Mutually exclusive expression pattern of keratin markers for differentiation and proliferation in circumscribed palmar hypokeratosis

Funding sources: none. Conflicts of interest: none declared. Dear Editor, Circumscribed palmar or plantar hypokeratosis (CPH) has been regarded as a benign clonal epidermal differentiation or keratinization disorder.1,2,3 CPH is characterized by a well‐demarcated annular depressed erythematous lesion on the thenar or hypothenar eminences of the palms and sole. Its distinctive histopathological features are an abrupt decrease in the thickness of the corneal layer and hypogranulosis in the affected lesion.2,4 Herein we present a case of CPH and also demonstrate a mutually exclusive expression pattern of keratin markers for differentiation and proliferation between the affected and unaffected epidermis. A 64‐year‐old woman presented with a 5‐year history of erythema with a sense of irritation on the left thenar muscle, which gradually extended into the periphery. Physical examination revealed a well‐defined, 3.6 × 3.0‐cm depressed erythema on the left thenar muscle (Fig. 1a). Microscopic examination of a biopsy specimen showed a ‘sharp step’‐like thinning of the corneal and granular layers in the affected skin lesion (Fig. 1c), leading to a diagnosis of CPH. To the best our knowledge, our case was the second largest CPH in the literature.4 Unfortunately, previous topical administration of corticosteroids, heparinoid cream and tacrolimus ointment for years resulted in no improvement. Therefore, with consent by the patient to undergo surgical treatment, after the possible risk was explained, the lesion was excised and covered with a full‐thickness skin graft from the lower abdomen (Fig. 1b). No recurrence was observed 2 years after the surgery, to the patient's satisfaction.