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Precise ‘presbyometry’ key to conventional IOL presbyopia correction

The precise manipulation of myopic defocus in each eye provides the necessary range of both distance and reading vision.

Issue: February 1, 2006

ByWilliam F. Maloney, MD

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William F. Maloney

This is the third column in a four-part series dealing specifically with conventional IOL presbyopia correction and highlighting the cases of two longtime patients. Ed and Maxine each had their presbyopia-correcting cataract surgery using this approach. Ed’s surgery, early in my experience, was back in 1985, and his wife Maxine’s was in 2005.

Together, they are a “poster couple” for the 20-year history of conventional IOL presbyopia correction. In today’s column, they provide us with an excellent opportunity for a careful study of their preop “presbyometrics” — those critical assessments that precisely determined the unique conventional IOL pairing that made each of their results a complete success.

Presbyometry

Although 20 years apart, these two presbyopia corrections were remarkably similar. As with all of these cases, each result depended heavily upon preop presbyometry. This is the process — both art and science — of measuring and then successfully employing just the right amount of myopic defocus in each eye to provide the specific range of both distance and reading focus each wanted to achieve.

Last month’s column described how my understanding of the use of myopic defocus evolved over the years. Eventually, I came to learn how to effectively employ myopic defocus for a full range of presbyopia corrections customized to each patient’s unique goals (Ocular Surgery News, Jan. 1, 2006, page 38). I noted that this evolution actually began several years before Ed’s procedure in 1985, when I slowly started to move away from the typical goal of emmetropia in both eyes and started to target a small amount of myopic defocus in the nondominant eye instead. My goal at the time was to enhance the intermediate focus. Gradually increasing the amount of myopic defocus, I eventually approached –1.5 D, which became my self-imposed limit at the time.

Pushing the presbyopia envelope

Distance emmetropia in the dominant eye combined with –1.5 D of myopic defocus in the nondominant eye was a powerful corrective tool that was readily adaptive and routinely provided excellent (zone 2) social reading. I was on to something important, and my surprised, happy patients, who quickly began telling others, knew it.

Later candidates increasingly inquired about the possibility of achieving even better (zone 1) reading vision. As I began to explore the options, I quickly learned what, for me, has become a central tenet in presbyopia correction. If a compromise is necessary — as it still always is with any IOL — reading restoration trumps distance for the cataract patient in almost every case. As I pushed the presbyopic envelope away from far distance toward full “fine-print” reading focus, the majority of these patients eagerly accepted the anticipated trade-off of full reading restoration for diminished far distance acuity. Why? Lifestyle. Active as seniors now typically are, few cataract patients are aviators or long-distance night drivers. Their world had gradually moved closer, and they were telling me that they wanted their vision to do the same. For me, the presbyopia envelope was shifting even closer, and now it was my patients doing the pushing.

Letting go of emmetropia

These early patients taught me an important presbyopia principle that still applies in most cataract cases — let go of emmetropia. Once I understood that these patients were willing to compromise some measure of distance acuity, I was free to shift the fixed 1.5 D of interocular difference toward the closer vision they wanted. I moved gradually toward targeting –0.5 D for the distance eye and –2 D for the near eye, thereby easily providing full (zone 1 and zone 2) presbyopia correction.

I had arrived at a new appreciation of the customizable versatility and the broad effective range of this approach. I also now knew that even those candidates seeking the absolute maximum (full zone 1) reading focus need never compromise more than –0.5 D of their far distance acuity. This meant that patients willing to use glasses for far distance activities could expect to eliminate them for everything else, including fine-print reading, computer work, shopping, reading restaurant menus and TV viewing, and that is how I counseled them at the time.

The compromise that wasn’t

It was at this stage in this story that I did Ed’s surgery in November 1985 – my seventh full presbyopia correction. My rudimentary preop presbyometry of that time determined that an IOL pairing targeting myopic defocus of –0.4 D for the distance eye and –1.96 D for the near eye would result in the full reading result Ed sought. His actual results taught me that the distance compromise we both expected was, in fact, no compromise at all. He has never needed those distance glasses or any glasses again.

In July 2005, with a good deal more confidence in my prop presbyometry and a precisely accurate outcome, I corrected Maxine’s presbyopia by targeting –0.38 D for her distance eye and –1.93 D for her near eye with the same result.

So why then doesn’t Ed or Maxine, along with over 90% of my cases such as theirs, need any glasses at all, even for far distance? Why does the distance myopia of up to –0.5 D typically remain undetected? Here again, I learned a central principle of presbyopia correction. When you cross over from distance IOL calculations into the world of reading vision, the biometrics change dramatically.

The metrics of accommodation

It turns out that our myopic defocus of –0.5 D enhances accommodation to a much greater degree than it diminishes distance acuity. Part of the art of presbyometry is understanding how to effectively exploit this asymmetric effect to great advantage. Here is the math, so simple that it is hard to believe it underlies such a vital clinical tool.

Distance vision lies within a range of focusing power averaging somewhere around 20 D, while the additional incremental accommodative effect we need for reading is in the 2 D range. These become two different denominators when we calculate the relative impact of introducing an additional –0.5 D of myopic defocus to this presbyometric equation. The distance vision is diminished by just 2.5% (0.5/20 = 0.025). However, at the same time, the available accommodative effect is increased from 1.5 D to 2 D, enhancing the near vision by 25% (0.5/2 = 0.25).

If the math does not persuade you, I suspect a simple refraction will. Place yourself behind a phoropter with your best distance correction in place and click in the –0.5 D of myopic defocus. My guess is that you will detect little decrease in your distance acuity (a 2.5% reduction in acuity is barely detectable in this setting).

Next, determine your full reading add, and click all but 0.5 D into the phoropter. Note your reading acuity at 16”. Now click in the remaining 0.5 D of that add. You will read much better, reflecting that 25% enhancement. You will also have seen the artful use of just the right amount of myopic defocus at work for yourself.

Next column

The final column of the four-part series — six steps to successful conventional IOL presbyopia correction — can be found in March 1 issue.

For Your Information:

• William F. Maloney, MD, is head of Eye Surgery Associates of Vista, Calif., and 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; e-mail: maloneyeye@yahoo.com. In the interest of objectivity, Dr. Maloney has no financial interest in any ophthalmic product and has no financial relationship with any ophthalmic company.