Refractive Surgery Outlook Feature: October 2010

A Refresher Course on Pseudoaccommodation

By Linda Roach, Contributing Writer

Accommodating intraocular lenses (A-IOLs) have become a fact of life in clinical ophthalmology. European ophthalmologists can choose from several A-IOLs with the CE mark. In the United States, however, the Crystalens is the only approved A-IOL. But this year, the FDA is expected to approve a second option for addressing presbyopia, the dual-optic Synchrony (AMO/Visiogen).

Meanwhile, objective studies of A-IOL vision using high-tech tools have begun to replace the subjective assessments previously considered the norm.1-7 The goal of such studies is to determine how much, if any, of the improvement in near vision reported from A-IOLs represents true accommodation versus pseudoaccommodation.

The term, "pseudoaccommodation," however, is sometimes used in contradictory ways. It is easy to become confused by the different ways in which speakers and authors use "pseudoaccommodation" and related terms, said refractive cataract surgeon Richard L. Lindstrom, MD, of Minneapolis.

A Welcome Development
George H.H. Beiko, BM, BCH, FRCSC, assistant professor of ophthalmology at McMaster University in Hamilton, Ontario, and a lecturer at the University of Toronto, is using dynamic aberrometry (COAS-HD ocular analysis system, AMO WaveFront Sciences) to examine the intraocular behavior of conventional IOLs and A-IOLs in response to near stimuli. Understanding the flood of new information that Dr. Beiko and others are gathering will be crucial for lens-based refractive surgeons.

"This trend is very good, because I think we're going to come across some technology that helps us give our cataract patients with presbyopia the best possible postoperative vision," Dr. Beiko said.

Although pseudophakic/IOL accommodation may differ in some respects mechanistically from accommodation by the natural lens, Dr. Lindstrom believes that it is an active process in response to a near stimulus, and therefore should be considered accommodation.

Not Every 'Pseudo-' is Equal
"The reason that people like the term 'pseudoaccommodation' so much is that we have the term 'pseudophakia,' Dr. Lindstrom said. "But the 'pseudo-' in both words also confuses people. An eye does not have to be pseudophakic to have pseudoaccommodation going on.

"If we started to talk more about depth of field or depth of focus, about active versus passive influences on the eye's near visual function, then it might be less confusing for people," he said.

With this in mind, here is a synopsis of the key terms, what they do and don't mean and thoughts from the experts about where the field is headed.

1.   Accommodation
A few persistent critics notwithstanding, the classic theory proposed by Hermann von Helmholtz for how the eye focuses on near objects remains the accepted explanation for accommodation in prepresbyopic eyes. Accommodation researcher Adrian Glasser, PhD, of the University of Houston gives this summary:

Accommodation is defined as a dioptric change in the power of the eye. The young phakic eye undergoes an increase in optical power with accommodation for seeing at near. This change is brought about by an increase in the optical power of the crystalline lens owing to a decrease in lens diameter, an increase in lens axial thickness, and an increase in curvature of the lens anterior and posterior surfaces. The physical change in the shape of the lens and the increase in optical power constitute accommodation to allow the eye to focus on near objects.8

Dr. Beiko is using the COAS-HD to track accommodation with dynamic aberrometry. The system records 10-second, detailed videos and measurements of the eye as it responds to a near stimulus. In Dr. Beiko's view, accommodation cannot be claimed unless the IOL changes its position, shape or refractive index. By this yardstick, A-IOLs look shaky, he said.

Minimal IOL Movement
"When you look at pseudophakic individuals, and virtually every intraocular lens out there, whether it's labeled as an accommodating IOL or not, you will find a few individuals in whom the IOL does move," Dr. Beiko said. "At most, it moves 500 microns, which will produce about 1 D of accommodative effect. That's the most we find with objective testing. And it's very inconsistent from patient to patient."

However, Dr. Lindstrom uses a more expansive definition for accommodation. In the age of lens-based refractive surgery, he includes other intraocular changes that enhance pseudophakic near acuity—even if they do not resemble what happens during natural accommodation.

