Why Axial Length Matters

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September 1, 2023

By Debbie Jones, FCOptom, FAAO, FCBLA

Axial length measurement should be considered an essential part of the assessment of patients at risk of developing myopia, as well as those who are already myopic. 

why axial length matters

Photo Credit: bluecinema, Getty Images

As eye care professionals embrace the need to manage myopia with more than just a simple spherical optical correction, it is important to understand that the examination of patients involves more than just an assessment of refractive error. The measurement of axial length plays an important role in the decision-making process when managing our young myopic and pre-myopic patients. 

Axial length (AL) is measured in millimeters from the anterior cornea to the retinal pigment epithelium and is the most significant contributor to refractive error and potential myopia-related visual impairment.1

There are two basic methods to measure AL, ultrasound and optical biometry.2 The use of A-scan ultrasound, which requires contact with the cornea, has been largely superseded by modern instrumentation that avoids contact and has improved accuracy and ease of use. There are a variety of instruments available to measure AL, broadly divided into stand-alone instruments and multifunction instruments. Multifunction instruments have the advantage of occupying a small footprint, while offering the option of AL measurement along with other clinical measures/assessments, such as autorefraction, topography, support for contact lens fitting, and dry eye assessment. In general, optical biometers are more reliable than ultrasound biometers as they produce more repeatable measurements compared to ultrasound biometers. For instance, a study reported a smaller test-retest difference for the IOLMaster optical biometer compared to A-scan (0.004mm versus 0.042mm).3

The AL of a newborn eye is approximately 16.8mm, with an emmetropic adult eye being 23.8mm.4,5 There is, therefore, natural growth to be expected during childhood. The typical changes in AL are 0.15mm to 0.20mm/year up to age 8, and then 0.06 to 0.15mm/year through to the early teen years.6,7,8 Progression outside of this rate should give cause for concern, as the peak rate of axial elongation may occur two to four years prior to evidence of a myopic refractive error.8 An AL of 23.07mm in a 6-7 year old and a change of 0.74mm across a three-year time period are outside the norm and suggestive of future myopia development.9 

Identifying those patients at risk of developing myopia by careful monitoring of axial length can result in discussions around the need for early intervention with simple lifestyle modifications, such as spending more time outside, in an attempt to delay the onset of myopia.10 It has been shown that those patients with a myopic refractive error prior to the age of 10 are at a greater risk of developing high myopia in their adult years, and therefore delaying the onset of myopia is hugely beneficial.11 The severity of myopia-related visual impairment increases with increasing axial length.1 For example, patients with an axial length ≥ 30mm have a 90 times greater chance of developing uncorrectable visual impairment compared to an individual with an emmetropic axial length.1

There are large data sets of normative axial length available,6,12 providing an opportunity to compare the patient in the chair with a matched data set (of both gender and ethnicity). This gives an indication of possible trajectory and risk of progression. Comparison to normative data is invaluable when discussing the need for myopia management with parents in being able to demonstrate the potential refractive error outcome with no myopia management intervention.6,12,13 The clinical application of myopia management can be simple to explain in terms that are easily understood by parents, relating to other health metrics. For example, children below the 50th percentile for AL for age are less likely to become myopic than those over the 50th percentile. Many of the multifunction instruments designed to measure AL have normative data sets included in their software, facilitating comparison of the patient being examined against matched norms, and they can also be used to monitor progression. The ability to print out AL reports for parents can improve their confidence in the myopia management process. 

As myopia management continues to be recognized as an important part of primary eye care, we can anticipate the availability of an increased number of myopia management products on the market. Therefore, monitoring AL is an essential part of assessing the success of the chosen myopia management modality. In the case of orthokeratology, it is the only effective way to assess efficacy of the treatment for a patient.14 Even for other modalities, it is imperative that the effect of the treatment on AL is as meaningful as it is for refractive error. Consistent with this, both AL and refractive error changes are reported primary outcome measures in clinical studies of new and existing myopia management modalities.15

There is no doubt that there is a benefit to undertaking myopia management, and measurement of AL is a major factor in managing myopia. Axial length measurement should be considered an essential part of the assessment of patients at risk of developing myopia, as well as those who are already myopic. ECPs must understand that myopia is more than just a myopic refractive error. 

To read more about the importance of axial length measurement and CARE (Cumulative Absolute Reduction in axial Elongation), please see Johnson & Johnson Vision’s previous article by Dr. Noel Brennan.


Debbie Jones, FCOptom, FAAO, FBCLA, is a Clinical Professor at the School of Optometry and Vision Science and a Clinical Scientist at the Centre for Ocular Research & Education (CORE) at the University of Waterloo. Her main area of clinical focus is in pediatric optometry and her main area of research activity is in the area of myopia control. Debbie has a keen interest in the eye care needs of indigenous peoples and has provided eye care to a remote community in Northern Ontario on a number of occasions. Trained in the U.K., Debbie has been a faculty member at the School of Optometry and Vision Science for 25 years. She is formally a partner in an award winning private practice in the U.K., has published articles in optometric journals, and regularly presents at optometric conferences worldwide. She is a Fellow of the British College of Optometrists, the British Contact Lens Association, and the American Academy of Optometry.



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