Bilateral Neural Crosstalk Between the Posterior Semicircular Canals in Patients with Unilateral Vestibular Schwannomas: Consequences for the Vestibulo-Ocular Reflex on

Background: A group of patients with untreated unilateral vestibular schwannoma (UVS) was observed in previous clinical trials, and the results indicated a reduction in the vestibulo-ocular reflex (VOR) on the ipsilateral (tumor) site of the lesion. However, in a subset of patients, a loss of VOR gain was also observed on the contralateral (non-tumor) side, suggesting the presence of contralateral neural crosstalk. Methods: To understand our previous clinical findings, the present study has expanded its population to investi-gate whether these unexpected findings are recognized in a significantly larger population of patients with UVS (n=640). Retrospectively, mean VOR gains of all semicircular canals (SCC) were obtained using video head impulse tests (vHIT) and compared between ipsi- and contralateral side of lesion. To eliminate any potential bias resulting from procedural effects, vHIT data was also obtained from a group of 72 healthy subjects. Results: As ex-pected, a VOR gain reduction was identified on the side of lesion in a substantial propor-tion of patients with UVS, varying ranging from 19.4% (anterior SCC) to 39.7% (posterior SCC). More interesting was the observation of a significant number of patients (21.9%) ex-hibiting a significant VOR reduction in posterior semicircular canal on the contralateral side, with a strong correlation with the ipsilateral side (r = 0.69). In relation to this phe-nomenon, our data further demonstrates that the influence of the superior branch of the vestibular nerve is less pronounced on the contralateral side compared to that of the infe-rior branch. Conclusion: Current data from a significant group of patients with UVS sup-ports bilateral commissural connectivity of inhibitory neural pathways between vestibular nuclei, as evidenced by a considerable VOR gain reduction in the posterior SCCs, facili-tating compensatory mechanisms. This suggests that in a significant proportion of UVS patients the contralateral side may also exhibit partial impairment, albeit undetected.