His key requirement: An active, finite intraocular response to the near stimulus, just as there is in natural lens accommodation. The response must improve the patient's near acuity and last only as long as the near stimulus does. By that definition, Dr. Lindstrom says, he also includes this sequence of events in his definition of accommodation: that the near stimulus activate the Edinger-Westphal nucleus resulting in ciliary muscle contraction and zonular relaxation; that the relaxation of the zonules (or transient pressure from the vitreous) causes anterior movement, arching or tilting of the IOL's optic and/or pupillary constriction; and that this results in better focus on near objects (even if only because of greater depth of field).

Another Definition
Ioannis Pallikaris, MD, PhD, of the University of Crete in Rethymno, Greece, leans in the opposite direction from Dr. Lindstrom. In any instance in which an A-IOL moves enough to alter the eye's refractive power, this should be called pseudoaccommodation—not accommodation, Dr. Pallikaris says.9

Even if an A-IOL appears to be accommodating, aberrometry might tell a different story, Dr. Pallikaris' group reported at the 2010 ESCRS meeting in Paris. The group built refractive difference maps (distance versus near targets) for eyes with two types of A-IOLs plus a control eye from a prepresbyopic emmetrope. Data were collected using the iTrace aberrometer (Tracey Technologies), said Dimitra M. Portaliou, MD, the paper's co-author.

The researchers found big differences in the patterns of refractive change, with the results of the A-IOLs not resembling each other or the control eye, she said.

2. Pseudophakic accommodation
Despite the "pseudo-" prefix, pseudophakic accommodation is not the same thing as pseudoaccommodation. The term, "pseudophakic accommodation," distinguishes between the natural accommodation of the crystalline lens and the accommodative movement of an IOL's optic in response to Edinger-Westphal signaling and the subsequent ciliary muscle/zonule responses.

Credit Depth of Field
If the position of a single-optic accommodating IOL shifts anteriorly along the visual axis, it adds refractive power to the eye and enhances near vision. This anterior shift of the principal plane of the lens is also part of natural accommodation, as is pupillar miosis, which increases depth of focus, Dr. Lindstrom said.

"A lot of purists think that you have accommodation only if there is a change in the shape of the lens, whether natural or artificial," he said. "But I personally believe that if other things happen to the IOL?including anterior shift, arching, tilting, the separation of two optics as in the Synchrony IOL, two optics displacing across one another as in the AkkoLens, an increase in refractive index electronically driven as in the Elenza lens, or even pupillary miosis or the creation of an artificial small-diameter aperture in response to near stimulus?then I would still consider that accommodation. The key point is that it is an active and transient improvement in near vision stimulated by the brain."

Dr. Beiko is one of the purists. And he is quite skeptical about pseudophakic accommodation with any IOL.

3.   Pseudoaccommodation
This is the predominant term defining A-IOLs and also the one that is most likely to cause confusion.

Dr. Glasser's definition is straightforward and widely accepted. It says:

Pseudoaccommodation facilitates functional near vision, not through a change in optical power, but through an increased depth of field of the eye. An increased depth of field of the eye can be due to ocular aberrations such as astigmatism, spherical aberration, or higher order aberrations. The decrease in pupil diameter that accompanies accommodation also acts to increase the depth of field of the eye.8

In this framework, pseudoaccommodation is seen as a multifactorial, primarily passive phenomenon of the entire optical system. Variables that contribute to pseudoaccommodation, such as corneal aberrations, astigmatism and pupil diameter, combine to lengthen the conoid of Sturm and increase depth of field.8 However, the relative influence each factor has on a person's functional near vision is unclear.

Always a Factor in Near Acuity
Pseudoaccommodation is present to some extent in every eye, whether it has undergone cataract surgery or not, Dr. Beiko said. "It is at work in phakic, aphakic and pseudophakic patients across the board."

"If you take these factors and you control them in a study, if you optimize them, you can get 1.5 to 2 D of depth of field with them," he said.

"That is what we have actually been measuring when we see improvement in near vision with any IOL. And if you look at the studies that reported more impact from A-IOLs than from comparable monofocal lenses, a lot of them did not control for depth-of-field factors like pupil size, corneal aberrations and so on."

