D3.22 - Fixed Drug Eruption in a Patient with Vitiligo: localized patch testing as a diagnostic tool

Background: Fixed Drug Eruption (FDE) is a delayed T-cell mediated hypersensitivity cutaneous adverse drug reaction. It is characterized by recurrent lesions in the same sites of skin following repeated exposure to the culprit drug. Polypharmacy can complicate diagnosis and management.

Objective: To describe a case of FDE in which localized patch testing proved to be a useful diagnostic tool.

Case description: We report a 65-year-old male with vitiligo who presented with generalized pruritus and erythema, approximately 24 hours after administration of multiple medications for renal colic. The medications involved were metamizole, ondansetron, paracetamol, metoclopramide, butylscopolamine, tamsulosin, and diclofenac. On examination, the patient exhibited painless erythemato-violaceous circular plaques on dorsal, abdominal, and inguinal areas, as well as a large plaque on the right thigh. The lesions were non-detaching and had no mucosal involvement. Skin biopsies of the lesions were compatible with toxidermia and capillaritis. FDE was suspected, and the patient was treated with oral and topical corticosteroids, antihistamines, and advised to avoid the suspected medications. 

Initial patch testing performed on the dorsum showed an “angry back” pattern with no positive results. Subsequently, the patient was referred to our specialty for further investigation. We performed targeted patch testing with the suspected culprit drugs on the site of the resolved largest lesion on the thigh, which revealed positivity for diclofenac at 48 and 96 hours.

Conclusion: This case highlights the diagnostic challenges of FDE in patients with polypharmacy and highlights the importance of targeted patch testing on previously affected areas for accurate identification of the culprit drug. Early diagnosis, drug avoidance, and symptomatic management are critical for preventing recurrence and improving patient outcomes.