Positional vertigo and ageotropic bidirectional nystagmus.

A strong paroxysmal positional horizontal nystagmus accompanied by symptoms similar to those of paroxysmal positional vertigo (PPV) can be observed in a small fraction of patients who have positional vertigo. This nystagmus may be a lateral canal variant of PPV. We evaluated nine patients who had episodes of prolonged, intense positional vertigo provoked by lateral movements of the head while in the supine position. The nystagmus appeared as horizontal and was directed toward the uppermost ear (ageotropic) when the head was rotated to either side (bidirectional). The duration of nystagmus lasted more than 1 minute in all the cases, although it presented a progressive decrease in the velocity of the slow component. The clinical and electronystagmographic features of this syndrome lead us to propose a different form of horizontal canal PPV associated with a paroxysmal positional ageotropic and bidirectional nystagmus, probably caused by a "heavy cupula" as a result of deposits of extraneous bodies (otolithic?) or by a cupula denser than the endolymph.