Dr. Beiko has assembled data on the levels of spherical aberration that are optimal in different types of eyes and hopes eventually that this can become part of "customized pseudoaccommodation" planning by cataract surgeons.

Harnessing the Data
"I've been pushing to get somebody to work on optimizing all these pseudoaccommodative factors, to give individual patients the best postop near vision as possible," Dr. Beiko said.

The only tools that ophthalmologists would need in order to use the data would be an infrared pupillometer and a good topography unit that also can measure higher-order aberrations, he said. He estimates the total investment at about $20,000.

"For instance, if we know that a patient already has 2 D of pseudoaccommodation before cataract surgery, how can we optimize the surgery to give them even more of it postoperatively? The information is out there already. Someone just has to put it together for the clinicians."

What about the hyphen?
And now let's settle a most vexing issue about pseudoaccommodation: punctuation. Specifically, the hyphen that sometimes sneaks into "pseudoaccommodation."

According to the American Medical Association's Manual of Style, medical terms that begin with "pseudo-" generally are not hyphenated.11

So keep your fingers away from that hyphen key!

References

1. Win-Hall DM, Glasser A. Objective accommodation measurements in pseudophakic subjects using an autorefractor and an aberrometer. J Cataract Refract Surg. 2009;35(2):282-290.
2. Beiko GHH. High-Definition Wavefront Aberrometry to Measure Accommodation. Cataract & Refractive Surgery Today Europe. April 2010. 63-64.
3. Trattler W, Neal D, Raymond TD. Measurement of accommodation in pseudophakic eyes with an objective aberrometer. Paper presented at: XXVII Congress of the European Society of Cataract & Refractive Surgeons; September 2009; Barcelona, Spain.
4. Bartoli EA, Bollini G, Bartoli E, Arpini L, Grignolo FM. Pseudo-accommodation study through aberrometric measurements and ray-tracing simulation. Paper presented at: XXVII Congress of the European Society of Cataract & Refractive Surgeons; September 2009; Barcelona, Spain.
5. Lopez-Gil N, Fernandez-Sanchez V, Legras R, Montés-Micó R, Lara F, Nguyen-Khoa JL. Accommodation-Related Changes in Monochromatic Aberrations of the Human Eye as a Function of Age. Invest Ophthalmol Vis Sci. 2008;49(4):1736-1743.
6. Harton P, Vukich J, Rivera R, Ginsberg B. Assessment of human crystalline lens changes and presbyopia-correcting IOL movements during normal accommodation. Paper presented at: XXVIII Congress of the European Society of Cataract & Refractive Surgeons; September 2010; Paris.
7. Gerten G. Simultaneous measurement of wavefront and shape of the crystalline lens during accommodation. Paper presented at: XXVII Congress of the European Society of Cataract & Refractive Surgeons; September 2009; Barcelona, Spain.
8. Glasser A. Accommodation: mechanism and measurement. Ophthalmol Clin North Am. 2006;19(1):1-12, v.
9. Pallikaris I, Portaliou DM. What is the real accommodation in accommodating IOLs? Paper presented at: International Society of Refractive Surgery Symposium at XXVIII Congress of the European Society of Cataract & Refractive Surgeons; September 2010; Paris.
10. Yeu E, Wang, L, Koch DD. Do Corneal Higher-Order Aberrations Increase Pseudoaccommodation? Cataract & Refractive Surgery Today Europe. April 2010.
11. Iverson C, Flanagin A, Fontanarosa PB, et al. AMA Manual of Style, A Guide for Authors and Editors, 9th Edition. Baltimore, Md: Williams & Wilkins; 1998:211.

Financial Interests
Dr. Beiko is a consultant to Abbott Medical Optics. He receives research funds from Lenstec.
Dr. Lindstrom is a consultant to and/or investor in several firms involved in dynamic aberrometry and presbyopic IOL development, including AMO, Bausch & Lomb, Alcon, NuLens, Calhoun Vision and Tracey Technologies.
Dr. Pallikaris has no financial interests to disclose.