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Eight Myopia Mysteries

2 CPD in Australia | 1G in New Zealand | 1 December 2018


 

By Dr. Kate Gifford

Despite increasing research into myopia, many mysteries remain about its etiology, its progression, and effective treatment strategies. This article groups common clinical treatments in an effort to explore what we do and don’t know regarding myopia control efficacy. It highlights the need to balance the available evidence with emerging knowledge when discussing options for myopia control with patients and their carers.

LEARNING OBJECTIVES

  1. An understanding of the key treatments and presumed mechanisms for myopia control
  2. An appreciation of the horizons of research understanding and its translation to clinical practice
  3. An understanding of the difference between the evidence base and anecdotal observations in myopia control efficacy, to ensure appropriate informed consent for parents and young myopic patients.

There is a lot we know about myopia control. Firstly, we know it is imperative to reduce the final level of myopia for an individual because this will reduce their lifelong risk of vision impairment from myopia-associated pathologies like myopic macular degeneration.1

Secondly, we know there are numerous options in the toolkit for myopia management – spectacles, contact lenses, and pharmacological options – with more on the horizon.

Finally, the sooner we start treating myopia the better – all paediatric myopes are progressors until proven otherwise, and will generally progress more quickly at younger ages.2

While no strategy yet guarantees 100 per cent efficacy, if we can reduce myopia progression by 50 per cent – the general mean efficacy of treatments currently available – we will reduce the incidence of high myopia across the population by 90 per cent.3

All of this evidence gives us the why, when, and how of myopia management. With increasing clinical research and industry interest in this area, it’s crucial for practitioners to understand the difference between clinical observations and the evidence base to ensure appropriate informed consent for paediatric patients and their carers.

Anecdotal observations drive research outcomes and ultimately industry innovations. Founding and facilitating the popular ‘Myopia Profile’ Facebook group some two years ago has seen the author observe an interesting combination of practitioner attitudes to the evidence – from reticence to accept credible studies to equating anecdotal observations with research outcomes. In another few years, an article like this may look very different – perhaps many of these ‘mysteries’ below will be proven. Until then, however, we have a responsibility to our patients to provide accessible information on the evidence base and to avoid over-promising results.

Mystery #1

What Drives Myopia Development and Progression?

Myopia development and progression is postulated to be due to numerous factors, including ethnicity, family history, time spent outdoors and on near work, and binocular vision status.4 Interventions, which have shown clinically significant efficacy for reducing refractive and axial progression of myopia, include progressive and bifocal spectacles, orthokeratology (ortho-K), dual focus and multifocal soft contact lenses (MFSCL), and atropine in various concentrations.5 The mechanism of atropine is unknown; the various mechanisms postulated for the optical treatments are best explained by the concept of simultaneous defocus – whether cast on the central or peripheral retina, contrasting myopic, hyperopic and/or emmetropic signals appear to control eye growth.

Numerous experimental models using animals have shown that an eye will grow in response to a hyperopic signal, and slow or halt growth in response to a myopic signal. Moreover, the eye will preference the more myopic signal rather than ‘averaging’ the signals via local retinal mechanisms.6 It is proposed that creating relative peripheral myopia will create a powerful ‘stop’ signal to eye growth, due to the sheer surface area of the peripheral compared to central retina. Ortho-K and MFSCLs significantly shift peripheral refraction – although not equivalently7 – and both seem to work for myopia control.5 However there is conjecture that shifts in relative peripheral refraction have anything to do with myopia progression.8 Similarly, on-axis simultaneous defocus occurs through the depth of focus created by ortho-K or MFSCLs, which feature significant shifts in spherical aberration.9,10 Changing spherical aberration in turn changes accommodative demand at near11,12 – just as a MFSCL supports accommodation in a presbyope, so it will have some effect on the young accommodating eye. One or all of these mechanisms could drive myopia progression, and by turn myopia control, in the individual patient. The ability to determine which mechanism could help to customise treatments and achieve maximum efficacy in the future.

Mystery #2

Do Progressive And Bifocal Spectacles Work?

Single vision spectacles have been used countless times as the placebo or control treatment in myopia control studies, indicating that they offer no efficacy benefit.2 Undercorrection of myopia has also shown no benefit and may even increase myopia progression.13 This leaves bifocal and spectacle lenses, which have shown unimpressive results for myopia control efficacy in several studies across an entire patient group.14-16 However, results have been clinically significant for children with two or more prism dioptres of near esophoria17 and accommodative lag of 0.50D or more at 33cm.15 In a well designed study, exclusively recruiting young myopes with demonstrated myopia progression, bifocal spectacle lenses with a +1.50 add were found to have a reasonable myopia control effect. This efficacy was increased with the inclusion of 3BI prism in each eye to balance the exophoric shift created by the add, with consistent results found across all phoria groups.18 While this single study does not necessarily indicate that bifocal spectacles are the first choice for spectacle lens myopia management – factors such as facial frame fit and acceptability of lens appearance will factor into this prescribing decision – the sum of these studies indicates the role of spectacle lens options which are likely to be the first line treatment, especially for younger myopic children and while introducing the concept of contact lenses.

