Ординатура / Офтальмология / Английские материалы / The Eye Care Sourcebook_Lavine_2001

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Esotropia

Esotropia, or crossed eyes, may appear any time from soon after birth to when a child is several years old. Infantile esotropia (or congenital esotropia), which is noticed right after birth, is caused by a developmental problem in the brain, which in some cases may be inherited. It is

normal for the eyes to wander a bit in a newborn infant, but persistent turning in of an eye is not normal. Children do not outgrow it. An

eye examination to determine the cause is essential. In some cases, the problem may be not with the brain but rather in the eye itself. If an infant’s eye sees poorly for any reason at all, it usually turns in, because this is the position of “rest” for an infant’s eyes. Normally, the eye muscles have to work somewhat to keep the eyes straight, because if both eyes see well but one turns in or out, double vision occurs. If one eye sees poorly, however, double vision is not recognized, so there is no stimulus for keeping the eyes straight. Poor vision in an eye can have many causes. It may even be due to a malignant tumor in the eye, which is, fortunately, a rare occurrence. An early eye examination will rule out such problems or diagnose them while they are easier to treat.

How do you know whether your child’s eyes are straight? Some infants have what we call pseudoesotropia. The wide bridge of their nose and the folds of baby eyelid skin that cover the white of the inner part of their eyes make it look as though they have esotropia. It may be obvious that true esotropia is present if the degree of crossing is large, but it

may be difficult for the layperson to tell in other cases.

Here is the way ophthalmologists determine whether both eyes are seeing well and

are straight. Hold your hand in front of one of the child’s eyes while holding something the child wants to look at with the other hand. If one eye has poor vision, the child will object when the good eye is covered. Obviously, in older children, vision tests can be performed. To test for straightness, do the Hirschberg test. Shine a penlight or very small flashlight at the child’s eyes in a dimly lit room so that the child looks at the light. The light causes a small light reflex to appear in each of the dark pupils. Normally, that light reflex should appear just a little off center in the pupil—displaced slightly inward toward

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the nose. In any case, it should appear in the same position in the two eyes. If the light reflex appears in the outer part of the pupil of one eye while the child is looking at the light, then that eye is probably crossed.

The examination for infantile esotropia includes an assessment of the health of each eye to rule out any structural problems or tumors. Dilating drops are instilled in the eyes so that the doctor can better examine the retina and optic nerve and estimate the refractive error. Most infants are mildly hyperopic (farsighted). We also determine whether the same eye is crossed all the time or whether the eyes alternate in that respect. If only one eye crosses, then that eye will develop amblyopia, commonly termed lazy eye.

Treating Infantile Esotropia

Surgery is the only treatment for infantile esotropia. Surgery can involve a weakening of the medial rectus muscle in each eye. The medial rectus muscle is the muscle that turns the eye inward. This type of surgery is called a recession. The muscle is severed from its attachment to the eyeball and then sewn back on several millimeters farther back, thereby “recessing” the muscle.

Why should both eyes be operated on if only one of the eyes is turning in? The answer is that we are really dealing with a relationship between the two eyes that is controlled by the brain and not with a problem of the muscles themselves. Many authorities favor this type of symmetrical surgery.

Alternatively, the medial rectus muscle in one eye may be recessed while the lateral rectus muscle, the one that turns the eye out, is resected. Resection is a procedure in which a segment of muscle where it attaches to the eyeball is removed and the remaining muscle is then sewn back on in the same place. This puts the muscle “on stretch,” which has the effect of strengthening it.

Timing the surgery is controversial, but it is generally done around six months of age. The infant will probably never develop an advanced type of depth perception, but the

goal is simply to get the eyes to fuse—to work together so that they remain straight. If the eyes can lock into this kind of position, then the surgery has been successful.

In some cases, the surgery may undercorrect or overcorrect the crossing. If that happens, then further surgery is necessary. In other cases, the eyes may

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not fuse but still look good from a cosmetic standpoint. If this occurs, the situation is simply carefully monitored to see whether the position of the eyes changes over time.

In some children, the eyes may seem straight for the first few years of life, but then intermittent crossing occurs. At first, it may occur only occasionally, especially when the child is feeling tired or sick. Then it may start to become more frequent. If crossing of an eye occurs even occasionally, it is time to get it checked out. This type of esotropia often

turns out to be what we call accommodative esotropia. Accommodation refers to the adjustments the eyes undergo when they shift their focus from distance to near. Three things

occur: (1) The ciliary muscle inside the eye changes the shape of the eye’s lens to allow it to focus close up. (2) The eyes converge: They both turn in so that they can focus together on the object they’re looking at. (3) The pupils become smaller to increase the depth of focus.

