Choosing an instrument to measure axial length

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?

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 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 atropine^8-10 and MiSight.¹¹

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 lenses^12-14 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.

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

In orthokeratology lens wear, a study comparing ultrasound and interferometry (IOLMaster) measurements of axial length found the latter to be more repeatable. The authors concluded that "measurements with the IOLMaster can replace the measurements from the two A‐scan ultrasonic biometers used, however, the reverse is not true."¹⁷

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

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

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.

1. There are many considerations in choosing an instrument to measure axial length, including affordability, size and ease of use.
2. Interferometry measurement instruments are more accurate than ultrasound measurements, but have traditionally been used by ophthalmology and been expensive for primary eye care.
3. 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.

• Case study: A mismatch between myopia and axial length

• Measuring the whole eye in myopia

• Six questions on axial length measurement in myopia management