How to Interpret Axial Length Measurements

May 2, 2022

By Thomas Naduvilath, Msc Biostatistics, PhD, Head of Biostatistics at Brien Holden Vision Institute

With recent advances in technology, the use of AL as a metric to evaluate and monitor refractive error status is gaining significance as the measurement techniques are rapid, non-invasive, and do not require cycloplegia.

The axial length (AL) of the eye is the distance from the cornea to either the retinal pigment epithelium or the internal retinal membrane (depending on the technique used to measure AL). When AL exceeds the focal length of the eye’s optical components, namely the cornea and the crystalline lens, the eye becomes myopic. 

While clinicians typically use refractive error assessment to identify myopia (-0.50D or worse commonly classified as myopic), refractive error assessment without cycloplegia results in a more myopic prescription in children.1 Furthermore, there is variability between instruments and techniques used to measure refractive error. With recent advances in technology, the use of AL as a metric to evaluate and monitor refractive error status is gaining significance as the measurement techniques are rapid, non-invasive, and do not require cycloplegia. Moreover, the repeatability of these measurements indicates higher accuracy compared to refractive error measurements.2 Thus, using AL measurements provides clinicians a quick and powerful means to classify normal versus abnormal eye growth leading to myopia. However, unlike refractive error classification of -0.5D for myopia, no such classification exists for AL, thus highlighting the need for guidance on using AL to assess the risk of myopia and monitor progression. 

Axial Length with Age and Gender
AL must be interpreted as a function of demographic factors such as age and gender. AL is known to increase in early childhood, followed by a slow growth until adulthood. AL from infancy to adulthood is reported to increase from 16.8 mm to 23.6 mm.3 Though a 1 mm elongation of AL is equivalent to a myopia shift of –2.00D to –2.50D in the absence of compensation, the increase in AL during childhood is offset by corresponding changes in lens and corneal power.4 Failure of these ocular compensatory mechanisms leads to myopia and high myopia and significantly increases the risk of vision loss. 

A European population of children and adults showed that AL increased with age until 15, after which AL continued to increase into adulthood in the top 50th percentile.5 Two Chinese urban populations showed that average AL increased from 4 to 18 years, where the rate of increase was higher in the younger ages and gradually slowed with increasing age.6,7 

Both Chinese studies showed that female children had significantly shorter AL and steeper corneal curvature compared to males. Similar gender differences in AL were observed in urban American children.8 

Axial Length Differences Between Ethnic Populations
Ethnicity is an additional source of variation in AL. A comparison of AL between populations indicates that AL for Irish children aged 6, 9, and 15 years was similar to German children.5,9 Similarly, AL for Chinese children in Shanghai was similar to Chinese children in Wuhan.6,7 However, when compared between European and Chinese children, the AL of European children was shorter by 0.37 mm at 6 years, which increased to 0.87 at 9 years and 1.48 mm at 15 years. These differences are likely to be reflective of the higher prevalence of myopia, educational workload, and time outdoors. However, analysis indicates that age and refractive-error-specific AL were higher in Asian children than Caucasians.

Use of Growth Curve Tools for Axial Length
As discussed in the previous sections, the primary sources of variation in AL arise from age, gender, and ethnicity. Normative AL data as a function of age, gender, and ethnicity represented visually as percentile curves help detect children at risk of myopia and monitor progression (for example, see percentile charts in the Oculus Myopia Master or Haag-Streit Lenstar Myopia). On an AL percentile chart, each curve represents a percentile across age from the reference population. These curves spaced at increments of 5%, cover the range from 5th to the 95th percentile of the reference population and can be used to rank the position of a child’s AL by indicating the percent of the reference population the child’s AL would equal or exceed. 

Consider the example of using AL length percentiles for a Chinese female child 7 years of age. The measured AL of 22.40 mm, when compared to the reference percentile chart for Chinese females, indicates that the child was at the 35th percentile at 7 years, meaning that 35 out of 100 females aged 7 years have shorter eye length, and 65 out of 100 females aged 7 years have longer eye length. In this example, while the eye grew to 22.64 mm at 8 years, the percentile remained the same at the 35th percentile, indicating a steady growth pattern. However, at 9 years, the AL measurement of 23.15 mm reflects the 45th percentile, indicating the child’s AL had shifted by 10 percentile points. The 10-point increase may suggest an unusual growth in eye length between 8 and 9 years, with an upward shift in percentiles indicating a higher risk of myopia, thus requiring further investigation and treatment if needed.


Patient Age in years RE Measured AL Age & Gender-Specific Percentile
Chinese Female, starting age @ 7 years 7 +1.50D 22.40 35th 
8 +0.75D 22.64 35th 
9 -0.50D 23.15 45th 


Consider another example of a European male child aged 9 years with a measured AL of 23.5 mm. Their measured AL places them at the 65th percentile, indicating that the child’s eye length was higher than 65% of males aged 9 years. Considering the child’s AL percentiles, refractive error status, and other risk factors, anti-myopia treatment was commenced. The AL increased to 23.6 mm and 23.65 mm in the following years at 10 and 11 years. While AL increased, the percentiles reduced to the 60th percentile, indicating an improvement compared to the child’s baseline percentile and the reference population. 


Patient Age in years RE Measured AL Age & Gender-Specific Percentile
European male, starting age @ 9 years 9 -1.00D 23.50 65th
10 -1.25D 23.60 60th
11 -1.25D 23.65 60th


As observed in these examples, the use of AL measurements and their percentiles can indicate the relative position of the child’s AL and may be used for myopia management. Additionally, follow-up evaluations of AL and percentiles can help monitor growth patterns or determine the efficacy of myopia management options if already instituted. 


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.  



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