Yellow nail syndrome: the nail that grows half as fast grows twice as thick.

We report a case of a 51-year-old man with yellow nail syndrome (YNS).1 During a 23-week period of study, the dynamics of thumbnail growth were compared between one affected thumb and the normal contralateral thumb. Longitudinal nail growth was normal (0.46 mm/week) in the normal thumb and double that of the affected thumb (0.23 mm/week). Thickness of nail at the free edge in the affected thumb (0.97 mm) was twice that of the normal thumb (0.57 mm). Within the nail plate in the dorso-ventral axis there were 50% more cells in the affected thumb (358) in comparison with the contralateral control (242). This case illustrates that rate of longitudinal growth does not necessarily reflect nail plate production.