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

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Fig. 8.5. Cupping of optic nerve disc caused by elevated ocular pressure

optic nerve’s job is to transmit all of this information to the brain, where it’s translated into images that make sense. The exact area where these fibers come together at the back of the eye, before exiting the eyeball, is called the optic nerve head, or disc. Within this optic disc, major retinal blood vessels enter and exit the inner part of the eyeball.

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What does all this have to do with glaucoma? Intraocular pressure elevations—such as those found in the majority of people with glaucoma—cause structural damage to these nerve fibers at the optic disc. Exactly why these fibers are so susceptible to damage here is not well understood. In normal aging, these fibers appear to diminish gradually anyway; but in people with elevated eye pressures and glaucoma, this loss is accelerated.

These million nerve fibers are somewhat loosely organized. The fibers responsible for central vision are generally located on the temporal side of the optic disc (the side nearest the temples). The ones responsible for peripheral vision are mainly found at the top, bottom, and nasal side of the disc, and it is theorized that these are the first to be damaged by elevated eye pressure. Unfortunately, these peripheral fibers can be damaged before any loss in vision can be noticed by the patient or discovered in an eye exam. When this loss of peripheral vision is detectable, the structural change that eye doctors see on the disc looks as if someone has excavated, or scooped out, part of the normally flat optic disc, making it resemble a cup (see figure 8.5).

At first this scooping is subtle and mild; in advanced glaucoma, however, the optic disc can become markedly excavated, as if someone had physically drilled out the center of its tissue. When this happens, as you can imagine, many nerve fibers have been lost and the loss of vision is severe.

Eye doctors often use the term cupping to refer to the relationship between the size of this scooping and the

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overall size of the optic nerve disc. The cup-to-disc ratio—basically, how much of the disc is damaged—is estimated by the eye doctor when he or she directly observes the optic nerve head using one of several techniques during an eye examination. Loss of vision—eye doctors often say “loss of visual field”—is found with increased degrees of cup-to-disc (C:D) ratio. The larger the C:D ratio, the greater the chance of visual field loss. A C:D ratio of .1 implies a relatively healthy optic nerve head with minimal cupping—an optic nerve head usually not associated with glaucoma. A ratio of .9, on the other hand, refers to a major degree of excavation and tissue loss, usually seen in advanced glaucoma. The C:D ratio and its progression over time are important measures for detecting and managing glaucoma.

Note: As always, there are exceptions. Some people have congenital forms of optic disc cupping that can mimic glaucoma. These normal variations, which are present from birth, aren’t associated with glaucoma; but they can confuse an untrained observer, leading to misdiagnosis and unnecessary treatment for glaucoma that doesn’t exist.

Classically, glaucoma’s damage to the optic nerve head leads to peripheral, or side-vision, loss. And unfortunately, as stated before, this can be very difficult to detect in the early stages. Many people try to check this on their own, by placing their fingers far out to the sides of their vision. But this isn’t a very precise gauge, and results aren’t nearly as accurate as those produced by today’s so-

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phisticated tests, performed by specially trained observers.

Many years ago eye doctors relied on manual visual field tests to measure side vision, a process known as perimetry. (Testing devices are called perimeters.) Early perimetry consisted of holding small test objects to the side of a patient’s vision, noting and then mapping what could or could not be seen. In recent years computers have automated perimetry testing, making it much more reliable; these sophisticated “visual field machines” can store vast amounts of information about someone’s range of vision, greatly improving our ability to analyze and interpret the test results.

Caution: Because these computerized perimetry machines are so popular, they’ve also been bought by eye care providers who aren’t terribly knowledgeable about how to use them or how to interpret their results— which means, unfortunately, that improper or inappropriate testing is not uncommon. Certain things can skew the test results, including testing patients while they are wearing their eyeglasses or checking someone’s side vision without placing the person’s proper eyeglass prescription in the perimeter of the testing field. Using pilocarpine, a kind of eye drop, can also cause misleading results.

In addition, some eye doctors may perform tests much more often than necessary. How often visual field tests need to be done is a question with no simple answer. In fact, there aren’t many simple answers to questions about

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Fig. 8.6. Visual field loss from glaucoma

Are You at Higher Risk for Glaucoma?

Risk factors include:

•Being over age fifty-five

•Having a family history of glaucoma

•Being very nearsighted

•Having diabetes

•Being African American

•Being Native American (according to some studies)

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glaucoma, including the question of deciding whether someone really has it. Although automated visual field testing is reliable for assessing someone’s peripheral vision, the value of this testing as a screening technique for glaucoma is debatable. Furthermore, visual field test results alone should never be used to diagnose or manage glaucoma.

Similarly, glaucoma screening tests—like those conducted at senior citizens’ centers, shopping malls, or health fairs—are of rather limited benefit in detecting glaucoma. (Unfortunately, the old adage “You get what you pay for” is largely true here.) The big problem with such events is that they’re often conducted by untrained observers, whose unenviable job is to screen a large number of people in a fixed amount of time, and who often rely on measurements of eye pressures and visual fields that are less accurate than those available in a doctor’s office. Therefore, these glaucoma screenings do not replace a comprehensive eye examination for glaucoma performed by a trained eye doctor. This is especially true for people considered to be at higher risk for glaucoma than the general population (see box).

