Underlying neurological mechanisms associated with symptomatic convergence insufficiency

Abstract

Convergence insufficiency (CI) is the most common binocular vision problem, associated with blurred/double vision, headaches, and sore eyes that are exacerbated when doing prolonged near work, such as reading. The Convergence Insufficiency Neuro-mechanism Adult Population Study (NCT03593031) investigates the mechanistic neural differences between 50 binocularly normal controls (BNC) and 50 symptomatic CI participants by examining the fast and slow fusional disparity vergence systems. The fast fusional system is preprogrammed and is assessed with convergence peak velocity. The slow fusional system optimizes vergence effort and is assessed by measuring the phoria adaptation magnitude and rate. For the fast fusional system, significant differences are observed between the BNC and CI groups for convergence peak velocity, final position amplitude, and functional imaging activity within the secondary visual cortex, right cuneus, and oculomotor vermis. For the slow fusional system, the phoria adaptation magnitude and rate, and the medial cuneus functional activity, are significantly different between the groups. Significant correlations are observed between vergence peak velocity and right cuneus functional activity (p = 0.002) and the rate of phoria adaptation and medial cuneus functional activity (p = 0.02). These results map the brain-behavior of vergence. Future therapeutic interventions may consider implementing procedures that increase cuneus activity for this debilitating disorder.

Figure 1

(A) Averaged vergence eye movement position (°) as a function of time (s) from BNC (solid blue line) and CI (solid red line) participants with ± one standard deviation (BNC blue shaded and CI red shaded) with the averaged peak velocity (°/s) as a function of time (s) from BNC (dashed blue line) and CI (dashed red line) participants. (B) Two-sample unpaired t-test of functional activity differences between BNC and CI groups during the vergence jump step fast fusional experiment showing V2, OMV, and right cuneus are significantly different between groups. (C) Linear regression of mean beta weight from right cuneus ROI using a 5 mm radius sphere as a function of peak velocity for BNC (blue circle) and CI (red triangle) participants. Linear regression (solid black line) analysis shows Pearson’s correlation coefficient of r = 0.33, p = 0.002 between right cuneus functional activity and vergence peak velocity.

Figure 2

(A) Group-level averaged phoria adaptation results of phoria (∆) as a function of time (min) using a 6∆ base-out (BO) prism for BNC (blue symbols) and CI (red symbols) groups ± 1 standard error of the mean. Exponential decay fit of BNC group (solid blue line) and CI group (solid red line). (B) Two-sample unpaired t-test of functional activity differences between BNC and CI groups during the phoria adaptation slow fusional experiment showing medial cuneus was significantly different between groups. (C) Linear regression of mean beta weight from medial cuneus ROI using a 5 mm radius sphere as a function of the rate of phoria adaptation (∆/min) to 6∆ BO prism for BNC (blue circle) and CI (red triangle) participants. Linear regression (solid black line) analysis shows the Pearson’s correlation coefficient of r = 0.25, p < 0.02 between medial cuneus functional activity and rate of phoria adaptation.