Progressives, bifocals, binocular vision and more
Are progressive addition lenses and bifocals created equal for myopia control? When do they work and when do spectacles have minimal efficacy? How should we pick which lens type to prescribe, and what's on the horizon for our non-contact lens wearing young myopes?
We know that single vision spectacle lenses provide no useful efficacy for myopia control, and in fact are used as control corrections in myopia control studies, demonstrating the ‘untreated’ progression of childhood myopia.1 Progressive addition (PAL) and bifocal spectacle lenses have shown reasonable research results for myopia control, and novel designs have been developed. It is important to be aware of the indications and evidence for spectacle lens myopia control, as this is likely the first correction we will prescribe – especially in younger children where the child (or perhaps more the parent!) is not ready for contact lenses. Spectacle lenses are also an important adjunct treatment in soft contact lens wearers – as a back up correction – and if atropine is being prescribed as a first line treatment. So let’s get into the detail.
There is conjecture about PAL or bifocal spectacles having any useful effects for myopia control2, 3 just the same as there is conjecture about peripheral refraction being a factor in myopia development and progression.4, 5 For the individual patient, there could be one primary driver to myopia development and progression – genetics, environment, peripheral refraction, accommodation – or it could be a combination. This is what makes myopia such a fascinating area of research and clinical practice.
Progressives or bifocals?
Progressive addition lens (PAL) studies for myopia control show negligible results when single adds are applied to all children, however when applied to children with esophoria and accommodative lag, the results become more impressive at 30-40% efficacy and start to approach that of contact lens studies.6, 7
By comparison, a three year bifocal study found a 40-50% efficacy for a +1.50 Add E-seg bifocal with 3 base-in (BI) prism incorporated. 8 Does this make the bifocal or the prism the magic sauce? Perhaps, or maybe it was more to do with the study design which firstly ensured that all participants were demonstrated myopia progressors in the year prior to study entry, and secondly by considering binocular vision. The use of the BI prism was designed to balance accommodation and vergence systems – not to reduce the response of either system. In a prior study, these authors had tested a combination of adds and BI prism, measuring accommodative lag and exophoric shifts. The +1.50 Add with 3 BI R&L ensured that there was no change to either lag or phoria once wearing the bifocal, indicating perhaps a different mechanism of action was at play (see more below!).9 The base-in prism ensured the exophoric children didn’t get more exophoric with the add, but wouldn’t have necessarily provided the orthoptic correction for esophoria which is typically desired in prescribing a near add.
So how well did the bifocal work? Cheng et al’s study investigated a standard bifocal with a +1.50 Add, and the same add with the 3BI prism in each eye. After three years of wear, they found a moderate myopia control effect – around 35% for axial length and 50% for refractive change – in children who were orthophoric and exophoric in their baseline single vision correction. They found a minimal effect in the baseline esophoric children, but they were a small group so there was less statistical power.8 When analysed by accommodative lag, the two year results showed the similar effect of both bifocal types in children with high accommodative lag (over 1D), but a better result with the prismatic bifocals in children with low accommodative lag.10
Which to pick in practice?
If you measure esophoria and accommodative lag in single vision correction, a progressive addition lens is an evidence based myopia control choice.
If you measure orthophoria, exophoria or normal accommodation (lag <1D) in single vision correction, a bifocal or prismatic bifocal is the better choice. A child with low accommodative lag (<1D) may respond better to the bifocal lens with prism to minimise the influence of the add on binocular vision function. Keep in mind, though, that if an add makes an exophoric child break down into intermittent exotropia, this condition has been associated with myopia progression.11
If not BV, how do they work?
The concept of simultaneous defocus is employed in theories of myopia development and progression. Instead of thinking of peripheral refraction, think of how a distance-centred multifocal will cast zones of clear retinal focus (the distance portion/s) and also zones of myopic defocus (the ‘add’ portion/s) across the retina. In animal studies, creating these conflicted zones of retinal defocus appears to influence the retina to pay attention to the more myopic plane, essentially halting eye growth, rather than the eye averaging the two planes.4 In PAL or bifocal spectacle lenses, the large zone of ‘add’ in the inferior lens creates a relative peripheral myopic shift on the superior retina. One study has found a relationship between the amount of relative peripheral myopia created by the inferior add zone and the myopia control effect of PALs.12
Even though the numbers of esophores were small in the bifocal study, the fact that they didn’t work well for the esophores in a study design where the bifocal add was counteracted with BI prism - to intentionally have a minimal influence on BV – indicates to me that binocular vision does play a role for some children, and contact lens research is confirming this.13, 14
Which add to use?
If you want to apply a single add, then most PAL studies have used a +1.50 or +2.00 Add, and the bifocal study I’ve described used a +1.50 Add. In practice, though, I prescribe an add based on their presentation. In children with esophoria and/or accommodative lag, for example, we need to manage their binocular vision as well as think about the best myopia management strategy. Individualising the add is important for optometric management and visual comfort, and could be responsible for the difference between what’s measured in research and what we observe in practice. You'll learn a lot more about how to do this in the next portal, which has a strong focus on binocular vision assessment and prescribing, including flow charts for prescribing adds based on esophoria and accommodative lag.
For more on how we can use spectacle lenses in myopia management, head to Spectacle lenses for myopia control - Part 2.