Is Axial Length Growth Significantly Different in Children Based on Ethnicity?

September 1, 2023

By Thomas Naduvilath, Msc Biostatistics, PhD, Head of Biostatistics at BHVI

Cultural and environmental differences between ethnicities, which can be significant and challenging to identify and quantify, could be the underlying reason for ethnic differences in myopia prevalence and progression. 

Photo credit: pondsaksit, Getty Images

Cycloplegic refractive error measurements are clearly preferred for identifying myopia onset in children. Still, axial length using interferometry and swept-source techniques is the preferred metric to assess myopia progression due to its advantage in terms of precision, reliability, and ease of use. Axial length measurements coupled with percentile charts further improve patient awareness and engagement in myopia management.

With the growing popularity of growth charts for axial length, an important question is the impact of ethnicity on axial length growth. Race and ethnicity, which are terms interchangeably used in myopia literature, have been reported as risk factors for myopia due to the higher prevalence and earlier onset of myopia in East Asian countries compared to European populations. Here it must be noted that the factor “race and/or ethnicity” comprises genetic and cultural variations.¹

Regarding the genetic aspects of ethnicity, genetic predisposition to myopia was not found to be different between Asian and European children, and environmental factors appear to explain a greater proportion of myopic risk compared to genetic factors.² On the other hand, cultural and environmental differences between ethnicities, which can be significant and challenging to identify and quantify, could be the underlying reason for ethnic differences in myopia prevalence and progression. 

A Deep Dive into the Research
A meta-analysis of 79 clinical trials showed that axial elongation decreased exponentially with age between 8 and 16 years. Age-specific axial growth was greater in myopic Asian children, between 26% and 34%, compared to myopic non-Asian children.³ Similarly, a meta-analysis of spherical equivalent progression also showed that Asian children progressed significantly more than non-Asian children.⁴ These ethnic differences in progression are likely to reflect regional and population differences, as meta-analyzed studies representing regions have a predominantly high proportion of a specific race/ethnicity, but also represent a more uniform exposure to myopigenic factors. 

Recently reported axial elongation in a large sample of urban Chinese children showed that age-specific axial growth in myopes at 6 years of age was 0.58mm/year, 0.33mm/year for emmetropes, and 0.29mm/year for hyperopes.⁵ In comparison, a cohort of 6-year-old children predominantly of European descent in the Netherlands reported lower axial growth of 0.34mm/year for myopes, 0.19mm/year for emmetropes, and 0.15mm/year for hyperopes.⁶ Their entire sample of myopes and non-myopes had an annualized axial growth of 0.21mm/year between 6 and 9 years, which was similar to a cohort of German children with an estimated annual axial growth of 0.20mm/year for 6-year-olds (Graph Grabber v2).⁷

Overall, similarities in axial growth were also observed between a cohort of children in Germany (0.115mm/year) and Northern Ireland (0.12mm/year).^7,8 Interestingly, a Singaporean sample of myopic children aged 7 to 9 reported an annual change of 0.34mm/year, which is similar to the subsample of myopes in the Netherlands.^6,9 Possible similarities within myopes across regions are likely to reflect children’s exposure to factors associated with myopia rather than the genetic factors of ethnicity.

Analyzing Myopia Progression
If ethnicity, in terms of genetic predisposition, is the cause of variations in axial elongation, then this should appear in studies that compare axial growth between ethnicities within the same region. The COMET study that recruited a large sample of American children across multiple sites showed that axial growth was not different between American children of European, Asian, or Hispanic descent. African American children had significantly lower axial growth than other ethnicities. However, when the group differences were annualized, African American children were lower by only 0.04mm/year in the 6-11 year age group.¹⁰ A more recent analysis of 6-9-year-old children from the Netherlands shows that children of non-European ethnicity were at a greater risk of axial elongation. Still, the difference was only 0.01mm/year and contributed only 5% to the overall risk score.¹¹

In an Australian cohort of urban 12 and 17-year-old children, progression was higher in Asian compared to Caucasian children, but when this was analyzed only in myopes, it was observed that there were no differences between the ethnic groups, suggesting myopigenic factors play a greater role.¹² Data on Singaporean myopic children aged 7 to 9 years also indicated that axial elongation was not different between Chinese and non-Chinese ethnicity children.⁹ Based on available reports, ethnic/racial differences may exist within a region, but it appears that these differences are small when compared to regional differences, which may broadly reflect differences in exposure to risk factors such as educational pressure, indoor environment, and lifestyle factors. 

