Ординатура / Офтальмология / Английские материалы / The Eye Book A Complete Guide to Eye Disorders and Health_Cassel, Billig, Randall_2001

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tion—vision—is one of the most important. What goes on with the eyes when we age can affect our vision just a little bit, or more seriously. Any disease, injury, or infection in the eyes, of course, can cause even more problems, affecting vision and also causing discomfort or pain. We’ll get to these specific problems later in the book. For now, let’s turn in the next chapter to a discussion of how everyone’s vision changes over time, and how these changes matter.

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to picture the eye as a camera. The cornea, the front surface of the eye, acts as a converging lens: it takes rays of light and bends and focuses them. Next, the light is focused more finely still by the lens, which sits just behind the iris (the colored part of the eye).

In a perfect eye, all of this focused light is then beamed back to the retina—what you might call the eye’s “film.” The elegant, complicated retina processes all the information it receives, breaking it down into countless elements, and then transmitting these elements in the form of electrical signals to the brain.

But as we get older, our eyes become less than perfect. Both the cornea and the lens change shape over time. Ac- cordingly—inevitably—our prescription changes, too.

Changes in the cornea: When we’re born, our upper eyelids are nice and tight and tend to press on our corneas. It has been suggested that this snug fit eases over time, and as it does, the cornea starts to reshape itself. What results is a mini-domino effect. The loosening of the eyelids causes the corneas to alter their contour, changing the way our eyes focus light—and, consequently, changing the prescription we need to correct this focus. Often this also causes us to have an increase in astigmatism (a focusing problem; see below) as we reach our forties and fifties.

Changes in the lens: Another issue is that the lens within the eye also changes throughout our lives, starting even before we’re born. As it grows, layer after layer of cells—like the rings of a tree or the skins of an onion—add themselves to the front surface of the lens.

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This growth pattern too has an impact on our prescription. Older layers of cells within the center of the lens become more compacted, making the lens nucleus more dense and the lens in general less flexible. Eventually these changes affect our ability to focus on near objects. (An increase in lens density is also the most common cause of cataracts. For more on this, see chapter 7.)

Refractive Errors

When you need to wear corrective lenses in order to achieve clear vision, you have what’s called a refractive error. Refraction is a physics term that refers to how light is bent as it passes through a lens. When your eye doesn’t do a good job of bending and focusing light onto your retina—and thus allowing you to see—then there’s an error in the eye’s “refraction ability.” (Refraction also describes the technique doctors use to determine what your glasses prescription should be.) Refractive errors for the eye are myopia, hyperopia, astigmatism, and presbyopia.

Myopia

Myopia means “nearsightedness.” A nearsighted eye can focus better on close objects than on distant objects. But this is a relative term, meaning that the degrees of nearsightedness vary considerably. Without corrective lenses, someone who is extremely nearsighted really only sees clearly at too close a distance to be functional, while people who are only mildly nearsighted can generally see well enough to perform most of their daily tasks.

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Fig. 2.1. (A) Normal eye, nearsighted eye, and farsighted eye; (B) nearsighted eye with and without corrective lens; (C) farsighted eye with and without corrective lens

The myopic eye is actually an eye that has too much focusing power. There are three possible reasons for this excess power: either the cornea has too much curvature, or the eye is too long from front to back, or the lens within the eye is focusing excessively. In all of these instances, light is focused in front of the retina (in other words, it undershoots its mark). Your prescription is designed to have the opposite effect, so that your corrective

Image not available.

Image not available.

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lens offsets the eye’s high power and allows the light to focus directly on the retina. Eyeglass prescriptions written for someone who’s nearsighted have a minus sign in front of the lens power. This indicates that the lens of the eyeglasses is effectively taking away the excess power of the eye. A nearsighted eye is not weak; instead, it’s too strong.

Hyperopia

Hyperopia is the technical term for farsightedness. As you might expect, a hyperopic eye is the opposite of a myopic eye in that it doesn’t have enough power to focus light precisely on the retina. Either the cornea or the lens doesn’t have enough curvature, or the eye is too short for light to be focused appropriately. The term accommodation refers to the way the eye muscle and lens work together to focus on something nearby—a book, for example, or a makeup mirror. Farsighted people use this same accommodative system to compensate for their lack of focusing power, in order to see at a distance. When farsighted people try to read, however, they must focus for their hyperopia as well as for the reading dis- tance—an effort that requires significantly more eye muscle power. For farsighted people, then, vision is relatively clearer at a long distance than up close, because it takes less muscular effort. For someone who is extremely farsighted, the world really is blurry at all distances. The hyperopic prescription is the opposite of the myopic prescription: there’s a plus sign before the lens power,

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indicating that the eyeglass or contact lens is adding more power to the eye.

Astigmatism

Astigmatism may be one of the most misunderstood and misused terms in our field; we’ve heard patients use it to describe everything from lid twitches to floaters. Actually, astigmatism is a refractive problem in which an eye doesn’t focus light evenly.

A short lesson in physics may help illustrate this. Imagine that you have a tiny light bulb, the size of a pinpoint. This light sends rays out equally in all directions, forming a sphere of light. If this light is focused through a lens that is also spherical—in other words, if it’s curved equally in all directions like a basketball—then this light, when focused onto a flat surface such as a wall, forms a circle, just like the round spot of light you create when you shine a flashlight at a wall.

A lens that has astigmatism isn’t curved evenly in all directions. Instead of a basketball, it’s like a football. In an eye that has astigmatism, usually the corneal curvature is greatest in one direction and least in the opposite direction. A football has less curvature across the ball than around it. Both directions—across the ball and around the ball—have some curvature, but it’s not evenly distributed, as it is on the basketball.

If you focus a pinpoint of light through a footballshaped lens, the light gets focused more around the lens than across the lens, and it exits as a stretched-out circle

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or oval. Because the eye’s variations in curvature are much more delicate, you really can’t see this football shape. But this subtle difference in curvature tends to blur images that we see, so that the world looks stretched out, just like that contorted circle of light.

If you have astigmatism, a good way to demonstrate this distortion is for you to look at the taillights on a car in front of you at night without your glasses (while someone else is driving, of course!). The red taillights will send off streamers in the same direction as your astigmatism. These streamers will diminish when you put your glasses back on, but they won’t fully go away, because of some scattering of the light at the edges of your glasses.

In eyeglasses, an astigmatic prescription is designed to create the opposite of the eye’s curvature, so that light is focused evenly onto the retina. In an astigmatic prescription, the power is referred to as cylinder power. The axis is specified to tell the optician at what angle to direct the cylinder power (either horizontally, vertically, or somewhere in between).

Presbyopia

The word presbyopia is derived from Latin and translates literally—though rather unflatteringly to those of us who develop it—as “old eyes.” The term describes the phenomenon that eventually befalls everyone: the loss of our ability to focus up close. Presbyopia generally starts anywhere from age thirty-five to fifty. You may first notice it when you’re trying to read the label on a bottle of aspirin or the fine print of a magazine ad.