Multiple Interventions

The Importance of Measuring Axial Length

August 9, 2019

By Fuensanta A. Vera-Diaz, OD, PhD, FAAO
Associate Professor of Optometry
New England College of Optometry

Why is it necessary to measure axial length for myopia management?
Myopia is caused by a mismatch between the optical power of the eye and its length. The development and progression of myopia in children is due to excessive axial elongation of the eye,1–6 with relatively stable corneal power throughout development.3,5,7–9

There is a strong correlation between the amount of myopia and the length of the eye in all individuals, although the relationship between the two is not linear and not constant throughout childhood.3,10 At certain points during development, axial length (AXL) progresses faster than the amount of myopia, and the reverse happens during other periods. AXL growth charts for children of different racial backgrounds are now available and can help assess whether a child or teenager is at high risk or low risk to develop myopia based on the current AXL (Figures 1 and 2).5,11

 

Figure 1. Growth chart depicting axial length (in mm) versus age for European study subjects, males (left) and females (right), with the risk of myopia in adulthood. The myopia percentage represents the proportion of myopia in halfway above and below the percentage line. From Tideman et al., 2018, Figure 2.5 Reproduced with permission under the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/).

Figure 2. Ocular growth percentile charts (axial length vs age). Sanz Diez et al., 2019, Figure 1.11 Reproduced with permission under the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/).

The main reason to provide myopia management treatments is to prevent the child’s eyes from growing too long. This is because the longer the eye, the higher the risk for associated ocular pathology, even though lower myopias with shorter eyes still have increased risk for these conditions.12–18 If it is decided that a myopia management treatment will be initiated, AXL, along with cycloplegic refraction, is the standard of care to monitor the effectiveness of the treatment.19,20 In addition to the reasons above, if orthokeratology (ortho-k) is chosen as the treatment for myopia management, AXL is the only measure that can be used to evaluate the progression of myopia. Ortho-k corrects the eye’s refractive error by reshaping the cornea; therefore refractive error cannot be used as an indication of treatment success.

Measuring AXL is therefore necessary to (1) determine the risk of associated pathology for an individual patient, (2) predict the risk for myopia development for an individual patient, and (3) evaluate the effectiveness of myopia management treatments.

Do I need to perform both cycloplegic refraction and axial length measurement?
Cycloplegic refraction is necessary to evaluate the progression of refractive error in children. For children who already have myopia, using 2gtt of Tropicamide 1 percent instead of cyclopentolate is accepted to measure cycloplegic (“damp”) refraction.19–21 Prescribing accurately is very important for myopia management in children, and, without cycloplegic refraction, we risk overminusing the child.
Both cycloplegic refraction and AXL measurements are necessary to evaluate a child’s myopia and/or its progression and are recommended every six to 12 months — six months if a myopia management treatment effectiveness is being evaluated.

How to translate axial length in millimeters to amount refractive error in diopters
Even though there is a strong correlation between AXL and the amount of myopia in diopters, the relationship between the two is not linear and not constant throughout childhood. The change in AXL in mm per diopter depends primarily on the child’s age and also on the amount of myopia. AXL correlation with diopters is stronger in older children and for longer eyes. It is important to consider this dynamic relationship between ocular growth and refraction and use both measures to evaluate myopia progression.

Based on a review of the currently available data, the estimation is that a 1 diopter change in refractive error corresponds to 0.28mm increase in AXL for children aged 6 to 7 years, and 1 diopter corresponds to 0.32mm for children aged 12 to 13 years.3,7–10 For adults, 1 diopter corresponds to 0.35 to 0.40mm.3,22

In the COMET study,1 where all children had myopia, 1 diopter of myopia was associated with 0.50mm of AXL in progressing myopes. The mm per diopter relationship increases with age and the amount of myopia. Cruickshank and Logan3 found that for children with low myopia (grouping all ages), a 1 diopter change in refractive error corresponds to 0.32mm increase in AXL, whereas for children with moderate myopia, a 1 diopter change in refractive error corresponds to 0.58mm increase in AXL.

Both measures are necessary as they are not interchangeable. When discussing with parents, it is useful to provide both measures and explain whether their child’s AXL is within norms, as described below.

