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HomemiequipmentMyopia Management: Why Eye Length Matters

Myopia Management: Why Eye Length Matters

Myopia management is one of the fastest growing areas in optometry today. The World Council of Optometry, in April 2021, passed a resolution advising optometrists around the world to incorporate evidence-based myopia management as a standard-of-care in their practices. In response to this, optometrists globally need to change their approach, from simply correcting vision, to managing myopia as an eye health condition. A major part of this is to carefully evaluate myopia progression by measuring refractive error and axial length whenever possible, then to have discussions with parents to explain what myopia is, the risks associated with this condition, and the interventions available to slow its progression.1

COMMUNICATING MYOPIA

To be successful in managing myopia, we must first change the way we communicate with parents about the need for myopia management.

In myopia, refractive change is only a manifestation of the increase in eye length

Figure 1. A graph showing axial length change for a -3.50D patient with well-controlled myopia progression treated with orthokeratology lenses for almost three years.

This begins with changing the way we describe the condition – we need to make a fundamental shift from describing myopia as a vision condition, to a disease state. Rather than using the common terms ‘short-sightedness’ or ‘near-sightedness’ – words associated with the symptoms of refractive error – we need to use the medical term ‘myopia’, which carries a greater weight reflecting its seriousness.

Make no mistake, myopia is an ocular disease – the irreversible elongation and stretching of the eye, from abnormal eye growth, leads to increased risks of pathology and visual impairment. No level of myopia is safe – every extra dioptre of myopia, every millimetre of eye elongation, heightens the risks. Eyes with axial length greater than 26mm have a 25% lifetime risk of uncorrectable vision loss, rising to 90% for eyes of axial length exceeding 30mm.2

Once the child’s parents understand what myopia is, practitioners can take the next steps to discussing treatment options available to slow myopia progression. While to most people, this means preventing the child’s glasses prescription from increasing, what we are actually doing is slowing abnormal eye growth – the root cause of the condition.

In myopia, refractive change is only a manifestation of the increase in eye length. Whether we prescribe optical or pharmaceutical interventions for myopia management, the aim of treatment is to slow or halt excessive eye growth.

What’s the best way to assess this objectively? To measure the length of the eye and to monitor its growth. I recently had the pleasure of presenting a workshop on this topic at the 2021 Global Myopia Symposium, where I shared my experience in using axial length in clinical practice.

OCULAR BIOMETRY

As an early adopter in using axial length in practice for myopia management, my clinic has been equipped with biometry instruments since 2017. Optical biometry has replaced ultrasound A-scan as the preferred method of measuring axial length, due to its ease of use, greater accuracy and repeatability, without the need for topical anaesthetics.

In my busy Melbourne clinic, looking after more than 1,500 myopic children across Victoria, we use the Zeiss IOLMaster 500 and the new Oculus Myopia Master to measure axial length. These instruments are indispensable and critical for our workflow in evaluating myopic patients and their management. Axial length measurements are taken for all our patients attending for myopia control assessment, and at each of their regular reviews. We also take baseline axial length for siblings of children already undergoing myopia management, as they have similar genetic and environmental risks of developing myopia. For adults with myopia, axial length can help assess their individual risks of ocular pathology.

AXIAL LENGTH VS. REFRACTION

Axial length does not replace refraction for diagnosing myopia. One axial length measurement means little, as this depends on the individual’s corneal curvature and lens power, and can vary greatly between individuals for any given refractive error. It is the change of axial length, and particularly the rate of change over time, that matters. Accurate refraction, with accommodation in a relaxed state, which may require cycloplegia on younger children with very active accommodation, is necessary to first diagnose the degree of myopia at baseline.

Figure 2. The Zeiss IOLMaster 500 optical biometer in action, measuring axial length on a young myope.

Axial length is the better metric to evaluate progression at follow-up visits. With optical biometers capable of measuring to 0.01mm, with a level of accuracy of +/- 0.04mm, axial length is a very sensitive measure of change. In comparison, the margin of error in subjective refraction is approximately +/- 0.50D, and is affected by accommodation and whether cycloplegia is used.

