INTRODUCTION Inverted follicular keratosis is a benign skin lesion that is common on the face and less frequently on the eyelids. It occurs in older individuals from the fifth decade on, and is considerably more common in males. It is frequently mistaken for a malignant tumor. These lesions arise from the infundibular epithelium of the hair follicle and therefore are related to epidermoid cysts. Inverted follicular keratosis may be an irritated form of seborrheic keratosis or verruca vulgaris.
CLINICAL PRESENTATION Inverted follicular keratosis presents as a small, solitary, well-demarcated, hyper-keratotic or wart-like keratotic mass most commonly on the upper eyelid and cheek, Rarely, it may be pigmented simulating a melanocytic tumor. This lesion may show scaling and exophytic projections presenting as a cutaneous horn. The lesion typically appears weeks to months before presentation, but sometimes may be present for many decades. Inverted follicular keratosis shows a growth pattern with epidermis extending over the base and sides of the lesion and then taking an abrupt inverted or downward turn towards the central epithelial mass.
HISTOPATHOLOGY This lesion resembles seborrheic keratosis except that the proliferating epidermis protrudes into the dermis instead of being exophytic. It is often classified as a subtype of seborrheic keratosis, distinguished by its endophytic growth and the presence of whorls of maturing squamous epithelial cells ("squamous eddies," illustrated on p. 47). There are variable numbers of horn cysts, and there may be an intense infiltrate of chronic inflammatory cells, as seen in the photomicrograph on the left.
DIFFERENTIAL DIAGNOSIS The differential diagnosis includes verruca vulgaris, papilloma, senile keratosis, seborrheic keratosis, cutaneous horn, keratoacanthoma, and squamous cell carcinoma.
TREATMENT Complete surgical excision is recommended, as recurrences are common with incomplete removal.
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