20 years of developing conventional IOL presbyopia correction

To meet the presbyopia challenge, elements such as precise manipulation of myopic defocus are employed.

William F. Maloney

My last column featured the presbyopia correction of two longtime patients. Ed and Maxine each had their reading vision restored using the conventional IOL approach. Ed’s procedure was in 1985, and his wife Maxine’s was in July 2005. Together, they bracket the 20-year history of presbyopia correction using conventional IOLs by incorporating just the right amount of myopic defocus to meet their individual reading goals.

If you were surprised to learn that the conventional IOL approach to presbyopia correction has such a long track record, you are not alone. Many readers were. I, too, was shocked when Ed reminded me that his surgery was all the way back in November 1985 and that I had told him then that he was my seventh presbyopia-correction case.

Ed’s reading result taught me early on that the conventional IOL approach could be successfully tailored to achieve even the most demanding reading vision goals. Maxine’s more recent success says loud and clear that we can now reproduce that same experience reliably and accurately for a wide range of candidates. Ed and Maxine were important milestones on my path to presbyopia correction.

Specific comparison challenge

Any surgeon experienced in refractive surgery knows that Maxine represented a major challenge — the specific comparison syndrome. She had watched Ed function smoothly without glasses for 20 years. Her tacit, mostly unconscious assumption was that she would achieve the same set of distance and reading vision capabilities. No length of optical explanations about patient variability was going to change that. Despite being a delightful, cooperative and positive patient, any deviation from Ed’s result was bound to disappoint Maxine, whether or not she ever voiced it.

Exact refractive accuracy is the hallmark of all successful presbyopia corrections, no matter which IOL you use. However, precisely duplicating Ed’s specific reading result for Maxine would require the utmost of control over each variable in the presbyopia equation to ensure that the selected target refractive outcomes of –0.38 D (right eye distance) and –1.93 D (left eye near) were achieved. Until recent years, I would not have felt certain of achieving the pinpoint refractive outcome necessary to meet this challenge. I have that confidence now because of these key elements of our preop approach to presbyopia correction.

Biometry

	Ed (1985) and Maxine (2005) are bookends on 20 years of presbyopia correction.
	Images: Maloney WF

It has been more than 3 years since I began performing biometry on all of my presbyopia patients using the Carl Zeiss Meditec IOLMaster. I am aware that this still represents a major departure from the status quo. I am also convinced that this change is essential to reaching this level of confidence. The IOLMaster is capable of remarkably accurate biometry, including keratometry. However, consistently achieving the maximum degree of accuracy requires subtle skill and a degree of interest in the outcome that is the surgeon’s alone. In my view, there is no alternative if we are to reach the rock-solid confidence that presbyopia corrections require.

Comparative astigmatism analysis

The new accuracy metrics of presbyopia require correction of even low levels of astigmatism. The degree and axis of the cylinder need to be accurately assessed and then compared using topography, wavefront, IOLMaster and manual keratometry. Consistent measurements across the board allow the high degree of confidence we need. Discrepancies must be reconciled. Surgical correction of any preop astigmatism, typically with peripheral astigmatic keratotomy, must be consistently accurate enough to reduce any astigmatism to less than 0.5 D.

Manipulating the myopic defocus

Myopic defocus is now regularly used as a supplemental source of reading magnification with all presbyopia IOLs, but it is central to the conventional IOL approach. Based on recent questions from readers, this is the least understood aspect of this approach, and I think a little historical perspective will help.

During the early years, well before Ed’s procedure in 1985, I began this process by targeting for emmetropia in one eye as usual but then slightly increasing the degree of myopic defocus in the fellow eye. My goal at the time was to slightly improve intermediate focus. Full reading focus seemed to me well beyond the limit of this approach, so at the time presbyopia correction was not even a consideration.

In time, however, these patients began to show me that even a little myopic defocus goes a long way — much more than with contact lens monovision. The difference was the –1 D effect of IOL pseudoaccommodation that I would soon come to fully understand and rely upon in this process.

I continued to gradually increase the myopic defocus employed in one eye. I passed –0.5 D and –1 D without creating any symptoms but was increasingly uneasy as I approached –1.5 D. Therefore, I determined that an interocular difference of 1.5 D would be my limit — sufficient to provide good intermediate focus yet still typically below the symptom threshold. At the time, that seemed as far as this approach could go.

Breakthrough — the distance compromise

In retrospect, the 1.5 D limit was fortuitous because it forced me to consider the other end of the focus spectrum — distance vision.

This 1.5 D of interocular difference did not necessarily have to begin at emmetropia. The distance eye could also be targeted for some myopic defocus. What an important breakthrough this realization turned out to be. If a patient sought full reading focus and was willing to sacrifice a slight amount of distance, then the same fixed 1.5 D of interocular difference could be flexibly tailored according to each patient’s unique goals.

A conventional IOL is one of the choices for correction of presbyopia.

Rather than fixing distance emmetropia as the automatic starting point, I began to slide the distance eye target toward –0.5 D and the near eye target toward –2 D. With pseudoaccommodation’s effect of –1 D, this proved sufficient to provide full fine-print (zone 1) reading. I realized that now even patients seeking the finest reading focus need never compromise more than –0.5 D of their distance acuity.

I began working with a range of IOL pairings with the distance eye targeted between –0.2 D and –0.5 D and the near eye targeted between –1.7 D and –2 D, always holding the interocular difference below the 1.5 D limit.

As I piece this timeline together, it was a few months later in November 1985 that I met Ed and performed his presbyopia correction with a conventional IOL pairing that targeted –0.4 D for the distance eye and –1.96 D for the near eye. Twenty years later, in July 2005, with a good deal more confidence in a precisely accurate outcome, I performed essentially the same procedure on Maxine using a conventional IOL pairing targeted for –0.38 D for the distance eye and –1.93 D for the near eye.

Typical of most cataract patients, they would eagerly trade a small amount of distance acuity for a solid restoration of both intermediate and fine-print reading focus. Why then does neither Ed nor Maxine need any glasses, even for distance? Because myopic defocus of less than –0.5 D most often goes unnoticed except for night driving, and cataract patients usually do only limited, local driving after dark. It turns out that the distance compromise side of the myopic defocus equation is most often no compromise at all. In the next column we will examine the optics that explain why.

Next column

In our February 15, 2006 issue, we will discuss how the 0.5 D impact is minimal for distance but huge for reading.

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.