Back to basics on axial length measurement

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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.

What types of instruments measure axial length?

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 versus optical biometry

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.

Optical biometry measurements are around 10 times more accurate than A-scan measurements. When it comes to measuring changes in myopia, optical biometry will be several times more sensitive than even a cycloplegic refraction. A-scan measurement does not achieve this additional sensitivity.5

How many measurements are required?

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.

How to use axial length data

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.

Final thoughts

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.

Further reading

Jeanne copy (1)

About Jeanne

Jeanne Saw is a clinical optometrist based in Sydney, Australia. She has worked as a research assistant with leading vision scientists, and has a keen interest in myopia control and professional education.

This content is brought to you thanks to an unrestricted educational grant from

References

  1. Mutti DO, Hayes JR, Mitchell GL, Jones LA, Moeschberger ML, Cotter SA, Kleinstein RN, Manny RE, Twelker JD, Zadnik K; CLEERE Study Group. Refractive error, axial length, and relative peripheral refractive error before and after the onset of myopia. Invest Ophthalmol Vis Sci. 2007.
  2. Fledelius HC, Christensen AS, Fledelius C. Juvenile eye growth, when completed? An evaluation based on IOL-Master axial length data, cross-sectional and longitudinal. Acta Ophthalmol. 2014.
  3. Rozema J, Dankert S, Iribarren R, Lanca C, Saw S-M. Axial Growth and Lens Power Loss at Myopia Onset in Singaporean Children. Invest Ophthalmol Vis Sci. 2019;60(8):3091-3099.
  4. Tideman JWL, Polling JR, Vingerling JR, Jaddoe VWV, Williams C, Guggenheim JA, Klaver CCW. Axial length growth and the risk of developing myopia in European children. Acta Ophthalmol. 2018.
  5. Wolffsohn JS, Kollbaum PS, Berntsen DA, Atchison DA, Benavente A, Bradley A, Buckhurst H, Collins M, Fujikado T, Hiraoka T, Hirota M, Jones D, Logan NS, Lundström L, Torii H, Read SA, Naidoo K. IMI - Clinical Myopia Control Trials and Instrumentation Report. Invest Ophthalmol Vis Sci. 2019 Feb 28;60(3):M132-M160.
  6. Olsen T. The accuracy of ultrasonic determination of axial length in pseudophakic eyes. Acta Ophthalmol (Copenh). 1989 Apr;67(2):141-4.
  7. Trivedi RH, Wilson ME. Axial length measurements by contact and immersion techniques in pediatric eyes with cataract. Ophthalmology. 2011 Mar;118(3):498-502. doi: 10.1016/j.ophtha.2010.06.042.
  8. Chan B, Cho P, Cheung SW. Repeatability and agreement of two A-scan ultrasonic biometers and IOLMaster in non-orthokeratology subjects and post-orthokeratology children. Clin Exp Optom. 2006 May;89(3):160-8.
  9. Rudnicka AR, Steele CF, Crabb DP, Edgar DF. Repeatability, reproducibility and intersession variability of the Allergan Humphrey ultrasonic biometer. Acta Ophthalmol (Copenh). 1992 Jun;70(3):327-34.
  10. Hitzenberger CK. Optical measurement of the axial eye length by laser Doppler interferometry. Invest Ophthalmol Vis Sci. 1991 Mar;32(3):616-24.
  11. Shen P, Zheng Y, Ding X, Liu B, Congdon N, Morgan I, He M. Biometric measurements in highly myopic eyes. J Cataract Refract Surg. 2013 Feb;39(2):180-7.
  12. Gifford KL, Richdale K, Kang P, Aller TA, Lam CS, Liu YM, Michaud L, Mulder J, Orr JB, Rose KA, Saunders KJ, Seidel D, Tideman JWL, Sankaridurg P. IMI - Clinical Management Guidelines Report. Invest Ophthalmol Vis Sci. 2019 Feb 28;60(3):M184-M203.
  13. Chamberlain P, Peixoto-de-Matos SC, Logan NS, Ngo C, Jones D, Young G. A 3-year Randomized Clinical Trial of MiSight Lenses for Myopia Control. Optom Vis Sci. 2019 Aug;96(8):556-567.
  14. Walline JJ, Walker MK, Mutti DO, Jones-Jordan LA, Sinnott LT, Giannoni AG, Bickle KM, Schulle KL, Nixon A, Pierce GE, Berntsen DA; BLINK Study Group. Effect of High Add Power, Medium Add Power, or Single-Vision Contact Lenses on Myopia Progression in Children: The BLINK Randomized Clinical Trial. JAMA. 2020 Aug 11;324(6):571-580.
  15. Zadnik K, Sinnott LT, Cotter SA, Jones-Jordan LA, Kleinstein RN, Manny RE, Twelker JD, Mutti DO; Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) Study Group. Prediction of Juvenile-Onset Myopia. JAMA Ophthalmol. 2015 Jun;133(6):683-9.

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