Chapter One

The Miracle of Eyesight

Macular degeneration is an eye disease that has been described, most recently by ABC's 20/20 TV program, as perhaps the most significant epidemic of the twentieth century, affecting 13 million Americans and millions more around the globe. The probability of being afflicted by macular degeneration increases with age, but this is not to say that younger people are not stricken. While the disease can result in a loss of one's central vision, the rate of loss differs widely.

    There are several forms of macular degeneration, but the most prevalent is age-related macular degeneration (ARMD), the subject of this book. Despite the name, ARMD can occur at a relatively early age. Arthur Schmidt, for example, is an attorney who was diagnosed with ARMD at the age of fifty-seven.

    Arthur first experienced "eye problems" decades ago in law school. He had trouble making out some of the words in his textbooks and often got headaches as he read. He reasoned, quite logically, that eyestrain and fatigue were the cause of his problems. At the student union he was referred to an optician. As soon as he got his first pair of reading glasses, his ability to read without difficulty was instantly restored, and his headaches vanished. Every few years, when the edges of letters grew fuzzy and the lines harder to read, he would visit the optician nearest his law office and get a slightly stronger prescription.

    Arthur had learned, without thinking about it, that if reading became more difficult, the proven solution was simply a new pair of spectacles with a stronger correction. He bemoaned the fact that his advancing years were requiring prescription changes ever more frequently, but he didn't give the matter any more thought. Whenever necessary, he visited the optician for a quick fix.

    Eventually, he realized that although he could read reasonably well with a brand-new pair of glasses, he could not read perfectly. It took him longer and longer to finish the materials necessary to prepare cases for his clients. Finally, when reading became just too difficult to ignore the problem any longer, Arthur took time out of his busy schedule to consult a good ophthalmologist, a medical doctor who specializes in diseases of the eye.

    Arthur Schmidt never suspected he had an eye disease. When his doctor gave him the diagnosis of macular degeneration, Arthur had no idea what a macula was and didn't know he even had one. (In fact, we have two maculae, one in each eye, at the center of the retina.) He had never spent even twenty seconds thinking about the health of his eyes. Like so many people, he took for granted that he had perfectly healthy eyes and that any vision problems could be corrected with glasses.

    Arthur's case is far from unusual. Macular degeneration is not a disease that is accompanied by fiercely debilitating symptoms at its onset. There is no eye pain--ever. Isolated as a discrete event, failure to read well, even with properly prescribed glasses, is an obvious sign of some difficulty that should be investigated. But Arthur assumed that as a healthy, middle-aged man, he could expect his vision to "slip a bit each year," as he put it, "just as my hairline did." He paid very little attention to what he failed to see when he tried to read, or how the lines appeared when he studied them carefully.

THE ANATOMY OF THE EYE

The eye is a remarkable organ. That is the plain and simple truth. When you think of trying to accomplish the ordinary things you do each day without eyesight, you realize immediately how much you depend on your vision. Despite its small size, the eye is one of the most complex organs in the human body, infinitely more complex than even the most sophisticated computer. To understand how macular degeneration and other eye conditions affect eyesight, you need to know how the eye works.

    I have been studying human eyes for years, but recently I've been astonished at how quickly we are increasing our understanding of the highly specialized role played by each component of the eye. A thorough knowledge of each component--both its structure and its function--provides us with clues for solving complex eye problems. This understanding is what enables medical science to make strides toward finding cures for eye diseases such as age-related macular degeneration (ARMD).

The Front of the Eye

The front, or anterior section, of the eye is composed of the cornea, the pupil, the iris, and the lens.

    The cornea is a transparent membrane sitting on the very outside of the eyeball. A resilient substance comparatively resistant to damage, the cornea is what is touched when you're accidentally poked in the eye. It is also what gets scratched when a particle lodges between it and a contact lens, creating a very painful but a generally quick-healing injury. The clear cornea is continuous with the white sclera that forms the rest of the outer layer of the eye. An additional thin membrane of clear skin known as the conjunctiva covers all of the eyeball except the cornea.

    The pupil, the opening at the center of the colored iris, limits the total brightness of incoming light that is focused by the lens behind it, onto the retina itself.

The Back of the Eye

The back, or posterior section, of the eye is composed of three layers of material. The outermost is the sclera, the membrane that maintains the basic shape of the eye. The second layer of the eye, sandwiched in the middle, is known as the choroid. It is a very thin layer comprising mostly blood vessels. The last and innermost layer is the retina, which contains all the nerve cells that communicate with the optic nerve. The retina is, in effect, the brain's telephone line.

