Vertical vestibular responses to head impulses are symmetric in downbeat nystagmus.

Downbeat nystagmus (DBN), a frequent ocular motor sign often caused by cerebellar lesions, is a fixational nystagmus with fast phases directed downward. Its precise etiology is not known. One hypothesis is that it is caused by a central imbalance of pathways of the vertical vestibulo-ocular reflex (VOR). Such an imbalance would cause not only ocular drift but also deficient and asymmetric vertical VOR responses. To test this hypothesis, the authors analyzed the functionally relevant VOR responses to head impulses in the roll, pitch, and yaw planes. Head and eye movements were measured with the search-coil method in 10 patients with DBN and 10 age-matched control subjects. Analysis revealed no gain difference in patients with DBN compared with control subjects. Specifically, upward and downward VOR responses in patients with DBN in the pitch plane were symmetric. These findings do not support the hypothesis of a vertical VOR imbalance and put into question the view that DBN is a central vestibular syndrome in the sense of vestibular dysfunction. Although DBN possibly involves vestibulocerebellar pathways, in the patients that we studied, DBN did not affect the immediate VOR responses in the high-frequency range that corresponds to natural head movements.