Binocular vision is a much neglected (and even maligned?) domain of eye care where I’ve had numerous colleagues say their professional excitement and learning opportunities have been reinvigorated through seeing the clinical imperative and application in practice. Not only does binocular vision assessment add so much more to your clinical picture, and make optometric life more interesting, it could be the secret sauce that helps us bridge the gap towards 100% efficacy… or at the very least answer questions in cases where we get lower efficacy than expected.
A colleague from the USA sums this up best in a message she sent me recently: “Kate, You are so awesome. I have watched your video for your students about myopia control and binocular vision and enjoyed it. However, I am an OD that never really paid attention to binocular vision in school and has not carried forward with it as an OD. So, I need help. Like, from the beginning help. I've read directions on how to do BV tests, but would love a video watching someone do them, geared to the primary care OD who is now doing OK. I know how to do distance and near phorias and maybe some other stuff, but not much. Then, how do you interpret and how do the results relate to OK?”
In the Expand your clinical skills portal you’ll find a bunch of useful resources to improve your BV diagnostic and management confidence. But let’s get clear on the ‘why’ of binocular vision in myopia management first with some quick facts.
1. BV can help us to identify children at risk of myopia.
Pre-myopes show a higher accommodative lag that their peers who do not become myopic, with the correlation becoming stronger after onset of myopia, indicating that this may be a feature and a cause of myopia.1 Children with higher response AC/A ratios have an increased risk of myopia development within one year of over 20 times.2
If we see esophoria and accommodative lag in combination with a lower than age-normal level of hyperopia (+0.75 or less at age 6-7 is the biggest risk for future myopia)3, then controlling binocular vision is our main management tool for these children, along with recommending more time spent outdoors.4
2. BV can help identify children who will respond to progressive addition spectacle lens treatment.
In myopia control studies of progressive addition spectacle lenses (PAL), children with esophoria in single vision spectacle control groups were found to progress more quickly, 5 and children with a larger baseline accommodative lag in the PAL groups showed statistically greater treatment effect.6 If we have a child with normal binocular vision, PALs and bifocals may not show much of a myopia control effect. But in the presence of esophoria and/or accommodative lag, PALs and bifocal lenses can show treatment effects approaching that of some contact lens options.7
3. BV is altered in contact lens wear, and could help to ‘power up’ contact lens corrections.
Two key research papers are relevant here. 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% reduction in axial elongation over twelve months compared to single vision soft contact lens wearing controls.8 This is an impressively high result compared to the average 30-50% seen in other similar studies.9
Secondly, children with lower baseline accommodative amplitude have shown a 56% better myopia control response to orthokeratology contact lens wear compared to normal accommodators, in a two year study.10 In this study, they separated the children by the mean accommodative amplitude into ‘below average’ and ‘above average’ accommodators. The children with ‘below average’ accommodation showed the bigger improvement in their amps in OK wear (around 4D more, compared to around 1D for the above average accommodators) and the better myopia control effect. My PhD work showed that OrthoK reduces accommodative lag and esophoria compared to single vision contact lens wear11, 12 – I found a correlation between the level of myopia and the reduction in accommodative lag in OK lens wearing children, but not in adults. Unfortunately, since my study was only 12 months long and the whole group did not progress, (my OK fitting worked so well! 😉 ) I was unable to correlate binocular vision measures to the myopia control effect.
4. BV can influence our prescribing choices and could influence contact lens designs in future.
In the Selecting an option – decision trees resource, the final flowchart combines decision about contact lens suitability and binocular vision status to comprehensively customise treatment to your patient. This is because:
- Esophoria and accommodative lag are likely to be improved by multifocal8, 13-15 and OrthoK contact lens wear11, 12, 16
- Exophoria can worsen when changing the myope (especially the higher myope) from spectacles to contact lenses. The myope reading through their spectacles experiences base-in prism at near, moving the image further away. When this myope changes to a single vision distance (SVD) contact lens correction, they will show an exophoric shift in contact lenses as they lose the base-in spectacle correction.17
There is conjecture, though, about whether non-presbyopes wearing a multifocal contact lens will accommodate normally through the lens,18 or whether they may relax their accommodation and use the ‘add’ of the lens.14, 15 Answering this question could be a big deal in achieving consistent, or better, efficacy of these treatments. Monocular modelling of optical quality through multifocal lenses has shown that smaller central distance optical zones could lead to the young wearer ‘using’ the add at near, resulting in hyperopic defocus from the distance portions of the lens, which is presumably a bad thing in the goal of myopia control. This modelling showed that larger central distance optical zones could encourage a normal accommodation, giving myopic defocus from the ‘add’ portions of the lens, being the goal of myopia control.19
In future, perhaps we will have a measure, or combination of measures of accommodation and binocular vision, to help us 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’s 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% generally seen in contact lens corrections.20, 21
The BV bottom line
Myopia has long been associated with inaccurate and insufficient accommodative behaviour at near and increased accommodative convergence in comparison to emmetropes2, 22-25 – detecting these conditions in both the at-risk emmetrope and myopic child can reveal the picture of myopia progression risk, and their management could provide added benefit to myopia control treatment.
Binocular vision status is additionally relevant to visual comfort – ensuring children have functional skills for reading and schoolwork26, 27 and acceptance of their correction. Research has shown that esophoria at near can slow down reading speed in children26 as vergence problems result in a child having to make far more fixations across a page when reading, each at risk of the vergence error.27
To expand your clinical skills and power up your myopia treatment strategy, have a look at Selecting an option – decision trees, where the final flowchart includes questions on binocular vision status; and make sure to check out the How To Guides to increase your BV diagnosis and management confidence.