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Presbyopia correction presents optical challenges
Some newer modalities take advantage of pupil size changes and other factors to provide good near and distance vision.
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Optical physics, not surgical skill, may be the greatest challenge in achieving successful surgical correction of presbyopia, according to an optics expert. But some recently developed approaches, which take advantage of physiological changes in the pupil and other optical factors, may provide the best results to date in the effort to correct presbyopia surgically, said Jack T. Holladay, MD, MSEE, FACS.
The prospective patient base for surgical correction of presbyopia is projected to grow rapidly in the coming decades. In 2005, the baby boomer generation falls between the ages of 41 and 59 years old. This population is estimated to be 90 million in the United States and 73 million in Europe, according to Harvard’sGenerations Policy Journal. By 2030, the over-65 U.S. population is expected to grow to almost 70 million.
To attract this patient base, a satisfactory surgical solution to presbyopia is needed, but that has been an elusive goal, Dr. Holladay noted.
This sixth installment in the ten-part Quality of Vision series reviews some of the pluses and minuses of current surgical options for correction of presbyopia and looks ahead to promising techniques and technologies for the future.
Alternate vs. simultaneous
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Spectacles and contact lenses can correct presbyopia successfully, but some patients desire freedom from spectacles and some are intolerant of contact lenses. For these patients, surgical solutions to date have involved compromise, Dr. Holladay said.
“Spectacles and contacts work,” he said. “With contact lenses and spectacles, you can actually make a true bifocal, where the patient looks through different regions of the lens alternately. That is contrasted with, say bifocal IOLs, which work simultaneously.”
Dr. Holladay said the difference between alternate and simultaneous viewing of distance and near is a key concept.
“In glasses and contacts with a discrete bifocal for alternate viewing, you can look through the distance portion and get a perfect image and you can look through the bifocal portion and get a perfect image for near, and you don’t have the other image falling on top of it to reduce the contrast,” he said.
In contact lenses, both simultaneous and alternate viewing designs are available, Dr. Holladay said, and the alternate designs seem to be more popular.
“There are more people wearing a bifocal rigid weighted contact lens with two different zones than there are wearing soft multifocal contacts that create a simultaneous image,” he said. “If you compare the image quality, with the alternate it’s always better than the simultaneous because there is no reduction in contrast, no haloes, no dysphotopsia, no rings, because you’ve got two different lenses that you put together.”
Multifocal IOLs
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The same approaches that are used to create simultaneous distance and near vision in contact lenses are also used in multifocal IOLs, Dr. Holladay noted. Diffractive and aspheric lens designs have been used in both modalities.
Recent multifocal IOL designs using these concepts have been more successful than the early generation of multifocals that appeared in the 1980s, but the new designs still carry the inherent problem of reduced retinal image contrast sensitivity, Dr. Holladay said. This is because the total amount of light entering the eye is divided between two or more images, near and distant and in some lenses intermediate, which are simultaneously focused on the retina.
“It’s always true that the amount of multifocality in a lens is proportional to the reduction in contrast sensitivity,” he said. “The current designs are more equally weighted for distance and near vision than earlier designs, but they still carry the tradeoff of reduced contrast. Problems related to multifocality, such as haloes around lights and dysphotopsias, are now minimized, but they can never be eliminated.”
Neural adaptation
The plus side for multifocal IOLs, Dr. Holladay said, is the neural adaptation that takes place over time in patients implanted with the lenses. By 1 year postoperative, he said, the patient’s brain adapts to the new way visual information is being delivered to it, and patients who were at first unhappy become more contented.
It is vital to perform bilateral implantation of multifocal lenses in order to allow the neural adaptation to take place, Dr. Holladay said.
“Always implant multifocals in both eyes,” he said. “By 6 to 9 months, the neural adaptation will kick in, and the amount of unhappy patients will dwindle down to less than 1% or 2%.”
Despite this neuroadaptive effect, he said, multifocal IOL implantation carries the risk of intraocular surgery plus an inherent sacrifice in contrast sensitivity, so there is “a tenuous risk-benefit ratio in a person with a clear lens,” Dr. Holladay said.
Monovision
Another surgical technique for presbyopia correction that has been adapted from contact lens practice is mono-vision LASIK.
“More contact lens wearers prefer monovision than multifocal simultaneous-vision contact lenses because of the loss of contrast and dysphotopsia with multifocal contact lenses, and they are much more difficult to fit,” Dr. Holladay said.
The monovision strategy can also be used with LASIK, correcting one eye for distance and the other for reading vision, he said, but the disadvantages of monovision include reduced stereoacuity and a resultant reduction in depth perception.
“That’s a big sacrifice. A person’s ability to function with one eye is less than half his ability with two eyes because of the loss of depth perception,” he said. “The synergy of the two eyes with equally good vision providing a three-dimensional image is the basis of stereopsis.”
An advantage of monovision LASIK in comparison with monovision contact lenses is that the patient cannot compare monovision to stereo vision, Dr. Holladay noted. When contact lens patients take out their lenses at night they are reminded of the world of stereo vision, and for some patients the monovision modality suffers by comparison, he said. Patients after bilateral LASIK or IOL implantation cannot make that comparison, and they finally adapt “with the tincture of time,” he said.
