Binocular adaptation to +2 D
lenses in myopic and emmetropic children.
(Adaptation binoculaire à une addition de +2,00 chez des enfants myopes ou
emmetropes)
Sreenivasan V, Irving EL,
Bobier WR.
School of Optometry, University of Waterloo, Waterloo, Ontario, Canada.
vsreeniv@sciborg.uwaterloo.ca
PURPOSE: To compare vergence adaptation to +2 D addition lenses in myopic and emmetropic children and to evaluate the influence of the accommodative-vergence crosslink (AC/A ratio) on this adaptation.
METHODS: Nine myopic and 11 emmetropic children fixated a near target at a distance of 33 cm. Measures of binocular and monocular accommodation and phoria were obtained during a 20-min near task with and without +2 D lenses. Response AC/A ratios were determined from the experimental results. Vergence adaptation was quantified by the magnitude of reduction in phoria and the percentage of completeness (PC, return of adapted phoria to habitual level) after the near task.
RESULTS: Myopic children
showed significantly higher AC/A ratios, which led to greater lens-induced
exophoria and a greater demand for vergence adaptation. Both refractive groups
showed significant vergence adaptation; however, myopes exhibited significantly
reduced (p < 0.01) magnitudes compared with emmetropes (myopes = 3.95 +/- 0.15
Delta; emmetropes = 4.41 +/- 0.08 Delta). The mean PC was also significantly (p
< 0.001) reduced in myopes (61.02 +/- 1.57) compared with emmetropes (76.6 +/-
2.10). There was a significant correlation between magnitude of adaptation and
AC/A in both the refractive groups; however, myopes consistently showed reduced
magnitudes compared to emmetropes. AC/A ratio influenced PC in emmetropic but
not myopic children. In the accommodation system, +2 D lenses eliminated the
accommodative lags observed in myopic children during natural viewing
conditions. These lenses resulted in a small over-focus (-0.24 +/- 0.27 D) at
the onset of near work, which decreased during sustained viewing through the
near add.
CONCLUSIONS: Myopic
children demonstrate reduced magnitude and completeness of vergence adaptation
to +2 D lenses. The magnitude of vergence adaptation varied
with AC/A in both refractive groups; however, the presence of myopia
differentiated the amount of adaptation for all AC/A ratios. Conversely, the
degree of completeness appears to be primarily associated with the type of
refractive error.
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