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

330 / OTHER EYE PROBLEMS

Descemet’s membrane. These middle three layers act as scaffolding, providing structural support to the cornea as it arches over the front of the eye. Last is the single layer of endothelial (inner lining) cells (also called the endothelium). Because this important layer touches the aqueous of the eye’s anterior chamber, it serves as a sort of “bilge pump,” keeping the cornea free of excess moisture. When this pump malfunctions, the cornea can swell, and this can distort or even damage vision.

The cornea normally does not contain any blood vessels. However, it is rich in sensory nerve fibers: under the epithelial layer alone are about seventy of them, which helps explain why the cornea is so sensitive to pain. The epithelial cells act as a protective blanket, like enamel on a tooth, insulating the nerve fibers from the world. When that blanket is frayed—or, continuing the tooth analogy, when the enamel is cracked or has a cavity—those ultrasensitive nerves react. Painfully. Even a small loss of epithelial cells can be excruciating, if it exposes these nerve endings.

Now let’s look at some common problems affecting the cornea.

Corneal Abrasion

Because of the abundance of nerves throughout its layers, even a slight injury or irritation to the cornea can result in a lot of discomfort or pain. An abrasion—a scrape of the epithelium, or outer surface—is the most common injury to the cornea. It can happen so easily—

THE CORNEA / 331

when the eye gets too close to a baby’s fingernail, for instance, or the corner of an envelope, or a tree branch. All of a sudden it feels as if there’s a hot poker in your eye. Other symptoms include redness, a feeling like there’s a piece of grit in your eye, and extra sensitivity to bright lights. Because it’s often difficult to see the actual injury with the naked eye, eye doctors rely on special fluorescent dyes, which target and highlight areas of damage, to help us determine the extent of the wound.

Fortunately, despite the severe discomfort and blurred vision that often accompany corneal abrasions, these injuries usually heal fairly quickly—sometimes in a matter of hours, sometimes within a few days—and don’t leave any lasting damage.

Treatment: Basically, the cornea must heal itself, and all we can do is provide the best conditions possible. (Think of skin injured by a scrape or burn; it hurts until your skin lays down new layers of cells, which insulate the nerves beneath.) Thus, the main treatment for a corneal abrasion is simply to patch the eye. It’s not quite as easy as it sounds—in other words, you shouldn’t try to do it yourself with an eye patch from the drugstore— because to be effective, the eye patch must immobilize the eyelid and prevent it from rubbing over the injured area. The epithelial cells need time to multiply and coat the injury, which means that the patch needs to be tight enough to keep the eyelid still.

It takes several eye patches—generally three—to create enough bulk to secure the lid. (Eye doctors either stack three patches over the eyelid or use two, with the

332 / OTHER EYE PROBLEMS

one directly on the eyelid folded in half.) The eye pads are fixed over the eyelid with at least four pieces of surgical tape, extending from the forehead to the cheek.

Sometimes, when eye patches can’t be tolerated or when the abrasion doesn’t appear to be healing, eye doctors apply a special “bandage” contact lens over the abrasion. ( Note: Because there is a risk of infection with these lenses, this should be done only by an eye care specialist very familiar with this technique.) Bandage contact lenses allow the patient to avoid having to cope with the nuisance of wearing a large and bulky eye patch and enables the abraded eye to see while it heals.

There are different philosophies on fitting bandage contact lenses, but most often a large, mediumto highwater, thin, disposable soft contact lens (see chapter 5) is used. The lens is usually fit slightly loose to avoid adhesion to the cornea, and the large size allows for maximum coverage of the eye.

Contact lenses made of collagen are also available for patching an eye. These lenses are fit similarly to the soft contact lenses, but the collagen material dissolves on the eye within twelve to thirty-six hours, depending on the lens thickness. One of the advantages of a collagen lens is the lubricating effect that the dissolving collagen provides. However, these lenses do have a tendency to fall out as they dissolve.

