Axial length (AXL) has been well established as the critical measurement in examining the progression and control of myopia. It is accepted as the gold standard in understanding efficacy of myopia control treatments and, as a clinical measure, it could be up to 10 times more sensitive to detect myopia progression than refraction.1 AXL also appears to be the key risk factor for lifelong myopia pathology; more so than refraction.2
However, most eye care practitioners don’t routinely measure AXL in clinical practice, mainly due to a lack of access to the instrumentation and its expense. How necessary is AXL in myopia management? How accurate is it and how can it be used to direct treatment strategies? The following translates research into clinical practice, to answer these, and several more, crucial questions on AXL.
The average axial length of an emmetropic eye is 16.5mm at birth3 and increases to around 23.5mm in adulthood.4 However outside that average, longer eyes are at greater lifelong risk of myopia associated pathologies and resultant vision impairment.
A population analysis of outcomes of more than 15,000 individuals found that an AXL of 26mm or more was associated with a one-in-three chance of vision impairment by age 75; and AXL of 30mm or more was associated with a 90% frequency of vision impairment. The same analysis found that while there was a strong correlation between AXL and spherical equivalent refraction, the latter explained about 70% of the variation in AXL. When both were considered in a model of risk, AXL maintained the significant association with visual impairment, while spherical equivalent did not.2
As a single indicator of disease risk in myopes of all ages, AXL is more robust than refraction
It can be useful to consider AXL like intraocular pressure (IOP). Increased IOP, like increased AXL, brings with it a greater risk of pathology and consequent vision impairment. While the relationship is clear across a population, there is no guarantee that an AXL of 26mm will lead to pathology. Similarly, higher IOP means more risk of vision impairment from glaucoma, but complicating the picture is normal tension glaucoma, or ocular hypertension, which may not advance to glaucoma.
In what could be considered ‘borderline’ elevated IOP or elongated AXL, the clinical measure is an imperfect diagnostic marker, but it is the marker which can be measured and ideally controlled with intervention. In the aforementioned population analysis, 26mm or greater AXL delineated a significant increase in vision impairment risk, and 26mm was equivalent to around 5D of myopia.2 The low-to-moderate myope though, cannot be presumed to have an AXL below 26mm, as variation exists due to the individual’s cornea and crystalline lens power. As a single indicator of disease risk in myopes of all ages, AXL is more robust than refraction – an annual retinal examination through dilated pupils may not be as necessary for a 4D myope with a 24.5mm AXL as it is for a 4D myope with a 26mm AXL.
IS AXL MORE ACCURATE THAN REFRACTION?
There are two main techniques for measuring AXL – ultrasonography (A-scan), which requires contact with the cornea; and laser interferometry, which is a non-contact, optical method of measurement. In calculating intraocular lens (IOL) powers for cataract surgery, one study found A-scan measured a consistently shorter AXL by 0.20 ± 0.24mm compared to interferometry, due to corneal indentation when using the former. This led to an over-estimated IOL power of 1.01 ± 0.96D.5
In application to myopia, the International Myopia Institute (IMI) Clinical Trials and Instrumentation Report1 explains that ultrasonography is limited in resolution to about 0.30D, whereas interferometry measurements have resolution of around 0.03D, making this technique an order of magnitude better as a measure of myopia progression. Cycloplegic autorefraction, by comparison, has a repeatability of ±0.21D.6
Even in a research setting, the IMI agreed that refractive error should be used in conjunction with AXL measurement to evaluate success of treatments.1 They noted that subjective refraction can be more variable, however AXL is not the only ocular component to change throughout childhood – the cornea and crystalline lens are also key anatomical sites of refractive change and contribution. Refraction provides a summary measurement of all ocular structures, and is routinely and universally measured for all myopes.
HOW WELL DOES AXL RELATE TO REFRACTION?
A variety of papers have shown that this relationship is complex. In the threeyear MiSight study,7 the correlation became stronger as the children got older – meaning the relationship was more variable in younger ages. Across the study, they found that a 0.1mm change in AXL corresponded to a 0.24D change in myopia, in both treatment and control groups, giving a ratio of 2.40D/mm. In the newly published BLINK study, investigating distance centred multifocals for myopia control,8 this ratio was 1.44D/mm in the high add (+2.50) multifocal contact lens wearing group, 1.55D/mm in the medium add (+1.50) group, and 1.63D/mm in the single vision contact lens group. This was despite similar age ranges, ethnicities and methodologies in both studies. While it’s not known if these ratios are statistically different, it highlights that there’s still a lot to learn about this ratio; with a large part of the complexity due to the variability in measurement of both refraction and AXL.
The likely future of judging the success of a myopia control strategy based on AXL will fall to percentile growth charts
HOW MUCH CHANGE IN AXL INDICATES MYOPIA PROGRESSION?
