Changing a myope from spectacle to contact lens wear can alter their binocular vision (BV) function. The myope reading through their spectacles experiences base-in prism at near, as demonstrated in the image above, which moves the image further away and decreases vergence demand. Looking away from the optical centre of the lens (while keeping the vertex distance constant) also reduces the power, effectively reducing the accommodative demand. When this myope changes to a single vision distance (SVD) contact lens, he or she must increase their accommodation, which is why the early presbyopic, moderate to high myope always needs an add in their contact lenses before they do in their spectacles – and the high myopes can sometimes survive presbyopia without ever needing progressive spectacles.
Hence myopes of any age must increase accommodation demand in a SVD contact lens correction; they will show an exophoric shift in contact lenses as they lose the base-in spectacle correction and have to converge more by themselves.(1) This can be beneficial to moderate-to-high myopes with esophoria in spectacles, where the spectacle-to-contact lens effect will be greater, and given that esophoria at near has been associated with a higher rate of myopia progression in SVD spectacles.(2) These theoretical calculations have been confirmed experimentally in young adult contact lens wearers, with the differential between spectacle and contact lens binocular vision function at near generally increasing with increasing contact lens power, although it varies between individuals.(1)
Sienna's story - the myope
Sienna first presented at age 16, with progressing myopia and complaints of unstable distance vision in her glasses. Cover test quickly revealed the problem – a significant divergence insufficiency esophoria, breaking into an intermittent esotropia, worse at distance than at near. She was keen on getting rid of her -4.50 glasses and into full time contact lens wear. In Sienna’s case contact lens wear served a double benefit through meeting her needs for refractive correction as well as partially remediating her binocular vision problem. Additional management was required in the form of vision training and base-out relieving prism over-specs for the classroom until her eye control improved. Four years later, Sienna is now studying at university and still wears her relieving prism spectacles sometimes in very large lecture theatres to give her binocularity the support required, but otherwise is thrilled being a full time orthokeratology (OK) lens wearer with no myopia progression noted in that time.
Simply changing from specs to contact lenses, even if they were SVD CL’s, would have done something to reduce Sienna’s esophoria. OrthoK (OK) and multifocal soft CLs (MFSCLs), which of course we’re much more likely to fit to a young myope, have been shown to additionally improve esophoria and accommodative lag in comparison to SVD CLs, although not as may be predicted by the labelled ‘add’ power of a MFCL, (3-6) or central refractive power of the OK treatment.(7-9) MFSCLs may even increase accommodative lag – some research has shown children and young adults may relax their accommodation and use the ‘add’ of the lens at near.(5, 6) So while a myopia controlling CL may have an additional effect on BV than a SVD CL, it may not be a clinically significant or predictable additional effect. Read about Oliver’s story, another esophoric myope, in The esophoric myope and contact lenses.
Brooke's story - the hyperope
Yes, I know this website is called ‘Myopia Profile’, but a case for hyperopia helps to paint the picture on both sides of emmetropia! Brooke has worn spectacles full time since she was 6, with her prescription at age 15 being R +4.75 and L +5.00. She’s been lucky enough not to suffer amblyopia or esotropia as many of her similarly hyperopic counterparts – with her distance refraction she exhibits a moderate exophoria at near. She was first fit with daily disposable lenses for tennis when she was 12, worn twice a week. An increase in schoolwork demand saw her convergent (base-out) fusional reserves reduce, which are required to keep an exophoria in check, and as her confidence increased she was keen for more full time wear of her contact lenses. In comparison to single vision spectacle lens wear, Brooke’s exophoria decreased and her convergence improved in contact lens correction, leading to a both a better aesthetic and binocular vision result in contact lens correction.
When a hyperope wearing spectacles looks at a near target, their convergence away from the optical centre of the lenses creates base-out prism in each eye, with the base being the thicker part of the lens at the centre (see picture above). This base-out prism moves the image inwards and so decreases the divergence demand. When this hyperope then switches to contact lens wear they have to diverge more by themselves, which if they are capable, will result in an esophoric shift.(10) In Brooke’s case, fitting her with contact lenses improved her satisfaction with her correction as well as her binocular vision function – symptomatic convergence insufficiency has been associated with reading behaviours of avoidance in children.(11)
BV matters for CL success
In summary, when switching from specs to contact lenses, the myope will show an exophoric shift and the hyperope an esophoric shift. There may also be effects on accommodation – more accommodative demand and hence more lag for the myope; less accommodative demand and hence less lag for the hyperope. Whether you’re fitting children or adults with contact lenses, your new contact lens wearing patient can show alterations in their BV and this is more likely in higher refractions. Research has shown a correlation between contact lens discomfort and binocular vision disorders, with the authors stating that a similar set of symptoms could lead a clinician to confuse a BV disorder for a dry eye diagnosis.(12) If you want your CL wearer to be successful and comfortable, it’s worth evaluating their BV.
ANNOUNCEMENT: I'm excited to reveal that we're currently working hard on an online educational course called 'Binocular Vision Fundamentals' - all about practical diagnosis and management, with a particular focus on myopia control. I will show you how to assess binocular vision, sytematically manage disorders and improve your clinical skills - this course is relevant to all paediatric practice, not just myopia. I expect it will be ready for release in April 2019. Sign up to our email list at the bottom (mobile) / bottom right (desktop) of this page for advance notice of release, and keep an eye out for updates in the Myopia Profile Facebook group!