The Options for Surgical Correction of Presbyopia: Where Are We Now?
Exciting inroads into our surgical approach to this problem.
Presbyopia: it's predictable, quantifiable, and inevitable. By the time most people turn 45 years old, they will have lost more than half of their ability to accommodate. By 75 there is virtually no accommodative potential left. Although presbyopia is almost universal and affects people in their most productive years, our most effective (although far from perfect) technological solution to age-related loss of accommodation remains the multifocal spectacle lens, a technology that dates back to Benjamin Franklin's invention of the first bifocal. But change is coming. Although the goal remains elusive, the year just past saw substantial strides in the search for a viable surgical treatment for presbyopia.
Moving Beyond Spectacle Lenses
In the past few decades, the optical industry has worked not only to improve multifocal spectacle lens technology but to provide corrective options in other modalities as well. Several good bifocal and multifocal contact lenses are now available. However, issues such as age-related ocular drying, adaptation difficulties, a complex and often time-consuming fitting process, and the lack of crystal clear vision at all distances have hampered widespread use. So, for many patients, bifocals or reading glasses for near vision remain the most convenient way of dealing with the loss of accommodation.
Attempts to correct presbyopia surgically have been slow to take off, but they have been given impetus by the acceptance of refractive surgery for myopia, hyperopia, and astigmatism. Once the ophthalmic community at large became comfortable with the idea of surgery on otherwise healthy eyes strictly for the purposes of correcting refractive error, it seemed a logical next step to develop a surgical solution to our most common vision problem. Of course, the enormous size of the presbyopic market is a powerful incentive now that manufacturers and surgeons know many consumers will opt for an elective procedure.
Getting at the Problem
Modern physiological studies confirm Helmholtz's theory that progressive hardening of the crystalline lens is at the root of age-related loss of accommodation. Ronald Schachar's recently proposed theory, which attributes loss of accommodative power to the growth of the lens with time and stretching of the ciliary muscles, is provocative and has stirred useful inquiry into the mechanisms of presbyopia. If nothing else, Schachar has made us rethink basic issues, and that has been fruitful. However, multiple independent physiologic studies failed to confirm his theory. In fact, those studies have done nothing but confirm Helmholtz, and for the purposes of ameliorating the effects of presbyopia, we have to consider that loss of accommodation is due primarily to a hardening of the natural lens.
The ideal treatment of the crystalline lens's loss of functionality would be either prevention or reversal of the hardening. We are far short of realizing either. So replacement or augmentation of the lens is our best hope of improving the problem at its source. This strategy has brought forth a number of different approaches that are currently being investigated.
Crystalline Lens Replacement
Clear lens extraction (also know as presbyopic lens exchange) ameliorates the presbyopic condition by replacing the natural lens with an artificial lens with different focal characteristics. A side benefit of clear lens exchange is that it eliminates the need for cataract surgery latter in life. The primary limitation of presbyopic lens exchange is that, at present, the optics of stationary intraocular lenses (IOLs) don't replicate the visual effects of accommodation as well as many patients would like.
Use of two monofocal lenses (one for near and one for distance) is of limited applicability because relatively few patients can successfully tolerate pseudophakic monovision correction. However, those patients who can tolerate monovision[EM DASH]e.g., individuals who have experienced contact lens monovision or those who have grown up with one emmetropic eye and one slightly myopic eyetypically find it quite satisfactory. It's just not a solution with broad applicability.
Bifocal and multifocal IOL designs have progressed but still involve significant visual compromise, including blur, glare, halos, and loss of contrast sensitivity . Although the designs can be refined, some degradation of vision is no doubt a consequence of simultaneous multifocal optics. Currently t he Array®lens (Advanced Medical Optics) is the only multifocal implant approved by the US FDA. Others are in development.
I believe that while highly motivated patients will accept the tradeoffs of multifocal IOLs, this is not a solution that will satisfy most surgeons in the long-run or crack open the consumer market. This realization is driving the development of accommodating IOLs that more closely mimic nature to restore focusing power to the eye.
The end of 2003 provided a watershed moment for ophthalmology when, for the first time, the FDA approved an accommodating IOL. The Crystalens by eyeonics, Inc., which was approved for the correction of aphakia in cataract patients, has hinged haptics that allow backward and forward movement along the eye's axis in response to pressure changes in the vitreous cavity caused by contraction and release of the ciliary muscle. A theoretical problem is that the modest anterior-posterior movement of the lens during pseudophakic accommodation limits the effective power change.
