At the completion of this CPD activity, optometrists will have developed their knowledge of axial length measurement in myopia management. Including:
- Understand the role of axial length as a predictor for myopic pathology
- Understand how the rate of axial length growth differs between emmetropes and myopes
- Discern the rationale for the use of axial length growth charts to determine the potential risk of myopia in paediatric patients
- Be acquainted with the main axial length measurement methods.
NOTE: Optomery Australia members can enter their details at the bottom of this article to have it automatically added to their Learning Plan.
Measuring axial length has not traditionally been commonplace in mainstream optometry. But as KIMBERLEY NGU and DR KATE GIFFORD point out, there are good reasons why it may soon become an indispensable part of effective myopia management.
Kimberley Ngu
B Optom
Clinical optometrist – Private practice
Senior optometrist – Royal Perth Hospital
Clinical educator – Myopia Profile Pty Ltd
Dr Kate Gifford
PhD BAppSc(Optom)Hons, GradCertOcTher, FBCLA, FIACLE, FCCLSA, FAAO
Optometrist, professional educator and clinician-scientist
Co-founder and lead educator – Myopia Profile Pty Ltd
Visiting Research Fellow – Queensland University of Technology
Myopia management has come a long way in recent times. More practitioners are now aware that simply prescribing single-vision distance spectacles is insufficient management and strive to do more for their patients. The treatment options available for myopia management include orthokeratology, myopia control contact lenses, spectacle myopia control lenses and low-dose atropine.1 As interest in myopia management grows, so does the interest in monitoring axial length in primary eyecare practitioners.
Axial length (AXL) has been well established as an important measurement for myopia control outcomes in research2 and is now gaining momentum in clinical use. Average AXL at birth is 16.5 mm3 and increases to approximately 23.5 mm in adulthood.4 As the long-term eye health consequences of myopia have been revealed by research, AXL has been found to correlate more closely with future risk of vision impairment due to myopia pathology, than does myopic refraction.5
The ‘line in the sand’ for a notable increase in risk for pathology is around 26 mm – an AXL more than 26 mm is associated with a one in four chance of visual impairment by age 75 years, and in those with an AXL of 30 mm or more, 90% suffer uncorrectable vision impairment by age 75.5
Typically, an AXL of 26 mm equates to approximately 5D of myopia.5 While the correlation between AXL and refractive error is strong, refractive error accounts for only 70% of variation in AXL.5 If we consider refractive error as the sum of power of the optical system, the power of the cornea and lens can influence the overall refractive error of an eye – meaning that a low-to-moderate myope could potentially have a long AXL masked by a low-powered cornea or lens. Hence, AXL is the strongest predictor for myopic pathology and can be several times more sensitive in measuring myopic progression than refraction.2
Axial length growth in emmetropes and myopes
The US-based Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study which studied emmetropic children of various ethnicities between 6 and 14 years old found that AXL growth was on average 0.10 mm/year.6 Specifically, the AXL growth in 6 to 9-year-olds, 9 to 12-year-olds and 11 to 14-year-olds was at a rate of 0.16 mm, 0.08 mm and 0.02 mm per year, respectively. This was corroborated by European data observing emmetropic AXL growth at a rate of 0.10 mm/year until 13 years of age, with AXL growth thereafter “minute and without practical implications”.7
A study of Asian 8-year-old emmetropes noted a growth rate of 0.12 mm ± 0.24 mm a year;8 while another European cohort of 9-year-old emmetropes observed 0.19 mm ± 0.05 mm per year growth in AXL.9 In a study of 12,780 children of various ethnicities, Chinese children were found to have a longer AXL compared to that of European children,10 but these differences only emerged after 9 years of age. Overall, the rule of thumb appears to be that 0.10 mm per year of AXL growth is to be expected in pre-teenage emmetropes.
