Conventional lens implants provide versatile approach to presbyopia correction

For some patients, conventional lenses may be the only way to go to provide the amount of accommodative effect they desire.

Issue: December 1, 2004

ByWilliam F. Maloney, MD

This is the third article in this series exploring the relative merits of the presbyopic IOLs. I have argued for an overall approach to presbyopia correction that inverts the status quo. Thus far, most of us have begun by selecting a preferred presbyopic lens implant and then proceeded by asking, “which patients are best suited for this IOL?”

My recent experience has taught me that in order to be broadly successful we need to become familiar with the merits of each presbyopic implant so that we can begin to ask the better question, “What IOL is best suited for this patient?”

Only then are we in a position to tailor the procedure and accurately deliver the precise range of near vision focus each patient seeks.

The surgeon’s due diligence

Selecting only the best IOL for each patient’s specific presbyopia profile (see Figures 1 to 3) dramatically improves our ability to tailor the outcome and more fully satisfy a wider range of candidates. But this customized approach requires more than merely reaching for our preferred IOL from the consignment shelf. It demands, first, that we surgeons do a great deal more ”due diligence” in order to fully appreciate the many nuances of each patient’s presbyopia profile.

It also requires that we actively research the merits and compromises unique to each IOL approach. This means digging beneath the marketing mantras and podium infomercials to get a hold of the hard data. It also means checking with trusted colleagues about their experiences, with emphasis on the limitations and compromises they have encountered with a particular IOL.

Only then can we say a solid “yes” to a particular IOL on behalf of this patient, since that ”yes” necessarily means a ”no” for the alternative lens implants. We surgeons are the de facto decision-makers in this IOL selection triage, and we need to know that we have done all that we must to utilize the IOL that will provide the most satisfactory individual results. In many cases, that IOL will be what is perhaps the most versatile approach — a conventional posterior chamber lens implant.

Many surgeons are still unaware that a conventional IOL can provide the desired reading vision focus for a range of candidates. In fact, there are some candidates for whom the conventional IOL is the only approach able to fully satisfy their specific reading needs. How does this conventional IOL approach deliver reliable reading focus?

The two elements that contribute to this approach are pseudophakic pseudoaccommodation plus that amount of supplemental myopic defocus sufficient to deliver the required accommodative effect.

Pseudoaccommodation plus myopic defocus

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Pseudoaccommodation is an optical property of all conventional IOLs. Although pseudophakic pseudoaccommodation has been repeatedly described in the literature since 1983 (see references below) this refractive tool remains poorly understood and almost never purposively utilized in the treatment of presbyopia.

Even the term itself has been the object of confusion. “Pseudoaccommodation” is regularly misapplied to the accommodative effect associated with both multifocal IOLs and accommodative IOLs. For our purposes, pseudoaccommodation refers to the partial accommodative effect is found with conventional lens implants.

The optical basis for pseudoaccommodation is still unknown, but we are closing in fast. Aberrometry is pointing to the spherical aberration inherent in conventional IOLs, together with the natural miosis that accompanies reading effort, as the principal contributing factors. Corneal aberrations may also play a less important role. But we do not need to identify the precise mechanism in order to put this accommodative effect to work in our presbyopia treatments.

The amount of accommodative effect delivered by pseudoaccommodation has been reported mostly between 2 D and 2.5 D. You may think this implausible at first glance. If this amount of accommodative effect is indeed present in conventional lens implants, then all of our patients should have been reading unaided long ago, right?

Well no, not necessarily. First, I suspect that 2.5 D, although not rare, is not reached consistently. I have come to rely on an accommodative effect of closer to 1.5 D and, to be conservative, I typically rely on only 1 D of pseudoaccommodative effect. This is hardly trivial within the 3 D metrics of presbyopia corrections.

There is also another curious phenomenon at work here. Most patients will not spontaneously come to read without glasses until they are guided to discover that they are able to do so. I discovered this curious phenomenon early in my presbyopia experience.

Myopic defocus

Myopic defocus is another key element in presbyopia correction.

Probably as a result of my experience with astigmatic keratotomy, the idea of an ever-wider range of uncorrected postop vision became a longer-term goal of my cataract surgery early on. To that end, I began to experiment with targeting a small amount of myopic defocus — initially no more than 0.5 D — hoping to slightly extend the range of uncorrected vision in cataract patients.

