Comparison of binocular vision indices in Parkinson's disease patients vs age-sex-matched healthy controls

To evaluate the differences in near point of convergence (NPC), fusional vergence, saccadic eye movements, versional eye movements, and heterophoria between patients diagnosed with Parkinson's disease (PD) and healthy subjects.

A cross-sectional comparative study was conducted, enrolling two cohorts: a PD group and a healthy control group. The PD group was recruited via non-random convenience sampling, while the control group was selected randomly from individuals without PD. All participants were screened according to predefined inclusion and exclusion criteria before undergoing a comprehensive optometric assessment, which included measurements of uncorrected visual acuity, corrected visual acuity, and objective and subjective refraction. Subsequently, binocular vision function evaluations were performed, covering NPC measurement, fusional vergence reserve assessment at both distance and near, saccadic eye movement testing, and versional eye movement and heterophoria assessment.

A total of 42 PD patients and 41 healthy controls were included in the final analysis. The two groups were well-matched in terms of sex distribution [29 males (69.0%) in the PD group vs 29 males (70.7%) in the control group, P=0.867] and mean age (55.3±9.6y in the PD group vs 54.9±9.8y in the control group, P=0.866). The prevalence of abnormal versional eye movements was significantly higher in the PD group than in the control group (23.81%, 95%CI: 12.05%-39.45% vs 7.32%, 95%CI: 1.54%-19.92%; P=0.025). Near exophoria was more prevalent in PD patients (61.90%, 95%CI: 45.64%-76.43%) than in controls (17.07%, 95%CI: 7.15%-32.06%), with a significant difference [odds ratio (OR)=7.99; 95%CI: 2.83-21.99; P<0.001]. The mean NPC was significantly greater (more receded) in the PD group than in the control group (9.01±3.74 cm vs 7.20±2.15 cm; P=0.007). A statistically significant positive correlation was observed between PD severity and NPC values (Pearson's correlation coefficient=0.309; P=0.046). Except for distance base-out break and distance base-out recovery values, all other fusional vergence parameters were significantly lower in the PD group than in the control group (P<0.05). The mean saccadic test score was significantly lower in PD patients than in controls (3.29±0.57 vs 3.78±0.42; P<0.001). Among all fusional vergence indices, near base-in blur yielded the highest area under the curve (AUC=0.877), with a sensitivity of 69% and specificity of 90%, followed by distance base-out blur (AUC=0.824, sensitivity=97.6%, specificity=66.7%), near base-out blur (AUC=0.814, sensitivity=76.2%, specificity=72.7%), near base-out break (AUC=0.749, sensitivity=78.6%, specificity=67.6%), and near base-out recovery (AUC=0.749, sensitivity=95.2%, specificity=50%).

PD is associated with significant binocular vision function impairment, with receded NPC and reduced near fusional vergence reserves being the most prominent disorders. These findings highlight the potential value of binocular vision assessment as a non-invasive biomarker for the early detection and clinical monitoring of PD.