What Can Corneal Refractive Eye Surgery Do for You?

Excerpted from
The Complete Book of Laser Eye Surgery
By Stephen G. Slade, M.D., Richard N. Baker, O.D., Dorothy Kay Brockman

A about a quarter of the world's population is unable to see well without corrective lenses. To begin to understand why, let's follow a glorious streak of sunlight as it bounces off your ethereal guest, the butterfly, and penetrates the amazing structures of your eye. Designed to focus light so that you can see clearly, your eye works rather like a camera.

Traveling at 186,000 miles per second, one brilliant bolt of light from the sun strikes the Monarch's gossamer wings, washing them with color. Determined to create a marvelous image for you, the reflected light rays are about to stream through your eye's dome-like window called the "cornea". Your primary light-focusing lens, your cornea is set in the leather-soft white of your eye, rather as a clear crystal is locked in a watch. Covering one-sixth of your eyeball, your oval cornea, which is only about one fiftieth of an inch thick, shields the inner workings of your optical system from the dangerous outside world.

A powerful fixed lens that refracts light, your cornea can be surgically reshaped to help you see nature's diaphanous "winged flower." A change of one one-hundredth of an inch (fifty microns) in the thickness of this transparent curved lens can easily make the difference between legal blindness and excellent sight. Not a simple structure that merely protects your eye, your corneal lens concentrates, or bends, light inward to focus the butterfly's image on the receptors of your "retina". Hugging the back of the inside of your eyeball, your multi-layered retina-rather like the film in a camera-is exquisitely sensitive to light. Composed of neurological tissue, your retina, which is designed to capture and transmit images, contains the "photoreceptors"-the "rods" and the "cones" that are named for their shape. Photochemically responsive to visual stimuli, they make up an important part of the light-gathering neural circuitry of your brain.

As the dazzling reflected light touches the tears that coat the outside of your cornea, its wet surface shines with the brilliance of a smooth polished jewel. This thin tear film covers your self-cleaning eye even when you aren't crying. Bending the light rays ever so slightly inward toward your eye's dark entrance, known as the "pupil," your tears-which are vital to the health of your cornea-keep it supplied with dissolved oxygen from the air. With every blink of your eyelid, the watery three-layered tear film bathes your eye's window-which has no blood vessels-with oily lipids, dissolved salts, glucose, and mucus. Your tears act as a buffer against minor irritations such as smoke and fumes and contain an antibacterial enzyme that discourages infections. Since your cornea is richly supplied with nerves, the tiniest speck of dust will cause you to blink and tear.

Continuing along their journey through your gem-like eye, the intense light rays-which now have been refracted, or bent inward, by your corneal lens-stream through your pupil into your delicate inner eye. The size of your pupil's dark "opening" is controlled by your colored "iris" rather as the f/stop, or aperture, of a camera is governed by the action of its diaphragm (see fig. 1). A double muscle, your doughnut-shaped iris automatically expands and contracts like a pleated accordion to regulate the amount of light striking your retina. The color of your iris determines whether your eyes are blue, brown, green, or hazel. Since you are outside on a sunny day, your pupil constricts to almost pinpoint size to protect your sensitive photoreceptors from too much light stimulation.

Racing between the cornea and the iris, the light rays now penetrate a clear watery liquid called the "aqueous humor." This fluid circulates in the front chambers of the eye, flowing through the "anterior chamber". Supplying vital nourishment to the lenses of your eye, this clear watery liquid-which is continuously replenished and drained from the eye's front cavity-is under a relatively constant pressure. A dynamic force, the "intra-ocular pressure" stabilizes when liquid drainage equals fluid production. If the liquid that nourishes the lenses fails to drain properly, the pressure within the eye can become too high. When this potentially dangerous condition remains untreated, the optic nerve that carries visual information to the brain can be damaged, leading to a disease called glaucoma.

