Myopia management consists of many varied approaches. To determine how well treatments are working, progression is judged with two determinants: change in refractive error and/or change in axial length. These two factors are fundamental as the ocular measures which we strive to 'control' in our myopic patients. However, another approach that offers a more individualized idea of a patient’s progression is the use of a model eye and axial length growth charts. This article explains how we might incorporate this information in our myopia management diagnosis and decision making.
What is a model eye?
You might remember the name Allvar Gullstrand from your optics lectures in university. Gullstrand was the scientist that illustrated the intricate geometry of the eye in the form of the schematic or ‘model’ eye and, in fact, won the Nobel Prize for it in 1911.1 The model eye shows all the refractive components of the eye and how they are inter-related in the adult emmetrope.
Of course, myopia management more often than not concerns children rather than adults, so an adapted Gullstrand model eye for children is more useful in clinical practice. Even more useful, is also incorporating growth charts, which provide age, gender and ethnicity-specific normative data points and analyses. How To Use Axial Length Growth Charts will help you learn how to incorporate them into clinical practice.
Why use a model eye?
Myopia is caused by many different components of the eye – it’s not just axial length. Assessing how the refractive ocular components contribute to myopia, and whether or not these are within the normal range by comparing it to a model eye, is innovative when we consider the more traditional methods of citing averages for axial length growth and refractive error change. By modeling the exact contributions each refractive component of the eye has on the resultant myopia, we can determine whether or not treatments are actually working on those components - especially axial length, being our key target for slowed growth in myopia control.2
Directly comparing corneal and axial length data can also determine the likelihood of future myopia with greater accuracy. The AL/CR ratio incorporates axial length and corneal radius and assists in predicting myopia – you can read more about it in our recent article Predicting Future Myopia from Axial Length.
Why is this useful? Large screening studies have found that the AL/CR ratio predicts myopic refraction better than axial length alone, by incorporating the power of the cornea.3 Furthermore, along with obtaining age and gender, it’s as accurate as measuring non-cycloplegic refraction and visual acuity to screen for myopia, which has important applications in large-scale population screenings for myopia.3,4
Why gender and ethnicity specific data?
Studies have revealed that larger axial length and AL/CR can be indicative of future myopia4 however, some key differences are found between certain populations. One study4 which evaluated data from over 14,000 children in China found that girls tended to have shorter axial length and steeper corneas; that is, a lower AL/CR ratio with disproportionately more myopic refractive error than would be expected. Here, gender-specific AL/CR charts would be useful in capturing girls that may become myopic in the future.5
But it’s not just gender differences we should look out for – it’s also differences in ethnicity. During early emmetropization, children actually have very similar axial length. After age 5-6, however, axial length differences start to emerge for gender and after age 9 for Asian compared to European eyes. This is why comparative axial length data should incorporate age, ethnicity and gender variables. Read more about this in How much axial length growth is normal? and How to use axial length growth charts.
How could we use model eyes in practice?
Utilization of a model eye doesn't mean one of those handy plastic models as shown in the image above! Instead, look at the image below. Since use of model eye data involves comparison between the individual and the model, software analysis is needed.
The image above is of the output from the Gullstrand Refractive Analysis System (GRAS) Module in the OCULUS Myopia Master, which models the power of ocular components to chart a child’s axial length, corneal power, crystalline lens power and refraction as contributions to their overall refractive error, and if their current refraction is more myopic (left side of central vertical line) or hyperopic (right side of line) compared to their age-matched peers. This can then be used in follow up visits to gauge the impact of interventions – another way of seeing if the power components are shifting away from accelerated myopic growth.
Further reading
About Jeanne
Jeanne Saw is a clinical optometrist based in Sydney, Australia. She has worked as a research assistant with leading vision scientists, and has a keen interest in myopia control and professional education.
This content is brought to you thanks to an unrestricted educational grant from
References
- Gullstrand A. How I found the mechanism of intracapsular accommodation. Nobel Lectures, Physiology or Medicine 1921, 414 (1901).
- 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. [Link to Myopia Profile Science Review]
- He X, Zou H, Lu L, Zhao R, Zhao H, Li Q, Zhu J. 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(2). (link)
- 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. 2021 Sep 16:bjophthalmol-2021-319431. (link)
- Tideman JWL, Polling JR, Jaddoe VWV, 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):127-136. (link) [Link to Myopia Profile Science Review]