Any contact lens has the additional benefit of acting as a drug delivery system for the eye. Eye drops prescribed for a corneal abrasion are absorbed by the contact lens and then slowly leach out from the lens onto the

THE CORNEA / 333

eye. This keeps the medication on the eye for a longer time and enhances the therapeutic benefit of the drops.

Recurrent Corneal Erosion

As we said before, most corneal abrasions heal fairly quickly, without causing permanent injury to the cornea, the eye, or sight. However, an ornery few don’t stay healed, apparently because the new blanket of epithelial cells doesn’t stick to the injured area. This problem is called recurrent corneal erosion.

When an abrasion is particularly deep or damaging and healing is inadequate, the epithelial cells simply slide off—even months or years after the initial injury. And unfortunately, losing the protective insulation of the epithelial cells, again exposing the nerves underneath them, hurts about as much the second time as it did the first.

But we do have some clues as to who might be prone to recurrent corneal erosion—and therefore we can try to prevent it from happening. People who have had corneal abrasions due to fingernails, paper, or plant matter seem to be predisposed to developing recurrent erosion. We also know that the epithelial cells, if they’re going to erode at all—and remember, in most people they don’t—tend to come loose early in the morning, usually when people wake up. Why? Because your eyes dry out as you sleep. When the epithelial cells aren’t secured to the cornea, they can be rubbed off by the simple act of opening your eyes in the morning. So if you have a history of eye discomfort when you get up—if you

THE CORNEA / 335

having an abrasion—except that the redness, the sensation of having a piece of grit in your eye, and the difficulty tolerating bright light are usually worse.

Although countless bacteria exist in and around the healthy eye, normally they’re effectively prevented from invading the cornea by the epithelium, which acts as a shield, and by the powerful bacteria-fighting agents in normal tears. But these natural barriers aren’t impenetrable. They can be eroded by such things as eye trauma, dry eyes (particularly the severe form found in Sjögren’s syndrome; see chapter 13), refractive surgery, improper eyelid function (a problem with Bell’s palsy; see chapter 10), contact lenses, and even viruses including herpes zoster (found in chicken pox and “shingles”) and herpes simplex keratitis (the same virus found in “fever blisters” on the lips). (Just as fever blisters often come about in response to physical or emotional stress, herpes keratitis can also reappear after months or even years.) Note: Viral infections of the cornea are very serious and can ultimately lead to scarring and permanent vision loss. In fact, herpes keratitis is the most common cause of corneal blindness in developed countries.

The slightest chink in the armor of the epithelial cells opens the door to the host of infectious agents crowding just outside—an unsavory cast of characters that also may include fungi and such bacteria as staphylococcus, streptococcus, and pseudomonas (often linked to corneal ulcers in contact lens wearers). Some bacterial strains are so nasty and virulent that once present in the eye, they can even grow directly through an intact corneal

336 / OTHER EYE PROBLEMS

epithelium. Contact lenses greatly increase the risk of corneal ulcers when there is an infection present or there has been an insult to the eye, and in such cases should be removed immediately. Contact lens wearers should see an eye doctor at the first sign of a red eye or persistent eye discomfort.

Treatment: Because corneal ulcers are so serious, and potentially sight-threatening, your job is to get treatment as promptly as possible. Your eye doctor’s task is to figure out what’s causing the infection, and how best to treat it—with antibiotics, antiviral, or antifungal agents. Be sure to take the entire dose of antibiotic, if that’s the treatment, since if you take less than the full dose, resistant microorganisms might grow and the infection could become much more difficult to get rid of.

Corneal Dystrophy

A dystrophy is an abnormal, possibly progressive, condition, often hereditary, usually present at birth. Many forms of dystrophy can affect the cornea, but the two most common are corneal epithelial basement membrane dystrophy and corneal endothelial cell dystrophy.

Corneal Epithelial Basement Membrane Dystrophy

Think of the basement membrane of the corneal epithelium as a slab of cement. On this cement, in nice, neat rows, are stacks of bricks—in this case, epithelial cells. The smoother the cement, the neater the stacks of bricks, and the better they serve as a wall against infec-

THE CORNEA / 337

tion and as a smooth surface that, like a clean windshield, allows clear vision.