Myopic defocus quickly became a useful tool, and by 1994 I was targeting for up to –1.5 D in the nondominant eye. Many of these patients easily achieved Zone 1 focus, but only after I started asking them to read. Rarely did one of these patients discover their reading capability independently.

I began testing for Zone 1 and Zone 2 focus in all of my postop cataract patients. The majority demonstrated significantly greater accommodative effect than expected. Myopic defocus of just 1.5 D often provided unlimited Zone 1 reading, and 0.75 D typically delivered Zone 2 focus. It was becoming increasingly clear that something over and above myopic defocus was at work here.

A literature search revealed three reports describing ”apparent accommodation” in pseudophakic eyes. Two were by Nakazawa from 1983 and 1984; the other was reported in 1990 by Ravalico. I realized that I was dealing with two separate elements; myopic defocus plus this unexplained accommodative effect that we now call pseudoaccommodation. The formula for correcting presbyopia with conventional lens implants was rapidly taking shape, but what about the numbers?

After steadily seeing that full reading capability (Zone 1), which requires 3 D of total accommodative effect, often required only 1.5 D and never more than 2 D of myopic defocus. I came to realize that I could rely on pseudoaccommodation for at least 1 D of the total accommodative effect I needed.

The following details my approach to this with conventional IOLs. Keep in mind that this represents an off-label use of conventional lens implants that must be included in the informed consent.

Presbyopia correction by the numbers

Measure the interocular defocus threshold with trial frame and loose lenses. (I will go into the details of this next month). This threshold is the practical limit to the amount of interocular myopic defocus difference that each patient will comfortably tolerate. (Typically the threshold is between 1.5 D and 2 D).
Using the Focus Zone chart (Figure 3) with the patient, select the desired reading goal.
Zone 1: Unlimited reading — requires 3 D of total accommodative effect
Zone 2: Social reading — requires 1.5 to 2 D of total accommodative effect
Apply this formula to determine the amount of myopic defocus required to obtain the necessary total accommodative effect: Accommodative total result = Pseudoaccommodation + Defocus
Select the specific IOL pairing that is below the interocular defocus threshold that also delivers either type of distance focus selected in the Focus Zone chart. When intermediate focus is selected over far distance, some myopic defocus is targeted for the distance IOL. This also decreases the interocular defocus needed.

The range of possible distance/near IOL pairings for each patient is almost unlimited, which allows this approach to deliver results tailored to an unusually wide range of candidates. The only caveat is for candidates who do a lot of night driving, especially if they have large scotopic pupils. This is a significant challenge for all presbyopic lens implants, but for different reasons. We will address the issues surrounding night driving and presbyopia correction in detail in an upcoming column.

Readily reversible

The secondary ”piggyback” insertion of a low power silicone implant (now available in 0.5-D increments with the Clariflex from Advanced Medical Optics) provides an accurate, uncomplicated reversal of the conventional IOL approach to presbyopia correction if necessary.

In the December 15 issue, look for a Spotlight on Presbyopia. In the January 1, 2005, issue, look for the Lens-Based Refractive Surgery column on the requisite preop testing for presbyopia correction.

For Your Information:
William F. Maloney, MD, an associate clinical professor at the University of California, Irvine, and head of Eye Surgery Associates, of Vista, Calif., is a well-known teacher of cataract and lens-based refractive surgery techniques. He can be reached at 2023 West Vista Way, Suite A, Vista, CA 92083; 760-941-1400; fax: 760-941-9643; e-mail: MaloneyEye@yahoo.com. Dr. Maloney has no financial interest in any products mentioned, and has no financial relationship with any ophthalmic company.
References:
Nakazawa M, Ohtsuki K. Apparent accommodation in pseudophakic eyes after implantation of posterior chamber intraocular lenses. Am J Ophthalmology. 1983;96:435-438.
Nakazawa M, Ohtsuki K. Apparent accommodation in pseudophakic eyes after implantation of posterior chamber intraocular lenses: optical analysis. Invest Ophthalmol Vis Sci. 1984;25:1458-1460.
Ravalico G, Baccara F. Apparent accommodation in pseudophakic eyes. Acta Ophthalmol. 1990; 68:604-606.
Elder MJ, Murphy C, Sanderson GF. Apparent accommodation and depth of field in pseudophakia. J Cataract Refract Surg. 1996;22:615-619.