Effects of Unilateral vs. Bilateral Subthalamic Nucleus Deep Brain Stimulation on Eye Movements

The purpose of this study was to determine the differential effects of unilateral and bilateral STN DBS on motor and cognitive aspects of eye movements. We studied 10 right-handed patients with Parkinson’s disease with bilateral STN stimulators and 10 sex- and age-matched healthy controls. The patients were tested after a 12-hour overnight withdrawal from their anti-parkinsonian medication under 4 conditions on 4 separate days. The conditions were OFF stimulation, RIGHT stimulator on, LEFT stimulator on, and BOTH stimulators on. For each condition, the patients were rated on a Parkinson’s disease clinical rating scale, the MDS-UPDRS part III, and they performed 2 eye movement tasks: a prosaccade task and an anti-saccade task. We found that unilateral STN DBS improved clinical ratings on the MDS-UPDRS, yet bilateral STN DBS had a significantly greater effect than unilateral stimulation. We found that unilateral stimulation did not improve saccadic eye movements. When BOTH stimulators were on, velocity and gain were significantly improved for the prosaccade and anti-saccade task, yet saccadic latency was not improved. Latency for the prosaccade task was significantly reduced when BOTH stimulators were on, but this reduction moved the average latency further from the average latency of the control subjects. There was no change in latency during the anti-saccade task when BOTH stimulators were on. Additionally, the amount of erroneous prosaccades in the anti-saccade task increased when BOTH stimulators were on but did not increase when only one stimulator was on. The increase in erroneous prosaccades when BOTH stimulators were on indicates that bilateral STN DBS impairs saccadic inhibition. In summary, unilateral STN DBS does not improve eye movements as it does limb movements. Although bilateral STN DBS improves certain aspects of eye movement control, it also appears to abnormally increase the reflexive response to visual stimuli and deleteriously affect the processes underlying saccadic inhibition.