In the child with accommodative esotropia, one of two problems (or a combination of the two) exists. First, the child may be highly hyperopic (farsighted). This means that when the eyes are at rest, vision is blurry because incoming light rays are focusing behind the retina instead of right on the retina. The eye then accommodates as just described (even though it may be looking at something in the distance) to focus the light onto the retina. However, this accommodation causes not only a change in focusing of the lens but also a convergence of the eyes, because these muscular activities are linked to each other. The eyes try to use their other muscles to oppose this convergence action, but if they are not able to do so, the convergence takes over, and one of the eyes turns in.

The other possibility is that the child is not highly hyperopic, only moderately so, but there is an abnormality in the accommodative mechanism such that a given amount of change in focus of the eye’s lens stimulates a much greater degree of convergence than it should. This effect can also overcome the effort assumed by the opposing eye muscles to keep the eyes straight, and the eyes then cross.

If the child with intermittent accommodative esotropia shows crossing only rarely, say, once a week, then nothing may need to be done about it. But if the crossing occurs at least once a day, the brain starts to employ some

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unhealthy adaptations in its attempt to avoid double vision. It begins to suppress the image coming to it from the crossing eye, and it starts to get used to the crossing of the eye. Ultimately, amblyopia can develop. We don’t like these brain adaptations to take hold because they may ultimately make it more difficult to keep the eyes straight and working together properly.

Eyeglasses are used to treat accommodative esotropia. Dilating drops temporarily paralyze the muscle of accommodation in the eye so that we can determine the full amount of hyperopia. An instrument called a retinoscope helps us make that determination. Glasses that correct the farsightedness are then prescribed. In some cases, it may even be necessary to prescribe bifocals to keep the eyes straight when they are looking at things up close as well as in the distance. The goal of the glasses is to keep the eyes straight at all times. This may help vision as well for the following reason. Some children with accommodative esotropia may purposely keep their vision a little blurry, because when they accommodate to see more clearly,

their eyes cross. Less accommodation keeps their eyes straight, but they can’t focus as well. Eventually, some children, especially those who overconverge their eyes with a given

amount of near focusing, may be able to have glasses of reduced strength or even be able to go without glasses. Children who have very high hyperopia (farsightedness) will probably continue to need glasses even if the hyperopia decreases somewhat with age, as it often does.

Amblyopia

Amblyopia (reduced vision) results from changes in the brain that occur as the brain suppresses (blocks out) what the crossed eye is seeing. It does this as a way of avoiding double vision. Unfortunately, an infant who remains amblyopic in one eye throughout childhood will have poor vision in that eye throughout life, as amblyopia can only be corrected in young children. The way we usually treat amblyopia is to patch the good eye. This forces the amblyopic eye to be used, and the vision should improve. We must be careful, however, not to patch the good eye too long, because the good eye can then become amblyopic! In a young infant whose visual acuity cannot be tested directly, the endpoint is to have the two eyes alternate—either one

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may cross at a given time, and there is no apparent preference for either eye. The eyes can be alternately patched until surgery so that the amblyopia does not recur before then. Below, we will discuss amblyopia that results from other causes.

Other Causes of Amblyopia

We have already mentioned that an eye that is esotropic much of the time may develop amblyopia. Amblyopia can actually occur in any situation in which one eye is not aligned with the other one. It

can even occasionally occur in cases in which there is minimal or virtually no deviation of one of the eyes. We call this a monofixation

syndrome—because of some unknown abnormality in the brain, the two eyes do not work with each other properly. Even though the eyes may appear to be straight, the brain still suppresses what one of them is seeing, and the result is amblyopia. This type of amblyopia responds to patching of the good eye just as the eye with a large esotropia does. Thus, even if a young child’s eyes are straight, it is important to measure the child’s visual acuity just as soon as that child is able to cooperate with eye chart testing. If amblyopia is not detected until the child is older, it may be too late to fully restore the vision in the amblyopic eye.

Amblyopia can occur for other reasons as well. If there is a large difference in the refractive error between the two eyes, the more out-of-focus eye may develop amblyopia. For example, if one eye is much more hyperopic than the other, or has much more astigmatism than the other, this eye may develop what we call refractive amblyopia. Prescription of appropriate eyeglasses is necessary, and a little patching of the good eye at

first may be needed as well.

An eye that is not permitted to see well because of some obstruction may also become amblyopic. One example would be a droopy

upper eyelid that covers most of the pupil of one eye. An eye injury that causes a cataract or a scar in the cornea can also cause amblyopia. Treatment in these cases is directed at removing the obstruction and performing some patching to improve the vision in the weaker eye as much as possible.

