Choosing an instrument to measure axial length
In this article:
The most common parameter to monitor myopia progression is change in refractive error. However, as clinicians around the world become more proactive with prescribing myopia control strategies, axial length data becomes an increasingly valued piece in the clinical picture. If you aren't currently equipped with the appropriate equipment, how does one go about choosing an instrument to measure axial length?
Why is axial length measurement so important?
The main 'why' behind myopia control is to reduce the rate of axial length elongation. Whilst we can normally expect a direct correlation between axial length and refractive error, this is not always the case. A reduction in refractive progression does not always correlate to reduced axial length progression. Axial length measurements are also a valuable tool in monitoring the success of orthokeratology treatment in reducing myopic progression, as refraction is intentionally altered. It can also change one's clinical management plan, such as in cases of a mismatch between myopia and axial length. An example of this is a low myope with long axial length, where a higher-than-expected risk for future pathology may lead to a more proactive management strategy.
As biometric instruments are not yet traditionally used in optometric practices, practitioners may wonder which ones would be the most reliable to help monitor treatment success in myopia control. RM (link) and LF (link) asked the Myopia Profile Facebook community this precise question about choosing an instrument to measure axial length.
The community's comments on their favourite instruments are summarised below.
Interferometry measurement instruments
Interferometry is a non-contact, optical biometry measure. Examples include the Zeiss IOLMaster, Haag-Streit LensStar and the OCULUS Pentacam AXL. These instruments have traditionally been used by ophthalmologists to measure corneal curvature and axial length and facilitate intraocular lens implant power calculations for cataract surgery. They have high repeatability and accuracy,1-5 and have been used in myopia control studies of orthokeratology,6-9 atropine8-10 and MiSight.11
Pros: high repeatability and accuracy, reliable, non-contact measurement technique
A-scan ultrasound biometers have been used in several myopia control research studies to monitor axial length - typically older studies on bifocal and progressive spectacle lenses12-14 and atropine.14-15 Ultrasound measurement requires anaesthesia and corneal applanation, so children may be intimidated by the drop instillation and/or the probe's proximity to the eye. There is a steeper learning curve in getting consistent and repeatable measurements due to applanation pressure and is dependent on the operator.
Interferometry versus ultrasound - which is more accurate?
In application to myopia, the IMI Clinical Trials and Instrumentation report explains that ultrasonography is limited in resolution to about 0.30D, whereas interferometry measurements have resolution of around 0.03D, making this technique an order of magnitude better as a measure of myopia progression. Cycloplegic autorefraction, by comparison, has a repeatability of ±0.21D.16
Measurement of axial length with non-contact interferometry instruments is more accurate and repeatable than by A-scan ultrasound biometry.
There are also a range of instruments that combine several functions in one, which offers increased convenience for the practitioner and patient. Some are designed specifically for myopia management, with additional testing capacity and software outputs to streamline both testing and clinical communication.
Axial length + OCT. The Optopol REVO takes axial length measurement by using OCT imaging across the entire length of the eye. It has good axial length repeatability, and its measurements are in good agreement with those obtained on the IOLMaster.18
Topcon ALADDIN and MYAH
Axial length + corneal topography + pupillometry + dry eye assessment + progression reports. Topcon has released the Aladdin and the Myah (known as Aladdin-M in USA), both of which measure axial length and corneal topography. The Myah / Aladdin-M also includes pupillometry and dry eye assessment capabilities. Reporting to track progression include refraction (extra input required) and axial length charts. The Topcon technology uses non-contact, interferometry measurement of axial length, and has been demonstrated to be comparable to the IOLMaster.19
OCULUS Myopia Master
Axial length + keratometry + autorefraction + progression reports + risk factor analysis outputs. The OCULUS Myopia Master is a multi-function instrument, featuring autorefraction, axial length measurement and keratometry. It also has software developed in cooperation with the Brien Holden Vision Institute that provides axial length growth chart outputs and provides risk factor assessment to support clinical communication. The take-home report for parents provides the results, and also provides support on understanding myopia. The report can be printed or sent via email, together with the date for the next consultation, directly from the software. The OCULUS technology uses non-contact, interferometry measurement of axial length, which has been demonstrated to be in agreement with the IOLMaster.20 Since the Myopia Master was designed especially for the Optometrist, the combination of measurements provide utility for all patients, not just myopia management, through the Autorefractor and Keratometer functions.
Pros: repeatable and accurate, non-contact measurement, multi-functional, space- and time-saving, more affordable than single-function interferometry instruments, designed for primary eye care
Take home messages:
- There are many considerations in choosing an instrument to measure axial length, including affordability, size and ease of use.
- Interferometry measurement instruments are more accurate than ultrasound measurements, but have traditionally been used by ophthalmology and been expensive for primary eye care.
- Multiple function instruments which employ interferometry, especially those specifically designed for myopia, provide useful clinical functionality and affordability for primary eye care practitioners, as well as tools for reporting and clinical communication.
Meet the Authors:
About Connie Gan
Connie is a clinical optometrist from Kedah, Malaysia, who provides comprehensive vision care for children and runs the myopia management service in her clinical practice.
About Kimberley Ngu
Kimberley is a clinical optometrist from Perth, Australia, with experience in patient education programs, having practiced in both Australia and Singapore.
This content is brought to you thanks to unrestricted educational grant from
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- Wang XG, Dong J, Pu YL, Liu HJ, Wu Q. Comparison axial length measurements from three biometric instruments in high myopia. International journal of ophthalmology. 2016;9(6):876. (link)
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