Normal physiological growth of the eye of around 0.1mm per year can be expected in children throughout their emmetropisation period. By comparison, progressing myopes may show 0.1–0.3mm per year additional axial elongation than their emmetropic counterparts, with increased growth evident up to three years before myopia onset and the largest difference in the year just prior to onset.9 Axial elongation varies by ethnicity, with Asian children showing a 44% greater rate of elongation than non-Asian children.10 This is in line with meta-analyses of refraction which also show greater refractive myopia progression in Asian children.11
The greatest variation in axial elongation is due to age. Younger myopes progress more quickly in terms of both refraction and AXL. For example, a study which evaluated AXL elongation by age in an ethnically diverse group (46% White, 26% African American, 15% Hispanic, 8% Asian, 5% Mixed) found that over the three-year study, axial elongation was as follows:12
- 1.08mm in children who were six to seven years old at baseline,
- 0.82mm in eight year olds,
- 0.68mm in nine year olds,
- 0.57mm in 10 year olds, and
- 0.45mm in 11 year olds.
This all makes using AXL as a single measure of myopia progression much more complex than the evident change in refraction. Put simply, if a myopic child’s AXL increases by 0.1mm in a year, this likely represents a halting of myopia progression, because the change is similar to what is expected in the normal emmetropisation process. If a myopic child’s AXL increases by more than 0.1mm in a year, this likely indicates some myopia progression – how much progression, and how this compares to expected efficacy of a myopia control intervention, is still being determined by science.
HOW CAN AXL BE USED TO JUDGE SUCCESS OF A MYOPIA CONTROL STRATEGY?
Studies of myopia control interventions generally return a percentage control rate to indicate how much the treatment has slowed the axial growth and refractive change compared to a control group. The difficulty with percentages, as applied to clinical practice, are numerous. Firstly, a percentage efficacy is not easily compared between studies. The duration of the study, the age of the control group and the type and methodology of the outcome measures all influence the final percentage outcome. Secondly, a percentage efficacy cannot necessarily be applied or expected across the potential several years of myopia control treatment, as most studies exhibit results of one to three years duration and most show the greatest absolute effect in the first year of treatment.13,14
Research is now moving towards understanding an absolute treatment effect of an intervention – the total cumulative effect of a treatment over time.13 At this stage, research is indicating that the total treatment effect possible, based on modelling of published studies (the longest being an orthokeratology study of seven years duration), is a final reduction in axial length of 0.47mm.13 This is likely equivalent to a little over 1D, and while a reduction in final myopia of 1D doesn’t sound impressive from a refractive point of view, analysis has shown that 1D less final myopia can reduce the lifelong risk of myopic maculopathy by 40%.15 As this is clarified, it will help clinicians understand how to appropriately discuss the efficacy of myopia control treatments.
There is no doubt that measurement of AXL will provide the more accurate indicator of myopia progression and control
The likely future of judging the success of a myopia control strategy based on AXL will fall to percentile growth charts. These have already been published for Dutch children16 and Chinese children,17 with variations in age, gender and ethnicity. Authors of the Dutch percentile growth charts emphasised that AXLs, on the 75th percentile or higher, are at risk of high myopia, and hence greater risk of vision impairment. These same authors recently reported using the 75th percentile delineator to determine individuals who would be treated with high dose atropine (0.5%). On follow-up every six months, the AXL was measured and plotted on the growth charts, with a reduced percentile result for an individual indicating successful treatment. The authors reported that plotting this “visualisation of the reduction in axial length percentile [was] an enormous stimulus for patients to adhere to treatment”.18
In developing these growth charts, the authors highlighted that more datasets would require additional analysis to ensure robustness, with gender and ethnicity both requiring specific consideration.16,17
When attempting to determine the success of a myopia control strategy in practice, most clinicians do not measure AXL and instead rely on refraction. To provide some guidance on how to do this on the basis of short-term changes in refraction, the practitioner education website www. myopiaprofile.com, of which the author is a co-founder, features communication tools and educational blogs. Specifically, the blog Gauging success in myopia management details how to balance analysis of scientific outcomes with clinical communication.
CAN I PRACTICE MYOPIA CONTROL WITHOUT MEASURING AXL?
There is no doubt that measurement of AXL will provide the more accurate indicator of myopia progression and control – as described above, optical biometry methods are likely to be five to 10 times more accurate than refraction.1,6 Moreover, in the case of orthokeratology, when the refraction is intentionally altered, the AXL may be the only parameter that can be used as a gauge of myopia control outcomes. Since the refraction-to-axial-length ratio seems to come unstuck with low dose atropine – where the refractive outcomes outshine the minimal control of AXL19 – knowledge of AXL progression provides the primary measure of myopia control success. This is why AXL is a required measurement for treatment comparisons in myopia control research, especially to enable evaluation across different studies and patient groups.
At this stage, however, AXL measurement is not required to safely and effectively practice myopia management. In the IMI Clinical Management Guidelines, AXL measurement was included as a ‘standard procedure’ but with the caveat that there is currently no established criteria for normal or accelerated axial elongation in a given individual.20 As AXL measurement technology becomes more accessible, and there is increased evidence to support normative and typical myopic patterns for AXL growth across a variety of ethnicities and populations, AXL measurement will become increasingly important in myopia prediction and management. Keep an eye on the research and available equipment, because AXL measurement may become the standard of care in our comprehensive myopia management of the future.
