DOES ONE NEED to measure axial length in myopia management? When investing time and energy to learn about myopia and setting up systems in a clinical practice, should one also invest in the newest tech for diagnosis and tracking?
From the vantage point of the International Myopia Institute (IMI), when the original Clinical Management Guidelines were published, it was important to provide guidance relevant to all settings of eye care (Gifford et al, 2019). Primary eye care varies significantly across the world in terms of use of diagnostic drugs, topical drug prescribing, and access to equipment.
The IMI wrote that “at this point, there are no established criteria for normal or accelerated axial elongation in a given individual.” Axial length measurement was recommended “[w]here available, [and] measurement with a noncontact device is ideal” (Gifford et al, 2019).
From an accuracy perspective, noncontact optical biometry is far superior to A-scan ultrasound measurement. The repeatability of optical biometry is around 0.04mm to 0.05mm in children (Carkeet et al, 2004; Chan et al, 2006), and measurement in 0.01mm steps is equivalent to a resolution of around 0.03D when considering tracking of myopia progression (Wolffsohn et al, 2019).
By comparison, the repeatability of ultrasonography is around 0.2mm to 0.3mm for measures of axial length. With a 0.1mm change in axial length being approximately equivalent to a 0.3D change in refraction, this makes it no more repeatable than measuring refractive error in tracking myopia (Wolffsohn et al, 2019).
As for the utility of the numbers, axial length appears to have a stronger correlation with the lifelong eye health risks of myopia than diopters. An axial length greater than 26mm is a key threshold for risk (Tideman et al, 2016), so a single measure of a young myope’s axial length can indicate how proactive a myopia management strategy should be. If you are not able to access or invest in this technology, working with a colleague who does to obtain annual measurements can add a lot to the myopia management picture.
As a repeated measure, axial length is a little more complicated to use than refraction when determining myopia progression. Since axial length increases slowly throughout childhood emmetropization by around 0.1mm per year (Mutti et al, 2007), this component of change could be considered “normal” and any additional change is related to myopia. It is not known whether this “normal” component of axial length growth in childhood is potentially pathological for the myopic eye, which is already longer than “normal.”
To support utilization of these metrics, newly available technology provides percentile growth charts—for which a reduction in the centile indicates myopia treatment success—and other analyses that help to attribute myopic change to various optical components of the eye (Tideman et al, 2018; He et al, 2023; He et al, 2015; Nkansah et al, 2024). A vital point here is that regular measurement of the cornea can ensure that any evident progression isn’t due to potential ectasia. This makes corneal topography (or keratometry) an important part of the regular exam for myopia management (Gifford et al, 2019).
The IMI Clinical Management Guidelines reasonably stated in 2019 that it is difficult to know what is normal for an individual in terms of axial length progression (Gifford et al, 2019). While this is still true, we now have a much wider understanding of how to use these metrics and the value of both single and repeated measures of axial length in diagnosing and tracking myopia.
Limited access to axial length measurement equipment should not stop you from getting involved in myopia management. Having this data, though, provides the fullest representation and understanding of the individual’s myopia.
REFERENCES
1. Gifford KL, Richdale K, Kang P, et al. IMI - Clinical Management Guidelines Report. Invest Ophthalmol Vis Sci. 2019 Feb 28;60:M184-M203.
2. Carkeet A, Saw SM, Gazzard G, Tang W, Tan DT. Repeatability of IOLMaster biometry in children. Optom Vis Sci. 2004 Nov;81:829-834.
3. Chan B, Cho P, Cheung SW. Repeatability and agreement of two A-scan ultrasonic biometers and IOLMaster in non-orthokeratology subjects and post-orthokeratology children. Clin Exp Optom. 2006 May;89:160-168.
4. Wolffsohn JS, Kollbaum PS, Berntsen DA, et al. IMI - Clinical Myopia Control Trials and Instrumentation Report. Invest Ophthalmol Vis Sci. 2019 Feb 28;60:M132-M160.
5. Tideman JW, Snabel MC, Tedja MS, et al. Association of Axial Length With Risk of Uncorrectable Visual Impairment for Europeans With Myopia. JAMA Ophthalmol. 2016 Dec 1;134:1355-1363.
6. 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 Jun;48:2510-2519.
7. Tideman JWL, Polling JR, Vingerling JR, et al. Axial length growth and the risk of developing myopia in European children. Acta Ophthalmol. 2018 May;96:301-309.
8. He X, Sankaridurg P, Naduvilath T, et al. 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;107:167-175.
9. He X, Zou H, Lu L, et al. 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:e0111766.
10. Nkansah EK, Lingham G, Loughman J, Kobia-Acquah E, Flitcroft DI. Biometrically defining myopia with the Refractive Mechanism Map: Relationship with myopia progression and treatment efficacy. Invest Ophthalmol Vis Sci. 2024 Jun;65:142.