Before you can see close objects, the light must be focused by another lens inside your eye called the "crystalline lens". You may be familiar with how the lenses of a camera produce a well-defined photograph by changing the distance between the lenses and the film. Far more refined than any mechanical device, your flexible lens fine-tunes your vision by instantaneously changing shape to focus light. Suspended directly behind your fixed corneal lens and your iris by fine string-like ligaments called "zonular fibers," your adjustable lens provides about one-third of the light-bending power of your eye. Your amazing crystalline lens, which is the size and shape of a piece of M&M's candy, but about two-thirds water, allows your eye to refocus instantly when you are young. As you glance from the distant fern to your nearby hand, this pliable lens-which can "accommodate," or change shape, becoming rounder-works to place close images on your retina. Adding more power to your focusing system, your crystalline lens thickens when special "ciliary" muscles inside your eye release tension on the zonular fibers that control the shape of your lens.

As we age, this lens gradually loses its flexibility and thus its focusing ability, leading to a condition called "presbyopia," which literally means "old eyes." After age forty or so, even otherwise normally-sighted people begin to have problems seeing the fine print of a phone book. Eventually the newspaper becomes blurred, and reading glasses or bifocals are necessary. With time-at around age fifty-five, depending on the individual-the lens loses much of its ability to accommodate.

Remember when your grandmother needed cataract surgery because her vision was poor? Around age sixty or older the crystalline lens, which is surrounded by a clear elastic capsule, may become cloudy. Examining the eyes with a special ophthalmic "slit-lamp" microscope, a doctor may see a noticeable light-filtering lens opacity called a cataract. It can become so dense that it blocks light's path to the retina. When the cataract interferes with the persons daily life, a surgeon can remove the clouded lens and replace it with an artificial "intra-ocular lens." Since this artificial lens has no ability to change shape, patients who have had cataract surgery wear reading glasses to focus up close.

Proceeding on the wondrous journey through your eye's delicate structures, the beam of light now penetrates the largest chamber of the eyeball. This dark space, pierced only by luminous images coming into focus, is filled with a transparent, jelly-like substance, the "vitreous humor". A relatively inert fluid, the vitreous helps hold your eye in its spherical shape. Although this viscous liquid is almost 99 percent water, it also contains hyaluronic acid-a natural shock absorber to help protect your retina from fast moving curveballs.

At last, the sun's speeding photons of light reflected off the butterfly approach your fragile retina. The self-restoring "videotape" of your living "TV camera" your retina is the most forward part of the nerve fibers of your brain. If you are nearsighted, or "myopic," like Karen, the particles of light will come to a focus in front of this wet, thin tissue, producing a fuzzy image of distant objects. If you are farsighted, or "hyperopic," the rays will reach a theoretical focal point behind your retinal curtain, forming an unclear picture. If you have "astigmatism," your central cornea is probably shaped like a football halved the long way rather than a basketball similarly cut. Since the astigmatic cornea is aspherical, the light rays are unequally bent. This means that they fail to meet at one point so that your eyesight is blurred at every distance. For those of you who are blessed with normal vision, the light will splash directly on your retinal receptors. Nevertheless, even if you have 20/20 vision, you will really never "see" the actual Monarch stroking the air. You will only sense its presence through the shafts of light that the butterfly's wings reflect.

Intent on completing their mission, the light rays carrying the Monarch's ever-changing image place it upside-down on the inside of the back of your eyeball. Capable of absorbing light, your retina's rods and cones-the photoreceptors, or nerve cells, named for their shape-convert the light energy into electrical signals that are relayed to your brain through your "optic nerve". Early in life, your clever brain learned to turn this flat inverted picture into a three-dimensional, right-side up image. Trained during infancy, your brain interprets the Monarch's coded message, allowing you to perceive far more than an insect attached to a plant. For you, the moving fingers of light paint a colored, animated work of art, a thing of beauty swaying in the soft breeze.

In order for you to discern the brilliant orange hues of the Monarch's wings, the three basic types of color-distinguishing pigments in your retinal cones must absorb the light rays.