## Classification of SJS and TEN **Key Point:** The distinction between Stevens-Johnson Syndrome and Toxic Epidermal Necrolysis is based purely on the extent of epidermal detachment and BSA involvement. ### BSA-Based Classification | Condition | BSA Involvement | Characteristics | |-----------|-----------------|------------------| | **SJS** | <10% | Limited mucosal involvement, sparse target lesions | | **SJS/TEN overlap** | 10–30% | Mixed presentation, intermediate severity | | **TEN** | >30% | Extensive epidermal detachment, confluent erythema | **High-Yield:** The 30% BSA cutoff is the internationally accepted threshold (Bastuji-Garin criteria) that separates TEN from SJS. This distinction has prognostic and management implications — TEN carries a mortality rate of 25–35%, whereas SJS mortality is 1–5%. **Clinical Pearl:** Both conditions represent the same pathophysiological process (CD8+ T-cell–mediated keratinocyte apoptosis triggered by drugs or infections); the difference is purely quantitative based on extent of skin loss. ### Common Culprit Drugs - Antibiotics (sulfonamides, beta-lactams, fluoroquinolones) - NSAIDs (especially oxicams) - Anticonvulsants (phenytoin, carbamazepine, phenobarbital) - Antiretrovirals (nevirapine, abacavir) - Allopurinol **Mnemonic:** **SCAR** = **S**ulfonamides, **C**arbamazepine, **A**llopurinol, **R**etrovirals — the "big four" culprits in SJS/TEN. 
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