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In “modified monovision”, which can be achieved with a new IOL, one eye predominantly manages distance vision and the other near vision—as with standard monovision—but the extended depth of field provides a degree of intermediate vision that is independent of spectacles.
Patients who present for cataract surgery are increasingly younger and more active and are therefore interested in a greater degree of spectacle independence without the limitations of standard monofocal IOLs. The loss of intermediate vision may have a particular impact on these patient’s lives, as they spend more time in front of computers and smart ‘phones.
To provide a balance between distance and near vision, the use of monofocal IOLs to produce a monovision set-up has been well established. Although we often see a high level of patient satisfaction with this approach, monovision has its limitations, including tolerance issues due to anisometropia, loss of binocular stereoacuity and asthenopia.1
By using Rayner’s RayOne EMV IOL (RAO200E; Figure 1), my patients are achieving an extended depth of vision without the limitations of standard monofocal IOLs or the side effects associated with diffractive multifocal IOLs. The lens offers an extended depth of field via controlled positive spherical aberration, which can be used with and without monovision to increase the patient’s range of focus.2
This lens has a non-diffractive optic and is therefore not expected to produce the side effects that are usually associated with diffractive IOLs such as night vision disturbance and loss of contrast. Although a high level of spherical aberration may induce night vision disturbances when certain thresholds are exceeded, there is a therapeutic range of spherical aberration in which night vision disturbances are not experienced because they are effectively filtered and processed by the brain.
In fact, while treating patients who had problems with night vision and loss of contrast in low light, which were common in patients with very small optical zones who had undergone laser surgery in the 1980s and 1990s, Prof. Dan Reinstein found that some degree of spherical aberration was therapeutic.3 At these levels of spherical aberration, depth of field was increased, but no night vision disturbances were experienced by the patient.
In my patients who receive this lens, my goal is to produce “modified monovision”. The additional controlled spherical aberration produces superior distance vision in the non-dominant eye and superior near vision in the dominant eye compared with what could be achieved with a standard monofocal IOL in a “standard monovision” setup (Figure 2).
I usually target plano in the dominant eye, and I choose a target refraction of −1.50 D in the non-dominant eye. This target for the non-dominant eye is consistent with an offset my colleagues and I have already used successfully in the settings of PRESBYOND and LASIK,4 which also combines spherical aberration induction and monovision.
The outcomes achieved with this strategy avoid patient intolerance and the other drawbacks seen with standard monovision. This approach provides a broad range of spectacle independence across the visual range, and the wide range of functional vision achieved allows for a better range of refractive error tolerance, without glare and halos or drop in contrast.
In patients who have already had corneal laser refractive surgery, I am sometimes hesitant to use this lens since they will likely already have a raised level of corneal spherical aberration. Raising it further may risk exceeding the therapeutic level. In these patients, I explain that their eyes already have increased depth of focus due to their previous laser treatment and that standard monofocals can be used to achieve “modified monovision”.
My patient interactions begin by establishing their daily visual needs and asking what they hope to achieve with their cataract surgery. In those with substantial cataracts, I explain that any option will result in improved quality of vision.
Beyond this initial expectation, the question then becomes the amount of spectacle independence that they hope to achieve with surgery. I review both monofocal and multifocal options and explain the potential limitations to multifocal IOLs such as the potential impact on night vision and loss of contrast.
I first discuss the option of standard monofocal vision with one eye set for near vision and one eye set for distance vision, without synergy between eyes, which is tolerated by about two-thirds of our patients. I explain that intermediate vision is not likely to be covered in this scenario.
I then discuss the concept of “modified monovision” as a strategy to overcome these limitations, with one eye set to predominantly manage distance vision and one eye to predominantly manage near vision but working in synergy to provide a better depth of field and some degree of intermediate vision that is independent of spectacles.
I feel that it is important to under-promise and over-deliver, and the fact that multiple strategies currently exist to improve vision in patients undergoing cataract surgery suggests that no solution is perfect for every patient. I explain to all patients that no presbyopic treatment can guarantee complete independence from glasses. For instance, most patients will continue to require spectacles for reading the small print of medicine bottles or watching live performances at far distances.
Overall, I have aimed for a target vision goal of −1.50 for the reading eye and have yet to find a patient needs follow-up laser surgery to address anisometropia. However, I do explain to my patients that a LASIK enhancement is a safe option to reduce the myopia if they do not tolerate it.
In contrast, if a patient does not tolerate a multifocal IOL then only lens exchange will remedy the situation. Overall, I have seen excellent levels of patient satisfaction in those who have received the RayOne EMV with this goal of modified monovision, and I have been very pleased with the results so far.
Surgeons who are not comfortable attempting a modified monovision setup and who do not typically manage presbyopia may still see promising results with a standard plano target with this lens. This is because the depth of field offered will likely enable the patient to read text of a reasonable size in good light.
As the surgeon develops confidence, with patient experience, they may begin to target some myopia in the reading eye to increase the range of spectacle independence achieved.