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Vidhyapriya Sreenivasan, Elizabeth L. Irving, William R. Bobier; Differences between phoria and vergence adaptation in myopic children with esophoria . Journal of Vision 2009;9(14):76. doi: 10.1167/9.14.76.
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We previously showed that myopic children with normal near phorias exhibit reduced phoria adaptation compared to emmetropes (V Sreenivasan, EL Irving, WR Bobier, 2009). Here, we report the adaptive behavior of myopic esophores when they sustain near fixation without and with +2D adds. Eleven emmetropic phoria-normals (EMM; 0–8△exo) and 8 myopic-esophores (ME; 2–8eso) between 7–14 years fixated a high-contrast near target (33 cm) with and without +2D lenses for 20 minutes. Phoria adaptation (PA) was quantified through changes in near phoria, measured using the modified Thorington technique. Tonic vergence adaptation (TVA) was determined by measuring tonic vergence (distance phoria through 0.5mm pupils) before and after the near-task. Accommodative adaptation was evaluated by measuring pre and post-task tonic accommodation using a 0.5 cpd difference of Gaussian target. No add condition: The mean baseline near phoria was 4.48 (eso) ±0.57△ in ME and −3.54 (exo) ±0.86△ in EMM. Following sustained fixation, the near phorias significantly (p<0.05) declined in both groups (EMM: Exo reduced by 1.09±0.3△; ME: Eso reduced by 2.19±0.49△). Tonic vergence showed a convergent shift (p<0.05) post-task in both groups (EMM: 1.54±0.6△; ME:1.16±0.52△). Accommodative adaptation was greater in ME compared to EMM (EMM: 0.18±0.12D; ME:0.47±0.10D). ME showed higher response-AC/A ratios than emmetropes (EMM:3.76±0.48△/D; ME:6.45±0.89△/D; p<0.01). Add condition: +2D lenses caused a divergent shift in the near phoria (EMM: 4.61±0.33△; ME:6.67±0.58△; p<0.01) in both groups. Sustained fixation significantly (p<0.001) reduced the divergent shift in both groups, but significantly less in the ME (EMM: 4.23±0.13△; ME:1.93±0.16△; p<0.01). Post-task tonic vergence showed a convergent shift in both groups (EMM: 1.10±0.30△; ME:0.54±0.60△) while tonic accommodation showed no change in EMM (0.02±0.08D) and a small increase (myopic shift) in ME (0.25±0.16D). When near fixation is prolonged with and without +2D near adds, TVA moved toward a more convergent position in both groups. However, PA shifted in the opposite direction when ME sustained fixation through their habitual correction. The shift in PA was possibly influenced by the differing accommodative parameters seen in ME. Reducing accommodative activity through near adds reduced this effect. PA can be opposite to the direction of TVA and appears to act primarily to reduce the phoria (orthophorization).
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