Thomas Bond's Book *
Keratin extends vertically from the base in a horn-like fashion.
The keratin horn (cutaneous horn, mucosal horn) is a localized reactive epithelial proliferation resulting in such extreme excess of surface keratin that the lesion clinically resembles a small horn. It is a nonspecific reaction but actinic damage is a common underlying process. Some authorities consider this to be a variant of the actinic keratosis. Oral mucosal examples are very rare, but the interface between the vermilion border and the skin of the lips is one of the more common regions of occurrence.
Clinical Features: The keratin horn is an elongated upward extension of yellow/brown keratin, often with an inverted cone shape (cornu cutaneum) and sometimes with a crater-like area at its base. It is a painless lesion which enlarges slowly and may reach a length greater than 50 cm., although most remain less than half a centimeter in height. With time the surface of the horn can become quite smooth and darkened, even ebony in appearance. A bulge or hyperplastic ring at the base of the horn should be viewed with suspicion, as some lesions develop squamous cell carcinoma of this site.
Pathology: This entity shows a considerable excess of layered surface orthokeratin, perhaps with a central keratin-filled crater at its base. The latter resembles the keratoacanthoma but the keratin horn lacks the sharply acute angle between the surrounding epithelium and lesional epithelium. Dysplastic epithelial cells may be seen in adjacent epithelium and the underlying stroma often contains scattered chronic inflammatory cells. The surface of the keratin horn may be embedded with bacteria and can be quite smooth or quite irregular. The granular cell layer is often pronounced.
Most cases are associated with actinic keratosis, but additional skin lesions reported in association with the keratin horn include the filiform verruca, seborrheic keratosis and squamous cell carcinoma, although the latter may simply represent malignant transformation of a benign horn lesion. On rare occasions trichilemmoma or basal cell carcinoma is seen in adjacent or underlying epithelium.
Treatment and Prognosis: The keratin horn should be excised by conservative surgery because of its potential for malignant transformation. A rolled border or hyperplastic ring around the base of the horn warrants extension of the surgical margins several millimeters into normal surrounding skin or mucosa. Occasional lesions recur.
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