AAO2019 Part 2 - We Know What we Don't Know

Welcome to Part 2 of the updates from the American Academy of Optometry 2019 Meeting. These updates all sprung from Mark Bullimore and Noel Brennan's fantastic session entitled Twelve Evidence Based Things That We Should Know About Myopia. 

Links to Part 1 and Part 3 can be found at the end of this blog.

Noel Brennan demonstrated with his own phone that when focusing on a device there is a large amount of the peripheral retina that is receiving visual information from around the phone - it is receiving peripheral myopic defocus. Furthermore, the timelines from device emergence to the myopia boom don't match, as the myopia epidemic started before introduction of smart phones in 2007-8. The use of handheld devices may indirectly lead to more myopia by virtue of less time spent outdoors. The long and short of it is that Noel is very much of the learned opinion that hand held devices are not a causative factor.

Noel presented a research summary on the topic:

• A 2019 questionnaire did correlate a significant effect of screen time on prevalence of myopia, but screen time was not defined.
• Hansen et al described a relationship between higher screen time and myopia, but only for lower myopia groups; the relationship was less significant in higher myopia.
• Liu et al reported similar outcomes, but reading and writing had a 5 times more potent influence on spherical equivalent refraction than did screen time. Noel suggested that concentrated reading and writing does not provide same peripheral myopia defocus benefit as a small hand held device, due to the differential surface area in the visual field.
• Cheng et al found that children who used the plus power in a multifocal progressed faster - indicating that accommodation is an important factor. While that may be the case, and Noel was indeed a co-author on that paper, he did discuss the difference between what we are measuring with accommodative lag and what may be truly happening with the accommodative system. An interesting thought on which to end this update.

Recent research update: Lanca & Saw published The association between digital screen time and myopia: A systematic review in January 2020, which concluded that the results are mixed, primarily due to methodology, and further studies with objective measures of screen time are needed.

This is potentially one of the most controversial topics; firstly that we are not as good at measuring refractive error as we think we are! Noel has labelled this as the illusion of great success - where we apply a treatment and see a reduction in progression - where analysis shows that on average, when standard degrees of variance are applied to a group of myopes with a recent history of fast progression, the following year shows less average progression. Also, refractive error is a blunter measurement tool than axial length, meaning that it is difficult to relate previous progression rate in an individual to likely rate of future progression.

An important point here is our inability to predict progression based on previous progression means that we should not restrict myopia control treatment to children demonstrating prior fast progression. The key take home message here is that all myopic children should receive myopia control treatment.

The absolute treatment effect appears to be a constant across age, progression rate and race, but percentage treatment is not. This is tricky to describe without a graph - but essentially, studies in younger children show lower % myopia control effects because the control group progresses more quickly; while the higher % treatment effects are generally seen in studies investigating older children. The absolute treatment effect, in comparison, is the total amount of millimetres of axial length, or dioptres of refraction, by which the treatment group showed a reduced amount at the end of the study. Interestingly, this doesn't appear to show a difference with age.

Percentage treatment effects are useful to evaluate in an individual study, as it shows the effect compared to a control group; but they aren't useful to compare across different studies. Different study durations and age groups are primary reasons why percentage comparison between studies can be misleading. Ethnicity also matters - Asian children tend to show higher percentage treatment efficacy, but again this does not follow through to a greater absolute treatment effect.

Noel explained that myopia control treatment efficacy is apparently like driving a new car away from the dealer - most of the effect (like most of the loss of value in your new car!) occurs right at the start. To attempt to reflect this complexity in understanding efficacy, Noel and Mark have coined the acronym 'The Cumulative Absolute Reduction in axial Elongation', or the 'CARE' factor, which they consider the Gold Standard for describing efficacy. They are in the process of writing a mammoth review paper on myopia control efficacy to help the field understand more.

In total, viewing all of the myopia control studies currently available (generally of 1-3 years' duration), there is no scientific evidence for being able to slow myopia by more than 0.44mm or 1.2D in total. The plotting of these studies appears to show a levelling off over time, so perhaps longer studies would show minimal additional gains over this amount. This analysis of the current data doesn't mean that we can't potentially provide greater myopia control efficacy now and into the future, just that the evidence to date doesn't support any greater effect. Noel and Mark concluded to be aware that myopia calculators predicting large reductions in myopia over a childhood may be misleading - they are useful to demonstrate myopia progression but demonstrating treatment effect should be undertaken with caution. You can read more about research on the accuracy of myopia calculators compared to real world progression data in our International Myopia Conference Update Part 2.

Atropine slows myopia progression, but the concentration of choice is unclear. How does atropine control myopia? It is effective in controlling experimental myopia in chicken studies, but the ocular anatomy of a chick and that of a small human are quite different. Another complication is that while 1% atropine is a readily available commercial preparation, almost all lower concentrations than 1% are currently compounded. While compounding pharmacy's processes are of course stringent and well controlled, there is no set standard for formulating these lower concentrations. In particular, variability in pH of the formulations, including transport and storage, can dramatically alter the form of the drug and hence could influence its efficacy. Developing stable, low-concentration, commercially prepared versions of atropine is a rapidly developing area - stay tuned for updates.