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

30 / INTRODUCTION

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Fig. 2.3. Farsighted eye with presbyopia, with and without accommodation

This is what happens: Remember the accommodative system, the muscle and lens system within the eye that enables us to focus on near objects? The ciliary muscles, part of this system, sit just behind the iris. When these muscles contract, they cause the lens to bulge forward and change the focus of the eye, so that we can see something up close (see figure 1.3).

Now, remember how the lens in our eye is always growing? Well, by the time we reach our mid-thirties, this constant thickening has taken its toll; the lens has lost the flexibility needed to adjust its shape. Consequently, we can’t accommodate—in other words, we can’t see close up—nearly as well as we used to. Over time, the closest point at which the eye can focus moves

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farther and farther away—which is why one day we start holding the newspaper at arm’s length to read it. But our arms are only so long, and eventually they’re just not long enough to let us read without the help of corrective lenses.

Because presbyopia is part of the normal growth and development process of the lens, it isn’t really what we think of as an “age-related degenerative change” to our body. That is, the ciliary muscles aren’t weakening, and no tissue is breaking down. This ongoing evolution of the lens is simply an inevitable part of aging, something we can’t change or control. There are no exercises or treatments that can stave off presbyopia, or even slow it down.

How will presbyopia affect you? It depends. If you’re mildly to moderately nearsighted, presbyopia means you can read comfortably without your glasses, because your myopia does the job of focusing for a near object in place of your ciliary muscle and lens system. Because you can read more easily without glasses, presbyopia might not even trouble you until your late forties or early fifties. If you’re farsighted, problems with presbyopia will likely occur much sooner; your ciliary muscles already have extra work just compensating for the hyperopia as well as focusing on objects up close. People with astigmatism often compensate by bringing reading material closer, which makes it relatively larger and easier to see. When presbyopia begins, you probably won’t be able to compensate as well for your astigmatism, and you might need a presbyopic prescription sooner.

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because so much of these early years is spent in school, the many hours a day spent reading and focusing up close probably add to the problem. All of the accommodation (discussed above) necessary for this focusing creates a situation in which the ciliary muscles are constantly contracted and the lens is constantly focused at near; eventually the eye can adapt to that contracted state—in other words, by getting used to focusing mostly at a reading distance—and become still more myopic.

Between ages twenty-five and thirty-five, the nearsighted prescription usually doesn’t change much. However, as you might expect, those of us whose work involves extensive reading, writing, or other up-close focusing may still need increasingly stronger prescriptions during this time.

After about age thirty-five, the eye changes that cause accommodation problems begin to affect our degree of nearsightedness as well. As the up-close focusing system starts to fail, myopia that was brought on by excessive accommodation actually starts to get better; often this trend continues into our late fifties and early sixties. However, because this decrease in nearsightedness is a sign of diminishing accommodation, this seemingly happy turn of events may simply turn out to be the first step on the road to needing bifocals. Thus, for many people in their sixties there comes, after years of enjoying milder prescriptions, a reality check of sorts: becoming more nearsighted again. The most common type of cataract, nuclear sclerosis, is a consequence of years of growth and subsequent hardening of the lens. (For more

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on cataracts, see chapter 7.) In people with myopia this hardening of the lens at first may tend to increase its focusing power, and once again we start getting more nearsighted.

Aging and Farsightedness

With hyperopia, again, there’s a typical lifetime cycle of progression. Up to about age twenty-five, farsightedness sometimes gets better on its own, as our eyes grow and develop. (If, for instance, the problem is that the eye is too short for light to be focused appropriately, this might resolve itself when the eye simply gets bigger.) Often there’s no need to correct this problem in someone younger than twenty-five, because the accommodative system has a tremendous ability to compensate for this kind of anatomical shortcoming.

But the eye changes that cause us to need bifocals can manifest themselves much earlier in someone who’s farsighted, and they can begin to affect our ability to read and focus up close as early as our twenties. As we get into our late thirties and early forties, when we begin holding the newspaper at arm’s length, we also begin to lose the ability to accommodate for our hyperopia. We begin having trouble with our distance vision and may require corrective lenses to help us to see better. Glasses prescriptions for distance vision often get stronger in our late fifties and early sixties.

For farsighted people the dawn of cataract formation may cause the opposite effect of the change that occurs with nearsighted people: a shift in the power of the lens

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within the eye causes an increase in the eye’s focusing power, which results in a decrease in hyperopia. That is, in a farsighted person, distance vision without eyeglasses can actually improve with early cataracts. This is known as second sight (see below and chapter 7).

