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

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sion. In other words, it’s not enough just to focus on something up close; the eyes must also be coordinated enough that we see a single, understandable image. With presbyopia, as we discussed above, this ability to accommodate diminishes. Also, the signal the brain sends—to tell the muscles to converge what we see into a single image—gets weaker over time. The sensory fusion system tries to overcome this problem and keep our eyes working together.

By the time our presbyopia reaches the point where our reading prescription does 100 percent of our focusing, the motor convergence system is basically kaput; for some people who had an underlying convergence problem before they developed presbyopia, the sensory fusion system isn’t up to the task of maintaining single vision at near viewing distances. The result? Reading becomes an arduous grind. The print swims, runs together, and doubles.

But there is hope. Fusion problems can be treated with eyeglasses, eye exercises, eye muscle surgery, or a combination of any of these. Eye exercises have proven helpful in increasing sensory fusion in some people, but often the sensory system can’t be built up enough to overcome the increasing weakness of the eye muscles. Eye muscle surgery is often a last resort, when all other attempts have failed to restore binocularity. Often the best solution is to incorporate into the glasses prescription a lens correction called prism. This isn’t the prism you might have played with as a child—the long, triangle-shaped glass

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that spun rainbows when light shone through it. In spectacle lenses, prism is used to bend and redirect the light coming through the lenses without changing the lenses’ ability to refract the light. That is, the prism doesn’t change the power of the prescription; it just helps compensate for the misalignment of the eye muscles. Prism can be ground into an eyeglass prescription so that it can’t be seen in the lens, and your view of the world with either eye by itself doesn’t change. But with it, your view of the world with both eyes together is much more comfortable and clear; the prism boosts the motor fusion system, so that your eye muscles don’t need to work as hard. At the same time, the prism eases the sensory fusion system, so that the images of both your eyes come together more completely for clear single vision.

Some Questions You May Have about Changes in Your Vision

It seems like my eyes keep getting worse. Will I just keep getting more nearsighted until I’m blind?

Relax. Most of the increases in nearsighted prescriptions occur during the growth and development phase of life. Then the changes slow down. It might seem that every time you go to the eye doctor you need something stronger, but that trend usually reverses itself between the ages of thirty-five and sixty. Myopia typically stops progressing and actually starts to reverse with the onset of presbyopia. However, early cataract formation (also called

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nuclear sclerosis, discussed above) sometimes causes a shift in the prescription, which can make your myopia worse.

You may have heard about a rare condition called pathological myopia, in which a nearsighted eye can progress to the point of blindness. The blindness occurs because the eye becomes so large and the retina becomes so stretched and distorted that it may eventually degenerate. The people most likely to develop pathological myopia are those who were very myopic as young children. But again, this is a very rare condition and is far from the normal course of the aging eye.

I never used to wear glasses when I was younger. What’s wrong with me now? Why do I keep needing stronger glasses every time I come in, and why can’t I see as well without my glasses?

This question is usually asked by someone who is hyperopic, or farsighted. (For a discussion of farsightedness, see above.) As your focusing system, which involves your ciliary muscles, begins to lose its ability to “accommodate” for reading, it also becomes less able to compensate for your hyperopia. If you’re farsighted, you’ve probably been using these ciliary muscles all your life to compensate for this relative eye weakness. Because of this, when you’re younger the ciliary muscles tend to stay somewhat contracted; as a result, any early eyeglass prescriptions you may have are weaker than you might need if these muscles were fully relaxed.

As you get older and—like everyone else—develop presbyopia, the muscle tone in these ciliary muscles

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starts to decrease. (Basically, these muscles get out of shape because they don’t have much luck trying to push an aging, inflexible lens to focus.) As this muscle tone becomes more lax, the prescription for distance vision increases as the latent hyperopia—which was there all along, but masked by the efforts of the ciliary muscles— begins to express itself.

Even if you don’t progress beyond a certain distance prescription for hyperopia, you may still notice a deterioration of your vision at a distance without your glasses. As the accommodation system (for a discussion, see above) fails and your vision without glasses is not compensated for by the ciliary muscles and lens system, you’ll probably find yourself relying on your glasses more and more to see at a distance. You might not need stronger glasses over time, but you just won’t see as well without them.

This is great! I used to need my glasses for reading, but now I don’t. Did my eyes get better?

Well, the best answer may be, Enjoy it while it lasts! The earliest onset of age-related cataracts, or nuclear sclerosis (discussed above), causes a shift in the focusing ability of the eyes. This shift tends to make nearsightedness worse and farsightedness better for a time.

But sadly, as noted above, this “second sight” is a relatively fleeting phenomenon. Eventually the cataract takes its toll on our vision; we notice that, with or without our glasses, we don’t see as well as we used to, and

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once again we need a new prescription or perhaps cataract surgery (see chapter 7).