In contrast, myopes experience greater rates of AXL growth. CLEERE showed that the fastest AXL growth occurred in the year just prior to myopia onset, whereby these future myopes had an annual growth of 0.33 mm, and then went on to progress at a rate of 0.20 to 0.27 mm/year without myopia control interventions.6
Another study in an ethnically-diverse population indicated average myopic growth of 0.30 mm annually, with 13 to 16-year-old progressing myopes increasing AXL at a rate of 0.17 mm/year.11 Tideman et al9 showed similar figures with 0.34 mm/year growth in 9-year-old European myopes, and Rozema et al8 showed annual growth of 0.30 mm/year in 7 to 9-year-old Asian myopes, slowing to 0.20 mm a year in the 12 to 13-year-old cohort. An analysis of the CLEERE data suggests AXL growth of 0.22 mm/year will identify fast myopic progressors, and guide more proactive treatment strategies.12
Whether emmetropes or myopes, data indicates that males have, on average, 0.5 mm longer AXL than females of the same age, with the gender difference emerging around age five and persisting thereafter.7,8 When it comes to myopia stabilisation, the average age whereby myopic eyes ceased growing has been found to be 16.3 years, independent of ethnicity, gender and family history of myopia. The average AXL measurement at stabilisation is 25 mm for females and 25.5 mm for males.11
Figure 1 provides a summary of this data on axial length growth in emmetropes and myopes, based on their age, for clinical reference.
Axial length growth charts
While averages are useful, not every child presenting in practice fits an average. In such instances, percentile growth charts are useful to determine a child’s specific risk compared to their peers and can help monitor the outcomes of myopia management interventions. Growth charts are commonly used in paediatric healthcare, and parents are well-acquainted with their use in plotting a child’s height, weight, and head circumference for example. Hence, they are an easy visual tool. Growth charts for AXL are useful in identifying both the risk of future myopia in adulthood as well as the risk of progression to high myopia.5
The literature, informed by predominantly two studies, currently suggests AXL growth charts to be differentiated by gender and ethnicity. A European chart set has been developed from combining three studies of Dutch and UK data of almost 13,000 individuals9 and an Asian chart set has been developed from a similar volume of data from Chinese children.10
In both cases, separate charts exist for males and females. In comparing the two studies, the ethnicity differences appear to emerge from around age 9.
Figure 2 from the open-access paper of Tideman et al 20189 shows the European data, with the central dark line indicating the 50th percentile, and risk rates overlayed on the right side of each chart showing the risk of myopia and high myopia by adulthood for each AXL centile line provided.
NOTE: For a better view of Figure 2, click this link.
Growth charts for AXL can be used to indicate risk of myopia or high myopia, as described above, as well as directing treatment strategies and gauging treatment outcomes. The Dutch research group who developed the European charts report now using these to identify proactivity of treatment. Children on the 75th percentile, who are at risk of high myopia, are prescribed 0.5% atropine treatment (along with photochromic, progressive addition spectacles with a +3.00 Add to manage side effects) while children with lower centiles are prescribed treatments with a lower side effect profile, such as optical or low-concentration atropine interventions. Treatment was deemed successful when the AXL percentile reduced over time.13
Myopia control studies and axial length
Reporting axial length outcomes in studies of myopia control interventions is considered the gold standard, as measurement by optical interferometry instruments is up to seven times more sensitive to change than cycloplegic refraction.2 Axial length outcomes also provide an accurate gauge of outcomes in orthokeratology and atropine myopia control studies, where refraction is intentionally altered and influenced by the treatment.14
A large-scale analysis of all myopia control interventions published up until 2020 indicated that orthokeratology and soft multifocal contact lenses reduced axial progression by around 0.2 mm in the first year of treatment and around 0.3 mm total after two years of treatment.14 The newest myopia controlling spectacles appear to follow these same trends,15,16 as does one study on 0.05% atropine17 and another on 0.02% atropine.18
This has led to the conclusion that “no single method of treatment shows clear superiority with the best of orthokeratology, soft multifocal contact lenses, spectacles and atropine showing similar effect”.14 This simplifies both treatment selection and expected average outcomes in myopia management, encouraging eyecare practitioners to commence treatment based on additional considerations such as “their own skill set, preferences of parents and children, ability of the child to adapt to the treatment, as well as availability of product and regulatory considerations”.14
Methods of axial length measurement
There are various instruments available to measure AXL and these can be divided into three main categories according to the measurement method: interferometry (or optical biometry), ultrasound, and OCT measurement.