Aging and Astigmatism

Astigmatism doesn’t change as much with age as myopia or hyperopia do. After the eye stops growing, astigmatism levels off; decades may go by before you need a new prescription. Then along comes presbyopia—and this plus changes in lid tension across the cornea can cause changes in astigmatism that may be for better or worse, depending on your particular case. Early cataract formation can also cause changes, as the cataract subtly changes the shape of the lens within the eye.

Aging and Presbyopia

After its onset, presbyopia produces a classic pattern of change over the next fifteen to twenty years. Initially your glasses prescription is doing part of the focusing for you. As your accommodative system changes, the glasses prescription increases, until eventually the prescription does all of the focusing for you.

Usually the first symptom of presbyopia is that we can’t seem to bring an object as close to our eyes, and keep it in focus, as we used to. Also, it takes longer to bring a near object into focus and then to shift our focus to look at something across the street or even across the room. (The cause of this delay is the ever-thickening, in-

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creasingly less flexible lens, which doesn’t do its job as fluidly and effortlessly as it once did.)

As presbyopia progresses, the point at which we can focus moves farther and farther away, until eventually anything inside of twenty feet looks blurry. While your presbyopia progresses to the point where your reading addition to your distance prescription does 100 percent of the focusing for you, your near range of focus tends to diminish until your vision is really only clear from about twelve to twenty inches.

What’s happening is that as you need more lens power in your glasses to compensate for your presbyopia, your ciliary muscles and lens are less flexible for range of vision, and your near working distance keeps inching closer, to where the spectacle lens focuses.

Those of us who spend a lot of time viewing objects at intermediate distances of twenty to forty inches—the computer monitor, for example—often need additional lens prescriptions. For many people the best solution is either a progressive addition, “no-line” bifocal that allows for multiple working distances, or a trifocal that adds another lens to your spectacles, designed particularly for intermediate-distance viewing.

Early cataract formation, or nuclear sclerosis, also affects presbyopia. Because this causes a shift in vision that can make people either more nearsighted or less farsighted, some of us suddenly find ourselves able to read without our bifocals (a delightful phenomenon often referred to as second sight). Unfortunately, this is a short-

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term improvement; as the cataract progresses, vision at all levels tends to get worse.

Binocular Fusion

Besides refractive errors, the eye is also subject to other changes that affect our vision. Remember, we don’t see with our eyes alone but with our eyes and brain (see chapter 1). Everything we see—image information beamed into each eye—is transmitted to the brain. In turn, the analyzed information from both eyes is then processed by the brain to allow us to view the world, as eye doctors put it, “binocularly.” The term binocular vision refers to our ability to use both eyes together to provide an enhanced, in-depth view of the world. (Most of us see better with both eyes together than with either eye alone.)

This ability for our eyes to work together is harder than it sounds, and it requires extensive coordination. Messages from the brain enable both eyes to move together when viewing and tracking the movement of any object. When our eyes work together in this manner, it’s called binocular fusion. Binocularity is a combination of motor and sensory fusion. Motor fusion is the mechanical ability of the eye muscles to work together; sensory fusion is how well the brain can turn these visual messages from both eyes into one coherent image.

Because the eye muscles from both eyes are rarely perfectly coordinated, it’s the sensory fusion that helps fine-

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tune any slight mechanical misalignment. As we age, our ability to maintain good motor fusion, or mechanical alignment, tends to weaken; the muscles start to weaken in the eyes, just as they do elsewhere in the body. If your eyes tend to cross, for example, this tendency can increase as you get older and the sensory fusion system starts to labor harder than usual to help keep your eyes working together. When this happens, unfortunately, people are all too aware of it. Headaches located in or around the eyes can become common; so can symptoms of the eyes “pulling” together, plus overall fatigue and eyestrain with any big visual chore such as a long time spent behind the wheel of a car or in front of a book. Finally, if the sensory fusion system becomes too overbur- dened—if it can no longer compensate for misaligned eye muscles by keeping the images fused—the result is double vision.

The most common fusion problem with age is also a consequence of presbyopia. When you read, both eyes have to turn in slightly, in order for them to focus together on the page. This process is called convergence. The simple act of looking at something up close, like a book, sets off a series of actions in the brain and eyes just so that you can focus the lens within your eye, to make the book appear clear, and to converge your eyes, so that the words appear single.

Remember the accommodation process that enables our eyes to change shape so that we can see up close? Each amount of accommodation is matched by the same amount of convergence to maintain clear and single vi-