What are the axial length norms for children and teenagers?
The norms for AXL of the eye during childhood vary significantly with age, and, to a lesser extent but still significantly, with gender and racial background. Table 1 shows a summary of AXL results from nine major studies.4,5,10,11,21,23–26 The average AXL from these nine studies is identified here: for ages 6 and 7, 22.75mm for girls and 23.05mm for boys; for ages 8 and 9, 23.29mm for girls and 23.65mm for boys; for ages 10 and 11, 23.76mm for girls and 24.09mm for boys; for ages 12 to 14, 23.80mm for girls and 24.25mm for boys. These data need to be used with caution as these studies vary in the children’s demographics (age, racial background) and the percentage of myopic children included (from none to 100 percent).

Growth charts of AXL with percentiles are now available for European5 (Figure 1) and Chinese11 children (Figure 2). From Tideman and Sanz-Diez data, the 50th percentile for 6-year-old children follows: European — 22.33mm, Chinese — 22.77mm; for 9-year-old children the 50th percentile is European — 23.05mm, Chinese — 24.02mm; and for 15-year-old children it is European — 23.40mm, Chinese — 24.69mm.

Caution must be taken when applying these data clinically, as many confounding factors affect these norms. There is a clear need to unify efforts among research groups to create comprehensive clinical ocular growth charts that include children of all ages and racial backgrounds. Collaborative efforts are necessary to develop a practical normative database that would allow clinicians to efficiently determine whether an individual child is at risk for myopia development or progression and whether a treatment is effectively managing myopia for a specific child.

Ocular elongation is accelerated during the years preceding the onset of myopia and slows down after myopia onset.5,10 Initially, the AXL progression is compensated by a gradual reduction in crystalline lens power, but the thinning of the lens eventually reaches a physiological limit, at which point myopia begins.10,27,28 Lens power loss is accelerated up to about one year before myopia onset; then it slows down.10 Corneal power is relatively stable throughout development, although corneal biomechanics may change during ocular elongation.29 No significant changes in corneal thickness (CCT) or corneal ratio (CR) occur during the childhood years.9,10 Myopia peak incidence is at 8 to 10 years and the average age of stabilization is at 16 years.4

Which instrument(s) should I use to measure axial length if I decide to purchase a biometer?
There are several options available to measure AXL and other biometric measures relevant to myopia management, such as lens thickness (LT). The instruments available are based on one of four techniques: (1) A-scan ultrasound biometry (e.g., A-2500 Sonomed, Echoscan US-800), (2) A-scan partial coherence interferometry (PCI) (e.g., IOLMaster 500, AL-Scan, Pentacam AXL), (3) A-scan optical low-coherence interferometry (e.g., Lenstar LS 900, Aladdin) or (4) B-scan swept-source optical coherence tomography (SS-OCT) (e.g., IOLMaster 700, Galilei G6, OA-2000, ARGOS).

There are advantages and disadvantages to each of these methods. In summary, A-scan requires anesthesia and is less repeatable in children (60um for AXL and 200um for ACD)30, but it is less affected by accommodation as the child can fixate at distance. When testing children, optical biometers are generally preferred as they have better repeatability. The IOLMaster 500 and the Lenstar LS 900 are the most widely used instruments for myopia management. Repeatability of the IOLMaster 500 is the order of 30-50um for AXL measures and 25-180um for ACD.31–35 The IOLMaster 500 does not measure LT or CCT, and it measures ACD via a slit lamp technique that may not be as precise. The repeatability of the Lenstar LS 900 is 35um for AXL and 40um for ACD.36 The Lenstar LS 900 measures from the front of the cornea to the internal limiting membrane but assumes a standard retinal thickness of 200um. When using the IOLMaster 500 for evaluation of AXL during ortho-k, it is important to consider that ACD may be affected by the treatment and calculate the difference AXL-ACD.37,38 For both instruments, the ACD measures may be affected by accommodation, causing falsely shallower ACD in some children.33,39 The repeatability of the IOLMaster 700 for AXL measures is 24um and for ACD measures 34um.40,41 Note that there are small but significant differences when measuring AXL, ACD, and LT with each of these instruments.40,42–47 It is therefore not recommended to interchange measures among them.

References:

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