A pilot study of disparity vergence and near dissociated phoria in convergence insufficiency patients before vs. after vergence therapy

Abstract

Purpose: This study examined the relationship between the near dissociated phoria and disparity vergence eye movements. Convergence insufficiency (CI) patients before vergence therapy were compared to: (1) the same patients after vergence therapy; and (2) binocularly normal controls (BNC).

Methods: Sixteen subjects were studied-twelve BNC and four with CI. Measurements from the CI subjects were obtained before and after 18 h of vergence eye movement therapy. The near dissociated phoria was measured using the flashed Maddox rod technique. Vergence responses were stimulated from 4° symmetrical disparity vergence step stimuli. The peak velocity of the vergence response and the magnitude of the fusion initiating component (FIC) from an independent component analysis (ICA) were calculated. A linear regression analysis was conducted studying the vergence peak velocity as a function of the near dissociated phoria where the Pearson correlation coefficient was computed.

Results: Before vergence therapy, the average with one standard deviation FIC magnitude of convergence responses from CI subjects was 0.29° ± 0.82 and significantly less than the FIC magnitude of 1.85° ± 0.84 for BNC (p < 0.02). A paired t-test reported that the FIC and near dissociated phoria before vergence therapy for CI subjects significantly increased to 1.49° ± 0.57 (p < 0.04) and became less exophoric to 3.5Δ ± 1.9 exo (p < 0.02) after vergence therapy. A significant correlation (r = 0.87; p < 0.01) was observed between the near dissociated phoria and the vergence ratio of convergence peak velocity divided by divergence peak velocity.

Conclusion: The results have clinical translational impact in understanding the mechanism by which vergence therapy may be changing the vergence system leading to a sustained reduction in visual symptoms.

Keywords: convergence; convergence insufficiency; divergence; fusion initiating component; near dissociated phoria; vergence therapy.

Figure 1

(A) The Dual-Mode model describing vergence as preprogrammed with feedback controlled subcomponents termed the fusion initiating component (FIC) and fusion sustaining component (FSC), respectively. These components lead into the plant, which represents mostly the extraocular muscles. (B) This model generated simulated convergence responses to 4° symmetrical steps where each colored trace represents a convergence eye movement. These simulated responses are similar to experimental data. (C) The simulated responses had known underlying sources. The average FIC and FSC from the known sources shown in plot (B) are plotted in blue (labeled using arrows). The ICA algorithm computed two sources plotted in red. The known simulation sources and the ICA sources are similar; validating ICA can be used to dissect experimental convergence responses into their underlying FIC and FSC sources.

Figure 2

(A) Ensemble convergence eye movement responses to 4° symmetrical step stimuli. Each colored trace represents a convergence eye movement. The left plot is responses from a binocularly normal control whereas the middle and right plots are from the same subject with CI before and after vergence therapy, respectively. (B) Results from the ICA of the ensemble responses from plots (A). The average of the ensemble eye movements (red), the FIC (blue) and the FSC (green) are plotted. The peak magnitude of the FIC (blue arrow) is reduced in the CI subject before vergence therapy, compared to the BNC, and increases to be more similar to the BNC after vergence therapy.

Figure 3

Group analysis of the average peak magnitude of the convergence FIC for BNC (gray bar), CI subjects before vergence therapy (light gray bar), and the same CI subjects after vergence therapy (black bar). Error bars represent one standard deviation. The average convergence FIC for CI subjects before vergence therapy is significantly reduced compared to BNC and then significantly improves post vergence therapy. The number of subjects studied is denoted under each bar.

Figure 4

Linear regression analysis of the vergence peak velocity ratio (average convergence peak velocity divided by average divergence peak velocity) as a function of the near dissociated phoria. Esophoria is denoted as positive and exophoria is denoted as negative. The BNC, CI subjects before vergence therapy and the same CI subjects after vergence therapy are shown in blue diamonds, green circles and red triangles, respectively. A significant correlation was observed (p < 0.01) and the Pearson correlation coefficient was r = 0.87.