Ungual seborrheic keratosis with longitudinal melanonychia: A case report

In this paper, we report the case of a 71-year-old man with an 8-year history of melanonychia on the right little finger, who referred to our hospital because the color of pigmented area had gradually darkened and the width had expanded. Physical examination revealed longitudinal melanonychia with brown color (4 mm in width). Dermoscopic examination revealed multiple white round clods and splinter hemorrhages. No micro-Hutchinson sign was observed. We performed a punch biopsy (diameter 3 mm) of the nail matrix for diagnosis. Histopathological examination revealed irregular acanthosis of epithelium of the nail bed and distal matrix, which consisted of basaloid cells without nuclear atypia. Several zones exhibited cell whorls reminiscent of squamous eddies. The whorls were composed of large pink cells arranged in an onion peel-like fashion. The basaloid cells tested negative for human papillomavirus in situ hybridization and p16 staining. The morphology of white round clods (milia-like cysts in metaphoric term) observed in dermoscopic analysis corresponded to squamous eddies in histopathology. Considering these features, ungual seborrheic keratosis (SK) with longitudinal melanonychia was diagnosed, which is an atypical site for SK. We suggest that the dermoscopic finding of milia-like cysts may be useful for the diagnosis of ungual SK.