Welcome to the final instalment of myopia content from the American Academy of Optometry 2019 meeting. These updates continue from Part 2, which detailed Mark Bullimore and Noel Brennan's fantastic session entitled Twelve Evidence Based Things That We Should Know About Myopia. This update then concludes with barriers to clinical implementation of myopia management, presented by Manbir Nagra. Links to Part 1 and Part 2 are at the end of this blog.
Axial length is the metric of choice for assessing efficacy
There is a clear disease association with increasing axial length. 95% limits of agreement +/-0.02mm corresponds to 0.05D, meaning that axial length measurement is more reliable and repeatable as a measure of myopia progression than refraction. When it comes to atropine or orthokeratology treatment, it becomes much harder to assess refraction, at least in the academic sense, because of the influence each has on refraction. While there is research and discussion on measurement of choroidal thickness, choroidal variation appears to be small relative to the increments of axial length change that are relevant in myopia progression.
More time spent outdoors is a great idea
It definitely delays myopia onset, but evidence for slowing progression is limited. We don't know whether outdoor effect is due to light, i.e. bluer or brighter, or the different dioptric visual environment when outdoors, where much of the visual field is distant and hence creating minimal dioptric demand compared to an indoor environment. Ian Flitcroft’s paper was discussed, a great paper if you haven't already read it, that shows large amounts of myopic defocus across the visual field when outdoors, compared to large amounts of hyperopic defocus when indoors. When the light is bright the visual system receives more detail in the optical 'message', i.e. there is more detail in the images being received focused on the retina. So, we don't know why, but outdoor time is a great idea, and not just for eye health as it also brings other health benefits too.
Rebound is a real possibility for all treatments
Rebound effects from the abrupt discontinuation of atropine is well known, and in fact led to the favouring of lower concentrations which show less rebound. But what about other treatments? Pauline Cho’s orthokeratology studies have found that there is some rebound over 7 months when OrthoK lens wear is ceased in children under age 14. My comments: the authors described this result as not a 'true' rebound as the rate of change was similar to standard rate of non treated progression in different children - it wasn't any faster indicating a loss of treatment, they simply commenced the same amount of progression as non-OrthoK wearing children. A take home message here from the research group was that it's important to follow children closely if discontinuing a treatment for any reason, and being ready to restart a treatment if myopia progression becomes apparent.
Barriers to clinical myopia control
Dr Manbir Nagra, academic optometrist from England, spoke on the barriers for practitioners practicing myopia control, and started her talk by stating how she finds it incredible to realize that 1mm increase in axial length equates to about -3D of myopia. The reason being that 1mm is so tiny, but even being so small it has an incredible impact on an individual’s lifetime ocular health. The eye doesn’t just elongate, it increases in height and its overall shape; it's not just a two-dimensional millimetre but a global increase in volume. This highlights the need to act early while the eye is still growing.
There are lots of stakeholders in myopia control, and it’s important to consider 'interest vs influence’ for these stakeholders. A practitioner might be willing to undertake myopia control, but if there is no buy-in from the practice owner then their ability to provide myopia control is limited. Research has shown the importance of the grassroots practitioner too, though, in putting centralized directives from larger groups into practice on the front line.
Scope of practice varies greatly across the world – most of Europe is restricted in use of therapeutic drugs, and in some parts of Europe an optometrist is not legally permitted to manage younger children or assess eye health independently. This influences how consistently and comprehensively research can be applied to primary eye care practice.
When it comes to parental views of myopia control, McCrann et al 2018 found 14% of parents thought myopia to be a cosmetic inconvenience only. For those that recognised the eye health concern of myopia, despite intending to make changes to visual environment to help reduce myopia progression, only a small measurable difference was actually seen.
In China, Zhou et al 2017 showed that active parental intervention made a more noticeable difference to treatment outcomes; helicopter myopia parents are helpful! Manbir's closing summary was that there are multiple stakeholders involved that need to come on board if we are to be effective in managing progression of myopia. My interpretation – we all need to continue doing our part to help expand the Resistance!