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

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Fig. 6.1. Radial cuts in cornea for a radial keratotomy

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Fig. 6.2. Nearsighted eye before and after radial keratotomy

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number of incisions. If a patient has astigmatism, AK is performed as well (see above), with incisions made to compensate for the cornea having more curvature in one direction than another. And that’s about it.

Recovery: Afterward, there’s no patching or bandaging. You’ll probably be given a combination of antibiotic and anti-inflammatory drops to help your eyes heal without infection or scarring—other than the scarring deliberately caused by the incisions—and to reduce pain from the nerves being exposed. It usually takes three months for your eyes to become fully comfortable and your vision to stabilize.

Excimer PRK

Before the excimer laser can be used to reshape or “sculpt” the cornea, the front surface of the cornea, the epithelium, must be removed. This is done either manually, by scraping the cornea (while it is anesthetized), or with the laser itself—or in a combination of both. The laser is then used to change the cornea’s curvature by vaporizing the tissue deep inside it.

Recovery: Because the corneal surface has been removed, you’ll probably be fitted with a protective clear “bandage” contact lens until the epithelium has time to regenerate, a process that takes about four days. You’ll be given antibiotic and anti-inflammatory drops to help your eyes heal. You’ll probably need some form of oral painkiller as well; stripping the epithelial surface exposes certain sensitive nerves, and the resulting pain can be severe until the epithelium regenerates to cover these

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nerves again. Because PRK alters the cornea more than RK does, recovery time is longer—generally about six months.

LASIK, or Laser ALK

LASIK evolved from a procedure called automated lamellar keratoplasty, or ALK, in which tissue from the central cornea was removed to reduce myopia. In ALK, the surgeon creates a flap in the epithelium (the top layer of the cornea) and carefully lifts it to the side. Then the surgeon removes a section of stroma (the tissue just beneath the epithelium) to reshape the cornea. The flap is then replaced and allowed to heal.

Although the idea behind ALK seems sound, in practice the operation’s success rate has proved disappointing; because it is so complex, there is much room for error. In recent years surgeons have sought to improve the technique with high-precision lasers, which brings us to LASIK (laser-assisted in-situ keratomileusis), or laser ALK.

The laser procedure is basically the same as ALK, except that tissue is vaporized, or photoablated, instead of cut out. It’s much less painful than RK or PRK, because corneal nerves are not left exposed, and the original corneal epithelium is preserved—so you don’t need to grow a new layer of cells over your cornea. Thus, it takes less time for your eyes to heal.

Recovery: You’ll still need to use antibiotic and anti-in- flammatory eye drops, but for a shorter time than with RK or PRK. Recovery time is about three months.

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Holmium LTK

The big difference with holmium LTK is that tissue is merely heated, not vaporized. First, you’ll be given eye drops to constrict the pupil and drops to anesthetize your eyes. Now for the procedure, which is fairly simple: The surgeon marks the cornea with an instrument to pinpoint precisely where to aim the laser probe. The laser then heats only those selected portions of the cornea, to shrink the collagen fibers around the cornea’s edges.

Recovery: You’ll be given antibiotic drops for the next two to three days while your eyes are healing. With this procedure, unlike the others, there’s little injury (or, as surgeons say, “insult”) to the corneal epithelium, and so most patients experience no pain, or only minor discomfort.

How Well Do These Procedures Work?

An easy question, but a complicated answer—mainly because most of these procedures are still too new and still evolving. The largest follow-up studies to monitor the results of refractive surgery have been done on patients who have received RK. One reason for this is simply the comparative volume of patients. Before 1995, RK was the refractive surgery of choice (actually, it was the only approved refractive surgery in the United States). The laser techniques have been “under investigation,” and have been done on limited numbers of patients in research centers, but they were not widely available until

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1995. PRK was approved by the Food and Drug Administration only in 1996. Therefore, the total number of patients undergoing laser procedures is still small when compared with the many patients in this country who have had RK.

What’s Considered Effective?

