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Fitting background    Soft contact lenses

Multifactorial

Are Multifocal Lenses ‘Down-and-Out’? Yes and No.

Presbyopia affects everyone eventually, including people who are already hyperopic or myopic. It is estimated that the number of presbyopes in the entire population who need some form of vision correction is 25%. I am only a 50+ beginner (which sounds much better than ‘old man’). And, as an expert witness in the field now, I have reservations regarding the term presbyopia. The etymology stems from the Greek πρέσβυς presbys, meaning "old," and ὤψ ōps, meaning "sight" (GEN ὠπός ōpos). I can assure you that to ‘young presbyopes,’ such as what I consider myself, this sounds like the equivalent of senile. Senile means (of a person) having or showing the weaknesses or diseases of old age. Indeed, senile miosis is a simple, gradual reduction in the size of the pupil that occurs with age in normal eyes and is caused by atrophy of the muscles controlling dilation of the pupil. While this information is in factually correct, and even relevant to both the eye care practitioner (ECP) and to the patients involved (it simply means ‘use more light!’ while reading) – using this in conversation with a ‘beginner presbyope,’ as one of my students once did, can be quite detrimental. By Eef van der Worp

Are we getting too soft?

Looking at the 2019 Eurolens Research Survey data on contact lens fits around the world, we see that the mean age of patients at fitting was in the early 30s (32.8 ± 14.9 years). But contact lens patients tend to be older at fitting in western Europe and in other developed markets (e.g., the average age at fitting was greater than 39 years in Denmark and Switzerland, for instance) while it is younger in developing and/or Asian markets (younger than 30 years on average in Japan, Mexico, Israel, and the Philippines). But many lens fits in western Europe start at presbyopic, or at least pre-presbyopic age on average at least. And age at time of fitting seems to be ‘creeping up’: in the Netherlands for instance the average age increased from 33 to 39 years of age in 20 years’ time the survey has been running).

Altogether, GP lenses (including standard, scleral and Ortho-K lenses) accounted for 14% of new fits (in which scleral lenses accounted for 14% of all GP lenses fitted) in the survey. Regarding multifocal lens fitting within the GP lens group, only 8% of GP lens fits are with multifocal designs, while another 3% are monovision.

Soft lenses in general made up 86% of contact lens fits in the survey. Spherical, toric and multifocal lenses accounted for 51%, 28% and 13% of soft lenses prescribed, respectively. Looking at just the presbyopic age group, multifocal soft lenses are fitted in 40% of cases. In addition, 10% to 15% are prescribed monovision soft lenses; this would suggest that close to half of presbyopes, when fitted with soft contact lenses, receive an appropriate correction for reading and far. The other half doesn’t and receives a distance-only correction.

Down and out

So, what is so hard about fitting multifocal soft contact lenses? Let’s first take a look at some technical facts. If we really want to understand multifocal lens success, the first thing to note (and to know) is that soft lenses tend to decentre on the eye – typically temporally, because of the flatter shape of the nasal sclera that we now know exists, and inferiorly, because of gravity and eyelid forces. On average, all lenses display a temporal-inferior decentration. So yes, literally seen multifocal lenses center ‘down-and-out’.

From a clinical standpoint, it is interesting to note that according to Cathleen Fedtke from the Brien Holden Vision Institute, certain multifocal contact lenses decentred more than others, which can primarily be attributed to differences in lens design and fitting parameters. Moreover, an association between multifocal contact lens decentration and higher-order aberrations was found. In other words, visual performance is negatively influenced by the lens decentration. It should be obvious to everyone, therefore, that for a multifocal lens to function on the eye, the optics of the lens need to centre. If the optics of the lens are decentred over the eye, the effectiveness of the lens can decrease quite dramatically as higher-order aberrations are induced, degrading the quality of the visual performance of the lens system.

A different angle

Unfortunately, in addition to the lens decentration, the pupils on average are slightly displaced, typically nasally and superiorly. In a study by Wildenmann and Schaeffel on 4mm pupils (the decentration varies depending on pupil size), the pupils were nasally decentred relative to the corneal centre by 0.18 ± 0.19mm in the right eyes and 0.14 ± 0.22mm in the left eyes. Vertical decentrations were 0.30 ± 0.30mm and 0.27 ± 0.29mm, respectively, always in a superior direction. What this means is that while there are individual differences, for a standard eye the error caused by temporal decentration of a soft lens is further magnified by the nasal decentration of the pupil. And, the described error caused by inferior decentration of the soft lens is typically magnified by the superior decentration on average of the pupil.

To top it off, the line-of-sight is another phenomenon to be considered; most individuals have a positive angle kappa. This sounds good, but the negative part of a positive angle kappa is that the visual axis is nasal to the pupillary axis. In other words, this further adds to the effect of a relative temporal optical displacement of the lens. All of this at least explains why multifocal soft lenses are not always successful.