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Exotropia

Exotropia, or turning out of an eye, is often intermittent at first but may sometimes become constant over time. It may be noticed only occasionally at first, for example, when a child is tired or sick, staring at something in the distance, daydreaming, or in bright sunlight. It may first be noticed when the child is only a few months old. As the child gets older, it may or may not cause symptoms. A common symptom would be a feeling of tiredness in the eyes. Some blurring of vision may occur as the child uses accommodation to try to control the eyes. The convergence that is part of accommodation keeps the eyes straight, but the change in focusing that occurs along with it causes the blurring.

Treatment of this disorder depends on how frequently it occurs, whether it is causing any symptoms, and how far the eye turns out. Orthoptic therapy, which involves the use of special eye exercises, may be tried if the degree of turning out of the eye is not very large. If the eye does turn out fairly far to the side, then surgery is necessary. Such surgery is not needed if the turning out is only occasional and not causing any symptoms.

Exotropia may also occur in children who originally had esotropia. The exotropia may occur immediately after surgery to correct the esotropia. This is called an overcorrection, and additional surgery

is probably required to straighten the eyes. Sometimes, a person who had a small esotropia as a child ultimately becomes exotropic as an

adult. Because such a person has never developed fusion (alignment of the eyes), the deviation of the eyes can change with age as the eye muscles change. Similarly, an adult who has lost vision in one eye often develops an exotropia in that eye, since that is the normal position for an adult eye at rest when there is no stimulus (such as double vision) to keep the eyes straight. In people whose eyes do not fuse or work together, surgery to correct exotropia can be done for cosmetic reasons, but there is no guarantee that the eyes will always remain straight thereafter.

Convergence Insufficiency During Reading

People who have a tendency to lose the alignment of their eyes when they are reading are said to have a convergence insufficiency. Their eyes are usually perfectly straight when they are looking at something in the distance, with no tendency

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for the eyes to deviate in either direction. However, when they are looking at something close up, the process of accommodation, in which the eyes not only change their focus but also converge, does not work properly. The eyes do not converge as strongly as they should, and it requires considerable effort to keep both eyes focused on the reading material. Occasionally, one of the eyes just “gives up” and turns out, losing its alignment with the other eye.

Convergence insufficiency is a very common cause of reading problems in adults. When they try to read, their eyes tire quickly. The words may start to run together or go double. Headaches and feelings of eye irritation may also occur. For people whose schooling or work requires them to read extensively, these symptoms can be disabling.

Convergence insufficiency is easy to diagnose. Even if the eyes are perfectly straight at the reading distance, there may be a tendency for them to turn out. We call this exophoria (described later in this chapter). Exophoria that may be part of convergence insufficiency is detected by having people focus at an object at the reading distance. As they do so, we alternately cover and uncover each eye. If an eye starts to turn out while it is covered but then returns to proper alignment when it is being used with

the other one, then exophoria is present. Another test is to have someone focus on an object at the reading distance while we move that object closer to the eyes. We see how

close we can move it without having one of the eyes start to turn out. Normally, we should be able to get fairly close to the nose. These tests, which are often part of a routine eye examination, make it easy to diagnose convergence insufficiency.

Convergence insufficiency is treated with eye exercises, not surgery. Orthoptists, technicians who specialize in eye muscle exercises, can prescribe appropriate exercises. A very simple but effective exercise is what we call pencil push-ups. With pencil push-ups, we try to strengthen the medial rectus muscles, the muscles that turn the eyes in and are therefore involved in convergence. To begin the pencil push-up routine, hold a pencil a few feet in front of your eyes while you focus on something on the wall across the room. Suddenly shift your focus to the tip of the pencil. As you do this, your eyes converge. Then focus back on the wall. Go back and forth like this a dozen times. Then bring the pencil an inch or two closer and repeat the process. Do

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about twenty-five of these sets every day. As the days and weeks go by, gradually move the pencil closer and closer to your face. With time, you will strengthen those medial rectus muscles, and your reading symptoms should diminish. Pencil push-ups are very effective, but do not expect them to bring about a permanent cure. If you stop doing them, your symptoms will probably return.

Occasionally, some people try the exercises and still obtain no relief. In such cases, we may have to prescribe prisms in reading glasses. Prisms are special lenses that change the direction of the light rays entering the eyes. Prisms do not strengthen the eye muscles, but they do reduce the degree to which the eyes need to converge at the reading distance. As a last resort, this can be quite effective as well.

