Blended vision implants provide customized correction of presbyopia

Pseudoaccommodation, which contributes to the presbyopic correction, is key to this method.

Last month we discussed the powerful appeal that permanent surgical correction of presbyopia can have for this patient population. I wrote from my own clinical experience how patients have taught me that the restoration of natural reading vision for a full presbyope often has more of an impact than distance correction for a myope. The 55-year-old man who had been bald since his early 40s said this of his presbyopic correction: “Imagine how I would feel if I woke up tomorrow with my full head of natural hair again … well, this is better.”

Not all patients are quite as pithy in their response to restored reading vision, but believe me, many are just as emotional. In contrast to the ametropic patient who often describes feeling like a new person with vision he never knew before, these patients describe a sense of regaining what was theirs, in some way feeling newly whole and once again their natural selves. A surgical solution to presbyopia will not be for the majority. Nevertheless, my experience thus far has shown that, provided our surgical skills are honed and we proceed at an appropriately measured pace, this approach could become one of the most successful applications of IOL technology to date. This month I want to focus on blended vision implants and the key role of pseudoaccommodation, which is thus far my procedure of choice for lens-based presbyopic correction.

(Source: Maloney WF)

Not monovision

This approach has been equated with contact lens monovision. I think this is an unfortunate association, for my experience has shown me that monovison is far less effective and for far fewer patients than blended vision implants. Although contact lens monovision can be very useful for younger partial presbyopes, it is not widely successful in the patient population we are considering here. Why? There are a few different reasons, but mainly because contact lenses lack the key element of pseudoaccommodation, a functional property of IOLs that is well documented, although not fully understood.

Monovision in a full presbyope requires at least 2.5 D (often 3 D at the contact lens plane) of anisometropic defocus to sustain full reading acuity. This degree of interocular difference is often sufficient to result in symptoms related to decreased contrast sensitivity and stereopsis and insufficient blur suppression. This is why I think that a contact lens monovision trial before blended vision implants is usually not helpful. I have had numerous patients be intolerant of this trial but readily accept blended vision implants, so I no longer consider it a useful predictor. The anisometropic defocus of 2.5 D to 3 D required for contact lens monovision is above the symptom threshold of too many patients to be widely successful. The most important reason for a significantly wider acceptance of blended vision implants is pseudoaccommodation, which contributes 1 D (± 0.25) to the presbyopic correction and thus reduces the required degree of anisometropic defocus to less than 2 D. This interocular difference is readily accepted by all but the most unusual fully presbyopic patient (more on identifying this patient in a moment).

Exploit pseudoaccommodation

I have found the pseudoaccommodation effect to consistently contribute 1 D (± 0.25). Let us call this my “personal” pseudoaccommodation amplitude, much like the surgeon factor in IOL calculations. This is higher than is generally quoted — 0.5 D is more typically cited — and certainly is more than I anticipated when I began this approach. However, over time I have become confident in this 1 D and routinely select the blended vision IOL pair on this basis.

I feel it is important for each surgeon to individually determine this factor because it may vary considerably among surgeons with slightly different techniques, or more likely, different IOLs. Also, it is important that you have solid confidence in the reliability of this factor because in order to fully exploit its contribution, the full anticipated pseudoaccommodation amplitude must be factored into the equation. In this manner, the degree of interocular power difference of the IOL pair is kept to a minimum and below the symptom threshold.

The overriding principle in this approach is to minimize the interocular power difference using only that necessary to achieve the required presbyopic correction after the pseudoaccommodation amplitude has been factored in. The concept is readily expressed in this simple formula:

Here the formula illustrates a fully presbyopic patient to be corrected with an IOL pairing for emmetropia in the D IOL and –2 in the N IOL. The remaining 1 D of presbyopic correction is contributed by pseudoaccommodation. Most presbyopes will readily accept 2 D of interocular defocus, but there is more that can be done to tilt the odds in favor of complete patient acceptance by customizing the implant pair more precisely to the patient’s individual visual needs.

