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Atanu Ghosh, Michael J. Collins, Scott A. Read, Brett A. Davis, Payel Chatterjee; Changes in Axial Length in the Nine Cardinal Gaze Directions over Time in Emmetropes and Myopes. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4457.
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To investigate the changes in axial length occurring during 15° shifts in gaze direction over 5 mins. We also investigated the relative effects of gravity and extraocular muscles on axial length changes at 15° and 25° downward gaze over 5 mins.
Ten emmetropes (mean - 0.31 ± 0.16 D), 10 low myopes (mean - 1.73 ± 0.53 D) and 10 moderate myopes (mean - 4.40 ± 1.30 D) aged from 18 to 30 years were recruited. Axial length measurements were taken from each subject’s left eye through a rotating prism, along the foveal axis, using the Lenstar optical biometer at 0 and 5 mins, in nine cardinal gaze directions, with 15° deviation. For all gaze conditions, the subject fixated on an external distance target with their optimally corrected fellow eye. Therefore in dichoptic view, an image of the external target was visible from the right eye and an image of the Lenstar’s target was simultaneously seen from the left (tested) eye. To further study the relative influence of gravitational force and extra-ocular muscles on the axial length changes in downward gaze, the Lenstar was inclined at 15° and 25° on a tilting table. Measurements of axial length were again performed over 5 mins with two test conditions, i) looking down with head tilt, and ii) looking down without head tilt.
Repeated measures ANOVA revealed a significant influence of gaze angle and time on axial length (p < 0.001), with the greatest axial elongation (group mean 17 ± 8 µm) in the infero-nasal gaze direction (p < 0.001) and a slight decrease in axial length in superior gaze (group mean - 11 ± 15 µm) compared with primary gaze (p < 0.001). Moderate myopes had a significantly greater axial elongation compared to low myopes and emmetropes in the infero-nasal gaze (p < 0.001). We also observed a significant axial elongation in 15° and 25° downward gaze without head tilt. However, there was no change in axial length in downward gaze with head tilt over 5 mins (p > 0.05).
The angle of gaze has a significant short-term effect on eye length, with greatest elongation occurring in the inferior nasal direction (such as typically occurs during reading). The elongation of the eye appears to be due to the influence of the extra-ocular muscles, since the effect was eliminated when head turn was used instead of eye turn.
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