Using axial length and corneal curvature percentile charts to predict myopia

Paper title: Normative data and percentile curves for axial length and axial length/corneal curvature in Chinese children and adolescents aged 4-18 years

Authors: Xiangui He (1,2,3), Padmaja Sankaridurg (4,5), Thomas Naduvilath (4,5), Jingjing Wang (1,2,3), Shuyu Xiong (1,2,3), Rebecca Weng (4), Linlin Du (1,2,3), Jun Chen (1,2,3), Haidong Zou (1,2,3), Xun Xu (1,2,3)

1. Shanghai Eye Disease Prevention and Treatment Center, Shanghai Eye Hospital, Shanghai Vision Health Center & Shanghai Children Myopia Institute, Shanghai, China.
2. Department of Ophthalmology, Shanghai General Hospital, Shanghai Jiao Tong University, Center of Eye Shanghai Key Laboratory of Ocular Fundus Disease, Shanghai, China.
3. National Clinical Research Center for Eye Diseases, Shanghai, China.
4. Brien Holden Vision Institute, Sydney, New South Wales, Australia p.sankaridurg@bhvi.org drxuxun@sjtu.edu.cn.
5. School of Optometry and Vision Science, University of New South Wales, Sydney, New South Wales, Australia.
6. Brien Holden Vision Institute, Sydney, New South Wales, Australia.
7. Shanghai Eye Disease Prevention and Treatment Center, Shanghai Eye Hospital, Shanghai Vision Health Center & Shanghai Children Myopia Institute, Shanghai, China

Date: Feb 2023

Reference: 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. 2023 Feb;107(2):167-175. [Link to open access paper]

Myopia and myopic progression are commonly established and monitored with refractive error assessment. Increasing myopia is principally caused by axial elongation and including axial length data as part of an examination, particularly in relative to corneal curvature, can provide additional information on an individual child's risk for myopia development and progression.

The purpose of this study was to develop age-specific and gender-specific axial length (AL) and axial length/corneal radius of curvature (AL/CR) percentile curves with the aim of estimating refractive error (RE) and probability of myopia for Chinese children.

This was a retrospective analysis using baseline data of AL (IOL Master), cycloplegic refraction and CR from 3 studies involving 14,127 healthy Chinese children aged 4 to 18yrs.

• The AL/CR ratio was calculated as the ratio of AL to the average of the flattest and steepest corneal curvatures.
• Percentile curves for AL and AL/CR were estimated for age and gender, and individual probabilities for myopia for specific ages and genders were calculated.

The accuracy of the curves was assessed by comparison to an independent validation data sample of 5,742 eyes of children aged 7 to 10yrs. The validation data had also been determined by cycloplegic refraction and IOLMaster.

The percentile curve prediction values for AL and AL/CR were compared to 1-year follow-up data from the validation sample to assess the difference in the predicted and actual values.

The results showed that:

• The mean averages for the baseline data were found to be SE -0.76D, AL 23.7mm, corneal curvature of 7.85mm and AL/CR ratio of 3.03.
• As age increased, the values for AL, myopic refractive error and AL/CR ratios also increased. The AL and AL/CR curves also aligned closely with the validation sample.
• Year-on-year changes were seen across all age groups other than <10^th centiles from 15yrs
• Compared to boys, girls had shorter AL, steeper corneal curvatures, higher myopic refractive error and a slightly lower AL/CR ratio. For a given AL and AL/CR value, girls were also more myopic
• The AL/CR percentiles were able to predict myopic refractive error more accurately than AL-only percentiles for all ages
• There was a higher risk of myopia from age 10yrs for both genders. Higher percentile positions gave a greater risk again, even with younger age.

• The chances of myopia increased from age 10yrs for both boys and girls, and higher percentiles demonstrated a higher risk of myopia, even for younger ages
• Axial length and the ratio of axial length to corneal curvature (AL/CR) were found to be dependent on age and gender. Having this reference percentile data in clinical settings can help to identify the risk of myopia development and progression, as well as providing for monitoring over time.
• In screening for myopia risk from these biometric measurements, using data based on the AL/CR ratio appears to increase the accuracy compared to axial length measures alone.

1. For a given AL/CR ratio, girls were found to have shorter AL, steeper corneal curves and higher myopia than boys. This difference was more apparent at onset of myopia. Where myopia was detectable at a cut-off AL/CR ratio of 3.0, female eyes were still more myopic than boys, suggesting that a shorter AL wasn't compensating for the myopic shift. Further research would explain why this seems to be the case for girls, and if differences in crystalline lens power may play a role.^1,2
2. There are some limitations to using percentile curves: there are no absolute values for direct comparison between eyes and individuals, and factors other than age or gender could be involved. For example, developing percentile curves based on country-specific data could increase the accuracy and relevance to different ethnicities. This study only included data from children living in China.
3. None of the children in the study had undergone myopia control, and the data was not longitudinal. Although this provides valuable baseline data to construct percentile curves, it also highlights the potential for future research into expected trajectories following different myopia control methods.

Title: Normative data and percentile curves for axial length and axial length/corneal curvature in Chinese children and adolescents aged 4-18 years

Authors: Xiangui He, Padmaja Sankaridurg, Thomas Naduvilath, Jingjing Wang, Shuyu Xiong, Rebecca Weng, Linlin Du, Jun Chen, Haidong Zou, Xun Xu

Purpose: To develop age-specific and gender-specific reference percentile charts for axial length (AL) and AL/corneal radius of curvature (AL/CR) and, to use percentiles to determine probability of myopia and estimate refractive error (RE).

Methods: Analysis of AL, cycloplegic RE and CR of 14 127 Chinese participants aged 4-18 years from 3 studies. AL and AL/CR percentiles estimated using Lambda-Mu- Sigma method and compared for agreement using intraclass correlation (ICC). Logistic regression was used to model risk of myopia based on age, gender, AL and AL/CR percentiles. Accuracy of AL progression and RE estimated using percentiles was validated using an independent sample of 5742 eyes of children aged 7-10 years.

Results: Age-specific and gender-specific AL and AL/ CR (3rd, 5th, 10th, 25th, 50th, 75th, 90th and 95th) percentiles are presented. Concordance between AL and AL/CR percentiles improved with age (0.13 at 4 years to >0.75 from 13 years) and a year-to- year change was observed for all except <10^th percentile from 15 years. Increasing age, AL and AL/CR was associated with a more myopic RE (r2=0.45,0.70 and 0.83, respectively). The sensitivity and specificity of the model to estimate probability of myopia was 86.0% and 84.5%, respectively. Estimation of 1-year change in AL using percentiles correlated highly with actual AL (ICC=0.98). Concordance of estimated to actual RE was high (ICC=0.80) and within ±0.50D and ±1.0D of actual RE for 47.4% and 78.9% of eyes, respectively.

Conclusions: Age-specific and gender-specific AL and AL/ CR percentiles provide reference data, aid in identifying and monitoring individuals at risk of myopia and have utility in screening for myopia. AL/CR percentiles were more accurate in estimating probability of myopia in younger children.

[Link to open access paper]