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V. Sreenivasan, R. Suryakumar, E. L. Irving, W. R. Bobier; Binocular Adaptation to Near Addition Lenses Differs Between Myopic Children and Emmetropes. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1006.
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We previously reported that adults viewing through +2D lenses at near exhibited the presence of tonic vergence adaptation that enhances convergence and reduces convergence driven accommodation1. This study compares the responses of myopic school aged children with emmetropes.
15 myopes (11 ±1.6 yrs; SE: -1.95 ± 1.97D); 13 emmetropic children, EMM (11 ± 2.3 yrs; SE: 0.6 ± 0.12D) and 11 emmetropic adults (23.2 ±2.3 yrs; SE 0.4 ±0.38D) were examined. The myopes were further divided into two groups: Esophores (ME: 3.3 ± 1.7Δ; N=6) and phoria normal’s (MN; -2.8 ± 2.8Δ; N=9) based on their near phoria. All participants fixated a near target at a distance of 33 cm for 20 minutes. The task was then repeated while binocularly viewing through +2D lenses. A PowerRefractor (Multichannel Co) recorded monocular and binocular changes in accommodation at 3, 6, 9, 15 and 20 minute intervals. Phoria measures (modified Thorington) were interleaved between accommodation measures.
Lens induced exophoria was not significantly different between the three children groups (EMM -5.65 ± 2.54Δ; MN 6.48 ± 2.44Δ and ME 6.25 ±1.78Δ; p>0.05). The time constant of exophoria reduction was found to be significantly delayed (P =0.02) in myopic children when compared to their age-matched emmetropic controls (EMM: 1.5Δ/min; MN: 2.3Δ/min; ME 3Δ/min) but the rate of adaptation, though slower in ME was not significantly different compared to the MN group. The gain of tonic vergence adaptation, (magnitude of phoria adaptation after 20 minutes/magnitude of the initial lens induced exophoria) was found to be significantly lower in the both the myopic groups (EMM:0.80; MN:0.64; ME: 0.42; P<0.001 ) with ME group showing significantly lower gain compared with the MN group (p<0.001). The reduction in binocular focus with +2D lenses paralleled the reduction in exophoria (correlation: r > 0.9; P <0.05) in all three groups. In addition, no significant difference (P>0.05) in time constant or gain was observed between the emmetropic children and the adults
Both myopic groups exhibited a longer time course and a reduced magnitude of phoria adaptation following lens viewing compared to the emmetropic controls. Phoria adaptation resulted in a mean close to orthophoria in the esophores but an increased exophoria compared to baseline in the normal phoria group.(1)Sreenivasan et al, Invest Ophthalmol Vis Sci 2006;47: E-abstract 1183.
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