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

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and keep the medication and preservatives concentrated on your eye much longer than is safe, and this may harm the cornea. If you use the drops ten to fifteen minutes before inserting your contacts, the drop should have provided its relief and dissipated, and your lenses should be safe to insert.

How can I tell if my lens is worn out?

It depends on how bad the lens is. The most common symptom is that your lenses will feel drier than usual, and you may not be able to wear them comfortably for as long as you used to. Often the dryness is so significant that there’s no relief, even with rewetting drops.

Quantity of vision—how many letters you can read on a chart—is occasionally poorer with a worn-out lens, but usually it’s the quality of vision that people notice first. Worn-out lenses provide less contrast; nothing looks crisp.

Many people find that they need to increase their use of cleaners when the lenses are wearing out. For instance, you might need to remove the lens in the middle of the day and clean it with your daily cleaner. Weekly enzyme cleaning won’t seem to have the same benefit when a lens is worn out, and you might need to increase the frequency of this cleaning regimen as well.

For RGP lenses the symptoms may be similar, but often the lens can be polished and effectively refurbished. A soft contact lens that is worn out should be replaced.

If you’re not sure whether your lens is worn out, see your doctor or lens fitter. Unfortunately, many people

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simply replace their lenses without having anyone take a look at their eyes—which is fine, unless there’s another problem that needs to be addressed. Say, for example, you have a contact lens allergy (not hay fever, but a reaction to your contacts; see above). Your symptoms may be similar to those you’ll have with a worn-out lens, and not treating the symptoms may be harmful.

Why do I have a red streak on my cheek after I insert my lenses?

Most likely you’re sensitive to the preservative sorbic acid (also called potassium sorbate). This is the preservative that has traditionally been used in solutions labeled “for sensitive eyes,” and while it is much less sensitizing than any of its predecessors, sorbic acid still causes reactions in as many as 5 percent of those who use it. But even if you are sensitive to this preservative, you still may not have any symptoms in your eyes, or you may experience only mild stinging. You can test yourself by placing several drops of a contact lens solution containing sorbic acid on the back of your hand and checking for any skin redness.

Why have my soft contact lenses discolored?

Soft contacts can discolor when they wear out. However, the culprit is usually a preservative in your contact lens solution. If you tend not to stick with one particular brand but buy “whatever’s on special,” and as a consequence mix various solutions containing different preservatives, these preservatives can interact. This can

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discolor your lenses, usually turning them yellow, red, brown, green, or gray. Also, if you use heat disinfection, check your saline solution: a saline that’s not recommended for heat disinfection may contain preservatives that will discolor contact lenses when heated.

Can hairspray “gunk up” my contact lenses?

Yes. Over time the sticky protein in hairspray can indeed build up on contact lenses and shorten their life span. So try not to use hairspray after inserting your contacts (or even to insert them in the same room after using hairspray, because the stuff can linger in the air for several minutes). Similarly, it’s a good idea to use waterbased mascara (such as Almay, Lancôme, or Borghese), hypoallergenic or “sensitive-eye” makeup remover, oilfree moisturizers, and soaps that aren’t lanolin-based (such as Neutrogena in the bar form).

Can my contact lens get lost behind my eye?

No. Your eye is very well sealed off by several safeguards, including your sclera (the “white” of your eye), conjunctiva (the clear layer over the sclera), and eyelids. There is one, smooth, continuous flow of tissue from the margin of your lower eyelid, across the eye, and to the margin of the upper eyelid (see chapter 1). If your contact lens goes where it’s not supposed to in your eye— above your cornea, for instance—it will just stay there, under your eyelid, until you get it out.

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What is orthokeratology?