Presbyopic LASIK
Several surgeons are pioneering techniques for multifocal LASIK for presbyopia, Dr. Holladay noted. A number of surgeons in Europe and North and South America are gathering increasing experience with these techniques, all of which involve creating an aspheric corneal surface similar to a zonal multifocal lens. In some of the techniques, reading vision is handled by the periphery of the cornea and distance vision is in the center of the cornea, while others reverse those roles.
The chief proponent of presbyopic LASIK in North America has been W. Bruce Jackson, MD, of Ottawa. Dr. Jackson’s technique, which parallels that of Luis Ruiz, MD, of Bogota, Colombia, is designed to steepen the central cornea to provide an area of near vision, while distance vision is handled more by the peripheral cornea.
But Dr. Holladay said he believed the technique works for additional reasons. He said the exaggerated prolate central cornea provides good distance vision and better near vision with smaller pupil sizes. But if the asphericity of the cornea is too great, the patient’s contrast sensitivity will suffer, he said.
Accommodating IOLs
Dr. Holladay said the eyeonics crystalens, the only accommodating IOL approved in the United States, works for the correction of presbyopia — but not necessarily for the reasons most people think it works.
“The term ‘accommodating IOL’ is a misnomer,” Dr. Holladay said, referring to the crystalens. “The IOL appears to move less than 1 mm axially, which is not enough movement to explain the amount of accommodation that clinicians have reported.”
The principal reason for the crystalens’ performance, according to Dr. Holladay, is its smaller-diameter optic. The crystalens optic is 4.5 mm in diamter, 25%, smaller than the 6 mm diameter of most IOLs implanted today, he said.
“The smaller optic provides a 25% greater depth of field,” Dr. Holladay said. “With a standard monofocal IOL, about 50% of people can achieve 20/40 equivalent near vision with both eyes. With the accommodating lens, that figure moves up to about 80%. Depth of field and size of the lens aperture is sufficient to explain that effect. But still about 15% of people implanted with the crystalens can’t read unaided.”
NearVision CK
NearVision CK is the technique for presbyopia correction introduced in 2004 by Refractec, the maker of the ViewPoint CK system for conductive keratoplasty.
Dr. Holladay said NearVision CK, which he called a “mini-monovision” technique, is the only corneal surgical treatment for presbyopia that does not sacrifice an equal amount of distance vision to gain near vision.
He said the CK treatment creates a ring at a nominal diameter of 7 mm where the cornea is actually flattened. That ring essentially forms an annular multifocal cornea, with a flat spot in the very center of the cornea, a steeper zone outside that and the flatter 7-mm ring.
In bright light, Dr. Holladay said, the CK patient’s pupil comes down to 3 mm or 4 mm, and the patient can use the steep zone of the cornea for reading. In very bright light the pupil comes down further and takes advantage of the depth of field provided by the very central flat zone. And in dim light the pupil dilates out past the flat zone to the steeper periphery and again allows reading.
“So what this has done is created an annular multifocal that utilizes the physiologic change of the pupil size to get the best results.” he said. “And it’s serendipitous. They didn’t work all that out ahead of time, it just turned out that way.
“This reduces the effect on the distance vision and contrast sensitivity and yet still gives you pretty good close vision,” he continued. “The only thing you have to do is make sure that the application of those spots is outside the patient’s scotopic pupil, to avoid haloes at night when the optical zone is smaller than the scotopic pupil.”
Dr. Holladay noted that patients must be tested for their tolerance of monovision before NearVision CK is employed, even though the reduction in distance vision may be minimal.
Intracorneal implant
Another new device for presbyopia correction that Dr. Holladay called “extremely promising” is a small-diameter, ultra-thin intracorneal inlay in clinical trials by Acufocus Inc.
The 3.6-mm-diameter, 15-µm-thick inlay is placed under a LASIK flap or in a corneal pocket after distance correction is performed with excimer laser ablation. The device has a 1.6-mm central clear aperture surrounded by an area that is shaded but not opaque — it lets about 10% of light through. The device creates a pinhole effect to provide both distance and near vision using depth of field, Dr. Holladay said.
In clinical trials in Mexico and Turkey, 100 patients have had the device implanted for 6 months, Dr. Holladay said, “and of those 100 people, all of them are 20/15 for distance and near uncorrected with no contrast loss.”
Dr. Holladay said the device works when the pupil is large in mesopic or scotopic conditions by allowing light to pass around it and through it. In a brighter situation, when the pupil goes down to 4 mm or below, “it ‘stops’ down the effective pupil size to 1.6 mm, the retina becomes a little more sensitive, and the distance and near vision are excellent,” he said.
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For Your Information:
- Jack T. Holladay, MD, MSEE, FACS, can be reached at Holladay LASIK Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200, Bellaire, TX 77401; 713-668-7337; 713-668-7336; e-mail: docholladay@docholladay.com.
Reference:
- eyeonics inc. can be reached at 6 Journey, Suite 125; Aliso Viejo, CA 92656; 949-916-9352; fax: 949-916-9359; Web site: www.eyeonics.com.
- Refractec Inc. can be reached at 5 Jenner, Suite 150; Irvine, CA 92618.
- Acufocus can be reached at 18 Technology Drive, Suite 154; Irvine, CA 92618; 949-585-9511; fax: 949-585-9545.
- The Age Explosion: Baby Boomers and Beyond. Harvard Generations Policy Journal. Winter 2004.
- Tim Donald, OSN Copy Chief, is writing the QOV series.