    A hardworking tissue, the retina actually uses more oxygen and nutrients per gram of tissue than almost any other part of the body. In order to secure the rapid flow of oxygen and nutrients it requires, the retina is held between two layers of blood vessels. One is a dense carpet of vessels supplying the neurons that process signals for the brain. The other is within the layer known as the choroid. These latter blood vessels supply oxygen to the retinal pigment epithelial cells (RPE cells) and to the light-sensing cells in the retina known as photoreceptors. The RPE and choroid are critical support tissues for the retina. The RPE is a thin layer of cells just under the photoreceptors that helps remove waste products from the retina, maintaining the health of the photoreceptors. The choroid also helps remove retinal waste and brings new supplies of nutrients and oxygen to the retina.

    As the figure on page 6 demonstrates, the retina takes up a good deal of the surface behind the eye, but the brain is really most attentive to only a small part of the data the retina sends to it. The macula, an area the size of a pinhead, is in the very center of the retina and is responsible for providing central focus.

    The macula enables you, for example, to see the football that is passed on the field, even from bleacher seats. While you watch the ball, you simultaneously see an illegal block downfield, but that event is being processed in the 97 percent of the retina that receives images in the periphery of your vision. The macula, although it represents only 3 percent of the retina, is its most essential component, because it is that tiny sliver that enables us to analyze fine detail.

    Certainly no part of the human eye is dispensable, but the orchestrated action of the many separate components within the eye is really the miracle of sight itself. Still, if one part can be said to be functionally harder working than other parts, it would surely be the retina with its multiple layers of vessels, neurons, and photoreceptors, all continuously active.

    The photoreceptor layer of the retina contains two types of cells, known as the rods and the cones. The rods are called into action for dim or dark situations. They account for your being able to see a black cat on a starless night, crossing a wooded field. The cones function in bright light or simple daylight. Cone cells also determine your ability to distinguish fine detail and to differentiate color. They account for your being able to see that same cat the next morning on your neighbor's porch, wearing a red and gray collar. The color-sensing cones are concentrated in the macula, and they aid in defining the sharpness of central vision. Rods and cones send their information to another group of cells, the bipolar neuronal cells, which in turn relay the data to ganglion neuronal cells. This is the last transmittal terminal on the way to the brain itself.

    The heart of our ability to fixate or focus on one object directly, the cat's pink nose, for example, is lodged in the fovea, a tiny group of cone cells at the very center of the macula. The fovea provides you an unobstructed view of the world directly ahead. In fact, all the blood vessels in front of it are pulled to the sides, like a stage curtain, to create crystal-clear vision. Trouble in the area of the fovea spells trouble in central vision and often in color differentiation. These symptoms frequently lead to a diagnosis of macular degeneration.

HOW WE SEE

Light from a distant object first hits the surface of the cornea, the clear part of the eye, on its outermost surface. Light travels through the cornea and is bent, the first step in the process of focusing the image. The light then travels across a fluid-filled area called the anterior chamber located in front of the lens. Next the light reaches the pupil and lens of the eye. The pupil serves as an aperture that adjusts the overall brightness of the incoming image. The light passing through the pupil is then further bent as it passes through the lens. The lens, unlike the cornea, has the ability to vary the degree to which it bends the light. This is what gives the eye the ability to focus on objects that are at different distances. The closer an object is to the eye, the more the lens must bend the incoming light to keep it in focus on the retina. The rays leaving the lens then travel across a second fluid-filled chamber in the back of the eye known as the vitreous. At this point, the image is ready to be received by the retina, the thin layer of neuronal tissue responsible for interpreting the focused image.

    The incoming image is actually flipped upside down as it traverses the lens, but the retina and the brain are able to restore the proper orientation as they process the signal. There are three different layers of cells stacked one upon the other in the retina. Surprisingly, the layer of photoreceptors that detects the light and converts it into electrical impulses is at the bottom of the stack. Thus light must travel through two layers of secondary processing neurons before reaching the light-detecting cells at the bottom of the retina. Photo-receptor cells convert light to electrical signals, which then leave the photoreceptors and backtrack through each of the other two retinal layers where the signal is processed and further sharpened to prepare for transmission to the brain.

    The eye functions much like a camera. Light entering through the cornea and through the lens is focused the same way that a camera lens focuses the "picture" of what's in front of it. The focused light or image is projected onto the retina, just as a "picture" is projected onto the film in a camera. The image captured by the retina is then sent to the brain through the optic nerve. This camera-like action is so swift in ordinary light, and normally so reliable, that we take for granted the very sophisticated mechanism that allows it to occur.

    In effect, the brain is the commentator for the eye's slide show, and the interpreter of the images that are received. A comprehensive understanding of the pictorial data is essential to vision, because without the brain's interpretive work on the photographic information, we would technically "see" but lack any understanding of what we're seeing. It would be like looking at the various shapes drawn on a sheet of paper without realizing they were the blueprints for a building.

    The delicacy of the eye and its many parts are both the reason for its miraculous functioning and the explanation for some of the sight problems that can develop, especially later in life. A machine with millions of parts is expected, periodically, to require some repairs; its very complexity compounds the chances that problems will occur with decades of constant use.

Copyright © 2000 Robert D'Amato and Joan Snyder. All rights reserved.