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K. A. Rose, J. Ip, A. Kifley, T. L. Maddess, P. Mitchell, I. G. Morgan, Sydney Childhood Eye Study, Sydney Myopia Study; Al/CR Ratio and Spherical Equivalent in Two Age Samples of Australian School Students. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1535. doi: https://doi.org/.
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To assess the relationship between axial length/corneal radius ratio and spherical equivalent refraction in Year 1 and Year 7 Australian students.
: The Sydney Myopia Study randomly selected 55 secondary schools. All Year 1 and Year 7 students were invited to participate. Cycloplegic autorefraction with 1% cyclopentolate was performed, and axial length (AL) and corneal radius (CR) were measured using the IOLMaster (Carl Zeiss, Germany). Spherical equivalent (SE) was calculated (sphere +½ cylinder) for the right eye only.
Of the 4108 students who participated (77% response rate), 3449 are included in this analysis. The mean refractive error for students in Year 1 (mean age 6.7 years) was +1.24D and in Year 7 (mean age 12.7 years) was +0.49D. The mean axial length was longer in the older children (Year 1 22.61mm, Year 7 23.38mm), but there was no significant difference in mean CR (Years 1 and 7, 7.78mm). The distribution of refractive error was highly kurtotic for both the Year 1 and Year 7 students, suggesting that active matching of the axial length of the eye to optical power of the cornea had taken place before Year 1. Plots of SE against the ratio of AL to CR show a triphasic relationship in the Year 1 sample. A similar pattern could also be seen in the Year 7 sample, although the fit was not as good. For refractions >+2.00D and <+0.50D, SE decreased by -2.7D for every millimeter increase in AL. However, in the intermediate zone, SE decreased by only 0.5D per millimeter increase in AL.
These results suggest that there is a tightly coordinated phase of refractive development, in which changes in some optical components effectively oppose emmetropisation, maintaining a state of mild hyperopia in which clear vision can be achieved through accommodation, despite continued axial elongation. Given the evidence in the literature for reductions in lens power with age, this seems the most likely basis for such tightly coordinated compensation.
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