In an earlier post on refraction and safety with multifocal soft lenses, I discussed how colleagues often as this question – if you’re fortunate to have access to multiple multifocal, dual focus or myopia controlling contact lens designs, which is best? In Part 1 I discussed considering refraction and safety, and in this second part I’ll discuss how these lenses compare based on efficacy and visual function. For simplicity, I’ll use the terminology of multifocal soft contact lens (MFSCL) for any lens with more than one optical focus power, including aspherical multifocal (ie. Coopervision Biofinity, Mark’ennovy designs, Visioneering Technologies NaturaVue) and dual focus (ie. Coopervision Misight) designs.
At present, there is no study directly comparing the different MFSCL types for their relative myopia control effects. As I’ve described previously to help understand myopia control efficacy, the percentage efficacy rates we see quoted in studies are difficult to directly compare to eachother, as the outcome depends on the control group – how old were they, what correction were they wearing, how much they progressed – and study factors such as duration and testing protocol. To keep things simple and evidence based, then, I refer to the 2016 meta-analysis which examined numerous lens designs, some commercially available and some not, which concluded that these lenses show an average of around 50% efficacy for refractive and axial length myopia control.1 This takes the pressure off you, as the prescriber, by choosing any lens design available and knowing that it should have a reasonable effect for that patient. The future of customised MFSCL fitting will see us learn more about which lens designs work best for which patient, based on individual factors such as their age, refraction or binocular vision status. Until this occurs, though, if the specific design has been evaluated in a peer reviewed study, it’s reasonable enough for you to consider all available MFSCLs as equivalent myopia control options.
Visual function: over-refraction
The final consideration. This is where things get really interesting, and where research will tell us more in future – including two student research projects I’m currently co-supervising at QUT, which will hopefully turn into publications in future. While exact lens specifications are proprietary information, variation in the size of the distance central optic zones in these lenses could influence everything from acuity and over-refraction to accommodation. When comparing a Coopervision distance centred multifocal to a Misight lens, the central distance zone of the former is much smaller. This makes sense, as the D centred MFs were designed for presbyopes, with smaller pupils, who need to use the add to read. Children have bigger pupils, though, so may need more minus in these D centred lenses for good acuity – indeed, the BLINK study which is investigating these lenses for myopia control, comparing +1.50 and +2.50 Adds, found that children generally needed an extra -0.50 or so on their distance retraction to get good acuity.2 By comparison, in the three-year prospective Misight study (first presented by Paul Chamberlain at BCLA 2017), the children were fit to their refraction and didn’t need over minussing. The Visioneering NaturalVue lens online fitting guide promotes fitting on best-spherical refraction, and a retrospective myopia control case series analysis did not comment on comparison between expected and final fit contact lens powers.3
Visual function: accommodation
Furthermore, if a child does not accommodate normally in a MFSCL this could influence their efficacy – if using the ‘add’ at near in these lenses, as does a presbyope, this could generate hyperopic defocus from the distance portion/s of the lens which could likely lead to an undesirable optical effect in myopia control. Modelling of this effect in a monocular situation indicates that smaller central optical zones could give higher likelihood of the child using the ‘add’ to read, causing hyperopic defocus in the central and peripheral retina. Larger optical zones could be more likely to encourage a normal accommodation response.4 While this data was monocular modelling – things could be different in a binocular system where vergence could influence accommodation – it does point future research on efficacy of these lenses as dependent on how the child uses them. Could this be what helps us bridge the gap between the average 50% efficacy, upwards to better results? In future, perhaps we will be measuring some aspect of accommodation (or several measures) and then selecting a specific lens design – maybe smaller zone lenses for normal accommodators and larger zone lenses for underactive accommodators? Time and research will tell.
Which is best?
The best lens to start with is the one you have available! It's clear that daily disposables win the safety prize, but myopic astigmats won't be suitable for these, so you'd be best to select a reusable toric MFSCL or OrthoK, which have a similar safety profile. It is reasonable to assume, until research reveals direct comparisons, that they're all similar in terms of efficacy. Finally, when it comes to visual function, the individual lens design could influence both acuity and accommodation. We still have a lot to learn about these lenses which is what makes it such a fascinating area of research and practice.