D1.401 - Progression from Immediate to Delayed Hypersensitivity: A Rare Case of Iodixanol-Induced Acute Generalised Exanthematous Pustulosis

Background: Acute Generalised Exanthematous Pustulosis (AGEP) is a severe cutaneous adverse reaction (SCAR) characterised by the rapid development of sterile non-follicular pustules on an erythematous base. While predominantly triggered by antibiotics, T-cell-mediated delayed hypersensitivity to iodinated contrast media (ICM) is a known but rare cause. We report a case of AGEP induced by iodixanol, an iso-osmolar dimeric ICM, following an intravenous provocation test.

Case report: A 75-year-old female with asthma, lichen planopilaris, hypertension and obesity was referred to our Allergy and Clinical Immunology department in 2024 for the evaluation of an episode of angioedema and cutaneous pruritus 30 minutes after a computed tomography scan with an unspecified ICM in December 2023. Skin tests (prick and intradermal, with immediate and delayed readings) for iomeprol, iopromide and iodixanol were negative. Given future imaging needs, an intravenous provocation test with iodixanol (Visipaque®), the safest ICM, was performed. The patient had not been exposed to any other new or high-risk concomitant medications prior to the procedure. Fourteen hours after administration, the patient developed pustules on an erythematous base, predominantly in skin folds. She was initially treated at a primary care centre with corticosteroids and oral antihistamines, maintaining this therapy until complete clinical resolution. Ten days later, she presented to an urgent Dermatology consultation, where the lesions were observed to have evolved with plaque and collarette desquamation, suggestive of resolving pustulation, without mucosal involvement. Given the high risk of cross-reactivity, all iodinated contrast media were strictly contraindicated, and the patient was advised to undergo alternative imaging modalities, such as magnetic resonance imaging or ultrasonography.

Conclusion: This case illustrates a phenotypic shift in hypersensitivity to ICM. The index reaction was a mild immediate response (angioedema within 30 minutes), whereas controlled re-exposure triggered a severe T‑cell‑mediated delayed reaction (AGEP). The negative results of the prick and intradermal skin tests did not predict that the patient might develop a SCAR with the provocation test, since the initial reaction pointed to a type I hypersensitivity. Although AGEP associated with ICM appears to be infrequently reported, this case underlines the need for caution and close monitoring during re-exposure, including with iodixanol.