Most clinicians understand the common risk factors for myopia development, such as family history of myopia,1 visual environment2 and binocular vision disorders.3 As more population data is added for verification, axial length growth charts are being further developed to determine if a child’s axial length is normal for their age, and to predict future myopia risk based on current axial length. This article explores how we can use the current axial length value, change in axial length, axial length growth charts, and another metric, the axial-length-to-corneal-radius (AL:CR ratio), to predict risk of a child developing myopia.
Predicting myopia from current axial length
A Singaporean study had an interesting finding - regardless of age, myopia tended to onset at a similar axial length of 24.08±0.67mm in boys and 23.69±0.69mm in girls.4 This provides a cut-off for prediction of myopia onset, and can be considered similarly to the refractive cut off of less than +0.75D manifest hyperopia at age 6-7 years indicating risk of future myopia.5
Predicting myopia by change in axial length
Given a natural emmetropization process is occurring during childhood, some axial length growth in children is normal. The amount of growth compared to age can identify children at most risk of myopia onset. The Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) Study, which collected data on children of diverse ethnicities over 11 years, found that those who remained emmetropic still had some axial length growth per year, around 0.1mm per year.6
The CLEERE Study found that axial length growth was fastest in the year just prior to myopia onset, with future myopes growing by 0.33mm on average. This slowed to 0.20-0.27mm per year after the initial myopia onset.6
Predicting myopia with axial length growth charts
Axial length growth charts can be used to predict risk of future onset of myopia, by comparing a child's current axial length to their matched age-normal results. Choosing the right axial length growth chart is important, as gender and ethnicity influence the percentile rankings.7,8 The percentile result, if above 50%, indicates increasing risk of developing myopia and/or high myopia.
Parents understand percentile charts in children's health - these provide a pictorial view of risk when accompanied by your explanation that higher percentiles are not the ideal outcome for childhood eyes, as they might be for childhood height! Learn more in How to Use Axial Length Growth Charts.
Predicting myopia with the AL/CR Ratio
The AL/CR ratio is a measurement which can also help to predict who is going to become myopic. The AL ratio is the comparison of axial length (AL) divided by the corneal radius (CR), both in millimetres (mm). A longer axial length in comparison the corneal radius generally indicates a greater risk of becoming myopic. Large screening studies have found that the AL/CR ratio predicts myopic refraction better than axial length alone, by incorporating the power of the cornea.9
Studies in both Chinese9 and European10 schoolchildren have shown that AL/CR ratios of more than 3 are highly predictive of myopic refractive error. New data has presented AL/CR percentile growth charts11 for Chinese children, concluding that AL/CR percentiles had a higher diagnostic accuracy for myopia than AL percentiles alone, and combining the two increased accuracy further. This was especially the case for children under age 10, where AL/CR ratio percentiles were significantly more accurate for myopia prediction than axial length percentiles alone.
AL/CR ratio measurement evidently requires measurement of both axial length and the corneal radius by either keratometry or corneal topography. Combination instruments specifically for myopia management include measurements of both ocular parameters, allowing for calculation of this ratio and display of percentile chart outcomes. Read more in Choosing an Instrument to Measure Axial Length.
Figure 3 from the open-access paper He et al 202111 entitled Age-specific and gender-specific percentile curves for axial length (AL) and AL/corneal radius (CR) curvature.
Detecting and managing risk of future myopia
Measuring axial length and using this data, as described above, can support detection of children at risk of developing myopia. This can be used in addition to the easily measured refraction risk factor of manifest hyperopia. The following cut points were identified from the CLEERE data as indicating risk of myopia onset by age 13 years.5
|Refraction cut point for future myopia risk (D)|
|Less than +0.75|
+0.50 or less
+0.25 or less
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- Jones LA, Sinnott LT, Mutti DO, Mitchell GL, Moeschberger ML, Zadnik K. Parental history of myopia, sports and outdoor activities, and future myopia. Invest Ophthalmol Vis Sci. 2007 Aug;48(8):3524-32. (link)
- Rose KA, Morgan IG, Ip J, et al. Outdoor activity reduces the prevalence of myopia in children. Ophthalmology. 2008;115(8):1279-1285. (link)
- Mutti DO, Jones LA, Moeschberger ML, Zadnik K. AC/A Ratio, Age, and Refractive Error in Children. Investigative Ophthalmology & Visual Science. 2000;41(9):2469-2478. (link)
- Rozema J, Dankert S, Iribarren R, Lanca C, Saw S-M. Axial Growth and Lens Power Loss at Myopia Onset in Singaporean Children. Investigative Ophthalmology & Visual Science. 2019;60(8):3091-3099. (link)
- Zadnik K, Sinnott LT, Cotter SA, et al. Prediction of Juvenile-Onset Myopia. JAMA Ophthalmology.2015;133(6):683-689. (link)
- Mutti DO, Hayes JR, Mitchell GL, et al. Refractive error, axial length, and relative peripheral refractive error before and after the onset of myopia. Invest Ophthalmol Vis Sci. 2007;48(6):2510-2519. (link)
- Tideman JWL, Polling JR, Vingerling JR, et al. Axial length growth and the risk of developing myopia in European children. Acta Ophthalmol. 2018;96(3):301-309. [Link to Myopia Profile Science Review]
- Sanz Diez P, Yang L-H, Lu M-X, Wahl S, Ohlendorf A. Growth curves of myopia-related parameters to clinically monitor the refractive development in Chinese schoolchildren. Graefes Arch Clin Exp Ophthalmol. 2019;257(5):1045-1053. (link)
- He X, Zou H, Lu L, Zhao R, Zhao H, Li Q, Zhu J. Axial length/corneal radius ratio: association with refractive state and role on myopia detection combined with visual acuity in Chinese schoolchildren. PLoS One. 2015 Feb 18;10(2):e0111766. (link)
- Tideman JWL, Polling JR, Jaddoe VWV, Vingerling JR, Klaver CCW. Environmental Risk Factors Can Reduce Axial Length Elongation and Myopia Incidence in 6- to 9-Year-Old Children. Ophthalmology. 2019 Jan;126(1):127-136. [Link to Myopia Profile Science Review]
- He X, Sankaridurg P, Naduvilath T, Wang J, Xiong S, Weng R, Du L, Chen J, Zou H, Xu X. Normative data and percentile curves for axial length and axial length/corneal curvature in Chinese children and adolescents aged 4-18 years. Br J Ophthalmol. 2021 Sep 16:bjophthalmol-2021-319431. (link)