Benign paroxysmal positional nystagmus.

A characteristic nystagmus profile of benign paroxysmal positional nystagmus was determined from analyses of horizontal and vertical electro-oculographic recordings in 32 patients. The vertical component was upbeat in both eyes (fast phase toward the ground in the head-hanging position), whereas the horizontal component was dissociated with the ipsilateral eye beating away from the down ear and the contralateral eye beating toward the down ear. The amplitude of the vertical component was larger in the ipsilateral eye. This dissociated nystagmus profile is consistent with a burst of excitatory activity originating in the posterior canal of the ear that is undermost at the end of the positioning maneuver.