease Control and Prevention (CDC) 11
• Document lesion morphology precisely, looking for the characteristic three-zone target configuration with central dark papule/vesicle, pale zone, and erythematous halo, as suggested by the MMWR Recommendations and Reports 11

Clinical Presentation and Differentiation of Erythema Multiforme

Cutaneous Lesion Distribution

• In affected individuals, erythema multiforme lesions typically begin on the ankles, wrists, or forearms and then spread to the palms, soles, arms, legs, and trunk, while most often sparing the face. 12

Distinguishing Features and Outcomes of Stevens‑Johnson Syndrome/Toxic Epidermal Necrolysis

• Stevens‑Johnson syndrome and toxic epidermal necrolysis are most commonly triggered by drug exposure and are associated with a mortality rate of approximately 5 %–30 %. 13

Distinguishing Typical and Atypical Target Lesions in Erythema Multiforme versus Stevens‑Johnson/TEN

Morphologic Features of Target Lesions

• Typical target lesions of erythema multiforme display three well‑defined concentric zones (dark central papule/vesicle, pale intermediate ring, and an outer erythematous halo) with raised, palpable borders, most often on acral sites. 14
• Atypical lesions seen in Stevens‑Johnson syndrome/toxic epidermal necrolysis consist of only two zones (dark central area surrounded by a pink ring), are flat without palpable elevation, have poorly defined irregular borders, and frequently coalesce into larger areas of dark erythema. [14][15]
• In Stevens‑Johnson/TEN the lesions progress to flaccid bullae and epidermal sheet detachment, with a positive Nikolsky sign, reflecting deeper epidermal injury. [14][15]
• Distribution of atypical lesions is more generalized, initially involving the upper torso, proximal limbs, and face, with rapid spread, whereas typical lesions remain predominantly on extremities. 15

Etiology and Prognostic Implications

• Erythema multiforme with typical target lesions is most commonly precipitated by herpes simplex virus infection or Mycoplasma pneumoniae, and carries a favorable prognosis with minimal mortality. 14
• Stevens‑Johnson syndrome/toxic epidermal necrolysis with atypical lesions is usually drug‑induced (e.g., sulfonamides, anticonvulsants, antibiotics) and is associated with a mortality rate ranging from 5 % to 30 %. 14

Mucosal Involvement

• Both conditions may involve mucous membranes, but in erythema multiforme the cutaneous lesions are typical targets on the extremities and mucosal disease is usually mild; in Stevens‑Johnson/TEN the flat atypical lesions are accompanied by early, severe, hemorrhagic erosive mucositis. [14][15]

Diagnostic Pitfalls

• Misclassifying cases with <10 % epidermal detachment as “major erythema multiforme” is erroneous when the skin lesions are flat purpuric macules or atypical flat targets; such presentations should be diagnosed as Stevens‑Johnson syndrome regardless of the extent of skin loss. 14
• True erythema multiforme is defined by the presence of elevated typical or atypical target lesions, primarily located on the extremities. 14