Practitioner method and judgement, and variability in patients’ subjective responses, also affect refraction accuracy. Therefore, axial length is about five to 10 times more sensitive, more repeatable and reliable, for monitoring myopia progression than using refraction alone. Both metrics go hand-in-hand; axial length assesses physical change, refraction assesses functional change.

Changes in axial length are highly correlated to changes in refractive error. A change of 0.1mm in axial elongation is approximately 0.25D in refractive change. We can use changes in axial length to confirm our refraction, and discrepancies can help us identify potential sources of error. For example, if a child’s axial length was found to have increased by 0.22mm in six months, and his myopia had progressed by -0.50D, from -2.00D to -2.50D, this finding would make sense. However, if his refraction was -3.00D, the mismatch between refractive change and axial length change would indicate possible over-estimation of his myopia, and perhaps over-accommodation. To use another example, if a child’s myopia has progressed by -1.25D in 12 months and her axial length has increased by 0.56mm, the refractive change correlates well with her axial elongation and confirms the progression is due to excess eye growth.

We are all aware that high myopia patients are more challenging to refract. Vertex distance variations between phoropter refraction, trial frame refraction and the patient’s habitual eyewear position, can affect the spectacle prescription. It is easy to unintentionally over-minus a high myope’s prescription, which then creates a false impression of myopia progression. Axial length helps us validate changes in refraction, improving our assessment of true progression. In cases of anisometropia, it is also helpful to confirm the physical differences between the two eyes, resulting in the anisometropic refraction.

INTERVENTIONS THAT INFLUENCE REFRACTION

Of the current myopia control interventions, orthokeratology and atropine influence the measurement of refractive error. In orthokeratology, by temporarily correcting vision via overnight lens wear, the true refractive status cannot be measured during treatment without an appropriate washout period, which is inconvenient for patients. Unaided visual acuity cannot be used as a guide for myopia progression, as treatment effect can vary slightly from day-to-day, and overcorrection can easily mask progression, delaying further interventions for fast progressors who may still progress on treatment. While estimation of refractive change is possible by performing subjective over-refraction with orthokeratology lenses on the eyes, this is less sensitive than measuring axial length, and patients may not always bring their lenses to their reviews. Notably, research studies on orthokeratology for myopia control rely solely on measuring changes in axial length to assess efficacy. In my experience fitting hundreds of orthokeratology patients each year, biometry is a necessary tool to properly manage myopia progression with orthokeratology.

Figure 3. Axial length measurement is necessary to accurately assess myopia progression in orthokeratology patients.

Atropine has a dose-dependent cycloplegic effect, which alters accommodative response. This can affect refraction assessment and comparation of pre- and post-treatment findings, particularly if cycloplegic refraction is not performed. Without measuring axial length, this may lead to an early favourable impression of treatment effect, even if axial elongation is occurring. Closer analysis of the ATOM2 study showed that, although 0.01% atropine may appear to slow myopia progression in terms of refractive error, it has no significant effect in slowing axial elongation.

MONITORING MYOPIA PROGRESSION

Parents who invest in myopia management treatments for their children expect to see outcomes. They will want to know if their child’s myopia is controlled or progressing, and by how much. It is important that practitioners set realistic expectations of treatment success and communicate outcomes clearly. Axial length data makes it possible to evaluate treatment outcomes systematically across the range of myopia control interventions. Once we have captured axial length data, we need to be able to interpret it, and use it to guide the child’s management plan. The best way to visualise progression is to plot axial length changes against time on a graph. Graphs are easy for parents to understand. They are also a great tool to motivate patients to maintain compliance to their treatment plan and to return for reviews.

Normal eye growth has been modelled in studies on school-children between six and 14-years,4 and recently on Chinese5 and European children.6 Some optical biometers now have growth curves incorporated into their software for axial length tracking and analysis. In myopia management, each child’s individual eye growth, specifically the slope of the curve, is compared to a ‘normal growth’ line. We can also compare the rate of eye growth between different interventions prescribed, and from before and after treatment is initiated.