Exophoria and Esophoria

A tendency for the eyes to turn in or out can be quite tiring, even if the eyes maintain their straightness all the time. A tendency for the eyes to turn out, as detected by a turning out of an eye when it is

covered up, is called an exophoria, while a tendency to turn in is called an esophoria. Very small degrees of exophoria or esophoria are considered normal. Even if larger degrees

exist, the esophoria or exophoria requires no treatment if it is not causing any symptoms. But if headaches, tiredness in the eyes, or occasional blurring or double vision is present, then something should be done.

As we examine the patient with an exophoria or esophoria that is detected on a routine examination, we proceed with additional testing if symptoms are present. In particular, we measure what are called divergence amplitudes and convergence amplitudes. These are done while the person is focusing on something in the distance and also at near. This testing, which is accomplished by placing special lenses called prisms in front of the eyes, measures the strength of the muscles that turn the eyes out (divergence amplitudes), as well as the strength of the muscles the turn the eyes in (convergence amplitudes). Depending on the results obtained, special eye exercises, often using prisms, can be prescribed to help strengthen the muscles and thereby alleviate the symptoms.

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Double Vision

Double vision is a troublesome symptom that cannot be ignored. People can generally function quite well with reduced vision in one eye and may, in fact, not even notice it for some time. Not so with double vision, which makes life difficult. There are many causes of double vision, so when someone has this complaint, we need to obtain more specific information.

Determining the Cause of Double Vision

We start by asking questions. Is it a true double vision—seeing two equally clear images of everything—or is it merely a “ghost image” type of double vision? With a ghost image, the two (or more) images overlap, and usually one of the images is clearer than the other. What happens if you cover up one eye (either eye)? Does the double vision go away, or does it remain? Are the two images side by side, or is one on top of the other? If there’s some of each, are they mostly side by side or mostly one on top of the other? Does the double vision go away when you move your head in any particular position? If the double vision has been present a while, have the two images been moving farther apart? If the double vision is intermittent, has it been becoming more frequent or staying about the same? Have you been having a headache or pain around either eye? Do you have any other symptoms, such as dizziness, when the double vision is present? Has either eyelid become droopy? Is the double vision present only when you are reading or only when you are looking into the distance? Does the double vision become worse toward the end of the day or when you are tired? You should try to determine the answers to all of these questions before you see your physician. As we discuss the different causes of double vision, you will see why these questions are important.

If the double vision remains even after you cover one eye, you know the double vision is not caused by an eye misalignment. The double vision, then, must be coming from just one eye or from each eye individually. Double vision from one eye does not usually signify a serious problem. It is generally a ghost image type of double vision rather than a true double vision. One of the causes we look for in this situation is a mild cataract in one or both eyes. Cataracts can present as a small opacity or cloudy spot in the lens of the eye, and this opacity can split the light rays entering the eye. Another possible cause is the line in bifocal glasses. If

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the glasses are out of adjustment, you may be looking through the bifocal line without knowing it, and this can create a split or double image. Astigmatism can cause a ghost image type of double vision if the astigmatism is not or cannot be adequately corrected. This may occur with either glasses or contact lenses.

If the double vision goes away when you cover either eye, then you know that it is caused by a lack of coordination between the two eyes. Let us now look at some examples.

Muscle Problems

In some cases, double vision associated with a lack of coordination between the eyes may be caused by a muscle problem. People who have an overactive thyroid gland, for example, often develop problems with their eyes. Inflammation and thickening of some of the eye muscles may occur, and these thickened muscles can tether the eye so that it cannot move freely in all directions. Rarely, a cancer from elsewhere in the body, for example, breast cancer, may spread to a muscle and have a similar tethering effect on the eye.

Myasthenia gravis is a disease that causes weakness of the body muscles, and the weakness is often most apparent after repeated use of a muscle or toward the end of the day. One form of myasthenia gravis affects primarily the eye muscles. When myasthenia gravis affects the eyes, the upper eyelid muscle is usually involved and causes a droopiness of the eyelids. Myasthenia gravis is therefore a disease that is always kept in mind when someone has what appears to be an eye muscle problem.

Direct injuries to the eye area can damage eye muscles and cause double vision. One type of injury called a blowout fracture occurs when a large, blunt object such as a softball strikes the eye. The eye itself may be spared, but fractures may occur in the bones of the orbit, especially below or on the nose side of the eye. One or more of the eye muscles may then become entrapped in the area of bone fracture, and this may tether the eye down and cause double vision.

Nerve Palsies

Nerve palsies may also cause double vision. Three separate nerves that come from the brain control the various eye muscles and hence the movement and alignment of the eyes. These are called the third (oculomotor), fourth (trochlear),

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