Flexure

In addition, lens flexure could interfere with lens optics and reduce success. Studies have shown that for a soft lens fit to work, the shape of that lens needs to be somewhat ‘deep’ on the ocular surface. If a soft lens is fitted completely aligned with the ocular surface – for example, it has the same sagittal height as the sagittal height of the eye – then the soft lens, influenced by tear film, eyelid pressure and blink forces, will move excessively on the eye; this will result in an unsuccessful lens fit that would be very uncomfortable to wear. Our current understanding is that the sagittal height of a soft lens on-eye likely needs to be somewhere in the range of 100–300μm ‘deeper’ to achieve a clinically successful lens fit. This may have consequences for the visual performance of that contact lens on-eye; lens flexure on the eye will cause a small change in lens optics from what the lens was designed to provide. For a simple spherical -2.50D correction contact lens, this does not seem to have a huge effect. But for multifocal lenses, and also for dual-focus myopia lenses, etc., it may have a considerable effect.

Lens optics

The good news here is that lens optics are becoming less of a mystery lately, as we have much improved instrumentation to analyse optical profiles of multifocal lenses. One of the main downsides of soft multifocal lenses that I’ve always had to communicate to students is that the vast majority of lenses have centre-near optical designs. To their disappointment and to their disadvantage really, there is not a lot of room to ‘play with’. But in all honesty, now that we see the different power profiles of all of these lenses (although they are mostly centre-near designs), that leaves us with a bit more leeway. Having access to custom-made soft lenses of course gives us ultimate flexibility in terms of whether the design is centre-distance or centre-near and even in terms of the size of the zones potentially. In addition, the improved back-surface lens design could control for lens decentration.

It is beyond the scope of this paper, but recent papers have shown that aberrations of the eye not only differ quite substantially with age, they also differ substantially per individual. If we could control for that, the future for multifocal (soft) lenses actually looks really bright.

Dropouts-dryness-dioptric

But there is more than ‘the technical stuff’. James Wolffsohn acted as a ‘live reporter’ from Aston University in Birmingham (‘the presbyopia research centre of the world’) for a recent ‘live talkshow’ on contact lenses that we recorded in the Netherlands. During that session, he actually diverted from technical issues as the main driver of multifocal contact lens success. Rather, he brought up the trilogy of dropouts, dryness and dioptric values used. Regarding the first: dropouts have been frequent topics of articles in GlobalCONTACT. One of the key factors is that dropout rates are highest (50%) in the first two months after lens fit. What this means is that the ECP absolutely must take control over the fitting process in the first period after the fit and stay in direct contact with the lens wearer. Second, among other things, tear film dynamics degrade after 45 years of age. ‘Treat dry eye first’ is James’ mantra. Then fit multifocals. Thirdly, he mentions practical tips with regard to lens power, such as ‘give them vision’ on the trial. In other words, make sure that you have the right dioptric value for the lenses on the first try or order a lens empirically. You want to make sure that the lens wearer experiences the ‘plusses’ of multifocals right away to optimize the chances of success. And make sure to incorporate cylindrical optical corrections from the start as well. It is also important to check vision using ‘devices’ that they use in daily life, not some artificial, 100% contrast letter chart with a construction lamp shining on it. ‘Real life’ situations best mimic the circumstances and increase the chances of success to determine the right dioptric value for far, near and intermediate.

It is time to refocus on multifocals

Multifocal lenses have always had very good cards in hand, with huge potential. Every lecture on the topic always starts with how many potential wearers there are. They might even have more potential for presbyopia than for myopia management. But guess what topic steals the show and gets all of the attention of these two? It is time to refocus on multifocals for presbyopia, in my view. While the focus within the modality is primarily on soft lenses, as explained, it is actually important to not forget about corneal GP lenses (with, of all lens modalities, the huge benefit of offering alternating lens optics), hybrids and scleral lenses.

Multifocals are not ‘down-and-out’, but there’s no easy fix either as it is indeed ‘multifactorial’. But if we realise and act upon that, then this could potentially be a huge opportunity for the industry – both on the manufacturers’ side and the eye care practitioners’ front, with the opposite of ‘down-and-out’ (financially) as a result. But we need to put multifocals back on the agenda. While the previous decade was surely the one for scleral lenses (see ‘The Decade of Scleral Lenses’ in a previous edition of Global Contact), now may be the decade of custom-made lenses, particularly for soft custom-made lenses where needed. This could have a gigantic effect on the success of multifocal lens fitting in practice.

References:

Fedtke C, Thomas V, Ehrmann K, Bakaraju RC. Association between multifocal soft contact lens decentration and visual performance. Clinical Optometry. 2016;8:57–69

Lampa, M. Utilizing technology to improve multifocal success. Contact Lens Spectrum. Sept 2020: 12

Morgan P, Efron N, et al. International Contact Lens Prescribing 2019. Contact Lens Spectrum, January 2020: 26-32

Van der Worp, E. Fit first – practical implications of multifocal soft lens design and fit. Optomety Today, Jan 2018:78-82

Wildenmann U, Schaeffel F. Variations of pupil centration and their effects on video eye tracking. Ophthalmic and Physiological Optics. 2013 Nov;33(6):634-41