Nonetheless, the fact that FDA approved an intraocular device that moves within the eye is momentous and opens the door to further innovation in this area. Visiogen, Inc. is developing a dual-optic accommodating IOL called Synchrony in which a high plus-power anterior lens moves forward in response to changes in the ciliary muscle, while a minus-power posterior optic remains stationary. The high power (on the order of +30 D) of the moving anterior lens increases the effective range of accommodation. The posterior optic is assigned the diverging power needed to produce emmetropia in a given patient.
I have little doubt that the ultimate solution to surgical correction of presbyopia will be an extremely sophisticated intraocular lens capable of focusing at whatever distance the patient requires. Significant progress in this direction has already been made. For example, a product designed for patients with severe retinal impairments, the Implantable Miniature Telescope (IMT), is now in Phase II/III clinical trials. Although not for cataract patients or normal presbyopes, the device is technologically significant because it proves that a two-element lens can work within the human eye. This, of course, has implications for the treatment of presbyopia. With the IMT, it is only be a matter of time until we progress from a fixed-focus telescopic lens that works within the eye to a telescopic lens that can move or change focus. A parallel line of development will give us IOLs with greatly enhanced features.
Also under investigation are scleral incision and implant procedures inspired initially by data that showed delayed onset of presbyopia in radial keratectomy patients and by Fyodorov's theory that radial incisions in the sclera would improve accommodative potential. Scleral approaches do, in fact, work. The problem is, we don't know with certainty why they work. Schachar's theory holds that anterior sclerotomy increases the equatorial space around the lens, giving it more room to move and, thereby, restoring accommodation. (In Schachar's view, it is lens growthwithout concomitant growth of other ocular structuresthat physically inhibits the movement necessary for accommodation. Sclerotomy would, in theory, give the lens more room for accommodation.)
Physiological studies have shown, however, that the lens does not have increased space to move and, additionally, does not move equatorially. So, although Schachar's theory predicts that scleral expansion will work, the theory isn't upheldleaving us searching for some other explanation. One suggestion is that the scleral expansion procedure induces multifocality, which gives the patients some near vision.
So, although we can't adequately explain why scleral expansion works, the fact is that it does an effect on near vision. However, the effects seen to date are neither long lasting nor highly predictable. Other downsides to scleral approaches include the danger of perforation, retinal detachment, hemorrhage, and ischemia. Also, implants raise the possibility of infection, migration, etc.
Products and techniques under investigation include scleral implants by Refocus Group, Inc., which received FDA approval in January 2004 to conduct Phase II clinical trials for the surgical treatment of presbyopia.
The Multifocal Cornea
Given the extraordinary acceptance of laser refractive procedures, it's not surprising that some are looking for the answer to presbyopic correction on the cornea. In addition, the relative safety and simplicity of lamellar procedures makes them attractive to surgeons. From customized ablations designed to produce multifocal corneal profiles to conductive keratoplasty (CK), a number of corneal options for presbyopia are being explored.
It is difficult, however, to create a multifocal cornea without also inducing aberration. The optical difficulties are much the same as those facing multifocal IOLswith the added disadvantage that corneal tissue, which heals, scars, and is subject to inflammation, is a far less stable and predictable refractive surface than a plastic lens. Even if we could control the cornea, the optics would be an issue. Contact lens manufacturers have tried for decades to create a satisfactory multifocal soft contact lens, without notable success. It's hard to make simultaneous multifocal optics work.
What we've discovered to date is that it is very difficult to apply multifocal corrections to the cornea with the degree of precision necessary to provide the quality of vision that will truly satisfy either surgeons or patients. Living tissue is simply too dynamic and unpredictable.
That is why some are looking at corneal inlays. The inlay would provide the needed near correction, with the cornea acting as a passive carrier. But inlays have had problems with biocompatibility and centration. In addition, an inlay is, in effect, just a different technology for creating a multifocal cornea, which, again, hasn't been very successful so far. A variety of inlay technologies are being investigated, but I, personally, am pessimistic about their long-run potential.
