Exploring the role of the electrically evoked Vestibulo-Ocular reflex in vestibular implant surgery

Vestibular implantation holds considerable potential as a therapeutic strategy for individuals with vestibular loss. A correct position of the stimulation electrodes is essential for effective implant functionality. For vestibular implants targeting the semi-circular canals, Vestibulo-Ocular Reflex testing (VOR testing) is used to evaluate functionality postoperatively. It might also be an effective tool for intraoperative decision making related to electrode placement. This study aimed to evaluate the predictive value of intraoperative VOR testing for the postoperative vestibular implant response. This would be the first step in exploring the usability of the VOR as intraoperative electrode placement indicator.

Nine patients with bilateral vestibulopathy were implanted with an investigational vestibulo-cochlear implant. Vestibulo-ocular reflexes (VOR) were electrically evoked using the implant, both intra- and postoperatively, and recorded using video-oculography. The correspondence between the intra- and postoperative measurements was evaluated. This included the presence of the VOR, activation threshold, eye velocity and alignment. Furthermore, the correlation between the intraoperative peak eye velocity and stimulation amplitude was assessed.

It was found that the presence of an intraoperative VOR has a high positive predictive value (1.0) for predicting the presence of a postoperative VOR. Additionally, the intraoperative VOR showed a high, though not perfect, negative predictive value (0.86) for predicting the absence of a postoperative VOR. The VOR activation threshold was higher, eye velocity was lower, and alignment differed on individual level, in the intra-operative measurements compared to the post-operative measurements. The intraoperative VOR peak eye velocity was related to stimulation amplitude.

Intraoperative VOR responses during vestibular implant surgery differ from postoperative responses, likely due to general anaesthesia. Despite this discrepancy, the presence of an intraoperative VOR is a strong predictor of postoperative VOR presence. Furthermore, intraoperative VOR eye velocity increases with higher stimulation amplitudes. These findings suggest that intraoperative VOR testing may aid in optimizing electrode placement during vestibular implant surgery.