It appears these lens options do work, although the mechanism is unclear. There is conjecture as to whether accommodative lag drives myopia progression, as it has been shown to be unchanged after myopia onset,16 but this does not discount other findings of improved myopia control efficacy by reducing it.19 An alternative theory is that the large area of plus power in the inferior field of vision creates a large area of relative peripheral myopia in the superior retina,16 however the vertical visual field of myopes exhibits relative peripheral myopia anyway, unlike the horizontal field which exhibits relative peripheral hyperopia.20 In clinical practice, prescribing not one single add to all children, but that specific to their binocular vision presentation, could yield different results but this has not been studied. The future of achieving better efficacy with spectacle lens myopia control could fall to novel designs, such as the Defocus Incorporated Multisegment Spectacle (DIMS) lens. This lens uses innumerable +3.50 add ‘lenslets’ across the lens, sparing a central distance optical zone of 10mm, to create a different type of simultaneous defocus to that presumed by progressive or bifocal spectacle lenses.21

Mystery #3

Is Low Dose Atropine Useful?

Low dose (0.01 per cent) atropine is the commonly accepted hero of myopia management, on the basis of the ATOM2 study which examined 0.5 per cent, 0.1 per cent and 0.01 per cent atropine and found that when rebounds were included, children treated with 0.01 per cent atropine showed the lowest refractive myopia progression over two years of treatment, a year of cessation, then another two years of treatment – around 50 per cent reduction in total. Its drastically reduced side effect profile, compared to higher concentrations, has also made it attractive.22 But does it actually work? Controversially, a 2018 analysis of the ATOM2 data has revealed that when compared to an historical control, 0.01 per cent atropine had no significant effect on axial elongation.23 This is rightfully concerning in prescribing a treatment in pursuit of not just refractive attenuation but axial length too, as reducing axial elongation appears to be the key risk factor in reducing risk of future myopic pathologies and uncorrected vision impairment.24 In addition to this finding, newer studies are showing similarly low efficacy of 12 per cent for axial length reduction and 27 per cent for refractive progression over one year of 0.01 per cent atropine treatment.25

If low dose atropine isn’t useful for axial length control, what about stronger concentrations? Concentrations of 0.5 per cent and 0.1 per cent have shown enormous reductions in accommodative amplitude – down from 16D at baseline to 2-4D after two weeks of treatment – which impair near acuity.22 Concentrations of 0.05 per cent and 0.025 per cent atropine have shown only a small reduction in accommodative amplitude of around 2D, but currently only one study has reported their myopia control efficacy over one year.25 The question of tapering dosage to reduce rebound risk is also unanswered – for how long and at what dosage is ideal? Moreover, a pilot study on young adults showed that twice weekly dosage, rather than nightly, improved patient satisfaction and did not alter accommodation amplitude, although the effect of this on myopia progression has not been studied.26 All taken, it would appear that atropine is not the ideal first line treatment for myopia management if a child is suitable for contact lens wear, or can be expected to respond to a spectacle lens treatment.

Mystery #4

Which Add To Use?

Various designs of dual focus and MFSCL have been evaluated for myopia control efficacy, with all showing useful results of around 30-50 per cent control for both refractive and axial length change.27 There is a belief that a higher add will create more simultaneous defocus (whether central or peripheral) and therefore a better myopia control effect, but this has yet to be proven beyond anecdotal accounts. The basis for this belief is likely that MFSCLs have been shown as less predictable in their induced peripheral refraction shift compared to ortho-K.7 A study undertaken on emmetropes showed that a distance centred MFSCL with an add in excess of +3.00D was required to significantly shift the peripheral refractive pattern from baseline.28 This could require an even higher add in myopes who tend to show greater relative peripheral shifts compared to emmetropes.29 The problem with this, though, is that children have been shown to not visually tolerate adds above +2.5030 and even with a +2.50 add, an extra -0.50 to -0.75D of central refractive power needs to be added, on average, to achieve good acuity.31

Experimental models in animals have indicated that treatment results from simultaneous defocus don’t appear to be graded – an all or nothing response means once the defocus reaches a certain threshold, the response is achieved.6 The Bifocal Lenses in Nearsighted Kids (BLINK) multi-site study in the USA is currently investigating distance centred MFSCLs with a +1.50 and +2.50 add to evaluate differences in myopia control efficacy and peripheral refractive shift.32 Lower adds in MFSCLs will likely be better tolerated by more children, leading to improved compliance – while it is likely there will be individual variability in the amount of simultaneous defocus required, if humans respond similarly to experimental models with an all or nothing result, then the lowest add that achieves the result will be ideal.

