Erythema Multiforme (EM)
Erythema Multiforme is an acute, immune-mediated condition often triggered by infections. It is clinically recognized by its characteristic lesion morphology.
Key Clinical Features
- Target Lesions: The classic “iris” or “target” lesion with three zones: (1) central dusky/necrotic area, (2) pale edematous ring, and (3) peripheral erythematous halo.
- Distribution: Predominantly acral (hands, feet, elbows, knees) and symmetrical.
- Triggers: Herpes Simplex Virus (HSV) is the most common trigger for recurrent EM. Mycoplasma pneumoniae is another frequent cause.
Comparison: EM vs. SJS/TEN
| Feature | Erythema Multiforme (EM) | SJS/TEN |
|---|---|---|
| Etiology | Infectious (usually HSV) | Medication-induced |
| Lesions | Typical “target” lesions | Atypical targets/diffuse necrosis |
| Mucosal involvement | Mild/Limited | Extensive/Severe |
High-Yield Exam Pearl:
- EM Minor vs. Major: EM Major includes significant mucosal involvement; however, it remains distinct from Stevens-Johnson Syndrome (SJS), which is almost always driven by drugs and involves widespread epidermal sloughing.
- Pathology: Look for interface dermatitis with necrotic keratinocytes in the epidermis.