Ординатура / Офтальмология / Английские материалы / The Eye Care Sourcebook_Lavine_2001

T H E E Y E C A R E S O U R C E B O O K

and sixth (abducens) cranial nerves. They may become palsied either individually or in combination. The double vision may come on suddenly or gradually, depending on the cause.

One of the most frequent causes of double vision from nerve palsies is diabetes. Any of the three nerves that control eye movement may be affected. The double vision is usually of fairly sudden onset and may be accompanied by pain around one eye. With a third nerve palsy, the upper lid over one eye usually droops, and the eye turns in a somewhat outward direction. Although third nerve palsies from other causes may

result in a dilated pupil in the affected eye, the pupil generally retains its normal size with a diabetic third nerve palsy. With a fourth

nerve palsy, the double vision is usually primarily vertical—that is, one image is higher than the other one. One of the images may also appear to be somewhat rotated. With a sixth nerve palsy, the affected eye may turn in, and the double vision is primarily horizontal, with the two images side by side.

These diabetic nerve palsies are usually seen in older individuals and are caused by a shutdown of the tiny blood vessels that nourish the affected nerve. However, a large percentage of elderly people suffer exactly this type of nerve palsy but are not diabetic. They simply have hardening of the arteries affecting their small blood vessels just as people with diabetes do. Hence, we sometimes use the term vasculopathic nerve palsy rather than diabetic nerve palsy. This indicates that there is a problem with the small blood vessels that can occur in anyone, not just in diabetics. The

good news is that these nerve palsies almost always get back to normal, although it may take two to three months. In the meantime, either eye can be occluded (covered) to get

rid of the double vision. An occluder can be a patch over one eye or a cover over one of the lenses in a pair of eyeglasses. Clear nail polish over one of the lenses often accomplishes the same thing.

If it appears obvious that a person’s nerve palsy is of the diabetic or vasculopathic type, additional testing may not be necessary. However, if the double vision becomes worse, or if any other symptoms are present or develop that suggest something more may be going on, a brain scan such as an MRI

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(magnetic resonance imaging) should be done. An aneurysm, a dangerous ballooning out of a weakened wall of an artery in the brain, could be bleeding or pressing on one of the cranial nerves. An aneurysm would require urgent surgery. Brain tumors may also cause nerve palsies, although generally these palsies and the double vision they cause are more insidious in onset, gradually becoming worse over time. These may be tumors that originate in the brain or have spread to the brain from elsewhere. An elevated fluid pressure around the brain from any cause may result in a sixth nerve palsy in one or both eyes, so we always keep that in mind as well.

Herpes zoster, or shingles, can occasionally cause nerve palsies as part of an attack that affects the head. Because of the other symptoms caused by herpes zoster, such as the characteristic rash, it is usually obvious that shingles is the cause of the problem.

Multiple sclerosis (MS) can cause double vision, either by causing nerve palsies or by affecting eye coordination centers in the brain. Eye movement disorders are very common in people who have MS, so if a person with MS develops double vision, the MS itself is the most likely cause.

When you go to the ophthalmologist because of a double vision problem, be sure you go to one who is conscientious enough to take the time needed to get to the root of the problem. A high-volume operator who spends

two minutes per patient and is mainly interested in screening for potential eye surgeries is not going to do justice to your problem. Determining the nature of the double vision problem and which nerves, if any, are affected can take time.

When you see an ophthalmologist because of double vision and a nerve palsy is suspected, careful measurements of the eyes must be taken. As you fix your gaze at objects both at distance and at near, the amount of deviation of the eyes is measured by holding special lenses called prisms in front of one of the eyes. Measurements may also be taken while the head is turned toward the left or right, while the chin is down or up, and while the head is tilted toward one side or the other. These measurements can not only help differentiate one type of nerve palsy from another but may also indicate whether a problem other than a nerve palsy may be causing the double vision.

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Transient Ischemic Attacks (TIAs)

An example of a brain problem that causes double vision without affecting the cranial nerves is a transient ischemic attack (TIA). A TIA represents a warning that a stroke may occur in the near future. It represents a temporary cutting off of the circulation to one part of the

brain. If the back part of the brain is involved in a TIA, skew deviation, a vertical separation of the eyes, may occur even though the cranial nerves are functioning properly. Besides

double vision, a TIA frequently causes dizziness. It is important to diagnose a TIA so that action can be taken to prevent a full-fledged stroke.

