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V. Sreenivasan, E. L. Irving, W. R. Bobier; Phoria Adaptation to Positive and Negative Lenses in Myopic and Emmetropic Children. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2014.
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We previously found that myopic children with normal near phoria (MO) and near esophoria (ME) exhibited reduced phoria adaptation to +2D add compared to emmetropic orthophores (EO). The overall adaptive pattern was ME<MO<EO1. Since individuals often show asymmetric phoria adaptation we examined whether an oppositely signed add (-2D) would reverse the pattern of adaptation between these groups.
8 emmetropic orthophores (SE:0.5 ± 0.2D; near phoria -1.73 ± 0.37Δ); 7 myopic orthophores (-1.8 ± 0.3D; -2.1± 0.67Δ) and 8 myopic esophores (-1.5±0.4D; 5.0 ± 0.8Δ) between the ages of 7-14 participated in the study. Participants fixated a near target (33 cm) for 20 minutes through +2D and -2D adds over their distance correction. Modified Thorington measures of near phoria were obtained at 2, 4, 6, 8, 10, 15 and 20 minute intervals. Phoria adaptation was quantified by the overall phoria change (magnitude) as well as the percentage return to the habitual level (completeness, PC) derived using an exponential decay function.
Response AC/A ratios were higher in both myopic groups (EO: 3.8 ± 0.3Δ; MO: 6.1± 0.4Δ;ME:6.9 ±1.3Δ; p<0.01), resulting in larger lens-induced phoria compared to emmetropes.-2D adds: Esophoria induced by the -2D add (EO: 4.9±0.5Δ; MO: 6.4±0.7Δ; ME:8.4 ± 0.9Δ) decreased significantly (p<0.05) in all groups with sustained fixation. The decay function showed higher magnitude (EO: 1.9 ± 0.6Δ; MO: 2.1 ± 0.8Δ; ME; 7.6± 1.1Δ; p<0.005) and greatest PC in ME compared to MO and EO (EO:36 ± 5%; MO:32 ± 4%;ME 92 ± 6%; p<0.001).+2D adds: Exophoria induced by +2D add (EO: 4.9 ± 0.3Δ; MO: 7.4 ± 0.5Δ; ME: 7.6 ±0.4Δ) reduced with time and the overall adaptive pattern was similar to our previous study1. Both myopic groups showed reduced PC compared to emmetropes (EO: 85 ± 2%; MO: 61±4%; ME 29 ± 3%; p<0.001). ME exhibited the least magnitude of adaptation (EO: 4.1±0.1Δ; MO: 4.5 ± 0.20Δ; ME 2.2 ± 0.1Δ; p<0.001) compared to MO and EO.Comparison between +2D&-2D adds: Both refractive groups with orthophoria show significantly (p<0.001) greater magnitude and PC with +2D compared to -2D add. An opposite pattern was found in ME where greater magnitude and PC was observed with -2D compared to +2D add (p<0.001).
All groups showed asymmetric phoria adaptation to +/- 2D adds. The asymmetry was reversed with myopia and esophoria but not myopia alone. Myopia was associated with a larger induced phoria for both add types and reduced adaptation to +2D but not -2D adds compared to EO. The capacity to adapt to near adds is dependent upon both the refractive state and phoria.(1)Sreenivasan et al, Invest Ophthalmol Vis Sci 2007;48:E-abstract 1006.
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