Ординатура / Офтальмология / Английские материалы / The Eye Book A Complete Guide to Eye Disorders and Health_Cassel, Billig, Randall_2001
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Therefore, suddenly experiencing flashes or floaters means you need to be evaluated by an eye care specialist, so that any holes or tears in the retina can be repaired before they cause permanent damage. Note: Flashes and floaters associated with retinal detachment usually happen only in one eye at a time, not both! (For more on this serious problem, read on.) Bilateral flashes (in both eyes) are an unusual phenomenon and are usually associated with visual forms of migraine, with or without an accompanying headache (see chapter 18).
A Detached Retina
We often describe the retina as the wallpaper lining the back of the eye, even though, as we’ve discussed earlier in this book, it’s infinitely more complicated than a single wafer of paper. For one thing, the retina isn’t one sheet, it’s an elegant, intricate network of layers. Its job is to take the light rays that enter the eye, convert them into nerve impulses, and then telegraph them, via the optic nerve, to the brain, where they’re decoded into images that make sense.
Simply put, we see because of the retina. An eye without a working retina is blind. Therefore, when the retina becomes detached, or unhinged from the back of the eye, it must be treated immediately, because the repercussions are so great.
Fortunately, there are some warning signs of a detached retina. At first, you may experience the sudden onset of flashes—bursts of light, like fireworks or
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Warning Symptoms of a Retinal Tear or
Detachment
•A gradual or sudden increase in floaters in one eye. Like pieces of lint on a movie screen, these intrude on vision and may resemble cobwebs, spiders, or even circles.
•A gradual or sudden increase in flashes in one eye. Fleeting bursts of light, like fireworks or lightning, these may be especially noticeable in the dark.
•The gradual or sudden appearance of a dark cloud or “curtain” over your field of vision, from any direction.
If you experience any of these symptoms, call your eye doctor immediately! These may be signs of a retinal detachment.
sparklers, that last only seconds at a time. Remember, the flashes we’re talking about happen only in one eye, not both eyes at the same time. Another cause for concern is the sudden development of annoying spots or specks in the eyes—floaters—which may resemble gnats, flies, spider webs, or even a recognizable shape like a ring or heart. (Typically, these are seen best against a solid background, such as the sky or a white wall.)
Floaters and flashes are most often due to a shift in the vitreous, the gooey jelly that fills the eye. Sometimes this
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vitreous shift leads to a break in the retina, in the form of a tear or hole. Then, gradually, the retina begins to detach, or peel off, from the back of the eye as the vitreous starts to seep through this opening. When this happens, patients can notice a slow loss of sight in that eye, as if a curtain were being drawn up or down across their vision. (This generally begins on one edge and may move centrally; moreover, if the detachment moves to affect the macular region at the back of the eye, central vision may also become affected.)
Ways the Retina May Become Detached
There are three basic things that can go wrong here.
Rhegmatogenous (or mechanical) retinal detachment:
In this most common type of detachment, a hole or tear develops in the sensory retina, allowing liquid vitreous (most of the vitreous is a gel; however, as we get older, some of it becomes more liquid) to seep through the sensory retina and sever it from the retinal pigment epithelium. Those at risk for this type of detachment include people with specific forms of peripheral retinal thinning such as lattice degeneration, or people with severe nearsightedness or aphakia (the lack of a lens in the eye). People with a history of blunt trauma—from boxing, for instance—or penetrating eye injury are also at risk.
Tractional retinal detachment: In this less common condition, fibrous scar tissue on the sensory retina’s inner surface contracts—just as scar tissue does elsewhere in the body—pulling the sensory retina away from the retinal pigment epithelium. This problem is seen in
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people with diabetic retinopathy after a vitreous hemorrhage that produces scar tissue. It also may occur after eye trauma, or even as a complication following surgical repair of a rhegmatogenous retinal detachment.
Exudative retinal detachment: In this form, there’s neither a hole for vitreous to pass through nor shrinking scar tissue to tug on the retina. Instead, fluid oozes from the choroid, through Bruch’s membrane, and accumulates under an intact retina, like a blister. This may happen as a response to inflammation or in such conditions as uveitis (see chapter 14) or age-related macular degeneration (see chapter 9).
Fixing a Detached Retina
How hard is the task of repairing a detached retina? Imagine trying to unfold a crumpled piece of Kleenex in a glass of water without tearing it. This delicate, precise surgery requires steady hands, high-powered magnification, and special instruments.
The good news is that remarkable advances have been made over the last several decades in our ability to treat each form of retinal detachment. We would like to illustrate this with a brief discussion of the surgical treatment of rhegmatogenous detachment.
Rhegmatogenous detachment: The goal here, clearly, is to reattach the sensory retina to the retinal pigment epithelium, and retinal surgeons do this by compressing the sclera, the “white” of the eye—either with a buckle or with tiny sponges—to force these two layers back together. (A buckle is often required to indent the scleral
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wall from the outside of the eye enough to put these two retinal layers on the inside of the eye back in touch with each other, so that they can reattach to each other.) Surgeons also may inject gas bubbles into the vitreous cavity to help push the sensory retina up against the retinal pigment epithelium. (Note: Because the eye may have to be specially positioned to make these bubbles “float” into the best position, some people need to spend hours in one position—on their side or even upside down—as the retina heals after surgery.) It’s almost always necessary to close the hole or tear in the sensory retina, and surgeons use lasers or cryotherapy (localized freezing techniques) to “spot-weld” around any retinal breaks. Sometimes this same treatment is used prophylactically on eyes prone to retinal detachment, to avoid a detachment before it occurs. Some people also may need to have excess retinal fluid drained from beneath the detachment so that the sensory and retinal pigment epithelial layers can reattach to each other. Modern microsurgical techniques using sophisticated small instruments for work inside the eye may also be used in reattaching the retina.