Long-Standing Muscle Imbalances

Some eye muscle imbalances may have always been present but only produce symptoms when a person reaches middle age or at least adulthood. For example, a convergence insufficiency may cause occasional double vision after you have been reading for a while, and this can be easily detected on examination. In someone who complains of occasional slight, vertical double vision, we may notice that the head is kept tilted a little toward one side. A fourth nerve palsy can make people do this because the head tilt can eliminate the double vision. The question, though, is whether the person just developed this fourth nerve palsy, indicating a possibly serious problem, or whether it has been long-standing. The solution to this dilemma is to look at old photos or even a driver’s license photo. If the head tilt is long-standing or even lifelong, we can breathe a little easier and be less worried that anything serious is going on. Prescription of some prisms in eyeglasses may eliminate the symptoms. Other long-standing muscle weaknesses or imbalances may similarly present later in life.

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Cataract

CATARACT IS A CLOUDINESS OF THE LENS OF THE EYE. IT IS neither a growth nor a film, but rather a condition in which the formerly clear lens has lost some of its transparency. A gradual loss of lens

clarity is a normal and expected part of the aging process, but in some people, the effect on vision becomes noticeable and starts to interfere with their daily activities.

Cataract can affect vision in many ways. Some people experience a general blurriness and may have more difficulty seeing under dim lighting conditions. It may become more difficult to read signs unless there is a great deal of contrast between the sign’s letters and the background. Glare is also a frequent complaint. Some people may say that they can barely see outside when it’s sunny. They may have a great deal of difficulty driving at night because of the glare from the headlights of oncoming cars. These headlights, as well as streetlights, may produce a starburst kind of effect, with halos and sparklers appearing around the lights. Some types of cataract may cause things to look double or triple. Unlike the double vision caused by an eye turning in or out, this is a “ghost image” kind of double vision that remains even when one eye is covered up.

Generally, cataract affects distance vision more than reading vision. If difficulty reading is the main problem, one must make sure that some other eye problem, such as age-related macular degeneration or dry eye syndrome, is

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not the cause. The hallmark of dry eye syndrome is a blurring of reading vision that is not present at first but appears shortly after one begins reading.

Cataract itself rarely causes any problems other than difficulty seeing. If, however, a cataract has become hypermature, meaning that the lens has become totally white and has partially liquefied, a complication called phacolytic glaucoma can occur. This problem is

marked by inflammation and high pressure in the eye. If that occurs, emergency cataract surgery must be performed. People who have a hypermature cataract have a white-appearing pupil and cannot even see a hand waving in

front of the eye. They can tell light from darkness, however. Recent studies have yielded important insights as to how cataract may be prevented or at least slowed. This is important, because if we could just delay the onset of visually disabling cataract by ten years, most people would be able to live out their lives without needing cataract surgery. To understand how to do this, you first need to know more about the structure and function of the lens of the eye.

The Crystalline Lens

The lens of the eye, often called the crystalline lens, is a clear disk located just behind the iris (see figure 3.1, page 10). It is suspended in place by spiderweblike threads called zonules. All light traveling through the pupil of the eye, the black circle in the center of the iris, passes through the lens, which refracts or bends the light rays so that they come to a focus on the retina in the back of the eye. In this respect, the eye is very similar to a camera, with the crystalline lens corresponding to the camera lens and

the retina corresponding to the film in the camera. However, the eye’s lens is even more sophisticated than that. When a person’s focus changes from an object in the distance

to one at near, a muscle in the eye contracts, changing the shape of the lens and allowing the eye to see clearly at whatever point the person’s attention is directed. This process, called accommodation, gradually declines as a person ages, eventually resulting in the need for reading glasses even for people with normal distance vision.

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The lens is a true marvel of nature. It is made up of cells whose unique arrangement and composition allow it to maintain a transparent state, at least until something goes awry and cataract develop. The entire lens is enveloped by a thin membrane called the capsule. The anterior capsule is the portion in the front of the lens, while the posterior capsule is in back. Along the inside of the anterior capsule is a single layer of cells called epithelial cells. These cells help regulate the transport of minerals and nutrients into the lens and produce energy to meet the needs of the lens. They also divide to produce the fiber cells that compose most of the lens. The fiber cells spread out and, beginning at a central core, form multiple shells, much like the layers of skin in an onion. This process begins before birth and continues throughout life. Thus, the oldest fiber cells are located near the center of the lens, while the ones laid down most recently are toward its outer portion.

