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Our Aging Eyes At an age when life starts to seem a little clearer, presbyopia, a widely deliberated disorder, blurs the picture Sabin Russell, Chronicle Medical Writer |
Monday, April 16, 2001
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It started on Claudia Fenelon's 40th birthday. "You suddenly realize," she said, "that your arms are too short." In dimly lit restaurants, the food was still good, but the menus were unreadable. In her Alameda garden, she was pulling up petunias instead of weeds. After a lifetime of 20/20 vision, Claudia - a corporate librarian - could no longer read a book without reading glasses. "I now have three pairs around the house, one in the car and one in the garden, and sometimes one on my head," said Claudia, now 45 years old. "I don't want to wear them around my neck. That's a dreadful stereotype for a librarian." As millions of Baby Boomers have discovered, one of life's little jokes is played out on every man and woman in their early 40s. It's called presbyopia (prez-be-O-pia) - Greek for "old eye" - and for many it is the first experience of the tangible decline that comes with aging. "The most extraordinary thing about presbyopia is that it happens to everybody," said Adrian Glasser, a physiologist at the University of Houston College of Optometry. "It is a remarkable failure of an organ system that occurs midway through the human life span." Research of Little Help Despite extensive research on the subject, little has changed in the basic understanding of presbyopia in 150 years. And vision experts readily concede there is much about it they still do not fully understand. "The living eye is especially difficult to study," Glasser said. The most widely accepted theory explaining presbyopia was laid out in 1856 by German physiologist and philosopher Hermann von Helmholtz, an intellectual giant who defined some of the precepts of 19th century science, and invented the ophthalmoscope, the instrument still used to inspect the interior of the eye. Helmholtz's theory says that loss of focus comes with loss of flexibility in the lens, the transparent disk that directs light to the back of the eye. The lens is suspended like a miniature trampoline, held rigidly in place by hundreds of taut fibers strung onto an outer ring of muscle. When that muscle tightens around the lens, it reduces tension on the fibers, and the naturally elastic lens bows out. The surface becomes rounder, more convex, boosting its focal power - allowing the eye to focus at close range. Presbyopia occurs, most eye doctors believe, when the lens loses its elasticity with age. It no longer springs out when the support fibers slacken. The eye gradually loses its ability to focus up close. Just why the lens becomes more rigid, however, remains one of those unsolved mysteries of presbyopia. Location a Factor Another head scratcher: Presbyopia reaches a stage where reading glasses are needed at different times in different places. In Ecuador, according to Dr. Edward Revelli, dean of clinical affairs at the University of California at Berkeley School of Optometry, presbyopia becomes apparent at about the age of 33. In Oslo, Norway: not until age 46 or 47. Diet or sunlight exposure could be factors, but no one really knows. The loss of focus proceeds from infancy at a steady, measurable clip. "When you are born, you can focus almost right up to your eye. But every year you live, you lose just a little bit of your focusing ability," said Dr. Bruce Mebine, a San Francisco optometrist. It all goes unnoticed until - typically around the age of 42 - the eye loses its ability to focus at a comfortable reading distance. The arms that hold the printed page, as Claudia Fenelon found, suddenly seem too short. "For people who have worn glasses their whole life, this is a small bump in the road. For people who have had good eyesight before, it is very disturbing, " Mebine said. In addition to the lens becoming more rigid, the ring muscle known as the ciliary body becomes weaker with age, reducing its ability to modify the shape of the lens. A third factor contributing to presbyopia lies in the internal structure of the lens itself. A magnified cross section of a human eye lens reveals a series of layers like the inside of an onion. In a young eye, according to Dr. Clifton Schor of the Berkeley School of Optometry, the density of material making up these bands decreases from the center toward the outer surface. Variations in density from band-to-band increase the ability of the lens to bend light. As we age, however, the density between bands becomes more uniform, decreasing the optical power of the lens itself. "It becomes more like a glass lens," he said. Competing Theory Recently, a Texas ophthalmologist, Dr. Ronald Schachar, has been stirring controversy with an entirely new - and vigorously disputed - explanation that would turn Helmholtz's theory upside down. At the heart of Schachar's theory is an assertion that Helmholtz got it all wrong: Schachar claims that fibers surrounding the rim of the lens increase focusing power by tightening, not loosening, as von Helmholtz observed. Schachar also contends that the diameter of the human eye lens grows naturally - about 20 microns, or a quarter the width of a human hair - every year. This growth gradually reduces the constant tension of the fibers connecting the lens to the muscle ring - and with it goes control over focusing. And Schachar disputes another axiom of optometry: "The lens does not get hard with age," he said. "That's just a story people have been pushing." Schachar has come up with a surgical procedure, currently in Food and Drug Administration-sanctioned trials, that he claims can reverse presbyopia. "He's really saying that Helmholtz was completely incorrect," said Dr. Edward Manche, a Stanford University ophthalmologist who has tested the surgery on five patients. Whichever theory is correct, presbyopia remains a vexing rite of passage, a kind of gateway to the golden years that most of us would prefer to put off indefinitely. "I spend a whole lecture with young interns teaching the psychological
aspects of presbyopia," said Berkeley's Revelli. "It's a real wake-up call to
people, that they are aging."
Why It Gets Harder to Read the Menu Presbyopia (Greek for "old eye") What is it? The lenses of your eyes lose their elasticity with age, making
it harder to focus on close objects. When does it occur? It happens to
everyone, usually becoming most noticeable when people are in their 40s. How
is it treated? People usually need reading glasses. The convex shape helps the
eye focus on close objects. Corrective surgery is being tested.
The lens is made of layers of clear, elastic tissue. As one ages the lens
flattens and hardens.
Ligaments pull and shape the lens in order to focus on objects
Normal vision focuses image on the retina
Presbyopia causes the eye to create a focal point past the retina
(The eye is a single-lens system, so images are projected upside down. The
brain perceives it right-side up.)
When you focus on...
...objects far away
Relaxed muscles keeps lens flat
Ciliary muscles keep tension on the ligaments causing the lens to flatten.
When you focus on...
...close objects
Muscles tighten lens becomes round
Ciliary muscles tighten and ligaments loosen, allowing the naturally elastic
lens to become rounder.
Sources: Adrian Glasser, Ph.D.; Atlas of Human Body by Frank H. Netter, M.D. John Blanchard / The Chronicle
How to Reach Us Comments, questions and suggestions for The Chronicle's Science page are welcome. Reach us by e-mail at science@sfchronicle.com, by fax at (415) 896- 1107 or in care of Science Page, San Francisco Chronicle, 901 Mission St., San Francisco, CA 94103. For updated science news throughout the week and links to science Web sites, go to sfgate.com/science/. E-mail Sabin Russell at srussell@sfchronicle.com. |
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