Subthalamic deep brain stimulation improves vergence eye movements in Parkinson’s disease

Abstract Background Visuomotor manifestations are increasingly recognized as a clinical dimension of Parkinson’s disease (PD). Among these, vergence dysfunction can cause double vision and impair everyday visual function, yet binocular control is rarely quantified objectively. Consequently, it remains unknown how vergence dysfunction relates to PD severity, and whether it is amenable to established PD treatments, including dopaminergic medication and subthalamic deep brain stimulation (STN-DBS). Methods We used a high-resolution binocular video-oculography paradigm to quantify conjugate and vergence eye movements in 24 people with PD and bilateral STN-DBS across three controlled therapeutic states (medication/DBS off/off, on/off and off/on), alongside 36 age-matched controls. Motor severity was independently assessed based on standardized videography using blinded MDS-UPDRS-III ratings by three independent assessors. Results Double vision was reported by 29% of participants with PD compared with 3% of controls (p= .005). In the OFF state, PD was associated with pronounced impairment of dynamic vergence, with convergence and divergence gains reduced by up to 65% and maximal convergence angles reduced by up to 42% compared with controls (all p≤ .006). Slow divergence latency was additionally prolonged (p= .001), whereas vergence stability did not differ significantly. Conjugate eye movements showed reduced vertical saccadic gain and fewer self-paced saccades, but largely preserved smooth pursuit. Vergence and vertical saccadic gain additionally scaled with MDS-UPDRS-III motor severity. STN-DBS improved slow vergence gain by up to 39%, maximal convergence angle by 20%, and increased vertical saccadic gain by 50%, whereas dopaminergic medication produced no significant improvement in these ocular motor parameters. Conclusion These findings identify dynamic vergence dysfunction as a visuomotor feature of PD and demonstrate amenability to STN-DBS. The pronounced modulation of both conjugate and vergence eye movements by STN-DBS suggests that impaired binocular motor control in PD is associated with dysfunction of subthalamic circuits.