Cutaneous Horn Arising from Chronic Plaque Psoriasis over the Ankle: A Rare Clinicodermoscopic and Histopathological Case Report

Abstract

Psoriasis vulgaris is a chronic immune-mediated inflammatory dermatosis that usually presents with erythematous scaly plaques, while cutaneous horn is an uncommon hyperkeratotic projection whose clinical significance depends on the nature of the underlying lesion. Horn formation over a psoriatic plaque is distinctly rare and may simulate premalignant or malignant pathology, warranting histopathological evaluation. We report a rare case documented in the Department of Dermatology, Venereology and Leprosy, Sree Balaji Medical College and Hospital, Chennai. A 50-year-old woman with an 18-year history of chronic plaque psoriasis presented with a horn-like growth over the lateral aspect of the right ankle of five months’ duration. The lesion had arisen over a pre-existing hyperpigmented scaly plaque and enlarged gradually without pain, bleeding, ulceration, or rapid growth. Examination showed multiple hyperpigmented scaly plaques over both feet and ankles, with a solitary firm yellowish conical keratotic projection measuring about 1 cm in height and having a broad base over a psoriatic plaque on the right ankle. Dermoscopy demonstrated compact keratin with regularly arranged dotted vessels over a light erythematous background, favouring psoriasis. Complete excision including the base was performed because of the possibility of underlying benign, premalignant, or malignant pathology. Histopathology revealed marked hyperkeratosis, parakeratosis, acanthosis with elongated rete ridges, and neutrophilic collections consistent with psoriasis, with no dysplasia or malignancy. The postoperative course was uneventful. This case highlights a rare manifestation of psoriasis and emphasizes that all cutaneous horns require excision and histopathological examination to exclude occult malignant transformation.