P06 Porokeratosis ptychotropica: a rare and recalcitrant genitogluteal eruption

Abstract A 45-year-old man with a history of psoriasis presented to our male genital dermatology clinic with a several-year history of pruritic, painful, hyperkeratotic red–brown plaques on both buttocks. He had previously been treated unsuccessfully with cryotherapy and topical imiquimod and podophyllotoxin for suspected condyloma acuminata. A 4-mm punch biopsy from the left buttock showed hyperkeratosis, hypergranulosis, papillomatosis and multiple columns of parakeratin with underlying agranulosis and dyskeratosis, consistent with cornoid lamella formation. A diagnosis of porokeratosis ptychotropica was made. He was treated with Dovobet® ointment and Trimovate® cream, with little improvement. He underwent four cycles of topical photodynamic therapy to his right buttock and two cycles to his left buttock, with minimal improvement. Oral isotretinoin was also trialled, but this led to lipid derangement prior to any observed benefit, so it was stopped. Porokeratosis ptychotropica is a rare and under-recognized variant of porokeratosis, characterized by epidermal keratinization that manifests as keratotic papules in the genitogluteal region. It is often misdiagnosed as psoriasis, tinea cruris or condyloma acuminata, as in our case. Typically, multiple cornoid lamellae are seen on histology (Lucker GP, Happle R, Steijlen PM. An unusual case of porokeratosis involving the natal cleft: porokeratosis ptychotropica? Br J Dermatol 1995; 132:150–1). The aetiology of porokeratosis ptychotropica is unknown, but there is potential for malignant transformation (Mazori DR, Shvartsbeyn M, Meehan SA et al. Transformation of porokeratosis ptychotropica into invasive squamous cell carcinoma. Int J Dermatol 2017; 56:679–80). It is commonly recalcitrant to treatment, with varying reports of success with topical retinoids, keratolytics, tacrolimus, calcipotriol, topical 5-fluorouracil, imiquimod, systemic retinoids, psoralen ultraviolet A, intralesional bleomycin, cryotherapy, photodynamic therapy, excision, dermabrasion and CO2 laser.