Morbilliform drug eruption and SDRIFE are not the same reaction wearing different clothes

Design

single-cell RNA sequencing, exploratory spatial transcriptomics and immunohistochemistry of lesional skin

Population

patients with morbilliform drug eruption compared with patients with SDRIFE

Primary outcome

lesional immune cell composition and spatial distribution

Effect

morbilliform: CD8+ cytotoxic and Th1 predominance with high CXCL9-11 and interferon-response genes; SDRIFE: limited CD8+ infiltrate, Th2-skewed CD4+ cells, immunomodulatory macrophages

Both follow a drug, both are erythematous, both are commonly beta-lactam related, and the difference between them has mostly been described in terms of where they appear. This study compared lesional skin from patients with morbilliform drug eruption against skin from patients with symmetrical drug-related intertriginous and flexural exanthema, using single-cell RNA sequencing, exploratory spatial transcriptomics and immunohistochemistry.

The immune landscapes diverged. Morbilliform lesions were dominated by CD8+ cytotoxic T cells and Th1-type CD4+ T cells, with high CXCL9-11 expression and upregulated interferon-responsive genes - a cytotoxic, interferon-driven picture. SDRIFE looked different: limited CD8+ infiltration, CD4+ T cells with modest Th2 skewing, and macrophages carrying an immunomodulatory rather than an effector profile. Spatial transcriptomics put T cells and macrophages in the superficial dermis in both.

This is a small mechanistic study with no outcome data and it does not change management today. What it does is support what the clinical pattern already suggests: SDRIFE is not a mild morbilliform eruption. If the inflammation is driven by a different programme, then the assumption that both carry the same risk of progression to a severe cutaneous adverse reaction, and that both need the same avoidance advice, is an assumption rather than a finding. It is worth holding the two apart in the notes rather than recording 'drug rash'.

• Record the morphology and distribution specifically - flexural and intertriginous symmetry is the SDRIFE pattern, not a variant of a morbilliform rash
• Photograph the distribution at first presentation; the pattern is the diagnostic feature and it fades
• Note the culprit drug class - beta-lactams dominate both - and give the patient the drug name in writing
• Do not extrapolate severe cutaneous adverse reaction risk from one pattern to the other
• Keep systemic symptoms, mucosal involvement and eosinophil count in the assessment regardless of pattern

Why it matters

It challenges the habit of treating SDRIFE as a morbilliform eruption in an unusual place.

Don't overread it

This is descriptive immunology in a small sample - it identifies different mechanisms, not different outcomes or different treatment.

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