Interferometry
Interferometry is a non-contact, optical biometry measure. This technology is routinely used by ophthalmologists to measure cornea curvature and axial length to enable intraocular lens power calculations for cataract surgery. It is the gold standard technique for axial length measurement in myopia control research2 and is now gaining momentum for use in myopia management by primary eyecare practitioners.
Instruments are now being released specifically for myopia management including additional functionality such as autorefraction, keratometry, corneal topography and/or pupillometry within the single instrument, plus software interfaces to chart outcomes. Optical biometry measurement is fast, non-contact, has a high repeatability and reliability, and is easy to perform which is an advantage for use in children.2
A-scan ultrasound
An older method to measure AXL is A-scan ultrasound. This involves anaesthetising the eye and applanating a probe onto the cornea. Its accuracy is more user-dependent and has a steeper learning curve in getting consistent applanations for repeatable AXL measures. It can also be intimidating for some children to have probe contact with the eye. In the context of myopia, when compared to interferometry, ultrasound gives a resolution of 0.30D while the former can measure up to 0.03D resolution, making ultrasound AXL measurement not any more accurate to track myopia progression than cycloplegic refraction.2
Some new instruments use optical coherence tomography (OCT) to measure AXL. The advantage of using OCT-based systems is that the whole eye can be visualised while doing measurements. From the standpoint of repeatability and accuracy, it is equivalent to interferometry instruments and the differences between the two systems have been reported as clinically insignificant.19
Using axial length measures in clinical practice
Measuring AXL in children can provide indication of risk of myopia and high myopia, supporting clinical decision-making and communication. Understanding a myopic patient’s AXL as a single measure provides an indication of their ocular disease risk across their lifetime. Even in myopic adults, knowing their AXL can help to direct frequency of ocular health review, where ‘high myopia’ defined by AXL (≥ 26mm) rather than refraction indicates necessity for annual retinal examination through dilated pupils.1
As a repeated measure, AXL provides a sensitive gauge of myopia progression, helping to track outcomes in line with both averages in research studies as well as outcomes specific to that individual. While a lack of access to axial length measurement should not be a barrier to eyecare practitioners commencing myopia management,1 it is likely to become an increasingly valuable measure and accepted standard over time.
References
1. Gifford KL, Richdale K, Kang P, Aller TA, Lam CS, Liu YM, Michaud L, Mulder J, Orr JB, Rose KA, Saunders KJ, Seidel D, Tideman JWL, Sankaridurg P. IMI – Clinical Management Guidelines Report. Invest Ophthalmol Vis Sci. 2019 Feb 28; 60 (3): M184-M203.
2. Wolffsohn JS, Kollbaum PS, Berntsen DA, Atchison DA, Benavente A, Bradley A, Buckhurst H, Collins M, Fujikado T, Hiraoka T, Hirota M, Jones D, Logan NS, Lundström L, Torii H, Read SA, Naidoo K. IMI – Clinical Myopia Control Trials and Instrumentation Report. Invest Ophthalmol Vis Sci. 2019 Feb 28; 60 (3): M132-M160.
3. 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. Invest Ophthalmol Vis Sci. 2007; 48 (12): 5454-5458.
4. Meng W, Butterworth J, Malecaze F, Calvas P. Axial length of myopia: a review of current research. Ophthalmologica. 2011; 225 (3): 127-34.
