Histopathology of the red lunula: new histologic features and clinical correlations of a rare type of erythronychia.

The lunula is the semi-lunar shaped area seen at the proximal aspect of the nail unit. It is the most distal aspect of the nail matrix that can be visualized through the transparent nail plate. The term red lunula describes erythema that partially or completely replaces the normal white color of the lunula. Red lunulae have been associated with a number of dermatologic and systemic conditions. However, the etiology and histopathology of the red lunula have not been characterized. We report a case of red lunulae of the fingernails in an adult male. Histopathologic examination revealed an increased density of benign-appearing and mildly dilated vascular channels present in the superficial papillary dermis of the nail matrix. To our knowledge, this is the first report of distinct pathologic findings seen on histologic examination of a red lunula. The increased vascularity of the lunula seen microscopically correlates with the clinically observed red color. These findings raise the possibility that systemic or local factors are responsible for angiogenesis within the nail unit in disorders associated with the red lunula.