Effect of convergence on the horizontal VOR in normal subjects and patients with peripheral and central vestibulopathy.

BACKGROUND

Vestibular compensatory eye movements provide visual fixation stabilization during head movement. The anatomic pathways mediating a normal horizontal vestibulo-ocular reflex (h-VOR), when lesioned, cause spontaneous nystagmus. While previous reports address the effect of convergence on different spontaneous nystagmus types, to our knowledge, a study of acute vestibular nystagmus suppression viewing near targets comparing patients with peripheral or central vestibular lesions has not been previously reported.

METHODS

We attempt to clarify potential vestibular and near-reflex interaction by comparing near and far h-VOR gain in 19 healthy controls, six patients with acute/subacute peripheral vestibular lesion (PVL), and one patient with unilateral vestibular nuclear lesion (VNL) in the pontine tegmentum.

RESULTS

The horizontal (h)-VOR in normal subjects increased with convergence in both eyes (P = 0.027, P < 0.001). In unilateral PVL patients, gain failed to increase in either direction (P = 0.25, P = 0.47). In contrast, when fixating at 15 cm, the h-aVOR in the VNL lesion, gain did not increase, and a right h-nystagmus developed. Even though we found inability to increase gain in PVL with near target fixation, this did not interfere with h-nystagmus suppression upon converging. Our VNL patient had normal h-nystagmus suppression viewing far distance targets and lacked near target h-nystagmus suppression.

CONCLUSION

We hypothesize that normal IO/flocculus pathway suppressed spontaneous nystagmus in PVL. Impaired h-VOR near adaptation in the medial vestibular nucleus was responsible for h-nystagmus direction with fixation block. Additionally, impaired viewing distance estimate contributed to near h-nystagmus suppression failure.