Back to basics on axial length measurement

Whether or not you are currently measuring it in your practice, axial length is an important dimension to be aware of in myopia management. New innovations and research in axial length measurement have allowed us to utilize this ocular measure in increasingly individualised ways to account for age, gender and ethnicity.^1-4 While this allows for advanced myopia progression assessment, it is first essential that the foundations of axial length measurement are understood. This article is a back to basics guide on axial length measurement.

There are two main types of instruments used to measure axial length: A-Scan ultrasound and optical biometry. A-scan, short for applanation ultrasound, has been used for many years and involves using high-frequency sound waves to measure axial length. Optical biometry, on the other hand, uses light waves to measure the length of the eye. These machines, although both capable of measuring axial length, are very different.

A-scan sends out 10mHz soundwaves and records the echoes that bounce back from ocular structures; this provides a picture and measurements of the eye. Axial length is measured as the distance between the anterior cornea and the inner limiting membrane of the retina.⁵ As the name suggests, A-scan ultrasound requires applanation: the machine uses a transducer which requires direct contact with the eye. The consequences of this technique is not isolated to the obvious problem of patient discomfort: A-scan typically yields artificially lower axial length readings, likely due to corneal compression from the direct contact of the transducer which results in reduced corneal thickness or reduced anterior chamber depth.^6-7 The repeatability of A-scan is around ±0.2mm to ±0.3mm.^8-9 

Optical biometry is based on optical partial coherence interferometry (PCI), and was developed due to the limitations of A-scan for measuring axial length.¹⁰ Biometry involves sending two laser beams into the eye; the reflections from ocular tissues are the basis of axial length measurements. Axial length is measured as the distance between the anterior cornea and the retinal pigment epithelium of the retina.⁵ Optical biometry provides a more accurate reading of axial length, with repeatability of ±0.04mm.^7-8 Given the need for corneal anaesthesia and the lower repeatability of A-scan, optical biometry is the preferred method of axial length measurement with better axial length readings yielded with biometry over ultrasound.¹¹ 

Learn more about different types of instruments in Choosing an instrument to measure axial length.

The International of Myopia Institute (IMI) Clinical Management Guidelines recommend that axial length measurements be taken every six months if available;¹² however, should a myope appear to be progressing faster, you may wish to shorten the review period to 3 months.

In research studies, axial length can be measured anywhere between 5 to 10 times per visit.^13-15 In clinical practice, the IMI guidelines do not have recommendations for this. Every machine is different, hence asking for recommendations from your equipment provider is a good idea. Repeated measures at follow-up appointments will give you a map of how your patient's myopia progression is tracking, and allows for objective measurement.

A one-time measure of axial length can also be incredibly insightful clinically, for both adults and children. For adults, a one-time measure is useful for indicating risk of diseases associated with increased axial length and may prompt you to give advice on the signs and symptoms of a retinal detachment. You may also then perform a retinal examination through dilated pupils and schedule an annual review of the peripheral retinae, as recommended by the IMI for patients with an axial length over 26mm and/or high myopia in excess of 5-6D.¹² For children, a singular axial length measure is helpful for indicated the urgency of myopia control and will guide you on what strategy to initiate.

Utilizing axial length data is valuable both to determine eye health risk, as described earlier, as well as judging the efficacy of a myopia control treatment. When determining how axial length growth is tracking in a child over time, it is important to compare data to matched information based on the child's age, ethnicity and gender. Children tend to have uniform axial lengths when they are very young, but after age 5-6 boys tend to show longer axial lengths than girls. After age 9, Asian eyes tend to become longer than European eyes. Read more about these topics in How much axial length growth is normal? and How to use axial length growth charts.

Axial length measurement is increasingly embraced in dedicated myopia management practices, but is still not yet widespread in primary eye care. However, even a one-time or annual measure of axial length can give great insight and direction in the management of your patient. In clinical settings where access to axial length measurement may not be imminent, consider referring to a colleague and/or a local ophthalmologist for axial length measurement, to enable addition of this vital clinical metric to your patient management decisions.

• How to use axial length growth charts

• Are you measuring the cornea in myopia management?

• How much axial length growth is normal?