Inverted follicular keratosis

Inverted follicular keratosis is a benign tumor originating from follicular infundibulum characterized by a solitary nodular lesion in middle to old aged individuals on head and neck ¹. Controversy exists regarding the recognition of inverted follicular keratosis as a distinct entity, or a variant of verruca vulgaris or irritated seborrheic keratosis. Inverted follicular keratosis is considered to be a variant of seborrheic keratosis ². However, some authors believe that inverted follicular keratosis is a different entity ³, ⁴. Inverted follicular keratosis often appears as solitary nonpigmented verrucous papule on the head-neck region in 90% of patients and is frequently seen in elderly males (middle-aged men affected twice as often as females ) ⁵. Inverted follicular keratosis generally presents as an asymptomatic, solid, white-pink, solitary papule smaller than 1 cm. Although inverted follicular keratosis generally presents in a solitary form, it was reported that multiple acral keratosis and inverted follicular keratosis were observed in a patient with Cowden syndrome ⁶.

It is difficult to clinically differentiate inverted follicular keratosis from viral warts, basal cell carcinoma (BCC), and squamous cell carcinoma (SCC) characterized with keratinized proliferation ⁵. The diagnosis of inverted follicular keratosis is generally established histopathologically because clinical differentiation from other lesions is difficult ⁷.

Figure 1. Inverted follicular keratosis

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Inverted follicular keratosis causes

The cause of inverted follicular keratosis is not exactly known. Although it was reported that it might be related to viral warts or seborrheic keratosis, some authors believe that inverted follicular keratosis is a different entity ³, ⁴. In most recent studies, human papilloma virus (HPV) was detected in cases with inverted follicular keratosis ⁹. Another study reported that inverted follicular keratosis is a neoplasm of follicular origin and in this aspect, it could be differentiated from seborrheic keratosis ¹⁰.

Inverted follicular keratosis signs and symptoms

Inverted follicular keratosis generally presents as an asymptomatic, solid, white-pink, solitary papule smaller than 1 cm.

Inverted follicular keratosis diagnosis

In inverted follicular keratosis, the diagnosis is generally established histopathologically, because its clinical features might resemble more frequently seen diseases such as viral warts, seborrheic keratosis, actinic keratosis, basal cell carcinoma (BCC), and squamous cell carcinoma (SCC). It is most difficult to differentiate inverted follicular keratosis from a seborrheic keratosis of the variety described as “irritated,” that sometimes mimics both benign and malignant conditions.

Inverted follicular keratosis histology

Inverted follicular keratosis has been described as an exo-endophytic proliferation of keratinocytes that is sharply circumscribed; the proliferation has a lobular arrangement showing basaloid cells at the periphery and keratinocytes with more squamous differentiation at the center. Some of the keratinocytes form concentric configurations termed “squamous eddies.” Additionally, hyperkeratosis and parakeratosis with occasional keratinous plugs can be seen ¹¹. Reflectance confocal microscopy features of inverted follicular keratosis have been previously described by Armengot-Carbo et al ⁵. These include epidermal projections, broadened honeycomb pattern, disarranged dermoepidermal junction, and looped vessels in the dermis. The authors added that these findings could also be observed in other lesions, such as squamous cell carcinoma (SCC) ⁵.

Inverted follicular keratosis treatment

The most common method in the treatment of inverted follicular keratosis is complete surgical excision ⁷. Following surgical excision, no invasive growth or metastasis have been reported in the literature ¹².

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