Six questions on axial length measurement in myopia management

Axial length (AXL) has been well established as an important, critical measurement in studies looking at the progression and control of myopia. It also has clear measures in assessing the risk profile for myopic degeneration; when the axial length reaches and lengthens beyond 26mm the risk of posterior pole complications and potential vision impairment is much higher.^1,2 For a look at the clinical use of single versus repeated measures of AXL, and the approach of the International Myopia Institute, read our blog Axial Length Measurement – A Clinical Necessity? Measuring AXL in clinic requires specialty equipment not found in most primary eye care practices. So how crucial is it? Is it required for practicing myopia control?

There is the simple formula you may have held as truth, that 1mm=3.00D. Easy? Not quite. A variety of papers have shown that this relationship is complex. In the three year MiSight study,¹ the correlation became stronger as the children got older – meaning the relationship was more variable in younger ages. Across the three year study, they found that a 0.1mm change in axial length corresponded to a 0.24D change in myopia, in both treatment and control groups, giving a ratio of 2.40D/mm. In the newly published BLINK study,² this ratio was 1.44D/mm in the high add (+2.50) multifocal contact lens wearing group, 1.55D/mm in the medium add (+1.50) group and 1.63D/mm in the single vision CL group. While we don’t know if those ratios are statistically different, it highlights how much we still need to learn about this ratio. A big part of the complexity is due to the variability in measurement of both refraction and axial length. Axial length measurement is considered the gold standard in the research context when investigating myopia control strategies, and is becoming increasingly important in the clinical setting where reducing axial elongation to reduce lifelong eye health risk is the key goal.³

However even in a research setting, the International Myopia Institute agrees that refractive error should be used in conjunction with axial length measurement to evaluate success of treatments.⁴ They noted that subjective refraction can be more variable, however axial length is not the only ocular component to change throughout childhood - the cornea and crystalline lens are also key anatomical sites of refractive change and contribution. Refraction provides a summary measurement of all ocular structures, and is routinely and universally measured for all myopes. Despite this, in the case of orthokeratology when the refraction is intentionally altered, the axial length may be the only parameter that can be used as a gauge of myopia control outcomes. Also, since the refraction-to-axial-length ratio seems to come unstuck with low dose atropine – where the refractive outcomes outshine the minimal control of axial length⁵ – knowledge of axial length progression provides the primary measure of myopia control success.

Yes, you can, but refraction may be more accurate and simpler – more on that shortly. On axial length, the Comparison of Ocular Component Growth Curves Among Refractive Error Groups in Children paper⁶ measured the ocular components and axial lengths of 737 children aged from 6 and 14, who were assessed at least three times each. With this data the authors evaluated relationships between the parameters and growth patterns where the end results were either hyperopia, emmetropia or myopia.

Children who became myopic had a similar axial length at age 6 to children who stayed emmetropic, although it quickly showed escalation over time (see the graph below). This highlights the importance of early intervention. Patients who were hyperopic and emmetropic had similar models of growth. The authors developed formulas for predicting changes with age for the children who stayed emmetropic, and compared this to the other refractive error groups for significant differences.

There is a question of the variability of axial length measurements, both between visits and between different instruments. Most present day studies employ non-contact, interferometry measurement instruments rather than the older generation A-scan applanation ultrasound technique. The IMI Clinical Trials and Instrumentation report⁴ explains that ultrasonography is limited in resolution to about 0.30D, whereas interferometry measurements have resolution of around 0.03D, making this technique an order of magnitude better as a measure of myopia progression. Compare this also to refraction – cycloplegic autorefraction has a repeatability of ±0.21D.¹⁰

Studies which evaluate axial length take multiple measurements and use the average in reporting. How many measurements are needed? Some studies measured 10 times,¹others measured 5 times.^2,8  The IMI Clinical Trials paper does not make a specific recommendation on this.⁴

If you are looking at a particular instrument, search the literature and/or ask the company if data has been published on repeatability of that instrument.

When the refractive error is intentionally altered to plano or nearabouts, the efficacy of orthokeratology for myopia control can be monitored via axial length measurement, without having to “wash out” the patient from the OrthoK to measure their full refraction. “Wash out” also requires ensuring the cornea has returned to topographical baseline to appropriately judge refraction change, presenting inconvenience to the patient and additional chair time for the practitioner. Clinicians may measure the refraction with the lens-on-eye, but since OK lenses can warp with time this could also prove inaccurate. Research has suggested that OrthoK's induced corneal changes will have minimal influence on repeated axial length measures, when comparing pre- and post-OK.¹¹

There is a similar complication with atropine treatment. When it comes to axial length, the one year LAMP study⁵ found a 12%, 29% and 51% control effect with 0.01%, 0.025% and 0.05% concentrations respectively, but the refractive results were a much more impressive 27%, 42% and 66% respectively. By comparison, the three year MiSight study showed a strong correlation between axial and refractive control, with similar percentages of just over 50%.¹ While comparing percentages across studies is problematic (and that’s a discussion for another blog or two or three!), the evident mismatch between atropine refractive and axial control additionally points to the value of axial length measurement.

It could dictate the level of proactivity needed. A child who is -3.00D and 25mm may need more proactive treatment and closer monitoring than the same -3.00D who is 24mm. We’re still learning if axial length is a better predictor of ocular disease risk than refraction – there are more prevalence studies available which have measured myopia by dioptres rather than axial length.³ One large-scale, multicentre analysis has indicated that axial length is more closely related to myopic complications than refractive error.¹²

Axial length can also determine other necessary eye health monitoring. For example, it is wise to schedule an annual fundus examination through dilated pupils for a patient if their axial length is over 26mm, regardless of their refraction, due to the increased risk of pathology. Here the data is striking – among people aged 75 years or older, cumulative risk of vision impairment or blindness from any cause increases from 3.8% in eyes less than 26mm to 25% in eyes longer than 26mm; and to more than 90% in eyes longer than 30mm.¹²

There is no doubt that measurement of axial length will provide the more accurate indicator of myopia progression and control – as described above, optical biometry methods are likely to be 5-10 times more accurate than refraction.^4,10   This is why axial length is a required measurement for treatment comparisons in myopia control research, especially to enable evaluation across different studies and patient groups. At this stage, however, it is not a requirement to safely and effectively practice myopia management. In the International Myopia Institute Clinical Management Guidelines, axial length measurement was included as a ‘standard procedure’ but with the caveat that there is currently no established criteria for normal or accelerated axial elongation in a given individual.¹³

As accessibility to axial length measurement technology becomes easier, and more evidence is developed to support normative and typical myopic patterns for axial length growth across a variety of ethnicities and populations, axial length measurement will become increasingly important in myopia prediction and management. Keep an eye on the research and available equipment, because axial length measurement may become standard of care in our comprehensive myopia management of the future.

• Axial length measurement - a clinical necessity?

• Axial length measurement in myopia management – how often and how much change is normal? A clinical case study.

• When you have low myopia and high axial length - A clinical case study.

• How reliable are growth charts for predicting myopia onset and progression?