This is a method of fitting a rigid contact lens to the eye that is flatter than the curvature of your cornea. The lens effectively flattens the eye while you wear it; then, when the lens is removed, the cornea remains flattened. (Because the lens is curved less than the cornea, it won’t be too tight.) If someone is nearsighted, flattening the cornea reduces the degree of myopia, or nearsightedness. In theory this means that after you remove the lens, you’ll be able to see without any correction. (However, the flattening may not be even on the cornea, in which case your vision can appear distorted when you remove the lens.) Once the desired effect has been achieved, contact lenses must be worn for at least a few hours every day, or worn overnight while sleeping, to maintain a flattened cornea. If you stop wearing the lenses, the cornea should simply return to its original shape without damage to the eye, and you won’t be any worse off than you were before.

This method of vision correction, said to lie somewhere between contact lenses and refractive surgery (see chapter 6), varies in effectiveness, and often the results are only short-lived. The safety and efficacy of molding the cornea remain controversial.

6

Refractive Surgery: Don’t

Discard Those Spectacles

Just Yet

Life without glasses—wouldn’t it be wonderful? If only there were some way to correct vision permanently, to fix the problem that made us need glasses or contacts in the first place. An intriguing thought, one that has fascinated doctors and patients alike for years. Is it possible? Yes, it’s called refractive surgery. Is it perfect? No. Is it for you? Maybe.

If you decide to undergo refractive surgery, it’s imperative that you go into it—pardon the pun—with your eyes wide open. Refractive surgery will most likely reduce your prescription, yes. But there’s absolutely no way to guarantee that it will provide you with even the same quality of vision that you had before surgery with your glasses or contact lenses. You may wind up needing something—glasses or contacts—afterward, to fine-tune your vision, and you’ll still face presbyopia later in life. If you wear bifocals, you’ll still need glasses or contacts to

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help you see up-close. If your potential surgeon leads you to believe otherwise, find another doctor.

A Little Background

The idea behind refractive surgery is, as its name suggests, to adjust the way your eyes refract, or bend, light, by changing the shape of the cornea (the front surface of your eye; for more on refraction and how the cornea works, see chapter 2). This was first attempted in 1885, by a Norwegian surgeon named Dr. H. Schiötz, in a procedure we now call astigmatic keratotomy, or AK. The patient’s trouble was severe astigmatism (a focusing problem, in which the eye doesn’t refract light evenly; see chapter 2), a result of previous cataract surgery. Dr. Schiötz decided to make an incision in the patient’s cornea to flatten it and thus counterbalance the astigmatism. The operation was successful, and refractive surgery was born.

It wasn’t until the 1940s, however, that refractive surgery expanded beyond astigmatism. The first procedures to reduce myopia, or nearsightedness, were done by a Tokyo eye surgeon named Tsutomu Sato. He found that if incisions were placed radially—like the numbers on a clock face—around the corneal surface, the cornea would flatten uniformly, and as a result nearsightedness would improve. Unfortunately Dr. Sato took a good idea and went too far with it, trying to do too much at once. He attempted to reshape the cornea from both the front and the back, and instead of seeing better, his patients actually wound up with worse vision than they’d had before.

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Dr. Sato’s theories were put aside until the early 1970s, when Dr. S. N. Fyodorov, of the Soviet Union, evolved a more prudent approach that improved myopia with significantly less harm to the cornea. A few years later Dr. Fyodorov’s procedure, called radial keratotomy, or RK, found its way to the United States. At first there was a flurry of interest. But many patients who underwent this procedure found that not only was their vision not fully corrected, sometimes it wound up being so distorted that even glasses couldn’t help! For some of these people, hard contact lenses saved the day, providing a smooth optical surface on the cornea and making adequate vision possible again. RK had other limitations as well, in that it couldn’t fully correct severe cases of myopia (and thus help the patients who needed the procedure most). As a result of these drawbacks, RK’s popularity faded in the 1980s. But the procedure was given new life in the early 1990s, as equipment and surgical techniques improved dramatically.

