Multifocality enhances image resolution but diminishes contrast sensitivity

Ongoing design improvements aim to reduce spherical aberration, nighttime glare and halo.

Roger F. Steinert

A thorough grasp of optical physics and lens design features is essential to optimizing patient satisfaction with multifocal IOLs, a surgeon said.

“Who is the right patient for a multifocal lens?” Roger F. Steinert, MD, asked at Hawaiian Eye 2010. “Somebody who clearly desires less dependence on glasses. In the absence of that, they’re probably not going to be as likely to be happy. You also have to educate the patient that there’s going to be this process that will take potentially several months to adapt to the new visual system.”

Clinicians should also be aware of contrast sensitivity and various pathologies, Dr. Steinert said in his presentation on multifocal IOL performance.

“Because of these issues, particularly contrast sensitivity, if there’s other vision-limiting pathology, particularly in the macula, you’re probably well advised to stay away from multifocal lenses,” he said.

The evolution of multifocal lenses began with the original diffractive-refractive design that Alcon acquired from 3M. More recent changes in diffractive step design have resulted in two versions of the AcrySof IQ ReSTOR IOL, with +3 D and +4 D additions, Dr. Steinert said.

Optics and neuroadaptation

Clinicians and optic designers need to understand one basic law of optical physics, according to Dr. Steinert: 50% of light energy is out of focus at any given time. “That is an unavoidable physical reality,” he said.

With multifocality, the in-focus and out-of-focus images add up and produce a luminance curve. The patient with a multifocal lens is able to discriminate the edge of an image but sees that image with reduced contrast sensitivity, he said.

Glare and halo are commonly associated with multifocal lenses, Dr. Steinert said.

“The reason is that you get the halos because, when the patient is looking in the distance and they have a bright light source, some of that light is going to go through the near portion of the multifocal lens, and it will be brought into focus in front of the retina and then diverge outward,” he said. “It’s that diverging outward that creates the perception of a halo. The IOL power does have a big impact on that in terms of where that halo is located.”

Multifocality depends heavily on neuroadaptation, Dr. Steinert said.

“This computer that we’re carrying that occupies a quarter of our skull is very, very good at improving edge definition and reducing out-of-focus aberrations,” he said. “We’ve seen plenty of that with laser vision correction as well. That’s why multifocal lenses can deliver functional simultaneous distance and near vision satisfactorily in most patients.”

Evolving optical design

Improvements in optical design include aspheric optics, which reduces spherical aberration, and apodization.

“Apodization is a technique that Alcon used to basically weight the reading toward the center and delete some peripheral rings and shift to more distance-dominant with the big pupil,” he said. “That hopefully reduces nighttime aberrations but does inherently compromise reading vision under dim illumination.”

Improvements to the Tecnis aspheric IOL (Abbott Medical Optics) have centered on diffractive rings around the back of the lens rather than the front, Dr. Steinert said.

“The idea is that perhaps that is working better in giving more depth of focus because it’s closer to the nodal point,” he said. “Certainly, they have better nighttime dim-light reading because they have more rings way out to the periphery, and the intermediate works pretty well.”

Dr. Steinert offered a pearl on reducing nighttime halo for a patient implanted with a multifocal lens: 0.5 D of myopic correction in the patient’s prescription for nighttime driving glasses will shrink the halo and make it less evident, he said.

“The idea is that you put a minus lens in front,” he said. “You’re going to defocus the distance slightly because you’ll be behind the plane of the retina slightly, but that will also bring that blur circle closer to the center and make the halo less obvious.”

There is no truly reliable test of patients’ ability to tolerate multifocality in the presence of a cataract, Dr. Steinert said. – by Matt Hasson

• Roger F. Steinert, MD, can be reached at The Gavin Herbert Eye Institute at University of California, Irvine, 118 Med Surge I, Irvine, CA 92697-4375, U.S.A.; +1-949-824-8089; fax: +1-949-824-4015; e-mail: steinert@uci.edu. Dr. Steinert is a consultant for Abbott Medical Optics and LenSx Lasers.