The author extends acknowledgement and thanks to optometrists Cassandra Haines, Kimberley Ngu and Connie Gan for authorship of parts of this source material, as previously published on www.myopiaprofile.com.
Dr Kate Gifford, PhD, BAppSc(Optom)Hons, FAAO is a clinician-scientist and peer educator in private practice in Brisbane. She holds four professional fellowships, almost 80 peer reviewed and professional publications, and has presented over 130 conference lectures around the world, primarily on clinical myopia management. Dr Gifford is the Chair of the Clinical Management Guidelines committee of the International Myopia Institute and lead author on their report. She is a co-founder and director of the education platform Myopia Profile.
Find Out More
• Six questions on axial length measurement in myopia management.
• Case study: Axial length measurement in myopia management – how often and how much change is normal?
• Case study: When you have low myopia and high axial length.
- Wolffsohn JS, Kollbaum PS, Berntsen DA, Atchison DA, Benavente A, Bradley A, et al. IMI – Clinical Myopia Control Trials and Instrumentation Report. Invest Ophthalmol Vis Sci. 2019;60(3):M132-M60.
- Tideman JW, Snabel MC, Tedja MS, van Rijn GA, Wong KT, Kuijpers RW, et al. Association of Axial Length With Risk of Uncorrectable Visual Impairment for Europeans With Myopia. JAMA Ophthalmol. 2016;134(12):1355-63.
- Axer-Siegel R, Herscovici Z, Davidson S, Linder N, Sherf I, Snir M. Early Structural Status of the Eyes of Healthy Term Neonates Conceived by In Vitro Fertilization or Conceived Naturally. Investigative Ophthalmology & Visual Science. 2007;48(12):5454-8.
- Meng W, Butterworth J, Malecaze F, Calvas P. Axial Length of Myopia: A Review of Current Research. Ophthalmologica. 2011;225(3):127-34.
- Goyal R, North RV, Morgan JE. Comparison of laser interferometry and ultrasound A-scan in the measurement of axial length. Acta Ophthalmol Scand. 2003;81(4):331-5.
- Moore KE, Berntsen DA. Central and peripheral autorefraction repeatability in normal eyes. Optom Vis Sci. 2014;91(9):1106-12.
- Chamberlain P, Peixoto-de-Matos SC, Logan NS, Ngo C, Jones D, Young G. A 3-year Randomized Clinical Trial of MiSight Lenses for Myopia Control. Optom Vis Sci. 2019;96(8):556-67.
- Walline JJ, Walker MK, Mutti DO, Jones-Jordan LA, Sinnott LT, Giannoni AG, et al. Effect of High Add Power, Medium Add Power, or Single-Vision Contact Lenses on Myopia Progression in Children: The BLINK Randomized Clinical Trial. JAMA. 2020;324(6):571-80.
- Mutti DO, Hayes JR, Mitchell GL, Jones LA, Moeschberger ML, Cotter SA, et al. Refractive error, axial length, and relative peripheral refractive error before and after the onset of myopia. Invest Ophthalmol Vis Sci. 2007;48:2510-9.
- Brennan N, Cheng X, Toubouti Y, Bullimore M. Influence of age and race on axial elongation in myopic children. Optom Vis Sci. 2018;95(Conference Abstract):180072.
- Donovan L, Sankaridurg P, Ho A, Naduvilath T, Smith ELI, Holden BA. Myopia progression rates in urban children wearing single-vision spectacles. Optom Vis Sci. 2012;89:27-32.
- 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;123(7):977-87.
- Cheng X, Brennan N, Toubouti Y. Modelling of cumulative treatment efficacy in myopia progression interventions. Invest Ophthalmol Vis Sci. 2019;60:10.13140/ RG.2.2.32179.68640.
- Brennan NA, Cheng X. Commonly Held Beliefs About Myopia That Lack a Robust Evidence Base. Eye Contact Lens. 2019;45(4):215-25.
- Bullimore MA, Brennan NA. Myopia Control: Why Each Diopter Matters. Optom Vis Sci. 2019;96(6):463-5.
- Tideman JWL, Polling JR, Vingerling JR, Jaddoe VWV, Williams C, Guggenheim JA, et al. Axial length growth and the risk of developing myopia in European children. Acta Ophthalmol. 2018;96(3):301-9.
- Sanz Diez P, Yang LH, Lu MX, 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):1045-53.
- CW Klaver C, Polling JR, Group EMR. Myopia management in the Netherlands. Ophthalmic and Physiological Optics. 2020;40(2):230-40.
- Yam JC, Jiang Y, Tang SM, Law AKP, Chan JJ, Wong E, et al. Low-Concentration Atropine for Myopia Progression (LAMP) Study: A Randomized, Double-Blinded, Placebo-Controlled Trial of 0.05%, 0.025%, and 0.01% Atropine Eye Drops in Myopia Control. Ophthalmology. 2019;126:113-24.
- Gifford KL, Richdale K, Kang P, Aller TA, Lam CS, Liu YM, et al. IMI – Clinical Management Guidelines Report. Invest Ophthalmol Vis Sci. 2019;60(3):M184-M203.