Although these cornea-shaping operations themselves were less than flawless, doctors knew that the basic ideas behind them were sound, and they kept exploring new techniques and refining procedures. One of these new techniques, developed in the 1980s, was called epikeratophakia. Epikeratophakia involved removing a section of cornea, freezing it, lathing it into a new shape, and then reinserting the newly curved section back into the cornea. Because of the complexity of this procedure, there were too many things that could (and did) go wrong, and it was considered too risky for elective surgery. (Actually, this

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technique proved most successful as a means of correcting vision in babies after the removal of congenital cataracts.)

By the late 1980s, lasers had come on the scene, designed for photoablation: selectively vaporizing the tissue at the center of the cornea, thereby sculpting the eye into a new shape. This technology gave rise to several other surgical procedures. The first, photorefractive keratectomy, or excimer PRK, is performed by a highly sophisticated computer-driven piece of machinery called an excimer laser. The computer, reading a topographical map of the cornea, selectively reshapes the cornea by telling the laser precisely which bits of tissue to remove. Like radial keratotomy, PRK can’t fix all vision problems; currently it’s limited to correcting myopia and some astigmatism. Laser-assisted in-situ keratomileusis, or LASIK (also known as laser automated lamellar keratoplasty, or laser ALK) also uses an excimer laser to vaporize tissue in the cornea. But this technique can correct more severe cases of myopia than PRK; it can also correct some astigmatism and moderate cases of farsightedness. Holmium laser thermokeratoplasty, or holmium LTK, uses a different laser, called the holmium:YAG. This exquisitely precise laser heats (instead of vaporizes) individual collagen fibers within the cornea, causing them to shrink permanently. As the collagen contracts, the central corneal curvature increases and adds more refractive power to the eye. This technique is used to treat patients who have hyperopia, or farsightedness. (People who are nearsighted already have too much refractive power.)

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Making the Decision to Have

Refractive Surgery

Realistic Expectations

So, today it’s surgically possible to correct myopia, astigmatism, and, to a lesser degree, hyperopia. But don’t be dazzled by hype or unrealistic expectations: none of these techniques is guaranteed to provide perfect vision or to prevent you from ever needing glasses or contacts again.

You’re still going to get older, and so are your eyes; nothing can change that. And because presbyopia—the trouble focusing close up that sneaks up on all of us eventu- ally—is inevitable, and because it’s a problem with the lens and ciliary muscles, not the cornea, refractive surgery on the cornea won’t do anything to prevent it, and you may still wind up needing a reading prescription. If you’re moderately nearsighted and wear glasses for distance vision only, you’ll have a trade-off: instead of needing glasses to see across the street, you’ll need them for reading. And if you spend your days in front of a computer, you’ll end up wearing glasses more than you did before surgery.

Aiming for Undercorrection

If you decide to have refractive surgery, your doctor will evaluate you to determine which technique (if any) is appropriate. (Sometimes, to correct more than one problem, more than one technique is used.) Most refractive surgeons err on the conservative side, aiming for

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undercorrection of your refractive error. Why? Because if your eyes are targeted for the ideal correction and the surgery works too well, you’ll end up with the opposite problem. In other words, if a surgeon “overshoots,” or overcorrects your nearsightedness, you’ll wind up being farsighted. Undercorrecting the problem leaves a margin for future action; you and your surgeon can decide together whether to try to enhance the surgery with a second procedure (which effectively doubles the risks involved in having had surgery to start with). But undercorrection also means that you’ll still have to wear glasses or contacts (although sometimes contact lenses aren’t possible if the cornea is considered too delicate after the surgery).

How the Procedures Are Performed

RK

In RK, the surgeon first anesthetizes the eye and then uses a diamond-bladed knife calibrated to reach a certain depth (about 90 percent of the thickness of the cornea) to make uniform incisions, or cuts (usually about four to eight in all), around the cornea. The more incisions, the flatter the cornea becomes; with milder cases of myopia, fewer incisions are needed. The length of the incisions also makes a difference: the longer the incision, the greater the correction. The patient’s age is still another factor: the older the patient, the greater the potential for refractive change with a given size and