Positional vertigo related to semicircular canalithiasis.

My clinical and laboratory observations support the theoretical concept that the mechanism of typical nystagmus, and most forms of atypical transient nystagmus, is hydrodynamic drag by gravitating free densities--most commonly displaced otoconia--in the endolymph of a semicircular canal; and that these "canaliths" have a significant mechanical advantage, by virture of the canal/ampulla cross-sectional differential, over densities acting directly on the cupula. Positional vertigo related to apparent canalithiasis (benign paroxysmal positional vertigo) is a common cause of incapacitation. The profile of the concomitant nystagmus localizes the semicircular canal involved. The canalith repositioning procedure, appropriately administered and targeted according to the observed nystagmus, provides a highly effective means for control of symptoms and a valuable resource for diagnostic evaluation of the more complex case. Surgery is rarely indicated.