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John Daniel Twelker, Joseph M Miller, Erin M Harvey; Native American schoolchildren tend to emmetropize to the spherical equivalent, regardless of level of astigmatism. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2744.
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A spherical eye has a focal plane located in front of, at, or behind the retina, depending ocular size and shape. The same is true for an astigmatic eye, however, the optics are more complex. There is an anterior and posterior focal plane, separated by the Interval of Sturm which increases with higher levels of astigmatism. We examine the hypothesis that the astigmatic eye emmetropizes to the spherical equivalent (SE), independent of low, moderate, or high levels of astigmatism.
Subjects were 777 participants in one or more NIH/NEI funded studies of visual development in Tohono O’odham children. Previous research has documented a high prevalence of with-the-rule astigmatism in this population. Cycloplegic autorefraction (Retinomax, KPlus or KPlus2, Nikon Inc., Tokyo Japan) was conducted at each study exam. Inclusion criteria included a baseline exam at age 5.5 years or younger, follow-up of at least 3 years, right eye autorefraction confidence ≥ 8, and only data from each child’s baseline and final study exams.
The mean age at the baseline exam was 4.17 years (SD=0.62), and at last examination was 11.33 years (SD= 3.24). Mean follow-up time was 7.15 years (SD=3.15). At baseline, in the right eye, 375 (48%) had low (<1.00 D), 294 (38%) had moderate (1.00 to 2.75 D), and 108 (14%) had high (≥3.00 D) refractive astigmatism.The median SE refractive error differed between refractive astigmatism groups (low=+0.75D, moderate=+1.25D, high=+1.13D, Chi-Square Test, p<0.001). At follow up, the median SE did not differ between groups, (low=−0.13D, moderate=plano, high=−0.13D), and as a group the subjects emmetropized over time regardless of level of astigmatism.
SE refractive error became less hyperopic in low, moderate, and high astigmatic groups over time. At the baseline visit, the anterior focal point was closest to the fovea, while over time the subjects tended to emmetropize to the spherical equivalent in all astigmatic groups.
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