Aspheric IOLs can mitigate glaucomatous visual loss

With reduced contrast sensitivity resulting from glaucoma, aspheric IOL implantation makes the most sense for the majority of patients.

Issue: March 10, 2008

ByThomas W. Samuelson, MD

Thomas W. Samuelson

Patients with glaucoma and concurrent cataract have inherent challenges when undergoing phacoemulsification compared with those with otherwise healthy eyes. Contrast sensitivity is adversely affected with aging, especially with cataract. In the glaucomatous patient, this becomes even more relevant, as a patient can still have excellent visual acuity but a pronounced reduction in contrast sensitivity.

Researchers have postulated that the basis for this is probably neuronal, as moderate to severe retinal ganglion cell loss may not affect visual acuity but may affect contrast sensitivity. Therefore, for glaucoma patients, it is generally a double hit to their contrast discrimination, with both the developing cataract and their glaucoma contributing to their deficit. The cataract removal will help improve contrast sensitivity, but the underlying deficiency on the basis of the glaucoma remains. As surgeons, we must ask how we can help these patients. Most of the time, contrast sensitivity is an optical phenomenon influenced by spherical aberration, the reduction of which is the main basis for the science behind aspheric IOLs.

Traditional IOLs have positive asphericity and give a pseudophakic eye a larger amount of positive spherical aberration. Aspheric IOLs, however, are available with differing amounts of negative sphericity, ranging from zero sphericity to –0.27 µm. The negative asphericity of the lens offsets the positive corneal sphericity, simulating the youthful eye.

When treating patients with compromised contrast sensitivity, it is in our patients’ best interest that we do whatever we can to restore it. Aspheric IOLs can enhance contrast sensitivity, which may offset some of the loss incurred with glaucoma and help give the patient better quality of vision.

Lens material choices

Biocompatibility of the lens material is especially important for glaucomatous patients undergoing cataract surgery. Typically, patients with glaucoma are more prone to perioperative inflammation secondary to medication toxicity, pupil manipulation, shallow anterior chambers and IOP fluctuations to varying extremes.

Acrylic lenses have been around for more than a decade and initially showed significant improvements over the early silicone materials in biocompatibility. The early-generation silicone materials were not biocompatible enough to use in patients prone to inflammation and resulted in numerous patients requiring prolonged topical steroid medications or YAG laser surface dusting to help eliminate giant cell membranes on the surface of the IOL. Despite these early limitations, the current-generation of silicone lenses has proved to be among the most biocompatible materials available, and their use is actually encouraged in high-risk eyes. Indeed, the bulk of the literature suggests newer-generation silicone lenses are at least as biocompatible as acrylic, and when both options are available, such as with the Tecnis (Advanced Medical Optic) platform, I prefer to use silicone over acrylic, even in high-risk patients.

Clinical pearls

In general, I would argue against using a multifocal IOL in a patient with moderate (or worse) glaucoma. Although the multifocals offer the versatility of near vision while retaining distance vision, there have been reports of decreased night vision and diminished contrast sensitivity. One of my truisms is to try not to further compromise an already compromised visual system.

A recent meta-analysis of eight randomized trials noted a loss of contrast sensitivity in multifocal patients in each of the studies.

There is scant literature concerning the influence of multifocals in glaucoma patients, but it seems prudent to use caution. Many glaucoma patients are already uncomfortable with their mesopic vision; multifocal lenses may further compromise them.

When surgically implanting aspheric lenses, we need to be vigilant about centration. A generalization is that the more negative spherical aberration in an IOL, the more important proper centration becomes. Some glaucoma patients are prone to decentration (they may have lax zonules due to exfoliation, for example). A good rule of thumb for the Tecnis is that it must be centered within 0.4 mm of the visual axis and tilted less than 7· from the visual axis to live up to its full potential.

Finally, it behooves us to remember that monovision achieved with bilateral aspheric IOLs remains a viable option in glaucomatous patients who desire spectacle independence for most of their activities, yet are accepting of spectacle use to achieve the highest quality visual function when necessary.

For more information:

Thomas W. Samuelson, MD, can be reached at Minnesota Eye Consultants, 710 E. 24th St., Suite 106, Minneapolis, MN 55404; 612-813-3600; fax: 612-813-3636; e-mail: twsamuelson@mneye.com. Dr. Samuelson is a consultant/scientific adviser for AMO.

Reference:

Leyland M, Zinicola E. Multifocal versus monofocal intraocular lenses in cataract surgery: a systematic review. Ophthalmology. 2003;110:1789-1798.