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Vidhyapriya Sreenivasan, Elizabeth L. Irving, William R. Bobier; Vergence Adaptation In Myopic And Emmetropic Children Under Open-loop Accommodation. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5711.
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We previously found that myopic children exhibit reduced vergence adaptation compared to emmetropes when they fixated through +2D add in the presence of an active stimulus to accommodation (closed-loop)1. Here, we examined vergence adaptation to base-out prism under conditions of open-loop accommodation.
18 emmetropic (Emm; 0.5 ± 0.1D) and 23 myopic (Myo; -2.0 ± 0.2D) children between 7-14 years participated in the study. Habitual tonic vergence (TV, distance phoria through 0.5mm pupils) was measured through best corrective lenses; following which 10 Δ base-out (BO) was added in front of the left eye. After ensuring fusion, TV was measured through the prism at frequent intervals when children sustained fixation (4M) through 0.5mm infra-red pupils. Vergence adaptation was quantified by the overall TV change (magnitude) as well as the percentage return to the habitual level (completeness) derived using an exponential decay function. Tonic accommodation was measured at 4 M using a 0.5 cpd difference of Gaussian target before and after the 20 min task.
The mean habitual TV (Emm: 2.6±0.5Δ; Myo: 1.7±0.6Δ;P=0.2) and the exo shift induced by the 10BOΔ (Emm:-8.8±0.3Δ; Myo:-8.6±0.3Δ;P=0.5) were similar between the groups. Both groups showed significant (P<0.001) reduction in TV with sustained fixation. The overall decay curves showed significant differences between the two refractive groups (P<0.005) such that myopic children showed less magnitude (Emm: 6.1±0.4Δ; Myo: 5.0± 0.4Δ) and completeness of adaptation compared to emmetropes (Emm: 69±2.5%; Myo: 58±2.5%). Post task tonic accommodation showed significant increase in myopes (-0.27± 0.08D;P<0.05) but not in emmetropes (0.11± 0.1D).
Myopic children show reduced vergence adaptation to BO prism under open-loop accommodation. This behaviour suggests that when fusional convergence is stimulated either through BO prisms or binocular viewing through plus adds, myopic children show reduced vergence adaptation. Tonic accommodation however shows a greater post-task shift in the myopes which could be due to a greater output from convergence accommodation resulting from reduced vergence adaptation or may be a function of a differing gain of accommodative adaptation in myopes.(1)Sreenivasan et al, Opt Vis Sci 2009;86;731-40.
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