Abstract
Trichilemmal horn, or trichilemmal keratosis, is a rare, benign neoplasm of the follicular epithelium exhibiting trichilemmal differentiation. Previous reports have described a clinical appearance consistent with a cutaneous horn or keratoacanthoma. The most common site of occurrence is the head, followed by the limbs. Patients over the age of 50 years are most often affected and approximately two-thirds of reported cases occurred in women. Three cases mention occurrence at sites of previous surgical treatment of pilar cysts. We report a case of a trichilemmal horn in a 27-year-old African American male. He presented with a two-year history of a lesion on his right posterior shoulder that had recently become painful. He denied a previous history of pilar cysts. Examination revealed a 1.4 cm keratotic plaque with few irregular yellow, verrucous projections of varying size arising from an erythematous, grey brown, crateriform base. A shave biopsy was performed. Histopathologic evaluation showed a verrucous profile with an endophytic process consisting of densely packed, eosinophilic hyperorthokeratosis, overlying an acanthotic and papillomatous epidermis. Along the base of the lesion were multiple dermal nodules with connections to the epidermis. The epithelial lining of both the squamous invagination and the underlying dermal nodules exhibited abrupt transition to lamellar keratinization without presence of a granular layer. On higher power, a basal layer of palisaded cuboidal squamous cells was appreciated. A granulomatous and sparse lymphoplasmacytic infiltrate, along with fibrosis, was present where some dermal nodules had ruptured. A complete re-excision was recommended. We present this rare case of trichilemmal horn to highlight some features that deviate from the typical characteristics suggested by the existing literature—male sex, age less than 50 years, and occurrence on the trunk.
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