Mystery #5

Are MFSCLs Better Than Ortho-K For Low Myopes?

Building on the previous mystery, ortho-K’s consistent peripheral refractive shift in a one to one relationship with its central refractive shift33 has drawn criticism for its simultaneous defocus propensity in lower powers. To use an example, a -1.50 ortho-K treatment will induce an approximate 1.50 shift in relative peripheral myopia – comparison to the availability of higher adds in MFSCLs has led some to postulate that ortho-K will provide lower myopia control efficacy for myopes under 2-3D. There are two logic errors in this assumption. Firstly, as aforementioned, the effect of the labelled add power of a MFSCL does not translate through to a consistently similar shift in relative peripheral refraction as it does for ortho-K. A study evaluating a +2.00 add distance centred MFSCL for myopes of -2.8D, on average, found no significant shift from the baseline peripheral refractive profile compared to ortho-K wear, which created a significant shift in a repeated measures comparison.7 A comprehensively designed industry project, where a MFSCL was designed to ‘mimic’ ortho-K in its anterior optical profile, was recently reported as having no significant benefit for myopia control, indicating that MFSCL and ortho-K present different optical properties to the eye.34 The authors fascinatingly postulated a possible mechanical mechanism for ortho-K (simply occupying space in the orbit?) but the peripheral refractive shift of the novel MFSCL was not confirmed to match that of ortho-K in a similar power. There is still a way to go before being able to define which lens type is best for a patient based on their level of myopia, except for myopia over 4-5D where ortho-K treatment becomes more challenging.35

Mystery #6

Do Smaller Ortho-K Zones Give Better Myopia Control?

Ortho-K treatment generally boasts a 4–5mm central zone of topographical flattening to correct myopia36 while a much smaller central optic zone in MFSCLs (proprietary information so not usually defined) has been shown to reduce acuity in the latter compared to the former.7 There is a belief that reducing the treatment zone of an ortho-K lens will increase the relative peripheral myopic shift achieved and therefore lead to a better myopia control effect. This assumption is likely based on a study where a larger pupil diameter was shown to result in a better myopia control effect of ortho-K than for those with smaller pupils (scotopic diameter of 6.43mm was the average and the delineation point). The authors speculated that larger pupils would allow for a greater peripheral surface area of defocus, but peripheral refractive shift was not measured.37 Anecdotal reports of better myopia control with smaller optic zone diameters in ortho-K are not new, but the evidence to show that these actually result in an altered topographical outcome is brand new, having been published in 2018.38 No research has yet shown that this alteration in topography outcome results in an alteration in peripheral refractive profile, although research is underway (Gifford, P – personal communication). The final kernel of truth will be if this alteration in topography and peripheral refractive profile then results in an improved myopia control effect, which would require a large longitudinal study. Since this is yet unproven, it can be considered that any ortho-K lens design will effectively control myopia by around 50 per cent on average,39 until the science proves otherwise.

Mystery #7

Could Binocular Vision Influence Contact Lens Efficacy?

It makes sense that when prescribing a MFSCL design to children, especially a design originally developed for presbyopes, some effect on accommodation could be expected. Esophoria and accommodative lag have been shown to improve in multifocal,22-25 and ortho-K contact lens wear,26-28 although not as may be predicted by the labelled add power of a MFCL, or central refractive power of the ortho-K treatment. Applying a +1.50D add in spectacles will have a predictable effect on binocular vision; this same +1.50D add labelled on a MFSCL may behave as a +1.50D, or +0.50D, or even have a minimal add effect. Some studies have shown that children accommodate normally through these lenses,29 while others have shown children and young adults may alternately relax their accommodation and use the add of the lens at near.24,25 None of the latter have yet correlated the accommodative response with the myopia control effect. The interaction between binocular vision and MFCL or ortho-K correction could prove the next frontier for both customising treatments and achieving better efficacy for myopia control.    

Two key research papers are relevant to understanding how binocular vision function could help to increase the efficacy of contact lens corrections. Firstly, fitting bifocal soft contact lenses to myopic children with esophoria at near, where the add was chosen to neutralise the associated phoria, resulted in a 70 per cent reduction in axial elongation over twelve months compared to single vision soft contact lens wearing controls;23 This is an impressively high result compared to the average 30-50 per cent seen in other similar studies.14

Secondly, children with lower baseline accommodative amplitude showed a 56 per cent better myopia control response to ortho-K contact lens wear, compared to normal accommodators, in a two year study.40 In this study, the children were separated by the mean accommodative amplitude into ‘below average’ and ‘above average’ accommodator categories. The children with ‘below average’ accommodation showed the bigger improvement in their amplitudes in ortho-K wear (around 4D more, compared to around 1D for the above average accommodators) and had a better myopia control effect.

