Presbyopia-State of the Art
Introduction and Foreward
by Robert M. Kershner, M.D., F.A.C.S.
©2002. Robert M. Kershner, M.D., F.A.C.S.
Director of Cataract and Refractive Surgery, Eye Laser Center, Tucson Arizona USA
Clinical Professor, University of Utah School of Medicine
Ik Ho Visiting Professor of Ophthalmology, Chinese University of Hong Kong
In the 1940s, Sato in Japan attempted to correct the refractive error of a myopic patient by making a series of incisions on the endothelial surface of the cornea. The treatment failed. Unfettered by history, Svyatoslav N. Fyodorov, the Russian ophthalmologist, discovered serendipitously that the tiny cuts made by shards from a broken eyeglass, allowed a thirteen year-old boy to improve his nearsightedness. Spurred on by this unusual finding, the surgeon then set out to create purposeful radial incisions into the corneal substance to flatten and reshape it. In so doing, he gave birth to what would become an entirely new field of ophthalmology, refractive surgery.
In 1976, researchers at International Business Machines in New Jersey, in their quest to find a new, more economical method for creating microchips for the burgeoning computer industry, created a new wavelength of laser light. At 193 nanometers they named this light excimer, for its origin from an excited dimer of argon and fluorine gases. Shortly thereafter, ophthalmic surgeons adopted this new concept to the emerging field of refractive surgery-laser reshaping of the cornea. With improved accuracy and speed, an entirely new field of laser refractive surgery emerged, laser assisted in-situ keratomilleusis or LASIK.
Such it is today. Myopia, hyperopia and astigmatism can be corrected with a combination of incisions, laser, or implantable intraocular lenses, thus eliminating the need for eyeglasses and contact lenses for millions of people the world over. But one unfulfilled need remains. Even with today's incredible advances, presbyopia remains the number one reason people need to wear spectacle correction following refractive surgery. A surgical cure for this refractive problem is desperately needed.
To understand the approach to the surgical treatment of presbyopia we first need to know what presbyopia actually is. Presbyopia literally means “old eyes.” What these "old eyes" refers to, is the loss of accommodation that occurs with aging. Accommodation is a process whereby the pupils narrow (miosis), the eyes focus together on the close object (convergence), and the power of the natural lens increases. This enables our eyes to see up close.
The anatomical and physiological changes that occur with accommodation increases the power of the lens. Does this increase occur by the lens moving axially or does the lens actually change shape? Research has shown that the lens changes shape with a forward bulging of it's anterior surface. This bulging of the lens increases its anterior curvature, which increases plus power. According to the theory of Helmholtz, the lens increases its anterior curvature by contracting the ciliary body in response to a blurred image on the retina. The contraction of the longitudinal muscle fibers of the ciliary body constricts the ciliary ring, relaxing the tension on the zonules of the crystalline lens. As a result, the lens becomes rounder with more plus power.
The measurement of the ability to accommodate, is known as the amplitude of accommodation. The amplitude of accommodation (A) equals the far point (R), minus the near point (P). As we age, there is a progressive loss of accommodative amplitude. There are several theories which explain this loss of accommodation. According to the standard accepted theory of Helmholtz, the human crystalline lens becomes more sclerotic with age, resists deformation, and therefore, is incapable of increasing its anterior lens curvature. Recently, Schachar proposed that the actual mechanism of loss of accommodation is due to the increased size of the crystalline lens. As the lens grows over time, crystalline fibers are laid down at the equatorial zone, much like the rings on a tree. By the age of 40, enough lens material has accumulated within the capsular bag to provide little room within the ciliary space for the lens to move. As a result, the lens becomes more globular, and is rendered unable to change power. According to Schachar, if his theory is correct, increasing the ciliary space may allow the restoration of accommodation.
The amplitude of accommodation decreases from birth to age 75. Most people notice the effects of a loss of accommodation by age 45. So predictable is the loss of amplitude of accommodation, that one can literally estimate an individual’s age by how much residual accommodation they have. At age 45, the individual has an average 3.5 diopters (D) of accommodation. By age 55, that has dropped to 1.75 D, by age 65 to 0.5 D, and by age 75 there is no measurable amplitude of accommodation.
Because of this relentless loss of the ability to focus up close, most people are forced to turn to spectacle correction in the form of reading glasses, bifocals or trifocals to function with this disability. Researchers have been working diligently to investigate possible surgical treatments to reverse the loss of accommodation which occurs with presbyopia.
Ophthalmologists presently have at their disposal, six different approaches to the treatment of presbyopia:
(1) spectacle correction with bifocal, trifocal or bifocal contact lenses,
(2) surgical incisions into the sclera overlying the ciliary body to enlarge the ciliary space and facilitate the movement of the lens (anterior sclerostomy),
(3) implantation of scleral expansion segments into four quadrants of the sclera overlying the ciliary body to increase the ciliary space,
(5) implantation of an intraocular lens with a bifocal, toric optic or with an accommodative lens (one which moves within the posterior chamber in response to ciliary contraction),
(6) Applying laser or thermal energy to the cornea to create an aspheric cornea.
The treatment of presbyopia with spectacles or bifocal contact lenses certainly has its drawbacks. Who hasn’t experienced the need to read something and having to go look for a pair of reading glasses? Spectacles are inconvenient. The incisional techniques are still in their early stages of development and the longterm results and benefits are not yet known. Incisions into the cornea or sclera may cause healing difficulties, unnecessary weakening, regression and long-term effects of exposing the uvea which may not yet be known. Implantation of scleral expansion segments pose new challenges in technique for surgeons and the potential acceptance of the cosmetic effect of the procedure by the patient. Bifocal intraocular lens implants have proven themselves to be less than satisfactory. The various styles of lens implants balance the challenge of improving range-of-vision along with the potential abberations created by splitting light into several points of focus. The toric intraocular lens, which is used to correct astigmatism has been shown to improve the range of focus in the pseudophakic patient. Further work in this area is ongoing. Surgical methods directed at increasing the ciliary space, show much promise and deserve continued attention. Laser or thermal alteration of the surface of the cornea may provide an additional method of restoring near focus. The challenges in altering this structure, however, pose some risks. Procedures such as thermal keratoplasty, which could steepen and create an aspheric cornea, may hold promise. In addition, excimer laser technology may evolve to a level where a bifocal optic could be etched into the surface of the cornea to improve range-of-focus. Work in this and future areas of investigation may someday provide the answer.
The future of presbyopic surgery may lie with an intraocular lens implant, which can either change position or increase power in response to some stimulus. New lens designs are rapidly being developed that could potentially restore accommodation with an implantable intraocular lens. Other modalities of presbyopia surgery could include microchips or materials that have not yet even been imagined
In this course, myself with our colleagues from around the world, share their knowledge of the state-of-the art in these important new areas of research. As the attendee studies the pros and cons of each technique, ask yourself, "what would I want for a solution to solve this perplexing puzzle for myself?" All of us who serve patients as they enter the age of presbyopia, need to stop and envision the ideal solution that may come. By furthering the knowledge in this field, together, we may finally achieve, the ultimate goal of today’s ophthalmic surgeon, the surgical treatment of presbyopia. When this noble endeavor is completed, we will have achieved true refractive correction of the eye, and made ophthalmic surgery truly a complete procedure. The final chapter to this new area of surgical endeavor however, has not yet been written. This course will be just the beginning.