A Rare Case of Transient Friction Melanosis of the Finger: A Case Report

Friction melanosis (FM) is an acquired pigmented disease that is caused by recurrent mechanical stress. There is no previous report explaining the presence of tiny brown-colored particles confined to the corneal layer. We describe a case of a rare form of FM of the finger that showed a relatively transient clinical course. A 17-year-old Korean female presented with a 5-month history of an asymptomatic localized hyperpigmented patch on the tip of the right index finger. The dermoscopic examination revealed homogenous globular pattern, which favored pigmentation over hemorrhage. Histopathologically, hyperkeratosis and acanthosis with lymphohistiocytic infiltration of the superficial dermis were noted on hematoxylin and eosin staining; however, there was neither a definite increase in melanophages in the upper dermis nor melanocytic proliferation in the basal layer. Per high-power field, multiple brown-colored tiny particles were scattered in the corneal layer. The particles were not dyed by Fontana-Masson stain, iron stain, and S-100. We questioned the patient about the presence of irritation and found that she had bought new shoes at the time of the onset. She was habituated to placing her fingers in her shoes while wearing them because they were slightly tight. The lesion disappeared spontaneously a week after the cause of friction was eliminated. Altogether, we encountered a rare form of FM that occurred in a rare location with a transient clinical course. Further cases on pigmentation restricted to finger tips might reveal the origin of the particles.