When the Eye’s Blood Supply Is Blocked
It’s simple: every single ounce of tissue, every tiny cell we have, needs oxygen to live.
Oxygen comes through the blood, and blood is piped throughout the body via our arteries and veins. Arteries deliver oxygen-rich blood throughout the body; like
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Image not available.
Fig. 15.1. Arteries and veins in the retina
rivers, they branch into ever-smaller streams to reach every part of us. Veins make the return trip back to the heart and lungs, where more oxygen is pumped into the bloodstream; then the arteries deliver it all over again.
Blood reaches the eye through the big aorta and carotid arteries, which basically transport blood up through the neck; then, a smaller vessel, called the ophthalmic artery, carries it up into the eye. This important artery branches into smaller arteries as it nears the eye. Some supply blood to the choroid layer underneath the retina’s pigment epithelium; others penetrate the optic nerve outside the eyeball and travel inside it to reach the eye’s innermost workings. Most crucial of these branches
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are the central retinal artery, which supplies oxygen-rich blood to the retina’s inner layers (these are known as the sensory retina), and the central retinal vein, which takes the oxygen-depleted blood out of the eye back toward the heart.
Blockage of the Retinal Blood Supply
These major retinal blood vessels, then, are pipelines —or, more precisely, lifelines—entering and leaving the eye. As you may imagine, a clog in one of these pipes can be devastating. Say the blockage is of an incoming, oxy- gen-bearing artery: deprived of oxygen, the retina immediately begins to react—to degenerate and swell. If the clog is in an outgoing retinal vein, the reaction is equally abrupt, like a sudden traffic jam on a major interstate— an almost-instantaneous backup of blood and fluid into the retina.
Retinal vessel blockage—either arterial (with incoming blood) or venous (involving outgoing blood)—is a significant cause of visual problems in all ages, but especially in people over sixty-five. Just how common are these blockages? Nobody knows for sure; one difficulty in making estimates is the fact that many people don’t notice a problem if it occurs in the nondominant eye or on the far edge of vision. Although their specifics differ, depending on whether an artery or a vein is involved, these blockages share one important common denominator for the retina: they mean big trouble.
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Retinal Artery Blockages
Before we discuss retinal artery blockages, picture a river with four branches, or a road with four forks. The main line, or river, is the central “trunk” of the retinal artery; each of the four arteries, which branch at the head of the optic nerve, supplies one quarter of the retina. These are the superior temporal, inferior temporal, superior nasal, and inferior nasal arteries (see figure 15.1). There are many variations, but this is the basic pattern.
As you may imagine, the degree of injury—and its effect on vision—depends on the location of the blockage, or “occlusion.” A clog at the central retinal artery, the main line, can cause an eye to lose all vision instantly. If the problem is in a branch artery, vision loss is confined to the particular quadrant of the visual field served by that branch. (Note: Remember from chapter 1 that the retina’s image is inverted and reversed by the brain. This means that a blockage of, say, the superior temporal retinal artery will cause a corresponding loss of vision in the inferior nasal visual field.)
In either case—whether it’s a central or a branch artery involved—the resulting loss of vision is usually sudden, painless, and complete. There’s an infarction, just as in a heart attack or stroke. Blood supply is cut off, and oxygen-starved tissue begins to swell, deteriorate, and then die. Almost always, this tissue loss is permanent.
As if this weren’t devastating enough, there are other implications here. Is this clogged artery a symptom of a really big problem in the rest of your body? In other
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Image not available.
Fig. 15.2. Central retinal vein blockage
words, is there an even worse infarction—a heart attack or stroke—waiting to happen? Also, could the same thing happen in the other eye, causing total blindness?
The most common cause of central and branch retinal artery blockages in older people are cholesterol plaques lining the carotid arteries, a result of atherosclerosis (“hardening of the arteries”). Atherosclerosis is also an important factor in heart attacks and strokes. In the most likely scenario, a piece of the cholesterol plaque breaks off, floats downstream through the ophthalmic artery, and makes its way into the eye through the central retinal artery. At each fork in the road, the passage becomes tighter. Because they’re so narrow, the central
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retinal artery and its branches in the retina are particularly prone to obstruction by these runaway cholesterol plaques.
Other causes of central and branch clogs in the retinal arteries include calcium deposits from damaged heart valves, leftover bits of a blood clot after a heart attack, and even foreign particles injected during IV drug abuse. Also, blood disorders such as sickle cell disease, other health problems such as migraines, collagen vascular diseases such as systemic lupus erythematosus, giant cell arteritis (see chapter 17), and even too-low blood pressure, by allowing these vessels temporarily to collapse, may cause these blockages. (In giant cell arteritis, the other eye is at especially high risk for a similar blockage.)
Fortunately, the chances of recovering vision are much better in branch blockages (80 percent of the time, vision returns to at least 20/40). However, even though they may recover much of their lost vision, many people with branch blockages do have some permanent vision damage.
Artery blockages in the retina are almost impossible to treat, because medical care must be almost immediate. Within the first hour after the artery becomes clogged, either the blockage must be removed or blood flow to the retina must be improved through the use of medications to dilate the arteries. After that, the lack of oxygen to the retina causes permanent and irreversible damage—and treatment days or even hours after the fact won’t help. Note: Some patients do recover a little of their lost vision