In this study, we aimed to differentiate the characteristics of video nystamnography, including vHIT, in iRBD patients from healthy controls, which in known as the prodrome of degenerative disease.

iRBD is known as the prodrome of alpha-synucleionopathies. Pons plays a pivotal role in the control of gaze. The VOR is a reflex stabilizing gaze during head movement. VOR gain in vHIT is calculate as the ratio of eye velocity to head velocity and reduced by VOR pathway disease.

Along 32 iRBD patients confirmed through overnight PSG, 16 patients with AHI of less than 15 were grouped in RBD group and 19 with AHI of 15 or higher as RBD with OSA group . 24 participants without sleep disturbance and underwent VNG were classified into normal group. The VOR gains as well as parameters of pursuit and saccade were measured. These results were compared by Kruskal-Wallis one-way analysis.

In the horizontal canal, the gain of normal group was 1.02 and the RBD group was 1.00 and the RBD with OSA group was 0.98 in right side, and it was measured as 0.98 in the normal, 0.85  in the RBD group, and 0.88 in the RBD with OSA group in left side. In the right anterior canal, values was 1.14 in the normal group, 1.02 in the RBD group, and 1.00  in the RBD with OSA group. In the left posterior canal, value was 1.10 for the normal group, 1.03 for the RBD group, and 0.97 for the RBD with OSA group. vHIT parameters were not different between RBD and RBD with OSA group.

We found that VOR gains in iRBD patients were reduced compared to normal controls. AHI did not affect the VOR gains iRBD patients.  This study suggests that VOR gains may be crucial determinants estimating brainstem dysfunction in iRBD patients.