Treating Glaucoma

Because glaucoma is a diverse collection of disorders with a common endpoint—a damaged optic nerve resulting in loss of vision—and because everybody is different, there is no one preferred form of treatment. In general, however, the basic goal of managing glaucoma

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is to lower someone’s intraocular pressure—and keep it lowered—sufficiently to prevent further nerve damage and loss of vision.

Frequent Monitoring

At first, when you’re beginning a new antiglaucoma medication or changing to a new regimen, you may require weekly or monthly monitoring of your intraocular pressures. Then, once a particular regimen seems to be doing its job—controlling eye pressure and stabilizing your visual field—your eye doctor will probably lengthen the time between visits to every three to six months or so. (Visual field testing may be performed less often, depending on such factors as your eye health, degree of vision loss, intraocular pressure readings, and the appearance of the optic disc. If you have severe glaucoma, this testing may be done more frequently.)

What’s the point of such frequent checkups? Eye pressure is not a constant. Your doctor needs to monitor your intraocular pressure readings while you are on medication and also will occasionally need to take a reading after asking you not to take your medication. Another reason for frequent monitoring is that many patients become resistant, over time, to the effects of a particular antiglaucoma medication; despite faithful use of the eye drops, their intraocular pressure gradually sneaks back up to where it was before treatment. For such patients it’s clearly important to detect this problem as soon as possible, so that they can be switched to a different type of medication. Also, your eye doctor needs to make sure

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that your vision isn’t deteriorating by using perimetry and by watching for any significant changes in the appearance of the optic disc.

Note: Because eye pressures fluctuate constantly, even from morning to afternoon, it’s a good idea to schedule your office visits at different times of the day. Also, be sure to report any changes in your general health, any other medications you may be taking, or any other visual or medical symptoms that you feel may be important.

Glaucoma Medications

Now, welcome to the world of eye drops.

There are two basic theories in lowering eye pressure. One is to reduce the production of aqueous fluid in the eye; the other is to improve the drainage of this fluid from the eye. All forms of antiglaucoma therapy are designed to do at least one of these.

Currently, the first line of treatment is to start glaucoma patients on antiglaucoma eye drops to lower the eye pressure. These drops can be used either alone or in combination with other eye drops. No matter what kind of eye drop you’re taking, here’s an important point to keep in mind: The eye can hold only about 20 percent of one drop. Therefore, it’s customary to place one drop in an eye at a time, not two drops. If multiple eye drops are to be given around the same time, then they should be spaced apart by about three to five minutes—so that they can be absorbed properly, and to minimize the chances of washing out one drop with the next.

If you’re taking a combination of eye drops, it may not

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matter much in which order you put them in. However, over the years we’ve found that taking the drops in one order rather than in the reverse order may reduce problems of burning or stinging associated with a particular drop. (This may be because one drop has more of an anesthetic or lubricating quality than the other.)

So, which drops will you take? Here’s a brief discussion of some of the more popular “antiglaucomatous” eye drops, their dosages, and potential side effects.

Pilocarpine

Pilocarpine, the first eye drop developed for treating glaucoma (developed nearly 120 years ago), comes from a South American plant and generally produces few allergic or toxic reactions in people who take it. Pilocarpine is commercially available in strengths ranging from 0.5 percent to 10 percent; most glaucoma patients are prescribed strengths of between 1 percent and 6 percent. (Most doctors feel that there’s a point of diminishing returns, in that concentrations above 4 percent don’t effectively reduce pressure any more significantly than lower concentrations. However, this can vary from patient to patient; patients with dark irises, for instance, often require higher dosages.)

After it’s administered, pilocarpine lowers intraocular pressure, or IOP, in about an hour, with its effectiveness peaking after an hour to an hour and a half and lasting about eight hours. It’s usually given as one drop four times a day. Pilocarpine is also available in a once-daily gel, and in a slow-release ocular insert that’s placed in the

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eye for a week at a time. You may want to discuss these alternatives with your doctor.

Pilocarpine’s most common side effect is miosis, or a small pupil. This can cause diminished vision, because less light will be able to get into the eye. Other important side effects in the eye include browache, induced nearsightedness, and retinal detachments. (For more on warning signs of retinal detachments, see chapter 15.) Systemic, or bodily, side effects are rare but can include increased salivation, sweating, nausea, vomiting, diarrhea, and trouble breathing.

There are many other antiglaucoma drugs that have a pharmacological action similar or related to pilocarpine’s. These include carbachol, physostigmine (Eserine), demecarium (Humorsol), isoflurophate (DFP, Floropryl), and echothiophate (Phospholine Iodide).

Epinephrine and Propine

Epinephrine is a hormone that’s found naturally in the body; dipivalyl epinephrine (Propine) is a synthetic, inactive derivative of epinephrine that seems to be more effective in the eye. The exact mechanism by which they lower intraocular pressure is poorly understood; however, they seem to have an effect on both aqueous production and outflow. Epinephrine is available in three different forms: hydrochloride, borate, and bitartrate. Propine is converted by enzymes into epinephrine, which then lowers IOP. Because Propine is effective in lower concentrations than epinephrine, less of it is absorbed into the rest of the body.