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Why axial length matters in myopia management

Three practitioners with a special interest in myopia management discuss why their colleagues should be prioritising axial length measurement, the importance of using software and data, and what lies ahead in optical biometry.

Assessing refractive error alone in managing myopia may have once been the norm, but the limitations of this method are now being acknowledged. Simultaneously, there is a growing awareness of the association between axial elongation and risk profile. 

To this end, measuring axial length using biometry is emerging as the standard of care – and optical biometry technology, typically used for cataract surgery work-up, has advanced with manufacturers making modifications for myopia management.

Improved technology means optical biometers have significantly better resolution than their predecessors, a high degree of accuracy and repeatability, and produce results quickly – an advantage when assessing young children with short attention spans.

Integrated software also alleviates the manual work of comparison and extrapolation, and allows optometrists to draw comparisons between their patient against normative data and growth curves.

And as experts in the field have pointed out, there are evidence-based reasons why measuring axial length may soon become an indispensable part of effective myopia management.

Subjective versus objective

Optometrist and clinic director of Eyecare Concepts, incorporating Myopia Clinic Melbourne, Dr Philip Cheng says the main limitation of not having a biometer to measure axial length is inaccurate monitoring of myopia progression and treatment response.

“Subjective refraction has a high degree of variability between tests, and subject to practitioner technique, equipment, vertex distance, patient response, accommodation – factors that can easily lead to differences in refraction of plus or minus 0.50 dioptres,” he says. 

“The variability in subjective refraction can be greater than the actual myopia progression that we are trying to measure. Atropine treatment can affect accommodation, which can appear to slow progression in refraction while axial length is still progressing. Optical biometry, as an objective measure, gives a reliable and consistent metric for monitoring eye growth and progression across all treatment modalities, eliminating guesswork and doubt.”

In addition, Cheng, a Fellow of the International Academy of Orthokeratology and Myopia Control and board member of the Orthokeratology Society of Oceania, says the only method of properly assessing progression in orthokeratology (orthoK) patients is to measure axial length, as the true refraction status cannot be measured due to corneal changes. 

“Without biometry, myopia progression in OrthoK patients may be masked until their daytime unaided vision is reduced from significant progression, delaying further intervention in these cases of fast progressors. Practitioners fitting OrthoK for myopia management but not monitoring their axial length changes are missing the full picture and are potentially not managing the patient as well as they should,” he says.

It’s a view shared by Ms Rebecca Dang, optometrist and UNSW lecturer, and Ms Zeinab Fakih, lead optometrist in paediatric services at Australian College of Optometry.

“The two main outcomes of myopia are refractive error and axial length,” Dang says. “Compared to refractive error, axial length has the advantage of being an objective measurement (therefore more repeatable in children) and directly related to the anatomy associated with sequelae of high myopia such as degenerative retinal changes.”

Dang, who is completing a PhD in factors affecting the adherence of multifocal contact lenses used for myopia control, says any change in refractive error is very small compared to the equivalent change in axial length, estimated to be approximately 0.25 D to 0.1 mm. 

“Considering these factors, a clinic without access to a biometer which instead relies on refractive error changes for myopia management will only be able to reliably detect larger changes in myopia progression before amending treatment,” she says.

Fakih adds that axial length has been found to be critical in a patient’s risk of ocular pathology. 

“Axial lengths greater than 26mm, irrespective of the dioptric myopic correction, have been shown to have a 25% chance of vision impairment by age 75. Without having access to a biometer, practitioners may miss these high risk myopes who, on refraction alone, may pass as having only mild-moderate myopia,” she says.

Furthermore, tracking axial length offers another metric with which the effectiveness of a myopia control therapy can be tracked.

Zeinab Fakih, lead optometrist paediatric services, in a consult in the Australian College of Optometry’s recently opened Myopia Clinic.

“It is not affected by how relaxed a patient’s accommodation is or on the type of myopia control being administered,” Fakih says.

“It is invaluable in the monitoring of progression in patients undergoing orthoK.  In the absence of axial length measurements, the clinician is reliant on over refraction, which can be variable, and requires the patient to attend with their contact lenses.”

Cheng, Dang and Fakih represent a profession moving towards more sophisticated myopia management, and their views highlight why the use of axial length measurement is growing in importance.

“I think eyecare practitioners are starting to see the limitations of assessing refractive error alone in managing myopia,” Cheng says.

“There is growing awareness that axial elongation in progressive myopia is associated with increased lifetime risks of uncorrectable visual impairment. [JWL] Tideman’s 2016 paper showed a 25% risk for axial length greater than 26 mm, so it is vital to measure the length of the eye to understand the individual’s risk profile and urgency of treatment.”

Recently published population studies from the Netherlands and China on axial length in emmetropic and myopic children are providing normative data and growth curves in Caucasian and Asian children. 

“This data will allow us to determine whether a child’s axial length and rate of elongation falls within a normal range or whether they may be at risk of becoming myopic, even before they develop symptoms and refractive changes. Preventing the onset of myopia, not just slowing progression, is set to be an important part of myopia management in the future,” Cheng says.

Cheng acknowledges optical biometers are a significant investment for a practice, with cost of entry of around $30,000 to $50,000, but takes a long-term view in regard to return-of-investment in practice technologies. 

