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B.–C. Jiang, S. Bussa; Changes in Vergence Responses When Viewing Through Near Addition Lenses Is Related to the Subject’s AC/A Ratio . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3674.
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© ARVO (1962-2015); The Authors (2016-present)
We previously reported that when a subject viewed an object through a +2 D addition lens, the accommodative response showed a lead and the near phoria shifted to an exo–phoric direction. The purpose of this study was to verify whether the phoria shift was related to the subject’s AC/A ratio.
Thirty subjects (10 emmetropic, 18 myopic, and 2 hyperopic) participated in the study. Accommodative responses and phorias with and without addition lenses (+2 D) at three working distances (33, 40, and 50 cm) were measured. The phoria was determined by the alternating cover test with prism neutralization and the accommodative response was determined with a Canon R–1 infrared refractor under binocular viewing conditions. For each subject, the phoria and accommodative response data at three working distances measured without addition lenses were used to compute the subject’s AC/A ratio. The phoria shifts caused by the addition lenses were plotted vs. the AC/A ratios for all subjects at three working distances, respectively. Regression analysis was applied to each plot to find the correlation between the subject’s AC/A ratio and the phoria shift.
The average AC/A ratio was 5.84 ± 1.97 (S.D.) (pd/D) and the averages of the phoria shift when the subjects viewed through the addition lens at three working distances, 50, 40, and 33 cm, were –4.9 ± 1.9, –6.4 ± 1.9, and –7.6 ± 2.4 pd, respectively. The corrections between the phoria shift and AC/A ratio were –0.43 (p = 0.017), –0.45 (p = 0.013), and –0.41 (p = 0.023) for the three distances, respectively.
The results suggest that the subject with a high AC/A ratio might have a more exo–phoric shift than the subject with a low AC/A ratio when both of them viewed through the addition lens. In addition, the nearer the viewing distance, the larger the exo–phoric shift. Based on these results, we suggest that when PALs are prescribed to slow myopia progression, one needs to consider not only the subject’s accommodative response, but also the vergence and the interaction between accommodation and vergence.
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