From the surface of the eye to the interior, about the only place we have not yet attempted to put a presbyopic correction is on the retina. But even here, we may soon see developments based on incredibly sophisticated work being done to create a microchip that functions as an artificial retina to restore sight in severely vision-impaired or blinded eyes. This could lead to implants capable of controlling various aspects of image processing, including accommodation. Although it sounds like science fiction, I believe we will see such breakthroughs in our lifetime if not within the next 10-15 years.
Treating Presbyopia Now
In addition to multifocal spectacles, contact lenses, and surgical monovision, we now have two FDA-approved intraocular treatments for presbyopia: the Array multifocal IOL and the Crystalens accommodating IOL. Those of us involved in clinical study may be able to offer other solutions on an investigative basis but, for the general public, these are the five basic options available. (The FDA may approve Refractec's application for an additional indication for the treatment of presbyopia by CK in early 2004.)
Selecting the right option requires a thorough understanding of each individual's vision requirements, lifestyle needs, goals, and expectations. For all patients, the necessary first step is the best refractive correction that can be achieved. Patients should be counseled carefully about the advantages and limitations of the various options. For example, I have treated a large number of pilots. Many of them are mildly hyperopic and have no problems until their 40s. Strongly motivated to avoid glasses, a hyperopic LASIK treatment will restore an acceptable range of vision (at least for some years) and make these patients very happy. The key is to match the procedure to the patient's needs.
For presbyopic individuals who do a lot of computer work, a monocular corneal correction can be a good solutionprovided the patient can tolerate monovision. For patients with a nascent cataract, cataract extraction and implantation of a multifocal IOL (supplemented by a pair of reading glasses for reading very small print) works well for patients who understand and accept that they will at times need to use spectacle lenses.
These are, however, far from perfect surgical solutions. So, when appropriate, I do not hesitate to tell patients that the best option for their particular needs is not yet available. I encourage them to be patient and not pursue surgical options that may preclude them from taking advantage of a better solution that will be available in a few years' time.
THE BOTTOM LINE
Surgical correction of presbyopia is in its infancy but making strides at an increasingly rapid pace. Today, presbyopes who want to reduce dependence on multifocal spectacle or contact lenses can opt for surgical monovision or an intraocular lens (IOL) procedure with either a multifocal or accommodating IOL. (Currently, there is only one FDA-approved device in each category, but more are on the horizon.) Many will be happy with currently available options, provided their expectations well managed and they are willing to accept varying degrees of visual compromise. This being the case, the search for a presbyopic procedure that will more closely approximate the eye's adjustable focusing powerand have broader market appealcontinues. The intrinsic drawbacks of scleral procedures, corneal inlays, and attempts to create a multifocal corneal profile with lamellar surgeries lead me to believe that the avenue most likely to generate the presbyopic solution we are searching for is highly sophisticated technology such as a telescopic lens implant or artificial image processor.
©2004. Robert M. Kershner, MD, FACS, is Clinical Professor of Ophthalmology at the University of Utah Moran Eye Center in Salt Lake City and is director of Eye Laser Consulting, Boston, Massachusetts, USA.
Intraocular Options: Crystalline Lens Replacement
*Eliminates need for later cataract surgery
*Pseudophakic monovision has limited applicability
**Patients who can accept compromise may be pleased
**Drawbacks of simultaneous multifocal optics limit potential
**One approved device, with more on the way
**Goal is to more closely replicate eye's focusing power
**2003 marked first FDA approval of IOL with moving parts
**Forthcoming designs include dual-optics
**Future may see variable focus telescopic lens implants
**Future may see flexible polymers injected into capsular bag
Corneoscleral Surgeries for Presbyopia
*Scleral incisional and implant procedures
**Short-term effects with variable predictability
**Potential for perforation, hemorrhage, etc., with incision
**Infection and migration risks with implants
**Mechanism by which correction occurs is uncertain
**Builds on acceptance of refractive surgery
**Relatively safe and simple
**Drawbacks of simultaneous multifocal optics
**Corneal tissue remodeling imprecise
**Drawbacks of simultaneous multifocal optics
The fact that FDA approved an intraocular device that moves within the eye is momentous and opens the door to further innovation
Although we can't adequately explain why scleral expansion works, the fact is that it does an effect on near vision
Selecting the right option requires a thorough understanding of each individual's vision requirements, lifestyle needs, goals, and expectations
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