Porokeratosis Ptychotropica: A Rare Form of Porokeratosis

Summary: A 72-year-old female presented with a 5 mm red to light-brown papule with a cutaneous horn on the left gluteal cleft. A shave biopsy was performed to rule out squamous cell carcinoma. Histopathology revealed prominent acanthosis with verrucous architecture; tiers of parakeratosis overlying dyskeratosis; and confluent parakeratosis forming a cutaneous horn. A diagnosis of porokeratosis ptychotropica (PP) was made. PP is considered a rare form of porokeratosis predisposed to body folds. Proposed associations include ultraviolet radiation, infections, trauma, and immunosuppression, though pathogenesis has not been elucidated. PP typically presents as red-brown, verrucous, scaly papules and plaques symmetrically distributed across the gluteal fold. Contrary to our case, lesions of PP are typically multiple or confluent, bilateral, and present in men. Clinical differential diagnoses often include viral warts, psoriasis, contact dermatitis, dermatophytosis, and condyloma acuminata. Histopathology characteristically reveals multiple cornoid lamellae throughout the stratum corneum. Our case highlights an unusual, mild clinical presentation of PP with classic histopathologic findings. Currently, no standardized treatments exist; surgery, CO2 laser, cryotherapy, and various topicals (eg. retinoids, 5-fluorouracil) have been used with variable effects. Emerging evidence for abrocitinib and topical cholesterol/simvastatin may be promising. Our patient has declined treatment thus far but plans to monitor closely.