By Paddy Kamen
Josh Josephson has a bee in his proverbial bonnet: “The question of whether to fit monovision or multifocal contact lenses is very important and in my opinion most practitioners don’t understand the issues involved.”
As the chairman of the ophthalmic devices section of the Standards Council of Canada and a founding member of the International Society for Contact Lens Research, Josephson knows whereof he speaks.
Monovision is a method of correcting presbyopia by using a contact lens corrected for distance in one eye (usually the dominant one) and another lens corrected for near vision in the other eye. Monovision works for the patient because the brain’s visual cortex chooses the focused image and ignores the blurred image. So when the patient looks into the distance the dominant eye does the lion’s share of the work and when focused at near, the corrected, non-dominant eye is focused and does the job.
Sounds good, but, according to Josephson, “Monovision wearers who « perceive » that they see well, do not realize how monovision can, under certain conditions such as low light and particularly in adds 1.5 D or greater, adversely affect their lives.”
As just one example, Josephson recounts the case of a woman who called him in distress. She had been prescribed monovision contact lenses and thought she was doing well with them — until she had to drive at night. “She had driven during the day from Toronto to New York Cit ybut when she started to drive home at night she simply couldn’t do it and had to pull off the road,” notes Josephson. “In low light environments, contrast sensitivity is reduced significantly and patients do not appreciate colours as they should be perceived. Further, when driving at night on a poorly lit road and moving into a curve in the road, if a car approaches from the opposite direction with their brights on, monovision wearers may be subjected to a shift of dominance in which the near-corrected (and distance vision blurred) non-dominant eye takes over for a brief period of time.”
According to Josephson, no active person with an add of 1.5 D or greater should be wearing monovision lenses unless they have compensatory eyeglasses for night driving.
Keith Harrison agrees. This contact lens specialist at Harrison Optical Services, located at the Toronto Western Hospital, would go Josephson one better and take the threshold for wearing monovision down to 1 D. “All too often I see patients who have been prescribed monovision contact lenses which don’t meet their needs. Sometimes they believe they are wearing multifocal lenses when they are not.”
“The biggest problem for those wearing monovision lenses is with binocular vision,” explains Harrison. “They have issues with depth perception and for those prone to migraine headaches or vertigo they are definitely not a good choice.”
In Harrison’s opinion, many practitioners simply don’t want to take the time to learn how to fit multifocal contact lenses properly. “For the practitioner it may seem easier at the start to prescribe monovision but you get diminishing returns. As time goes by you’re giving up too much distance vision or too much near vision. Certainly, monovision won’t meet the patient’s needs as they age and then they have to adjust to something new.”
How Much Chair Time is Enough?
Harrison has a professional dispute with the concept that contact lens fitters should always be aiming for less chair time in their dealings with patients. “I believe we have driven people into refractive surgery and the problems attendant on that, simply because we haven’t taken the time to fit them properly with contact lenses. There is so much emphasis from manufacturers on reducing chair time and I think it does the practitioner and the patient a disservice.”
Harrison advises practitioners to figure out what it actually costs them to maintain the office, take their professional training into account, determine how much money they feel they should be making and charge accordingly. “The most important thing is to determine the real wants and needs of the patient and that takes time. We’re getting paid to fit them so let’s charge what it takes to make a profit and do the best job possible for the patient.”
“It does take time for a patient to learn how to use multifocal lenses,” adds Harrison. “The practitioner can’t just say, ‘here they are, go and try them’. You have to have them back for a follow-up appointment in order to refine all aspects of the lenses for the patient to the best possible level of performance. When you dispense, everything may look fine, including the positioning and alignment on the cornea, but that’s only after 30-60 minutes. It’s a whole other thing when the patient has been wearing those lenses for a full day. By checking the patient after a week or two of wear, with the lenses being on for six-to-eight hours, you can really tell how the adaptation is going and you may have to make adjustments to fit, material or design.”
Harrison is a big fan of multifocal contact lenses. “I can help most people see as well as they did 15 years ago. That’s a big turning back of the clock.”
The Inherent Limitations
The limitation of all bifocal/multifocal lenses is that they are pupil-sharing lenses, says Josh Josephson. “They all work on the principle of simultaneous imagery. Light from both distance and near areas of the lens enters the eye simultaneously. If you’re looking into the distance, you have some light focused on the Rx for distance and when looking at intermediate or near, a certain amount of light is present for those areas. The amount of light devoted to each image is restricted to about 50 per cent of what it might be (depending on zone diameters) with a single vision lens. The brightness of the retinal image affects a person’s perception of how « clear » the image appears to be. The impact of this situation can vary somewhat from patient to patient. With alternating image bifocals, when they work as designed, this isn’t significant. However, even with those designs there is a small amount of pupil-sharing that typically occurs when looking down and reading, so they may have less of an impact than with distance viewing.”
Randy Kojima, director of technical affairs for Precision Technology Services, and instructor at Pacific University College of Optometry, notes that compared with ophthalmic glasses, multifocal contact lens designs are, “trying to pack all the power the patient needs into a tiny zone. For example, an average pupil for a 55-year-old presbyope in normal illumination is 3-3.5 mm, which means you need all the power packed into this small zone. It appears that early presbyopes do quite well in multifocal contact lenses but above +2.00 D one needs more magnification within a tinier zone and it becomes a bigger challenge to provide the extremes of far, near and intermediate vision. It’s like ophthalmic in that the bigger the add, the tougher it is to adapt. Having said that, multifocals are helping many patients with their presbyopia,” notes Kojima.
“Materials and designs for delivering the optics have improved over the past five years. Lenses with more oxygen and wetability are improved over older-generation materials. And the design options have given us improved distance and near vision. Some of them have really amazing outcomes while others achieve only near or distance but not both. And most are in between.”
More Research Needed
Kojima is of the opinion that neither rigid gas permeable (RGP) nor soft contacts are perfect — yet. “We want multifocal contacts to be more predictive and consistent. For instance, are certain pupil sizes less than ideal for this modality? Are certain Rxs contra-indicated? Are certain corneal shapes less likely to achieve ideal outcomes? More research needs to be done by true scientists in multifocal lenses. In the contact lens industry, there is a lot of trial and error, rather than research-based advancement. The more research that goes into optimizing these designs, the better we can advance this option.”
Harrison agrees with Kojima and adds, “We also need more practitioners who are willing to invest the time in their patients and practices to determine what can truly be a success for each individual.”
