Eye axial length: a crucial measurement for myopia control

Including measuring eye axial length is now mainstream for myopia control, as relying solely on refractive error is insufficient.

As a parent, we want nothing but the best for our children, especially when it comes to their health. When my son was diagnosed with myopia, I was determined to learn everything I could about the condition and how to prevent it from progressing.

One of the most important things I discovered was the significance of measuring eye axial length, which is the length of the eye from the cornea to the retina.

As infants grow, their eyes transition from being hyperopic to emmetropic through a process called emmetropization. During this process, their eyes elongate naturally, allowing light to be properly focused on the retina.

However, in cases of myopia, this process occurs more rapidly and overshoots the emmetropic level. This means that children who become myopic experience significantly more axial elongation.

In this article, I will share with you what I learned about the importance of measuring eye axial length for myopia control, how it is measured, and what the research says about the rate of eye growth.

What are the potential complications that can arise from abnormal eye axial length?

A recent review study found that people living with myopia had a higher risk of developing myopic macular degeneration, retina detachment, cataracts, and glaucoma.

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In high myopia, the elongation of the eyes may stretch and thin the retina, causing the cells to die slowly. Additionally, this strain makes the retina more susceptible to tears or even detachment.

The anatomical changes in myopic eyes may also cause excessive production of vascular endothelial growth factor (VEGF), a protein that leads to abnormal, leaky blood vessel growth and, ultimately, myopic macular degeneration.

Medical experts speculate that the increased risk of glaucoma in myopic individuals is due to structural changes to the optic disc caused by the elongated axial length of the eye, leading to damage to the nerve cells in the optic nerve head.

Furthermore, two possible explanations exist for why increasing axial length increases the risk of cataracts. Firstly, the elongated eyeball may lead to insufficient nutrient delivery to the back of the lenses, causing them to lose clarity. Secondly, the biomechanical stretch and damage to the retina cells may cause a cloudy lens due to the production of various biochemical by-products.

Why is measuring axial length necessary for myopia treatment?

This topic has been heavily debated in the medical community, as measuring eye axial length is a relatively new paradigm and is becoming the mainstream practice for myopia control. According to the latest IMI Clinical Management Guidelines report, it is recommended that axial length measurements be taken at least every six months if available.

But why is measuring axial length important? 

While we can generally expect a direct correlation between axial length and refractive error, this is not always the case. In fact, a reduction in refractive progression does not always correlate to a reduction in axial length progression. This means that simply assessing refractive error over time may not be enough to determine whether a chosen treatment is effectively slowing myopia progression.

Published literature has shown that relying on refractive error results alone can be misleading when evaluating the effectiveness of myopia treatment. For example, in an important atropine study called ATOM2, the results showed positive refractive error outcomes for different concentrations of atropine evaluated.

However, a closer look at the axial length revealed a different story; the change in refractive error and axial length were not well correlated. Unlike the higher concentrations, 0.01% atropine had a minimal axial length control effect but was shown to be effective at refractive control.

Since axial length is a key driver for the development of myopia pathology and vision impairment, relying solely on refractive control may not be sufficient.

Furthermore, if orthokeratology (Ortho-K) is the chosen treatment strategy, measuring axial length is actually the only method to monitor the treatment's effectiveness.

How to measure the axial length of the eyeball?

How is the eye axial length of the eye measured? There are two ways.

• Interferometry is a type of optical biometry that uses light waves to measure the length of the eye. This method has been used for many years by eye doctors during cataract surgery to facilitate intraocular lens implant power calculations. One of the main advantages of interferometry is that it is a non-invasive method, meaning that it does not require any contact with the eye. Instruments that use interferometry, such as the Zeiss IOLMaster, Haag-Streit LensStar, and the OCULUS Pentacam AXL, are easy to use on children and have high repeatability and accuracy.
• A-scan ultrasound biometry, on the other hand, is mainly used in research studies and is not typically preferred for children in clinical settings nowadays. This method requires anesthesia and corneal applanation, which can be intimidating for children.

What is the normal axial length of the eye?

According to published research, emmetropic children (without myopia) experienced an annual eye axial growth rate of around 0.09 mm. However, this rate varies with age, with the most significant growth observed in younger children.

How fast does eye axial length grow in myopic eyes?

Without proper myopia control measures, myopic children usually experience a faster increase in axial length. For instance, a study showed that children between the ages of 8 and 11 experienced a substantial increase in axial length every year. However, the increase slowed down in children between the ages of 13 and 16.

While every child's situation is unique, based on the research findings mentioned above, I set off aiming to minimize my son's axial length growth rate to as close to 0.14 mm as possible. To my surprise and delight, during the first eight months of treatment, we were able to achieve no increase in axial length.

Effective myopia control treatment should reduce the rate of change in axial length to significantly less than 0.3 mm per year. Depending on the specific intervention chosen, it may even be possible to bring the rate of change down to a level that is similar to or better than the rate of growth observed in emmetropic children.

For an overview of the change in axial length for different treatments, you can find the summary chart that I created on axial length reduction for different treatment options.

Keep in mind that the effectiveness of each treatment may vary depending on various factors, and it's crucial to work closely with a qualified eye care professional to determine the best treatment option for your child.

Key Takeaways

In conclusion, measuring eye axial length is a crucial step in effectively managing myopia in children. Research studies have shown that abnormal axial length can lead to serious eye conditions such as myopic macular degeneration, retina detachment, cataracts, and glaucoma.

While relying solely on refractive control is not sufficient, measuring axial length is becoming the mainstream practice for myopia control. With the latest technology, measuring axial length is now easy, non-invasive, and accurate.

By monitoring the axial length and slowing down the rate of abnormal eye elongation, it is possible to prevent the progression of myopia and reduce the risk of associated eye conditions.

With the right intervention, it is possible to bring the rate of change down to a level similar to or better than that observed in emmetropic children.

Let's work together to protect our children's eyesight and prevent the progression of myopia.

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