The fiber cells produce proteins called crystallins. These proteins are laid down to produce a crystal-like arrangement that results in the transparency and other special properties of the lens. All told, protein constitutes about 98 percent of the weight of the lens, and the crystallins constitute about 90 percent of this protein. Each fiber cell is responsible for producing its own protein, and aging takes its toll on the oldest cells, which lose their ability to produce new proteins. This is important, because it means that proteins that have become damaged or altered in some way can no longer be replaced.

Types of Cataract

As the lens ages, the oldest fiber cells, the ones located closer to the center of the lens, change in such a way that the appearance of the lens, as viewed by an examining physician, changes. This center portion of the lens, called the nucleus, becomes a little less transparent. When a person develops cataract, we usually classify it in terms of the part of the lens that has become opacified. Therefore, when the nucleus becomes cloudy or even yellowed, we call that nuclear sclerosis or nuclear sclerotic cataract. When the outer layers of the lens, called the cortex, develop cloudy areas, we call that cortical cataract. Still another type of cataract develops just inside the posterior capsule, the part that covers the back surface of the lens. This is called posterior subcapsular cataract, a type often more rapidly progressive and more visually disabling than the other types. It is useful to use this classification system, because the

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causes of one type of cataract may be different from those of another type. In reality, however, many people have the mixed type of cataract—that is, the cloudy areas are in more than one region of the lens.

How Cataract Occurs

Cortical cataract can occur following a change in permeability of the lens. Recall that the epithelial cells along the inside surface of the anterior capsule of the lens help maintain the chemical balance of the lens. The lens tries to maintain a very low concentration of the mineral sodium and a high concentration of potassium inside itself. This is

accomplished by a “pump” mechanism in the epithelial cells. When the system malfunctions, however, potassium can leave the lens, and sodium accompanied by chloride can

stream in. The buildup of sodium and chloride (salt) in the lens pulls water into the lens. This excess water can liquefy the lens fiber cells and create spokelike cloudy areas in the lens’s outer layers.

The entrance of large amounts of sugars into the lens can cause similar problems. In diabetics, for example, sugars and related substances called sugar alcohols can build up in the lens, drawing water in with them and causing cataract. It is well known that diabetics develop cataract at a much younger age than do nondiabetics. A sugar called galactose, which is derived from milk sugar, can do the same thing. Cataract of this type occurs in infants who lack an enzyme to break down galactose. They may also occur in adults who have relatively low levels of one of these enzymes and who consume dairy products. More about this later.

Another way that cataract can form, typified by the nuclear sclerosis type, involves damaging alterations to the crystallin proteins in the lens fiber cells. Oxidation and other chemical reactions cause linkages to occur among individual crystallin molecules. Eventually, huge clumps of crystallin proteins, called aggregates, form. These protein clumps are very different from the original crystallins, which have to assume a very precise configuration to maintain the clarity of the lens. These chemical changes in the proteins can also result in the formation of colored substances. Thus, the typical nuclear sclerotic cataract appears as a diffuse cloudiness of the entire nucleus of the lens, often with a yellow or brownish color.

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Whatever process is involved in the formation of cataract, oxidation is probably the main underlying mechanism. Oxidation is a common type of chemical reaction. A good example of oxidation is the rust that forms on things made of iron. To a chemist, oxidation

means that the molecules of a substance lose electrons. When one substance becomes oxidized, another substance becomes reduced—

that is, it gains electrons. Antioxidants are substances that help prevent other substances from becoming oxidized. They do this by becoming oxidized themselves. Although we think of rust as something undesirable, oxidation in the body is not all bad. It does have some beneficial functions, helping the body store energy and supporting the immune system. However, excessive or uncontrolled oxidation in the body can be bad.

Oxidation results in the formation of free radicals, unstable, highly reactive molecules that can trigger damaging chain reactions and attack the DNA, protein, and essential fatty acids in cell membranes. We think this damage can trigger a number of diseases, not only cancer, but also certain eye diseases like cataract.

