Vergence Adaptation And Convergence Accommodation In Convergence Insufficiency

Abstract

Purpose: : Our group previously showed that the capacity to tolerate steady increases in base out (BO) prism (positive fusional vergence; PFV) correlated with an increased rate and magnitude of vergence adaptation¹ in binocular normal adults. We concluded that vergence adaptation increased blur tolerance by reducing the excessive convergence accommodation resulting from the base-out prism. Reduced levels of PFV is a common finding in convergence insufficiency (CI) as is reduced vergence adaptation². Here we examine whether the reduced vergence adaptation in these individuals is accompanied by a reduced ability to control convergence accommodation output compared to normals.

Methods: : Ten participants (mean age = 17.4±2.3 yrs) with convergence insufficiency (CI) were examined. Participants showed reduced PFV at 40 cm or inadequate levels to compensate for their high exophoria (Sheard’s criteria) at near (mean BO to blur in group=12±0.9Δ). Testing followed our previous experimental design for (n=11) binocularly normal adults¹. Patterns of vergence adaptation to a 12 Δ BO while viewing a difference of Gaussian target were measured at 3 min intervals over a 15 min period using brief phoria measures (Thorington technique). Concurrent measures of convergence accommodation were also taken using a PowerRefractor (Multichannel system,Germany) for 5 sec at each 3 min interval. The magnitude of change in vergence adaptation and convergence accommodation was quantified in each CI participant using an exponential decay function and then compared to normal data. As tonic changes in accommodation could confound the CA measures, this was assessed in each participant following testing.

Results: : The data of two participants who were unable to fuse through the 12 Δ BO could not be included in the averaged results. When exponential decay patterns of vergence adaptation and CA were compared to our binocularly normal participants, patients with CI showed significantly reduced amplitude of decay of vergence adaptation (CI=2.9±0.8Δ; Normals=6.6±0.4Δ; p=0.01) and convergence accommodation (CI=0.23±0.1D; Normals=0.510±0.02D; p=0.03). Tonic accommodation did not show any significant difference (p=0.72) before and after sustained prism viewing in the CI group (Change=-0.05±0.1D), similar to binocular normals (Change=0.01±0.2D; p=0.8).

Conclusions: : The reduced capacity of vergence adaptation found in patients with convergence insufficiency results in higher levels of convergence accommodation which may correspond to the reduced base-out to blur response observed during clinical positive vergence testing in the group.1) Thiagarajan et al Optom Vis Sci. 2010;87:487-93.2) North RV, Henson DB. Am J Optom Physiol Opt. 1982;59:983-6.

Keywords: binocular vision/stereopsis • vergence • accommodation