Optometry and Vision Science

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Browsing Optometry and Vision Science by Subject "Accomodation"

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    Binocular Adaptation to +2 D Lenses in Myopic and Emmetropic Children:
    (Wolters Kluwer, 2009-06) Sreenivasan, Vidhyapriya; Irving, Elizabeth; Bobier, William
    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 Δ; emmetropes = 4.41 ± 0.08 Δ). 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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    Effect of heterophoria type and myopia on accommodative and vergence responses during sustained near activity in children
    (Elsevier, 2012-03-15) Sreenivasan, Vidhyapriya; Irving, Elizabeth; Bobier, William
    The influence of phoria-type and myopia on changes to vergence and accommodation during prolonged near-task was examined in 53 children. Participants were classified into phoria and refractive categories based on near phoria and cycloplegic refraction respectively. Measures of near phoria, binocular (BA) and monocular accommodation (MA) were obtained before and during a 20 min task when children binocularly fixated a high-contrast target at 33 cm through best corrective lenses. Vergence adaptation and accommodative adaptation were quantified using changes to near phoria and tonic accommodation respectively. The direction and magnitude of vergence adaptation was modified by the phoria-type (p < 0.001). Emmetropic exophores displayed convergent (less exo than baseline) adaptation while esophores showed divergent shifts (less eso than baseline) in phoria upon prolonged fixation. Myopic children also followed a similar pattern but showed greater divergent (or less convergent) shift (p < 0.001) in vergence adaptation for all phoria categories compared to emmetropes. Phoria-type also influenced the pattern of BA vs. MA (p < 0.001) such that exophores showed BA > MA while esophores showed MA > BA in both refractive groups. Accommodative adaptation was higher in myopes (p = 0.010) but did not demonstrate a significant effect of phoria (p = 0.4). The influence of phoria-type on vergence adaptation and the pattern of BA vs. MA relates primarily to the varying fusional vergence demands created by the direction of phoria. The greater divergent (or less convergent) shift in vergence adaptation seen in myopes (compared to emmetropes) could be attributed to their higher accommodative adaptation. Nevertheless, the adaptive patterns observed in myopic children do not appear to explain their high response AV/A ratios identified as a risk factor for myopia development.