What are the key clinical features that differentiate Stevens-Johnson Syndrome (SJS) from Toxic Epidermal Necrolysis (TEN)?

Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN) are severe cutaneous adverse reactions primarily differentiated by the extent of epidermal detachment. SJS involves less than 10% of body surface area (BSA) with epidermal necrosis and mucous membrane involvement, whereas TEN is characterised by more extensive skin detachment exceeding 30% BSA. Cases with 10-30% BSA involvement are considered SJS/TEN overlap.

Clinically, both conditions present initially with prodromal symptoms such as fever, malaise, and upper respiratory tract symptoms, followed by rapid onset of painful erythematous macules that progress to blistering and epidermal sloughing. However, the severity and extent of skin involvement are key differentiators: SJS shows limited epidermal necrosis and detachment, while TEN demonstrates widespread full-thickness epidermal necrosis leading to large areas of denuded skin.

Mucous membrane involvement is common in both but tends to be more severe and extensive in TEN. Additionally, systemic symptoms and complications such as sepsis and multi-organ failure are more frequent and severe in TEN due to the larger skin barrier loss.

Histopathologically, both share similar features of keratinocyte necrosis, but the degree and extent correlate with clinical severity. The distinction is primarily clinical based on BSA involvement rather than histology alone.

Thus, the key clinical feature differentiating SJS from TEN is the percentage of epidermal detachment: <10% for SJS, >30% for TEN, with overlap in between. This classification guides prognosis and management strategies.

These distinctions are consistent with UK NICE guidelines on drug allergy and severe cutaneous adverse reactions ¹, and are supported by recent literature emphasising a unifying spectrum of drug-induced epidermal necrolysis with gradations based on skin involvement (Owen and Jones, 2021; Ramien et al., 2022; Abtahi-Naeini et al., 2024).