How a child accommodates through a MFCL could influence its efficacy for myopia control. Monocular modelling of optical quality through multifocal lenses has shown that smaller central distance optical zones (such as in a presbyopic MFCL design) could lead to the young wearer ‘using’ the add at near, resulting in hyperopic defocus from the distance portions of the lens both centrally and peripherally – presumably a deleterious outcome for myopia control. This modelling also showed that larger central distance optical zones (such as in ortho-K) could encourage a normal accommodation, giving myopic defocus from the ‘add’ portions of the lens, being the presumed goal of myopia control.31

In future, perhaps we will have a measure, or combination of measures of accommodation and binocular vision, to help practitioners pick the best lens design for the individual – perhaps normal accommodators get one type of lens, while below average accommodators get a different type. It is likely that one single add – as employed by some myopia controlling soft lenses – won’t give us everything we need to bridge the gap to higher efficacy than the average 50 per cent generally seen in contact lens corrections.32,33

Mystery #8

Does Adding Atropine Increase Efficacy?

The mechanism of atropine is not yet understood, although it is assumed to be different to that of the optical interventions. Whether these mechanisms are additive or not is of great interest to clinicians keen to offer the best strategy to fast progressing young myopes. One study has pointed to a possible mechanism of combining treatments, investigating the interaction of atropine and retinal defocus on subfoveal choroidal thickness – choroidal thinning has been measured as a short term response to various myopigenic stimuli, being a likely precursor for eye growth.41 In Taiwanese children aged six to 14 years with low to moderate myopia, one hour of myopic defocus caused subfoveal choroidal thickening while hyperopic defocus caused thinning. After treatment with 0.3 per cent atropine for at least a week, hyperopic defocus no longer caused thinning, indicating that the atropine had abolished the thinning response while not interfering with the thickening response to myopic defocus. This indicates that myopia control could be enhanced by a combination of optical and pharmacological methods, however the study used a concentration of atropine not typically used clinically, which carrys a high side effect profile for mydriasis and reduced accommodation.

The aforementioned study, where ortho-K appeared to provide better myopia control efficacy for children with larger pupils,37 has led to extrapolations that using atropine to increase pupil size could improve the efficacy of ortho-K or even MFSCLs. It’s important to note though, that the lower doses of atropine have a minimal effect on pupil size and the direct relationship of pupil size to myopia control efficacy has not been explored – the study described above simply divided the group in half based on pupil size and didn’t correlate increasing size with increasing myopia control efficacy. 

Two studies of combining atropine with orthokeratology have been published in 2018. One investigated 0.125 per cent and 0.025 per cent in combination with ortho-K and reported an improved response, but only reported final axial lengths without any baseline comparisons, rendering the results invalid.42 The second was a small study with 20 children wearing ortho-K and 20 wearing ortho-K in combination with 0.01 per cent atropine treatment. Over one year, these authors found a 52 per cent better myopia control effect with the combination treatment, but declared the results not definitive due to a lack of statistical power and age skew in the ortho-K wearing only children, who were younger and hence more likely to progress.43 A larger and more rigorous study is being undertaken in Hong Kong at present with results expected next year.44 No studies have yet investigated MFSCLs used in combination with atropine.

Keep An Eye On The Future

These eight mysteries demonstrate the fascinating area of research and clinical practice that is myopia control. They should not discourage the practitioner from taking action to manage childhood myopia, by having conversations with parents and patients about the options for both at-risk and current myopes, and using the various clinical tools available to support clinical myopia management. Instead, these mysteries should encourage practitioners to balance the available evidence with new knowledge to provide appropriate informed consent opportunities for parents and patients, and to keep a curious eye on the future.  

 

   

Dr. Kate Gifford PhD, BAppSc(Optom)Hons, GCOT, FBCLA, FIACLE, FCCLSA, FAAO is a clinical optometrist, peer educator, and researcher in contact lenses, binocular vision and myopia control. Dr. Gifford holds four professional fellowships, has been published in 45 peer reviewed and professional publications, and has presented over 100 conference lectures throughout the world. In 2017, Dr. Gifford was awarded the inaugural BCLA President’s Award for Excellence for her peer education work, and also named the QUT Young Alumnus of the Year. After completion of her PhD in 2018, Dr. Gifford is now a Visiting Research Fellow at QUT and the Chair of the Clinical Management Guidelines Committee of the International Myopia Institute.  She practises at Gerry & Johnson Optometrists in Brisbane, Australia. 

 

 

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44. www.clinicaltrials.gov/ct2/show/NCT02955927

' it’s crucial for practitioners to understand the difference between clinical observations and the evidence base to ensure appropriate informed consent '