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Axial Length in Myopia Management – Why does it Matter?

Updated: Apr 26, 2023

The myopia ‘epidemic’ is real. Undoubtedly, the overwhelming evidence points to lifestyle changes being the major cause of this burgeoning global problem [1]. Only a relatively small proportion of myopia can be completely linked to genetic causes.

It is estimated that half of the World’s population will be myopic by 2050 [2]; this may equate to around 5 billion people by that date.

Worse still, up to a billion of those people will have high myopia. The implications are worrying; many of those high myopes face sight impairment and even severe sight impairment [3].

Thankfully, there is also an enormous amount of evidence that shows it is both possible and prudent to undertake a range of therapeutic approaches to slow or even halt the progression of myopia [4],[5], particularly in children, before it develops to high levels [6]. Doing nothing is not an option – the time to act is now.

Many practitioners are already providing myopia management to their patients. Most of these clinicians do not have access to biometry (I conducted a webinar with 600 optometrists, and 94% didn’t have access to biometry). Does this matter?

The general consensus is ‘Yes’.

Key Myopia Management Measurements and Questions

When carrying out myopia management, certain questions, tests and measurements need to be conducted [7]. These include the following:

  • Questionnaires asking about family history of myopia, the patient’s history of myopia, and lifestyle questions, including how many hours doing any form of near vision tasks, how much time spent outdoors, etc. All practices have access to this.

  • One of the most effective treatments for myopia is Orthokeratology (Ortho-K). These special 'night contact lenses' reshape the cornea (the front of the eye) during sleep so there is no prescription required during the day. Once this therapy is being used, you cannot measure the patient's refraction, so axial length measurement is essential in such cases.

  • Measure the patient’s refraction (often using drops to partially paralyse the eye’s internal muscles – cycloplegia). All practices have access to this.

  • The curvature of the patient’s cornea (keratometry). All practices have access to this.

  • The axial length of the patient’s eye. Most practices don’t have access to this, though devices are already available.

  • Use of some form of algorithm to predict the risk of myopia progression and to plan therapeutic interventions [8] and hopefully show a drop in the rate of progression (flattening the curve). Multiple websites and APPs already provide this function, as do many of the myopia management devices on the market.

  • Myopia is a disease of axial growth of the eye. We should really call the therapy 'axial length management'.

  • Eye drops to partially paralyse the eye muscles are required to conduct refraction properly. These are not needed when measuring axial length.

It's clear that there is a considerable lack of biometry to measure the axial length of the eye in the optometry market.

Why is Refraction not the best measure of myopia progression?

The patient’s refractive correction has historically been considered as the measure of their myopia (or hyperopia). This seems perfectly logical, of course. It is, however, the power of glasses or contact lenses required to correct a patient’s vision. However, the refraction does not represent the axial length of the eye.

Let’s look at what Myopia is. First, we’ll look at a normal, Emmetropic eye. As you can see, parallel light from a distant object is brought into focus on the retina in an eye which has Emmetropia (perfect focus).

Normal Eye

In Myopia, as can be seen in the image below, the light is brought into focus in front of the retina. This is caused either by several potential reasons. Myopia is normally subdivided into either Refractive or Axial (the most common form).

Myopic Eye

In refractive myopia, normally the curvature of the Cornea and/or Lens is too steep (too strong) or also the refractive index of the eye is too high.

In axial myopia, the length of the eyeball from the Cornea to the Retina (the axial length) is too long – this is by far the most common form of myopia [9].

There are occasions where the refractive error of the eye (the patient’s spectacle prescription) does not accurately represent the axial length of the eye [10]. This could be due to the cornea being excessively steep or, more often, unusually flat. This may lead to incorrect prognosis about the risks to the patient’s future visual health.

The expansion of the eye during axial myopia is what leads to long-term damage in myopia sufferers [11]. In refractive myopes, these risks are minimal or even non-existent.

This is one major reason why measuring the axial length is paramount to accurate, repeatable, and valid myopia management.

Myopia Therapies

There is a range of therapies that have been clinically proven to slow the progression of myopia.

Amongst these are two increasingly valuable treatments:

Otho-K (Ortho-keratology) – this is known as ‘night lens’ therapy for lay people. These hard contact lenses are worn during sleep, and they reshape the cornea to remove all or part of the spectacle prescription during the waking hours (leaving the patient's spectacle or contact lens free in many cases during the day). It has also been shown that these night lenses also help to slow myopia progression substantially. However, once a patient starts using such lenses, the refractive error (spectacle prescription) can no longer be measured. In these patients, measuring axial length is essential.

Low-dose Atropine and other potential eye drops. There are some countries where these special eye drops are used to partially paralyse the focusing power of the eyes, leading to a reduction in the mechanical impetus for eyeball growth. However, using such drops can also mean that the measured refractive error is changeable and not repeatable. Again, axial length is essential in these patients. These drops will become licenced and available in the UK and more countries over time.

To conclude it is very clear that most practitioners understand that measuring axial length is truly important and valuable in myopia management.

If you're a parent whose child has been diagnosed with myopia or if they are struggling with their distance vision, we hope you found MyopiaFocus helpful. Please join our community or sign our petition to get the government and NHS to recognise myopia as an ocular disease/severe ocular condition and fund myopia management for children.

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