Granular parakeratosis associated with the use of laundry sanitizers containing benzalkonium chloride: a case report.

Background: Granular parakeratosis (GP) is a rare benign inflammatory dermatosis that preferentially involves the axillae and other intertriginous, friction-prone skin sites and typically presents with pruritic erythema and scaling. Cases associated with benzalkonium chloride (BAC) exposure have rarely been systematically summarized, and further clinical evidence is needed. Herein, we report a case of generalized GP potentially associated with long-term exposure to a BAC-containing laundry sanitizer, with the aim of expanding the clinical spectrum of cutaneous adverse reactions associated with BAC. Case presentation: A 43-year-old Asian man presented with a persistent rash on the trunk and intertriginous areas for more than 1 year. He had a long-standing history of eczema with possible underlying skin-barrier dysfunction but no confirmed diagnosis of atopic dermatitis. He had regularly used a commercial laundry sanitizer containing BAC (0.5 g/L) for clothing disinfection since June 2020, at a frequency of 1-3 times per week. The first rash appeared in July 2021, approximately 13 months after the start of exposure, initially involving the groin and subsequently spreading symmetrically to the neck, axillae, antecubital fossae, lateral waist, back (with sparing of the spinal midline), and undergarment-covered areas. Fungal microscopy and periodic acid-Schiff (PAS) staining were negative. Histopathology showed epidermal hyperkeratosis with characteristic granular parakeratosis, intracorneal pustule formation, neutrophil aggregation, and superficial dermal perivascular inflammatory infiltrates. Direct immunofluorescence for IgG, IgA, IgM, and C3 was negative. In August 2023, the patient completely discontinued the BAC-containing laundry sanitizer, repeatedly rinsed all garments with clean water, and used topical emollients for skin-barrier repair. The lesions gradually subsided and achieved complete clinical remission in September 2023, approximately 1 month after the intervention. No recurrence was observed during 1 year of follow-up. Conclusions: Exposure to BAC-containing laundry sanitizer may be a potential trigger for generalized GP in susceptible individuals. A history of eczema and associated skin-barrier dysfunction may increase susceptibility to irritant exposure. Avoidance of suspected irritants together with skin-barrier repair may be effective in the management of BAC-associated GP. Further prospective studies and confirmatory testing are needed to clarify the causal relationship.