[Acral circumscribed hypokeratosis of plantar and dorsum of the finger localization: two cases].

Introduction: Palmar or plantar circumscribed hypokeratosis is a remarkable clinicopathologic entity described in 2002. It consists in a well demarcated decrease in thickness of the stratum corneum, that can be clinically mistaken for Bowen's disease or porokeratosis. We present a classical plantar localisation and a more original case on the dorsum of the finger, together with a microscopic and immunohistochemical study.
Case Reports: Case 1. A 65-year-old man was seen 15 years after a first consultation for a well demarcated 1.5cm erythematous lesion localised on the border of his left foot. The biopsy, then misinterepreted as keratosis sulcata, was reviewed. It showed a sudden and well demarcated decrease in thickness of the stratum corneum, overlying a slightly acanthotic epidermis, associated with dilated capillaries in the papillary dermis. HPV immunostaining was negative. Case 2. A 75-year-old woman had a well demarcaated erythematous lesion of the dorsum of her right index finger, lasting for months without significant evolution. A first biopsy showed pale and haloed keratinocytes that could be interpreted as koilocytes. She was therefore treated by cryotherapy, 5-fluro-uracile and imiquimod, that proved unsuccessful. A second biopsy showed a sudden and major decrease in thickness of the stratum corneum, overlying an area containing a few pale keratinocytes with perinuclear halo. HPV immunostaing was negative and Ki67 positive cells were slightly decreased in number when compared to lateral normal skin.
Discussion: Our first case is typical of plantar hypokeratosis characterised by its long evolution, typical semiology and well demarcated anomaly of the stratum corneum. Our second case is original as it shows that the disease can also affect the dorsum of the fingers. Acral circumscribed hypokeratosis is therefore a better name for this condition. We did not find any arguments in favor of a viral cause or an increased proliferation of keratinocytes. As often described in other cases, the lesion can remain unchanged for decades, which confirm its benign evolution. Topical treatments are generally ineffective. The pathogenesis of this localised hypokeratosis remains mysterious.