First published: June 29, 2018
Updated: August 16, 2021
Binocular vision is a much neglected (and even maligned?) domain of eye care - even though 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 higher myopia control efficacy, or at the very least answer questions in cases where we get lower efficacy than expected.
1. BV can help us to identify children at risk of myopia
Pre-myopes show a higher accommodative lag than 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. Children with higher response AC/A ratios have an increased risk of myopia development within one year of over 20 times.
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 strongest risk factor for future myopia), then controlling binocular vision is our main management tool for these children, along with recommending more time spent outdoors.
Intermittent exotropia (IXT) has also been associated with a higher prevalence of myopia - 50% of children with IXT are myopic by age 10 and 90% are myopic by age 20.
Read more about this in How to identify and manage pre-myopes.
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, and children with a larger baseline accommodative lag in the PAL groups showed statistically greater treatment effect.
If we have a child with normal binocular vision, PALs may not show much of a myopia control effect. But in the presence of esophoria and/or accommodative lag, PALs can show treatment effects approaching that of some contact lens options. Overall, though, bifocals seem to be more effective than PALs across various BV presentations. Learn more about how PAL and bifocal spectacle lens prescribing in myopia relates to binocular vision in When to prescribe spectacles for myopia control.
3. BV is altered in contact lens wear, and could be related to efficacy
Three 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 neutralize the associated phoria, resulted in a 70% reduction in axial elongation over twelve months compared to single vision soft contact lens wearing controls.
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. Study participants were separated 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.
Finally, a study investigating a soft contact lens designed with positive spherical aberration for myopia control (think of it as a type of 'distance centred' multifocal) showed that children wearing this lens relaxed their accommodation compared to a single vision control lens. This reduction in accommodative response was correlated with a reduced myopia control effect in the test lens, but no such association was found in the control (single vision) lens. This study was the first to link accommodation with myopia control efficacy in soft contact lenses, with the relationship in OrthoK indicated previously.
If you want to deep-dive into the science on this, check out this research summary entitled Spherical aberration, accommodation and multifocal soft contact lenses. If you're interested in the clinical side of this, check out Which multifocal soft lens? Efficacy and visual function.
4. BV can influence our prescribing choices for comfort and acceptance
To comprehensively customize treatment for your patient, managing their binocular vision can influence visual comfort as well as myopia control efficacy. We know that:
- Esophoria and accommodative lag are likely to be improved in orthokeratology wear.[12,13]
- Multifocal contact lenses may also cause a small exophoric shift but may slightly increase accommodative lag depending on their design.[14-16]
- The dual focus concentric contact lens design (MiSight) appears not to alter accommodation or phoria.[15-17]
- The new generation of myopia controlling spectacle lenses (eg. DIMS Hoya MiyoSmart) and H.A.L.T technology (Essilor Stellest) also don't alter binocular vision function.[18,19]
- Exophoria and accommodative lag can worsen when changing the myope (especially the higher myope) from spectacles to contact lenses.
Why does this matter? Firstly symptoms of binocular vision problems can be confused with dry eye symptoms - this has been shown in young adult myopic CL wearers. This could be extrapolated to children; although the link between binocular vision disorders and reduced reading speed and comprehension could be more concerning.[22,23] Secondly, if a child does not accommodate normally through a multifocal contact lens, it could influence the ideal optics required for myopia control efficacy.
Learn more about how to factor binocular vision into your prescribing decisions - for both visual comfort and myopia control efficacy - in Selecting an option: Clinical Decision Trees.
The BV bottom line
Myopia has long been associated with inaccurate and insufficient accommodative behaviour at near and increased accommodative convergence in comparison to emmetropes.[4,25-26] 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, to ensure children have functional skills for reading and schoolwork and acceptance of their correction, and to avoid dry eye-type symptoms in young adults.
If binocular vision function is normal, then selecting a myopia control option is uncomplicated - choose the best treatment that you have available, that suits the child and family. If binocular vision is abnormal, though, managing this as well as managing myopia may require different strategies.
Watch these Myopia Profile YouTube videos on binocular vision techniques:
Read more about accommodation, binocular vision and myopia:
Want to learn more about binocular vision?
Check out my online course Binocular Vision Fundamentals, which starts with my two-system approach to BV assessment and diagnosis. Stepping through understanding of the accommodation and vergence systems, the course then covers clinical tests, diagnostic criteria, prescribing and management. Once this foundation is set, it moves onto clinical communication and the importance of BV in myopia management. Always with a laser sharp focus on the clinical applications.
Included are video examples of assessment techniques and chairside infographic summary downloads to reference in practice.
You can enroll on the first two modules for free, with the full course priced at US$140 if you decide to continue. Reduced course fees by 30% and 50% are available by application for practitioners residing in lower income countries - check out the course page for more information.
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