Oxidative chemical reactions could damage the membranes of cells near the surface of the lens, altering their permeability, and could bring about changes in the crystallin proteins of the lens, causing their clumping. Hydrogen peroxide, a powerful oxidant, is naturally present in the aqueous humor, the fluid surrounding the lens. Oxygen itself under the right conditions can promote oxidation, as can ultraviolet light, a component of sunlight. To protect itself from oxidation and eventual cataract formation, the lens utilizes a number of protective mechanisms.

How the Lens Protects Itself from Oxidation

The aqueous humor and the lens itself are extremely rich in ascorbic acid (vitamin C). Vitamin C has a number of important functions in the body, including its ability to be a powerful antioxidant and scavenger of free radicals. It helps regenerate vitamin E to its active, reduced form. Another antioxidant in the eye is glutathione. Glutathione not only helps eliminate linkages between proteins caused by oxidation; it can also detoxify hydrogen peroxide with the help of the enzyme glutathione peroxidase. This enzyme contains an unusual

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amino acid, selenocysteine, in which the sulfur in the amino acid cysteine has been replaced by the mineral selenium. Although glutathione itself becomes oxidized, its active form is quickly regenerated by the enzyme glutathione reductase. This enzyme is a flavoprotein,

meaning that it contains a chemical called

FAD (flavine adenine dinucleotide), one of whose precursors is the vitamin riboflavin. It is aided in its action by the coenzyme NADPH

(nicotinamide adenine dinucleotide phosphate), which is derived from the vitamin niacin.

All this sounds very complicated, but the point is that many substances derived from the diet help make up the lens’s oxidation defense mechanisms. This suggests to us that dietary intervention might be useful in preventing or at least retarding the progression of cataract.

Preventing Cataract

As most people realize, surgery (removal of the eye’s lens) can be performed if the cataract becomes debilitating. People are bound to ask, “If cataract surgery is so simple and successful, why bother trying to prevent cataract?” The answer is that although most cataract

surgery is successful, any surgery takes its toll on the eye and can result in complications. As we shall see, loss of vision or even the entire

eye from infection or from other problems is a small but ever-present risk. Other complications can develop as well, sometimes years later. For example, any cataract operation, no matter how smoothly performed, increases the risk of retinal detachment, a serious, vision-threatening problem requiring major surgery. It could occur a month, a year, or ten years after the operation. The eye is never quite the same after cataract surgery. Therefore, prevention is the best policy.

Many studies have examined people’s food intakes and supplement use to see whether any specific nutrients or dietary patterns might help prevent cataract. Most of these have been what we call retrospective studies: obtaining dietary histories on people who already have cataract and comparing the findings with a group of similar people who don’t have cataract. This assumes that

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the answers people give on these questionnaires accurately reflect what they eat and have eaten for many years in the past. Clearly, such an assumption is not always valid. Another type of retrospective study involves measuring the levels of various nutrients in the blood. However, the blood levels today may not be what they were years ago when the cataract was developing, and blood levels of a given substance do not necessarily correlate with the levels of that substance in the eye. Furthermore, even if we find that people with cataract have a lower level of a certain nutrient than do people without cataract, it doesn’t necessarily mean that the lower level of that nutrient caused the cataract. To really determine whether consuming more of certain foods or taking certain supplements decreases the risk of cataract, long-term prospective studies need to be done. In this type of study, people are randomly assigned to two groups, each of which is placed on a different dietary or supplement regimen. Obviously, it is easier to get people to take a supplement than it is to get them to change their dietary habits.

Eating a poor, unbalanced diet has long been recognized as a risk factor for cataract. The fact that people of low educational or socioeconomic status are at much higher risk of cataract may well reflect inadequate nutrition. India is a country in which cataract is a major cause of blindness, and the incidence of cataract there is much higher than in the United States. Studies in India have suggested that deficiencies of protein, B vitamins, and other nutrients greatly increase the risk of developing cataract. However, overt deficiencies of this nature are relatively uncommon in Western countries, where people generally have plenty to eat.

Daily Vitamins: Panacea or Placebo?

Would simply taking a daily multivitamin/multimineral supplement reduce the risk of cataract? If people did have any vitamin or mineral deficiencies that put them at higher risk for cataract, we might

expect that such a supplement would be beneficial. Many studies have looked at this question, and the results have been mixed. Some groups of people studied seemed to benefit

from this type of supplement, whereas others did not. In some cases, a protective effect was seen for one type of cataract but not for another. In other

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