5. Tideman JW, Snabel MC, Tedja MS, van Rijn GA, Wong KT, Kuijpers RW, Vingerling JR, Hofman A, Buitendijk GH, Keunen JE, Boon CJ, Geerards AJ, Luyten GP, Verhoeven VJ, Klaver CC. Association of Axial Length With Risk of Uncorrectable Visual Impairment for Europeans With Myopia. JAMA Ophthalmol. 2016 Dec 1; 134 (12): 1355-1363.
6. Mutti DO, Hayes JR, Mitchell GL, Jones LA, Moeschberger ML, Cotter SA, Kleinstein RN, Manny RE, Twelker JD, Zadnik K; CLEERE Study Group. Refractive error, axial length, and relative peripheral refractive error before and after the onset of myopia. Invest Ophthalmol Vis Sci. 2007 Jun; 48 (6): 2510-9.
7. Fledelius HC, Christensen AS, Fledelius C. Juvenile eye growth, when completed? An evaluation based on IOL-Master axial length data, cross-sectional and longitudinal. Acta Ophthalmol. 2014 May; 92 (3): 259-64.
8. Rozema J, Dankert S, Iribarren R, Lanca C, Saw SM. Axial Growth and Lens Power Loss at Myopia Onset in Singaporean Children. Invest Ophthalmol Vis Sci. 2019 Jul 1 ;60 (8): 3091-3099.
9. Tideman JWL, Polling JR, Vingerling JR, Jaddoe VWV, Williams C, Guggenheim JA, Klaver CCW. Axial length growth and the risk of developing myopia in European children. Acta Ophthalmol. 2018 May; 96 (3): 301-309.
10. 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 May; 257 (5): 1045-1053.
11. Hou W, Norton TT, Hyman L, Gwiazda J; COMET Group. Axial Elongation in Myopic Children and its Association With Myopia Progression in the Correction of Myopia Evaluation Trial. Eye Contact Lens. 2018 Jul;44(4):248-259.
12. Hernández, J., Sinnott, L., Brennan, N., Cheng, X., Zadnik, K., & Mutti, D. Analysis of CLEERE data to test the feasibility of identifying future fast myopic progressors. Invest Ophthalmol Vis Sci. 2018; 59: E-abstract 3388.
13. Klaver C, Polling JR; Erasmus Myopia Research Group. Myopia management in the Netherlands. Ophthalmic Physiol Opt. 2020 Mar; 40 (2): 230-240
14. Brennan NA, Toubouti YM, Cheng X, Bullimore MA. Efficacy in myopia control. Prog Retin Eye Res. 2021 Jul; 83: 100923.
15. Lam CSY, Tang WC, Tse DY, Lee RPK, Chun RKM, Hasegawa K, Qi H, Hatanaka T, To CH. Defocus Incorporated Multiple Segments (DIMS) spectacle lenses slow myopia progression: a 2-year randomised clinical trial. Br J Ophthalmol. 2020 Mar; 104 (3):3 63-368. doi: 10.1136/bjophthalmol-2018-313739.
16. Bao J, Yang A, Huang Y, Li X, Pan Y, Ding C, Lim EW, Zheng J, Spiegel DP, Drobe B, Lu F, Chen H. One-year myopia control efficacy of spectacle lenses with aspherical lenslets. Br J Ophthalmol. 2021: 318367.
17. Yam JC, Jiang Y, Tang SM, Law AKP, Chan JJ, Wong E, Ko ST, Young AL, Tham CC, Chen LJ, Pang CP. 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 Jan; 126 (1): 113-124.
18. Cui C, Li X, Lyu Y, Wei L, Zhao B, Yu S, Rong J, Bai Y, Fu A. Safety and efficacy of 0.02% and 0.01% atropine on controlling myopia progression: a 2-year clinical trial. Sci Rep. 2021 Nov 15; 11 (1): 22267.
19. Wylęgała A, Bolek B, Mazur R, Wylęgała E. Repeatability, reproducibility, and comparison of ocular biometry using a new optical coherence tomography-based system and another device. Sci Rep. 2020 Sep 2; 10 (1): 14440.
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