A successful result for most studies is uncorrected vision of 20/40 or better. This is the minimal level of vision required by most states in granting a driver’s license without a restriction for glasses or contact lenses. So basically, with 20/40 vision you could see well enough to drive a car. However, for most of us, seeing at a level less than 20/20 is not ideal; we don’t feel that our vision is as clear as it should be. And many RK patients, even those with results considered to be “successful,” still need glasses at least part-time. (Most of these people, however, feel that this is less of a compromise than wearing glasses full-time.)

The most widely publicized study on refractive surgery so far is the PERK (Prospective Evaluation of Radial Keratotomy) study, sponsored by the National Eye Institute at the National Institutes of Health; it was started in the late 1970s and has been updated as surgical techniques have changed. Because it’s been around the longest, the PERK study’s findings give a better sense of what happens to an RK patient’s vision over time. The most recent findings show that results of RK surgeries are still very dependent on the amount of myopia that

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someone has before surgery. The more myopia, the less likely for uncorrected vision after surgery to be better than 20/40. For patients with mild myopia, 92 percent maintained 20/40 vision or better at five years after the surgery. For those with moderate myopia, the number drops to 86 percent, and for those with high myopia, 72 percent had 20/40 vision or better. And 64 percent of all RK patients with any degree of myopia wore neither glasses nor contacts five years after surgery. Some people with RK required eyeglass prescriptions because their corneas continued to flatten after the surgery and they became much more farsighted over time.

Early studies on the laser procedures show slightly better results. However, the patients these studies repre- sent—those who enrolled in research programs—were carefully screened to meet certain criteria, so they may not be representative of the general population; therefore, the results may be slightly skewed.

Surgical Complications

Your doctor has probably told you this, but it’s worth repeating: with any type of eye surgery, there is a risk of loss of vision. With RK, the main complication is perforation of the cornea by the knife used to make the incisions. While vision loss from a small perforation is rare, it can happen. Perforations of the cornea can lead to endophthalmitis (an infection of the entire inside of the eye), a breakdown of the cornea, corneal infections, and traumatic cataracts. The second major complication happens

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when the incisions aren’t placed accurately; this can result in irregular, and sometimes uncorrectable, astigmatism.

One way to measure a procedure’s safety and efficacy is to compare patients’ best “corrected” level of vision (with glasses or contacts, if needed) after surgery with their best corrected level before surgery. After RK, 98 percent of patients have been reported to maintain the best level of corrected vision they had before surgery. This still leaves 2 percent of patients with potentially worse corrected vision after surgery (even though they are very likely to have better “uncorrected” vision—that is, to see better without glasses or contacts—than they did before surgery).

Other postoperative complications from RK include pain, glare, and vision fluctuations. There is also an increased risk of corneal damage from trauma due to a structurally weakened cornea. While all are fairly common, these complications usually resolve over the first few days to months as the eye heals. However, occasionally the glare from corneal scarring and vision fluctuations can persist for years. Vision can fluctuate because the cornea has been effectively weakened by the surgery (with less tissue to support it, it’s not as sturdy as it used to be). For someone who has undergone RK, a closed eyelid during sleep tends to flatten the cornea slightly. Upon waking, there is less myopia. By the end of the day, as some of this flattening goes away, the myopia tends to get worse.

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Laser procedures seem to have fewer complications than RK, but there are a few. The primary symptoms are pain, hazy vision, halos around lights, and glare. As with RK, these symptoms usually go away within a few days to months; however, haze, halos, and glare can persist beyond the usual six months after surgery. PRK and LASIK both alter tissue in the central cornea (as compared with RK, which leaves the center of the cornea alone). It has been reported that, for between 1 percent and nearly 9 percent of PRK patients, scarring may persist in the central cornea indefinitely. With LASIK surgeries, corneas have been shown to stabilize in as little as one month with less risk of scarring than with PRK.

Refractive surgery, when it’s most successful, will give you the best vision soon after the surgery is performed, but whether or not you have the surgery, the vision you have today is not the vision you’ll have ten years from now. Surgery to your cornea won’t affect what happens to the rest of your eye (the lens in particular), which will keep right on changing over your lifetime—and therefore, so will your vision.

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