It’s official: I’m a senior citizen. Should I worry about my ability to drive?

Yes. For many of us, however much we hate to admit it, getting older means we don’t see as well to drive as we used to.

Relax—we’re not suggesting that you turn in your driver’s license the first time you have trouble seeing a traffic light or road sign. But your ability to drive is something you’ll need to consider, with unflinching honesty, from now on. Next to the notorious sixteento twenty- four-year-old age group, drivers older than sixty-five are second in the number of auto accidents per vehicle-mile traveled. Nobody knows exactly how many of these accidents are due to poor vision, but it has to be a factor.

Currently, more than thirteen million American drivers are age seventy or older—and this is just a drop in the bucket compared with what’s going to happen over the next two decades, as more baby boomers hit the bifocal and cataract years. By the year 2020 an estimated thirty million drivers will be over age seventy.

Most states (forty-two so far) require vision testing for a driver’s license renewal; most likely, all states will adopt such policies soon. A recent study showed that between 1985 and 1989, states that did not require vision testing for driver’s license renewal had significantly more fatalities each year among older drivers than states with mandatory vision screening.

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What does this mean to you? Mainly, just be careful and use your common sense. If you have any trouble seeing at night, limit your driving to daytime. If your peripheral vision is not great—from glaucoma or diabetic retinopathy, for example—do your best to avoid busy intersections, where side vision can be particularly important. If glare is a problem, consider purchasing glasses with special antireflective coatings (see chapter 4).

Why do I see double, and is there any help for me?

Double vision is often a result of the weakening of the extraocular eye muscles, the muscles that move the eyes. These muscles can lose their ability to keep the eyes working together as they should. When this happens, your right and left eyes aren’t working well together; each eye sees a slightly different image. The brain can’t put these two images together, and you see double. Close one eye; if you can lose one of the images by closing either eye, then your double vision is caused by this kind of muscle imbalance.

In rare circumstances certain diseases can affect the positioning of the eyes. An overactive thyroid gland, for example, can cause the eyes to shift within their sockets, resulting in a misalignment of the eye muscles. Certain tumors can affect the nerves in the brain that control eye muscle movements, and this may also lead to double vision.

If you still see double after closing one eye, then your double vision is caused by one of three other factors be-

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sides the relatively common muscle imbalance and the rare diseases that affect vision.

First, your double vision might be caused by cataracts, which occasionally develop at an inconvenient spot within the lens of the eye so that light that enters the lens is scattered by the cataract and splits into two images (see chapter 7).

The second possible reason is a sudden increase in astigmatism, which can also happen with the development of a cataract. Uncorrected astigmatism distorts an image so that it’s stretched in one direction. Occasionally this will appear as a clear image with a second “ghost image” next to it—kind of like what you see when you have bad TV reception.

The third reason can be the most serious of the three: age-related changes within the macula, the center of the retina that provides our best vision, can also cause a splitting of the image.

If you have double vision—even if it goes away for a time—seek immediate attention! Usually the problem is indeed related to the eyes; however, it’s also extremely important to rule out any other problems that may be jeopardizing your general health.

What does 20/20 vision mean?

The top number 20 refers to the testing distance of the eye chart. Optical infinity is a term that describes the minimum distance that an object needs to be from your eye so that it can be seen without any accommodation

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effort, or focusing of your eye muscles. Twenty feet is the minimum distance between an object and the eye to be effectively at optical infinity; that’s why this has become a standard testing distance for eye charts. The bottom number represents the size of the letter or target that you view. Years of research have yielded an average-size letter that you should be able to see at a given testing distance if your eyes have normal vision. (Technically, a 20/20 letter subtends 5 minutes of arc at 20 feet.) This letter size was given the designation 20. When you mathematically divide 20 (the size of the letter) by 20 (feet) you get a result of 1, so that 20/20 vision becomes an index of normalcy. If, for example, you have 20/40 vision, then 20 divided by 40 equals 1⁄2, and it could be said that your vision is half as good as it should be. Another way to interpret 20/40 vision is to say that you need to double the size of a 20/20 letter, or double the visual angle that a 20/20 letter makes, in order for you to be able to read that letter at twenty feet. Very simply, a person with 20/40 vision can see an object clearly from twenty feet that a person with “normal” eyes can see clearly all the way from forty feet.

Eye charts were designed by analyzing all of the letters in the alphabet and categorizing each letter according to how difficult it is to read at the testing distance. The letter L, for example, is not confused with many other letters and is therefore easy to spot at a distance. The letter E, however, can look like a B, F, P, R, S, or Z at a distance, and it’s usually the first letter that is missed on a line.

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Each line on an eye chart contains letters from easy and hard categories, so that if you misread a letter on a line, the specialist performing the test can determine the significance of your error. Missing a hard letter like E is not as significant as missing an easy letter.

P A R T I I

Getting to 20/20