Are Axial Length & Percentile Curves the New Standard?

May 1, 2023

By Thomas Naduvilath, Msc Biostatistics, PhD, Head of Biostatistics at BHVI

It has been recommended that axial length be considered the primary outcome for determining progression and that practitioners adopt axial length measurements in managing myopia.

axial length and percentile curvesRefractive error measurements are the common approach to monitoring myopia progression. However, with more advanced techniques to measure biometry parameters, there is increased motivation toward using axial length, which is the distance from the anterior cornea to the retina along the line of sight. 

Accuracy and Reliability
The increased interest in axial length measurements compared to refractive error is driven by the improvement in accuracy and reliability in a rapid and non-invasive manner using interferometric axial length technology and optical coherence tomography.
1 This provides several advantages when examining young children, especially in environments where a cycloplegic assessment is not feasible. Myopia management using orthokeratology is assessed exclusively using axial length measurements, as the treatment induces changes in the refractive components independent of axial length.2 Additionally, changes in axial length correlate strongly with refractive error changes, though the absolute values show a weaker correlation.3 Thus, it has been recommended that axial length be considered the primary outcome for determining progression and that practitioners adopt axial length measurements in managing myopia.4,5 Moreover, FDA lists both axial length change and refractive error change as primary effectiveness endpoints for clinical trials that assess myopia control devices but favors the use of axial length. 

Percentile Curves to Monitor Progression
To aid clinicians in the use of axial length, population-specific percentile charts have been developed as a tool to monitor the risk and progression of myopia.
6,7 These charts are helpful to identify those patients experiencing higher than expected axial elongation, which is a function of age, gender, and ethnicity, and accordingly commence or alter myopia management strategies.8 On a typical axial length percentile chart, a curve represents a percentile across age from the reference population.

Plotting a child’s axial length on an age-specific curve provides a rank relative to the reference population that indicates the percent of the population the child’s axial length would equal or exceed. Plotting follow-up measurements of axial length on percentile charts can be used to monitor axial length growth or the efficacy of myopia treatments by assessing the child’s percentile changes over time. 

Despite technological advancements in the measurement of axial length, using an optical biometer as a gold standard remains a challenge, particularly with the high cost differential between instruments. Moreover, refractive error under cycloplegia continues to be the definitive indicator of myopia classified as -0.50D or worse. Still, no such classification of axial length is agreed upon, though 23.85 mm has been suggested to be the cutoff for myopia onset.4 While high myopia is categorized using refractive error, multiple thresholds using axial length have been suggested, such as 25.5, 26.0, and 26.5 mm. To summarize, refractive error measurements are preferred for categorizing and identifying myopia onset, but axial length measurements are preferred for monitoring progression once myopic.

Jobson's OrthoK Research Report

Only 10% of respondents have an optical biometer. Access the full report here

Efficacy of Different Treatments
Certain myopia control treatments, such as OrthoK, use axial length measurements as the only means of assessing efficacy. However, pharmaceutical treatments such as atropine drops and pirenzepine ointment for myopia control tend to demonstrate efficacy using refractive error only. Studies on low-dose atropine did not significantly reduce axial length progression despite showing a significant reduction in refractive error progression.
9 With the increasing popularity of these pharmaceutical treatments for myopia control in children, replacing axial length in favor of refractive error is likely to be resisted by researchers and clinicians. Axial length needs to be retained as the preferred measure of efficacy if the ultimate objective of myopia control is to reduce the risk of long-term myopia-related diseases rather than short-term gains in refraction. 

Myopia researchers and the FDA are promoting axial length as the preferred metric to assess myopia progression, which has a clear advantage over refraction measurements in terms of precision, reliability, and ease of use. Axial length measurements coupled with percentile charts further improve patient awareness and engagement in myopia management. 


Thomas Naduvilath, Msc Biostatistics, PhD, is the Head of Biostatistics at BHVI and a Conjoint Associate Professor at the School of Optometry and Vision Science, University of New South Wales.  


To read more from Dr. Naduvilath on axial length measurements, click here



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