Implications
Using population-based growth curves for axial elongation is a valuable tool for monitoring myopic and non-myopic children. Still, axial growth data suggests that growth percentile curves cannot be interchanged between regions such as Asian and European countries. Slight ethnic differences within a region are likely to get smaller over time. Myopia management needs to be tailored to suit the patient’s exposure to various myopigenic factors such as younger age, parental myopia, excessive near work, and indoor environment. 

Thomas Naduvilath, Msc Biostatistics, PhD, is the Head of Biostatistics at BHVI and a Conjoint Associate Professor at the School of Optometry and Vision Science, University of New South Wales.

References:

1. Morgan IG, Wu PC, Ostrin LA, Tideman JWL, Yam JC, Lan W, et al. IMI Risk Factors for Myopia. Invest Ophthalmol Vis Sci. 2021 Apr;62(5):3. 
2. Tedja MS, Wojciechowski R, Hysi PG, Eriksson N, Furlotte NA, Verhoeven VJM, et al. Genome-wide association meta-analysis highlights light-induced signaling as a  driver for refractive error. Nat Genet. 2018 Jun;50(6):834–48. 
3. Shamp W, Brennan NA, Bullimore MA, Cheng X, Maynes E. Influence of Age and Race on Axial Elongation in Myopic Children. Invest Ophthalmol Vis Sci. 2022 Jun 1;63(7):257 – A0111. 
4. Donovan L, Sankaridurg P, Ho A, Naduvilath T, Smith EL, A. Holden B. Myopia Progression Rates in Urban Children Wearing Single-Vision Spectacles. Optom Vis Sci. 2012 Jan;89(1):27–32. 
5. Naduvilath T, He X, Xu X, Sankaridurg P. Normative data for axial elongation in Asian children. Ophthalmic Physiol Opt  J Br Coll  Ophthalmic Opt. 2023 Sep;43(5):1160–8. 
6. Tideman JWL, Polling JR, Vingerling JR, Jaddoe VW V., Williams C, Guggenheim JA, et al. Axial length growth and the risk of developing myopia in European children. Acta Ophthalmol. 2018 May;96(3):301–9. 
7. Truckenbrod C, Meigen C, Brandt M, Vogel M, Sanz Diez P, Wahl S, Jurkutat A KW. Longitudinal analysis of axial length growth in a German cohort of healthy children and adolescents. Ophthalmic Physiol Opt. 2021;41(3):532–40. 
8. McCullough S, Adamson G, Breslin KMM, McClelland JF, Doyle L, Saunders KJ. Axial growth and refractive change in white European children and young adults:  predictive factors for myopia. Sci Rep. 2020 Sep;10(1):15189. 
9. Saw SM, Chua WH, Gazzard G, Koh D, Tan DTH, Stone RA. Eye growth changes in myopic children in Singapore. Br J Ophthalmol. 2005 Nov;89(11):1489–94. 
10. Hyman L, Gwiazda J, Hussein M, Norton TT, Wang Y, Marsh-Tootle W, et al. Relationship of age, sex, and ethnicity with myopia progression and axial elongation in the correction of myopia evaluation trial. Arch Ophthalmol. 2005 Jul;123(7):977–87. 
11. Tideman JWL, Polling JR, Jaddoe VW V, 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):127–36. 
12. French AN, Morgan IG, Burlutsky G, Mitchell P, Rose KA. Prevalence and 5- to 6-year incidence and progression of myopia and hyperopia in Australian schoolchildren. Ophthalmology. 2013;120(7):1482–91. 

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