Binocular dysfunction in Parkinson's Disease: Decoding near triad dynamics and divergence deficits

Parkinson's Disease (PD) is a progressive neurodegenerative disorder that often affects the oculomotor system, causing strabismus and vergence impairments, particularly convergence insufficiency when focusing on nearby objects. This study evaluated eye deviation, vergence, accommodation, and pupil responses in PD during converging and diverging gaze shifts, correlating these findings with neurologic severity using the Unified Parkinson's Disease Rating Scale (UPDRS). We recruited 19 participants with varying severity of PD and 10 age-matched controls. 26 % of PD participants (PD Group 1) exhibited disparity-driven convergence and divergence responses comparable to controls, with expected miosis during convergence and mydriasis during divergence. In contrast, 74 % of PD participants (PD Group 2) showed reduced disparity-driven convergence and divergence, with diminished miosis during convergence and mydriasis during divergence. 37 % of PD participants exhibited increased exodeviation at near (30 cm) during binocular viewing in addition to reduced disparity-driven convergence. Blur-driven vergence was more significantly reduced than disparity-driven vergence in both PD participants and controls, likely due to presbyopia. Accommodation, assessed through changes in refractive error during disparity-driven and blur-driven vergence movements, was comparable between controls and PD patients. Our study indicates that PD disrupts disparity-driven convergence and divergence while sparing accommodation and blur-driven vergence. These findings offer insight into the pathophysiology of binocular dysfunction in PD, highlighting the involvement of neural structures such as the deep cerebellar nuclei and the supra-oculomotor area in vergence deficits and strabismus. Future studies can use these metrics to evaluate the effectiveness of targeted neuromodulation therapies in alleviating binocular dysfunction in PD.