Myopia control – science or luck?

Mounting evidence from clinical studies published in the last 10 years demonstrate the efficacy of orthokeratology (OrthoK) in slowing myopic progression and controlling axial length increases in children.^1,2 OrthoK has also been shown to be a safe and efficacious means of correcting refractive error.³ But, even with our best intention to slow myopic progression with OrthoK, none of us is truly prescribing OrthoK for myopia control – we prescribe it to correct refractive error, and the myopia control effect we have observed in follow-up exams has been a lucky side effect.

What causes myopic progression? Studies have attempted to find causative factors related to genetic, environmental, and behavioral influences including parental myopia, time spent outdoors, and hours of near work (reading, computer, video games).¹ In addition, there are optical factors. From the extensive work of Smith et al.,^4-6 we have been convinced that relative peripheral hyperopia stimulates the eye to increase in axial length. This is what we create optically when myopia is corrected with spectacle lenses. He also showed that changing the relative peripheral refraction to a myopic status stops the signal for increasing axial length. This change in peripheral refractive profile has been shown to influence axial length changes in both research and clinical arenas. Clinically, as we monitor our patients with OrthoK over time, we observe that the majority of children show little or no change in their axial length and refractive status. But, some children still show myopic progression. When we see myopia progressing in children wearing OrthoK, we may make changes to the OrthoK lens parameters or add additional myopia control strategies (such as atropine) in an effort to control further refractive and axial length changes. Myopia still progresses in some children using OrthoK because we are not yet accurately prescribing for the myopia progression component of our intended treatment, as we are yet to fully understand the mechanisms of both myopia progression and myopia control.

Meta-analyses of studies on myopia progression in children using OrthoK shows a reduction in myopia progression of 40% to 60% compared with children corrected with single vision glasses.^1,2 Recently, a study at University of Michigan of 97 long term OrthoK wearing patients showed that 67% showed little or no change in axial length over a 3-year period. Although impactful and impressive, those numbers also tell us that some patients still progress. Ideally, we would like to be able to totally stop myopic progression and increasing axial length. With further research using new technology, that may be possible.

We need to turn to science not luck! Until we understand more of the mechanism, treating myopia progression with OrthoK is a lucky side effect in many patients as we correct their refractive error. However this does not mean we shouldn’t offer OrthoK to our progressing myopes, and discuss this evidence- based benefit with our young patients and their parents. We still have a lot to understand about the myopia control effect of OrthoK, and how it may be optimized for each individual myope. The same is true of low dose atropine, where some children still progress significantly despite impressive overall group results.⁸

An analogy of this treatment side effect can be drawn to glaucoma. What is the definition of glaucoma? It is an optic neuropathy – damage to the optic nerve fibers that causes loss of vision. But, when we treat glaucoma, do we treat the optic nerve? Not at all – we are treating eye pressure, at the anterior part of the eye. Whether it is a beta blocker, a prostaglandin analog, or a carbonic anhydrase inhibitor; none of our current treatments, medical or surgical, actually treat the optic nerve. They treat intraocular pressure. Efforts to find neuroprotective treatments have not been proven, and none are being used at this time. Similar to OrthoK, many patients with glaucoma treated with pressure- lowering eye drops show good control of their disease, but not all of them. Even with maximum pressure-lowering efforts, some patients still have progression of their disease.

We have the technology and ability to do better with myopia control – we just need more understanding of the mechanisms to match treatments to the individual myope. Back to peripheral refraction, the optical changes created by OrthoK create a myopia control effect, even though the primary purpose is to correct refractive error. To attain even better myopia control for every patient, we need to analyze peripheral refraction as carefully as we analyze corneal topography. We need to find the answer to two important questions.

1. What is the exact amount of relative peripheral myopia that will stop myopic progression?
2. What is the exact location on the retina of the relative peripheral myopia to create ideal myopia control?

In other words, we need to know where the critical spot in the retina is and, once determined, precisely how much myopic defocus to create at that point. Once we know that, we can then create individualized optics in corneal reshaping designs (or soft lenses and possibly spectacles) for each patient to create the precise peripheral myopic defocus at the ideal position on the retina.

As we learn more about myopia control, ultimately it may be the case that the peripheral retina is only playing part of the role. OrthoK has been shown to alter on-axis optics like spherical aberration, which affects accommodation, and this effect can be simulated to some degree in multifocal soft contact lenses. Perhaps this may become a bigger part of the picture in future.

As clinicians, we should continue to prescribe OrthoK for the myopia control effects we have experienced, and have been proven in the literature, to date. But rather than relying on fortunate side effects, new and innovative research should focus on determining a scientific way to prescribe optical systems (OrthoK, soft lenses, or spectacles) for the individual myope which will totally arrest myopic progression.