Optical ‘dampening’ of the refractive error to axial length ratio:implications for outcome measures in myopia control studies


Purpose: To gauge the extent to which differences in the refractive error axial length relationship predicted by geometrical optics are observed in actual refractive/biometric data.Methods: This study is a retrospective analysis of existing data. Right eye refractive error [RX] and axial length [AXL] data were collected on 343 6-to-7-year-old children [mean 7.18 years (SD 0.35)], 294 12-to-13-year-old children [mean 13.12 years (SD 0.32)] and 123 young adults aged 18-to-25-years [mean 20.56 years (SD 1.91)]. Distance RX 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 was measured using the Zeiss IOLMaster 500. Optical modelling was based on ray tracing and manipulation of parameters of a Gullstrand reduced model eye.Results: There was a myopic shift in mean MSE with age (6-7 years +0.87 D, 12-13 years -0.06 D and 18-25 years -1.41 D), associated with an increase in mean AXL (6-7 years 22.70 mm, 12-13 years 23.49 mm and 18-25 years 23.98 mm). There was a significant negative correlation between MSE and AXL for all age groups (all p <0.005). RX: AXL ratios for participant data were compared with the ratio generated from Gullstrand model eyes. Both modelled and actual data showed non-linearity and non-constancy, and that as axial length is increased, the relationship between myopia and axial length differs, such that it becomes more negative.Conclusions: Optical theory predicts that there will be a reduction in the RX: AXL ratio with longer eyes. The participant data although adhering to this theory show a reduced effect, with eyes with longer axial lengths having a lower refractive error to axial length ratio than predicted by model eye calculations. We propose that in myopia control intervention studies when comparing efficacy, consideration should be given to the dampening effect seen with a longer eye.

Publication DOI: https://doi.org/10.1111/opo.12457
Divisions: College of Health & Life Sciences > School of Optometry > Optometry
College of Health & Life Sciences > School of Optometry > Optometry & Vision Science Research Group (OVSRG)
College of Health & Life Sciences
College of Health & Life Sciences > School of Optometry > Vision, Hearing and Language
Additional Information: © 2018 The Authors. Ophthalmic and Physiological Optics published by John Wiley & Sons Ltd on behalf of College of Optometrists. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Funding: College of Optometrists, UK
Uncontrolled Keywords: axial length, emmetropisation, myopia, ocular development, refractive error
Publication ISSN: 1475-1313
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Related URLs: https://onlinel ... .1111/opo.12457 (Publisher URL)
PURE Output Type: Article
Published Date: 2018-05-01
Published Online Date: 2018-04-24
Accepted Date: 2018-03-19
Authors: Cruickshank, Fiona E
Logan, Nicola S (ORCID Profile 0000-0002-0538-9516)



Version: Published Version

License: Creative Commons Attribution

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Version: Accepted Version

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