“When we purchased our first biometer five years ago, as one of the early adopters in measuring axial length for myopia management in private practice, we did so to provide the highest level of comprehensive myopia care to our patients. Investing in our patients’ care has been instrumental to our clinic’s success and growth, and today we are proud to be looking after well over 2,000 children across Melbourne for myopia management as a leader in this space,” he says.

Dang also notes early myopia control studies used refractive error as the preferred primary outcome, as optical biometers had limited resolution which proved refractive error superior. 

“However, improved technology means optical biometers have significantly better resolution making them the outcome of choice especially in addition to the factors I mentioned previously,” she says.

Software an effective communication tool

Hardware, in terms of optical biometers, is only part of the solution. Of equal importance is easy-to-use, integrated software to help optometrists assess, but also extrapolate, patient data for parents.

“Hardware should be easy and quick to use, especially on younger kids with short attention spans who are unable to sit still for long,” Cheng agrees. “It needs to have a high degree of accuracy and repeatability – but software is important to help eyecare practitioners understand the data and to communicate this to parents.”

Cheng believes software should be customisable to the practitioner’s needs to communicate the message and be able to show changes clearly, in terms of the raw data and in the trend of axial elongation.

“Practitioners need to have confidence in the data provided to make the best management decisions for the patient. The changes can be small (0.1 to 0.2 mm between reviews) and it’s important to visualise trends to act promptly in changing or adding treatments as required to best control an individual’s progression. Incorporation of growth curves and normative data helps with interpretation of results and guide management,” he says.

“Software is also an effective communication tool to show parents why their child needs myopia management as well as demonstrating treatment results, so they can understand how their child’s treatment is working, and why changes in treatment strategy might be made. Software should ideally be developed in consultation with experienced myopia management eyecare practitioners to understand their needs in a clinical setting.”

Dang, at UNSW, goes one step further and highlights the importance of integrated databases.

“Axial length measurements should be analysed in context of the patient by comparing to normative databases, which allow clinicians to stage for risk,” she says.

“A certain axial length could represent an eye of average length in a female child of European ethnicity but could represent an above average eye length in a male child of East-Asian ethnicity. A change of 0.1 mm axial length in one year may be high for a 15-year-old, but below average for a 5-year-old.”  

Dang continues: “Integrated software allows clinicians to conduct these otherwise arduous manual comparisons quickly and automatically, which allows them to make accurate management decisions faster.”

Fakih, at the Australian College of Optometry which recently opened a myopia clinic, also says having clinician-friendly software is critical to ensuring that optometrists can promptly measure a patient’s axial length and also compare the result to previous records. 

“Software that allows trend analysis to monitor rates of changes or stability would certainly be advantageous,” she says.

Innovation to be expected

Last year alone, Essilor’s Stellest myopia control lens launched in Australia and New Zealand, CooperVision released its MiSight 1 day contact lenses in higher prescriptions, low-dose atropine EIKANCE secured the first myopia TGA approval, and a three-way alliance between Haag-Streit, its Australasian distributor Device Technologies and HOYA Vision Care offered Australian optometrists greater access to the Lenstar Myopia optical biometer. In 2023, Rodenstock has entered the race with its MyCon lens with peripheral defocus areas.

This is in addition to HOYA’s MiYOSMART defocus lens launched in October 2020, and contact lens based interventions from Visioneering Technologies and mark’ennovy introduced in recent years. 

If these advances are any indication, many would agree the myopia management space is ripe for yet more innovation, and Cheng, Dang and Fakih share in the anticipation of what’s ahead.

For Fakih, having a system that more easily plots axial lengths over a particular time frame would allow the practitioner to monitor change more easily. 

“Changes greater than the expected yearly growth of 0.1 mm each year would be flagged and require follow ups. It would also allow parents to visualise a trajectory if no myopia control was initiated,” she says.

“Having the ability to also plot a child’s axial length against age-normal would allow easier visualisation of a child’s risk of myopia and long-term risk of ocular pathology.”

Fakih continues: “Myopia is increasingly being recognised as a true public health issue, and no longer considered as merely a refractive concern. As the importance of myopia control gains traction, it is hoped that partnerships with industry will make biometry more specific to myopia and increase accessibility in the profession.”

Cheng and Dang also believe industry is on the cusp of further change.

“Many manufacturers now have optical biometers with integrated myopia management software, such as Oculus Myopia Master, Lenstar Myopia, Myopia Expert 700. With the successful integration of these into clinical practice, I’m sure other manufacturers will follow and expand on the support they offer,” Dang says.

Cheng agrees: “I think we will see more manufacturers enter the biometry space for myopia management, perhaps some using their existing, proven biometers for cataract pre-op and adding specialised software for myopia management.”

But he cautions “all-in-one partnerships” may potentially create bias towards one particular treatment. 

“There is no one-size-fits-all approach in myopia management – each child’s situation is unique and should be managed individually, with a discussion on all available treatment options taking into account many different factors including progression risk profile, compliance, child/parent preferences and lifestyle,” he says.

“Practitioner decision making is still the most important part in myopia management, which takes skill and experience, and no machine can replace that, but having good data from a reliable biometer certainly helps with this process. Optical biometry is one of the most valuable tools we have in our clinic; such is the importance that we have not one but two biometers to ensure we capture reliable and repeatable data for all our patients, and I certainly can’t imagine practising myopia management without one.”  

More reading

Why Jim Kokkinakis uses the OCULUS Myopia Master

Myopia Management: Implementing an evidence-based Standard of Care

Management of high myopia

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