Customizing the IOL pair

It is fairly unusual for me to target the D IOL for emmetropia. Typically, I target for slight myopia, usually less than 0.5 D. This retains good functional distance acuity, increases intermediate acuity and further reduces the interocular power difference required to address the presbyopia. However, the blended vision IOL pair is always selected specifically for each patient’s particular visual needs and expectations. If a given patient has a specific need of maximum distance acuity, emmetropia is targeted for the D IOL. The chart of focus zones that we have developed (Figure) is very useful. It depicts the focus zones in a way that patients grasp instantly. It also makes them an active participant in the process of selecting their uncorrected vision zones.

When I am sure the patient clearly understands what will and will not be in focus within each zone, he is asked to select which three contiguous zones he prefers for the uncorrected portion of his vision correction. An avid reader will likely select zones 1 to 3, while the active athlete will tend toward zones 3 to 5. The majority of patients select zones 2 to 4. The patient who persists in asking why he cannot select all 5 zones does not yet understand the compromise inherent in this approach. He also may be identifying himself as one of the few non-candidates. These individuals will always concentrate on what they will be missing instead of what they will be gaining with this approach. No matter how beneficial the net gain may be, these patients still view compromise as a loss.

This chart has been invaluable in identifying this personality type because it requires patients to actively participate in the compromise. I immediately and unequivocally advise these patients that they are not candidates for this approach. I again stress that these patients are quite rare, but when you encounter one, I advise you to do the same.

Trust me on this one

Much like gravity, pseudoaccommodation is a theory adopted to explain a phenomenon that we consistently encounter. Whether this extended range of focus is a result of spherical aberration, corneal astigmatism, some degree of optic “travel” or some other unidentified property of the IOL, we do not yet know.

Nevertheless, I do know that pseudoaccommodation is a reliable tool that, if fully exploited, significantly enhances our ability to address presbyopia with blended vision implants. Can you rely on it to the same extent that I have described here? Although the amplitude of the pseudoaccommodative effect may vary with the surgical technique and IOL used, I have little doubt that you will not find my experience to have been unique.

Granted, I seem to be a voice in the wilderness with my enthusiasm for this approach, but as I have said, that is mostly because no new IOL technology is involved. There is no “vested interest” encouraging this technique. I think that is the principal reason why there is little room at the podium for this approach to presbyopia correction.

But do not take my word for it. Explore this phenomenon for yourself. Determine the pseudoaccommodative amplitude that your approach reliably and consistently provides, then apply it first in your cataract patients and when ready, in your fully presbyopic refractive lens exchange candidates.

Next month:

Multifocal and diffractive IOLs for presbyopia.

Here are two examples of customized blended vision IOL pairings:

A 59-year-old woman who reads several hours daily and works on a computer. Preop refraction: Right eye –6.5, left eye –7.25 spherical equivalent. Postop target: Right eye –2.5 (dominant eye), left eye –0.75 (note that the dominant eye is used for near in this myopic patient). Focus zones selected: 1 to 3. Results: Excellent reading and working vision. Wears glasses only for night driving even though she anticipated wearing them for all outdoor distance. Results exceeded her expectation (which is fairly typical because this approach allows closer to four zones of uncorrected vision, not the three expected). A 53-year-old man with an active lifestyle. Preop refraction both eyes: +1.65 spherical equivalent. Postop target: Right eye –0.25 (dominant eye), left eye –1.65. Focus zones selected for uncorrected vision: 3 to 5. Result: Functional distance acuity with reading vision for allbut zone 1. Both of these patients were very happy with their uncorrected vision. The particular blend of the selected implant pair was customized to their specific needs. I think of this almost infinitely variable selection as being similar to blending colors to achieve a unique custom shade of paint. I could easily add an additional 20 patients to these examples without duplicating an IOL pair. This customized correction, which for me is an indispensable benefit of blended reading vision implants, depends heavily on the added flexibility afforded by pseudoaccommodation.