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Fiona E Cruickshank, Nicola S Logan; Does optical theory predict the observed reduction of the refractive error:axial length ratio with increasing axial length?. Invest. Ophthalmol. Vis. Sci. 2017;58(8):2400.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the variance in the correlation between refractive error and axial length in a cross-sectional, multi-racial sample of school children and young adults.
Right eye refractive error and axial length data were collected on 343 six-to-seven-year-old children (mean 7.18 (SD 0.35) years), 294 12-to-13-year-old children (mean 13.12 (SD 0.32) years) and 123 young adults aged 18-to-25-years (mean 20.56 (SD 1.91) years). The ethnic variation of the participants was as follows: British Asian 56.4% (95% confidence interval [CI], 52.9-59.9), white 24.7% (95% CI, 21.6-27.8), black 11.3% (95% CI, 9.1-13.6), mixed 4.2% (95% CI, 2.8-5.6), other 2.1% (95% CI, 1.1-3.1), and East Asian 1.2% (95% CI, 0.4-2.0). Distance refractive error was measured with the Shin-Nippon NVision-K 5001 infrared open-field autorefractor. Child participants were cyclopleged prior to data collection (1% Cyclopentolate Hydrochloride). Myopia was defined as a mean spherical equivalent [MSE] ≤-0.50D. Axial length [AXL] was measured using the Zeiss IOLMaster 500.
There was a myopic shift in mean MSE with age (six-to-seven +0.87D, 12-to-13 -0.06D and 18-to-25 -1.41D), associated with an increase in mean AXL (six-to-seven 22.70mm, 12-to-13 23.49mm and 18-to-25 23.98mm). There was a significant negative correlation between MSE and AXL for all age groups (all p <0.005). The correlation coefficient increased with age (six-to-seven rs -0.37, 12-to-13 rs -0.48 and 18-to-25 rs -0.68), with 1mm of axial expansion having a greater effect on refractive error in children rather than an adult population (six-to-seven -3.58D/mm, 12-to-13 -3.10D/mm and 18-to-25 -2.49D/mm). MSE:AXL ratios for participant data were compared with the ratio generated from Gullstrand model eyes (p <0.001). Linear regression adjusted for age showed a 1mm increase in AXL for a 0.05 decrease in MSE:AXL ratio (p < 0.001).
Optical theory predicts that there will be a reduction in the MSE:AXL ratio with longer eyes. The participant data although adhering to this theory show a reduced effect, with eyes with longer axial lengths having less myopia per unit of axial expansion than predicted by model eye calculations. These differences need to be considered when assessing the efficacy of myopia control interventions based on axial length changes as the principal outcome measure in clinical trials.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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