100383 - When the sun is not the only culprit: Persistent Th2-driven hydrochlorothiazide-induced photoallergic dermatitis responding to dupilumab

Drug-induced photoallergy is a delayed T cell–mediated hypersensitivity reaction that may persist after withdrawal of the offending agent. Thiazide diuretics, particularly hydrochlorothiazide, are recognized causes of immunologically mediated photosensitivity. In severe cases, chronic inflammation may be sustained by ongoing Th2 pathway activation and resident memory T cells in the skin.

We report an 82-year-old man with pruritic eczematous lesions predominantly affecting photoexposed areas, progressing to erythroderma despite repeated systemic corticosteroid courses. Skin biopsy showed lichenified eczematous dermatitis. Given concomitant treatment with hydrochlorothiazide and fenofibrate—both potential photosensitizers—standard and drug-specific patch testing was performed. Patch tests with hydrochlorothiazide (1% and 10% in petrolatum) were positive at 48 hours without irradiation, whereas fenofibrate was negative, consistent with a type IV hypersensitivity reaction. Hydrochlorothiazide was identified as the causative agent and permanently discontinued.

Despite drug withdrawal, the patient developed persistent corticosteroid-dependent disease, consistent with a chronic self-sustaining inflammatory process. Considering the eczematous phenotype and possible Th2 polarization, dupilumab (600 mg loading dose followed by 300 mg biweekly) was initiated. Rapid and complete clinical remission of erythroderma and lichenification was achieved, allowing full withdrawal of systemic corticosteroids. Remission has been maintained after one year of continuous treatment.

This case illustrates severe hydrochlorothiazide-induced photoallergic dermatitis with persistent disease after drug discontinuation. Compatible clinical findings and positive patch testing without irradiation supported the diagnosis without photopatch testing. In chronic refractory cases, targeted IL-4/IL-13 blockade may represent an effective and sustained therapeutic strategy.