Answer lies in attention to candidate selection, counseling, and IOL positioning
Houston- Certain dysphotopsias are unavoidable with diffractive multifocal IOLs since light scatter and image defocus are inherent to the optics. The potential for those problems need to be explained preoperatively so that patients can decide if they are willing to accept them as a trade-off for the benefit of reduced spectacle dependence at near, said Jack T. Holladay, MD, MSEE.
However, IOL position also affects additional light scatter and subsequently the development of dysphotopsias. The latter problems are under the surgeon’s control and can be minimized by paying attention to angle kappa when considering candidates for a multifocal IOL and performing the implantation, explained Dr. Holladay, clinical professor of ophthalmology, Baylor College of Medicine, Houston, TX.
“Image defocus and 18% light scatter are unavoidable with diffractive multifocal optics and those phenomena give rise to nighttime halos and reduced contrast,” he said. “Nevertheless, many patients will tolerate those dysphotopsias in exchange for being able to function at near without glasses.
“However, light scatter will be further increased if the incoming rays are not concentric with the diffractive rings, that is the diffractive rings are not concentric with the pupil,” he continued. “In that situation, patients may be dissatisfied because they perceive the resulting loss of contrast as foggy, hazy, or waxy vision.”
Dr. Holladay noted that if a diffractive multifocal IOL is simply centered in the capsular bag, the optic center is likely to end up on the average 0.3 mm temporal to the pupil center. The incoming light will not be concentric with the lens (rings) and an asymmetric diffraction pattern will result with increased light scatter.
While it is possible to manipulate the haptics so that the lens centers on the pupil, some recent studies show positioning a diffractive multifocal IOL so that its center aligns with the visual axis also ensures that light enters the center of the lens perpendicularly, thereby minimizing forward scatter and image defocus.
(Image courtesy of Jack T. Holladay, MD, MSEE)
Since the visual axis lies about 0.35 mm nasal to the center of the pupil in the average eye, there is a paradox in that it is not possible to center the lens on both the visual axis and the pupil. Splitting the difference by centering the IOL halfway between the visual axis (Purkinje image 1) and the center of the pupil is actually the optimal way to position a diffractive multifocal IOL to minimize forward light scatter and reduce additional loss of contrast (Figure), said Dr. Holladay. This location for the IOL may be attained by first placing the IOL haptics vertically and then ‘nudging’ the superior haptic nasally using a Sinskey hook or similar instrument until optimal centering is achieved, he said.
As a caveat, he noted that it is important to look at angle kappa first to decide if a patient is a good candidate for a multifocal IOL, because in eyes with a large angle kappa (i.e., distance between the pupil center and visual axis > 0.7 mm) there is no optimal place to position the lens in order to reduce forward light scatter.
“If you put the lens halfway between the visual axis and the center of the pupil in an eye with a large angle kappa, the center of the lens will be far enough away from the pupil center and from the visual axis that the patient will still complain of glare and waxy vision,” Dr. Holladay said.
He added that while many surgeons don’t realize it, angle kappa is provided on the readout from both optical biometers that now are being used by about 90% of surgeons (IOLMaster, Carl Zeiss Meditec; Lenstar LS 900, Haag-Streit).
To achieve the desired slightly nasal positioning of the multifocal IOL, Dr. Holladay recommended placing the multifocal IOL with the haptics oriented vertically and then using a Sinskey hook or similar instrument to push the superior loop towards the nose.
“The inferior haptic will not rotate, the lens will shift nasally, and it usually stays there postoperatively,” Dr. Holladay said.