Basically, epithelial basement membrane dystrophy is a problem with the cement. It usually occurs in adults between the ages of forty and seventy, is slightly more common in women than men, and seems to be hereditary. The problem here is that the basement membrane becomes abnormally thick and irregular, forming a telltale pattern (as seen under the high magnification of the slit lamp) of ridges, cysts, and whirls—thus the descriptive name for this condition, “map-dot-fingerprint dystrophy.” This causes the epithelial cells to buckle, break down, become “unstuck,” and slough off.

Symptoms range from mild irritation to severe pain and redness in the eye. Because the underlying problem doesn’t go away, and because symptoms are identical to those of recurrent corneal erosion, epithelial corneal dystrophy can even be thought of as a cause of recurrent corneal erosion.

Here too, as in recurrent corneal erosion, symptoms are usually worst in the early morning. Remember, while we sleep, when our eyelids aren’t constantly blinking and applying new coats of lubricating tears, our eyes naturally become a little dry. But without that extra lubrica- tion—if epithelial cells are poorly stuck to the cornea al- ready—opening the eye is somewhat akin to scraping sandpaper across a layer of varnish; the eyelid rubs these cells right off. Ouch!

Treatment: Because this problem is so similar to recurrent erosion—and, in difficult cases, often just as frus-

338 / OTHER EYE PROBLEMS

trating—the treatment is much the same: keeping the eye properly hydrated so that the epithelial cells stay put.

The first line of attack is usually ointments—artificial tears and hypertonic saline preparations—at bedtime and drops during the day. An eye patch or “bandage” contact lens (described above, under “Corneal Abrasion”) may also be necessary.

Because your eye, like your skin, responds to your immediate environment, it may also help to make your home and office more humid—with cool misters or vaporizers, or even a fish tank.

If the extra humidity and lubrication fail to stop these cells from falling off, the next step may be surgery—lasers or other techniques. For example, some people have been helped by surgery that gently clears away some persistently “unsticky” cells to make room for new, more adherent cells.

Corneal Endothelial Cell Dystrophy

Corneal endothelial cell dystrophy (Fuch’s dystrophy) is a bilateral condition (one that affects both eyes); it usually manifests itself in people in their forties and fifties, is slightly more common in women than men, and is often hereditary.

Remember how the cornea’s endothelium acts as a pump? Well, in this disorder the pump slowly fails. And as it does, the excess moisture that used to be siphoned away starts to build up. The cornea swells and becomes less transparent, and ultimately, vision can deteriorate.

The first symptom of this corneal swelling is usually

THE CORNEA / 339

blurry vision that’s particularly noticeable when you first wake up. Here as well, too much moisture is a bad thing. During the day, when your eyelids are mostly open, water evaporates from the cornea; it’s also removed by the pumping action of the endothelial cells, which siphon off excess water. All of this moisture removal helps keep the cornea clear. But when you’re asleep, only one of these water-removing processes continues. There’s no evaporation, because the eyelids are closed, so the endothelial cells have to work extra hard to keep the cornea dehydrated. In endothelial cell dystrophy, however, because the pump isn’t operating at top form, excess water accumulates. Many people with this problem wake up with markedly swollen corneas and blurred vision—both of which improve gradually during the day, as the evaporation process commences again.

Note: Eye surgery, particularly cataract surgery, can hasten the deterioration of the endothelial cells in people who have this dystrophy. (Eye surgery is stressful anyway, but particularly when these cells are already vulnerable.) Surgery can cause severe corneal edema, which may even result in the need for a corneal transplant to restore someone’s vision. So if you have a problem with blurred vision in the morning, make sure your doctor knows this before you have any kind of eye surgery.

Treatment: The best way to treat endothelial cell dystrophy is to identify, as soon as possible, the people at risk for developing it. If you have a family history of this disorder, tell your eye doctor. Regular examinations will be very important as a means of detecting early changes