What is the ideal treatment outcome? To use a phrase that became common during the COVID-19 pandemic, we are aiming to flatten the curve – in this case, the curve of eye elongation. Myopia progression occurs when the rate of eye elongation exceeds normal emmetropic eye growth. Occasionally, we may see effective treatment appearing to halt eye elongation; this situation is easy to explain to parents – if the graph is a flat line, there is no eye growth occurring, the myopia is very stable. More often, there will be some eye elongation (a slight upward slope), particularly in younger children whose eyes should still be growing.

It is estimated that ‘normal’ eye growth in emmetropic children is 0.1mm per year,7 greater in younger children and less for teenagers, and also varies with ethnicity. Stable myopes are those whose refraction remains stable, and their axial length growth falls within the expected amount per year. Mild progressors have axial elongation slightly greater than normal eye growth, around 0.3mm per year, equating to around 0.50D per year of myopia progression.

Fast progressors are those whose eye growth occurs at a much faster rate than normal growth (a steep upward slope). These are individuals with eye elongation of 0.5mm per year or greater (more than 1.00D of annual progression), and this is an indication that a change of treatment strategy, or combination therapy, may be required to more effectively slow the progression. As longer eyes are at statistically higher risks of myopia-related pathology, children with longer-than-average eye length, and those approaching 26mm or greater, may need to be treated more aggressively. A tailored treatment approach, closely observing the individual’s axial elongation, is the key to optimising myopia control efficacy, with the aim of keeping the child’s eventual level of myopia as low as possible.

Myopia regression is rare but can sometimes occur with treatment. Not to be confused with pseudomyopia, myopia regression is a sustained reduction in measured axial length over time, appearing as a downward slope, which eventually plateaus and stabilises. Myopia reduction should not be discussed as a possible treatment outcome, as this would lead to unrealistic expectations. When it occurs it is a welcome unexpected bonus.

CONCLUSION

Optometrists, as primary eye care practitioners, are best-placed to provide myopia management to our paediatric patients. We can all do something – every practitioner in every practice setting has access to some intervention options. While measuring axial length is very helpful in improving clinical judgement, decision-making and ultimately the patients’ treatment outcomes; the lack of ocular biometry should not be a barrier to starting myopia management. For practitioners aiming to provide the highest standards of myopia care, particularly those fitting orthokeratology lenses, incorporating axial length assessment into their practice will take their myopia management services to the next level.

I anticipate that, in time, axial length will become a standard measurement in optometric care, just as optical coherence tomography is now considered standard-ofcare for early detection and management of glaucoma. In the future, children may have complete biometric measurements as part of their comprehensive eye examinations and be monitored on gender-specific and ethnicity-specific growth charts – similar to height and weight percentile growth charts that parents are familiar with. This means any unusual eye growth can be detected early, and optometrists may have the tools available to not only manage myopia progression, but to intervene even before a child becomes myopic.

Philip Cheng B.Optom, GCOT, FIAOMC is the clinical director of The Myopia Clinic in Melbourne. He has a particular interest in myopia management and orthokeratology. 

References 

  1. worldcouncilofoptometry.info/wp-content/ uploads/2021/04/10650-CVI-WCO-Standard-of-Care- Resolution_r08v01_UK-English.pdf. 
  2. 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(12):1355-1363. 
  3. Khanal S, Phillips JR. Which low-dose atropine for myopia control? Clin Exp Optom. 2020 Mar;103(2):230-232. 
  4. Zadnik K, Mutti DO, Mitchell GL et al. Normal eye growth in emmetropic schoolchildren. Optom Vis Sci. 2004 Nov;81(11):819-28. 
  5. Sanz Diez P, Yang LH, Lu MX et al. 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. 
  6. 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(3):301-309. 
  7. Mutti DO, Hayes JR, Mitchell GL et al; 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.