Don't be blind-sighted by glaucoma: with early detection and treatment, you can often protect your eyes against the profound and permanent vision loss from glaucoma.
"I'll never forget the beautiful image of Earth as I looked from the Space Shuttle Discovery," said former Senator John Glenn. "If my glaucoma had not been caught in time, I never would have seen such a sight."
The American hero and recent spokesperson for EyeCare America's campaign to prevent blindness, Glenn numbers among the three million Americans age 40 and older who have glaucoma, with millions more at high risk for the disease.
In the early stages of the disease, there typically are no symptoms--no pain, no noticeable vision loss. Unfortunately, by the time symptoms begin to surface, permanent damage to the optic nerve and vision loss have already taken root, highlighting the critical importance of early detection and treatment.
Each new study and genetic discovery reinforces the role of routine, comprehensive eye exams and early treatment as keys to controlling progression of the glaucoma.
Family history, previous trauma, ethnic background, steroid use, and underlying medical problems, such as hypertension or diabetes, play pivotal roles in the development of the disease.
All too often, a family member loses vision from glaucoma, only to discover that a parent or relative also suffered from the disease, yet never spoke about it. If a family member has glaucoma or other eye disease, you are at greater risk for glaucoma and should undergo a thorough eye exam with dilated pupils and close inspection of the optic nerve.
To discover the latest technologies and treatments for the disease, the Post spoke with Indiana University's Darrell WuDunn, M.D., Ph.D., associate professor of ophthalmology.
Post: What is the current definition of glaucoma, and how has it evolved?
WuDunn: Twenty years ago, we thought that glaucoma was defined by elevated intraocular eye pressure (lOP). If you had elevated eye pressure, you had glaucoma. Today, we know that pressure is just one aspect of glaucoma. In fact, we now define glaucoma as a characteristic optic neuropathy, so the definition doesn't even include intraocular pressure. We do, however, consider IOP as the most important risk factor for glaucoma, so people with higher pressure are at higher risk for developing glaucoma. We also now know that some people never have elevated pressures, yet develop a form of the disease called normal tension glaucoma.
Post: Could you discuss normal tension glaucoma, where a major diagnostic markers for glaucoma is absent?
WuDunn: Normal tension glaucoma is not associated with pressures above normal, with 21 milligrams of mercury (mmHg) regarded as the upper limit of normal. There is nothing magical about a pressure of 21, other than the fact that about 95 percent of the population has pressures below that. Think about pressure as a continuum where the higher the pressure, the more potential damage to the optic nerve.
In most people, pressure varies tremendously from day to day, hour to hour. If I measured your pressure right now, it might be 18 mgHg. This afternoon, it could be 21; and this evening, 24.
In normal tension glaucoma, we assume that pressure never goes above 21, but we do not know if elevated pressure occurs, because we don't check pressures 24 hours a day. We check it once in the office, then perhaps at a different time on a different day. We may have some patients stay in the office throughout the day to monitor and check their pressure every two hours, but even then, we are still checking pressure only between 8:00 a.m. and 5:00 p.m.
We miss measurements of eye pressure when people wake up and while sleeping. In some patients, we may never measure high lOP, but they could experience pressures above 21 at other times.
Many people suspected to have normal tension glaucoma may have had elevated eye pressure in the past due to other conditions--inflammation in or trauma to the eye, for example. Certain transient types of glaucoma can cause elevated eye pressure and appear as damage to the optic nerve from glaucoma. Even though their pressure may appear normal, most likely their eye pressure was elevated in the past, which is how they developed glaucoma.
Normal tension glaucoma is very hard to diagnose definitively. We at times toss people into a kind of wastebasket diagnosis of normal tension glaucoma, because we don't know their history and assume what exists now is a reflection of what is going on, but it may not be.
If we use the definition of glaucoma as an optic neuropathy, the difference between normal tension glaucoma and high tension glaucoma isn't that distinct.
Post: If normal tension glaucoma is such an elusive diagnosis, how can people detect early signs of the disease at an earlier stage to prevent vision loss?
WuDunn: The key for early diagnosis is a good ophthalmic exam, which includes looking at the optic nerve. A good exam is the most important thing we do in diagnosing any type of glaucoma. We know when pressure is elevated, there is a higher risk--so we may pay more attention. But doctors need to look at the optic nerve to determine whether areas appear suspicious for glaucoma.
Post: Would the visual field test be useful for screening in a patient with a family history of glaucoma, including normal tension glaucoma?
WuDunn: In general, we don't recommend using visual fields as a screening tool for glaucoma because it is very time-consuming. However, for individuals who are at high risk for glaucoma, such as those with a family history of glaucoma, the visual field test may be useful. The decision to use visual field testing usually is based on the appearance of the optic disk during the eye examination. If the optic disk looks suspicious for glaucoma, then we proceed with visual field testing. If the optic disk appears healthy, then the visual field test is extremely unlikely to show any abnormalities, even if the individual has strong glaucoma risk factors such as elevated pressure or a family history. For a patient that has already been diagnosed with normal tension glaucoma, visual field testing is important in monitoring for progression of the disease.
Post: Is measuring the visual fields also important for early diagnosis of glaucoma?
WuDunn: Yes, absolutely. Whenever we are suspicious that an individual has glaucoma, even early glaucoma, we recommend visual field testing. Now, certainly there are individuals who have pre-perimetric glaucoma and so they have glaucomatous optic nerve damage but not yet have abnormalities on standard visual field tests. However, we rarely catch patients at this pre-perimetric stage. Most individuals with suspicious-appearing optic disks but normal visual fields are considered "glaucoma suspects." Only those in which we can detect or document a change in the optic disk do we call pre-perimetric glaucoma.
Post: Does glaucoma show up on the visual field test?
WuDunn: Standard visual fields are not very sensitive. An individual may lose up to 40 percent of his or her optic nerve fibers before defects will show up on visual field testing. There are other, more sensitive types of visual field tests, such as short wavelength automated perimetry (SWAP--also known as blue-on-yellow) that detect damage sooner than standard visual fields, but their use has not yet become the standard of care.
Post: What is short wavelength automated perimetry (SWAP), and how does it aid in diagnosis?
WuDunn: SWAP is a type of visual field test that uses a blue spot on a yellow background. Conventional visual field testing uses a white spot on a white background. The "blue-on-yellow" spots stimulate a certain subtype of nerve fibers that are usually damaged in glaucoma sooner than other subtypes, such as those stimulated by white-on-white spots. Hence, in many cases SWAP defects will occur before standard perimetry defects and will often predict future white-on-white defects.
Post: Is SWAP widely available, Al and does it aid in diagnosing glaucoma at an earlier stage?
WuDunn: It is still not widely used. Most newer visual field machines can perform SWAP. It's just another option on the menu of field tests. Except for the spot and background color, SWAP uses basically the same software and hardware as standard perimetry. Hence SWAP is widely available and can be a very useful aid in diagnosing and monitoring early glaucoma. Patients generally find SWAP tests more difficult to do than standard visual field tests, and ophthalmologists find the test more difficult to interpret. I use the test infrequently, but I think it is most useful in monitoring ocular hypertension and glaucoma suspects.
Post: Could you tell us about other new diagnostic tools to measure the thickness of the cornea?
WuDunn: In my practice, the device called a pachymeter is becoming more important in terms of glaucoma. Traditionally, we measure intraocular pressure with a device that basically pushes on the eye. If the wall of the eye itself is stiff when we push on it, it will feel like there is a lot of pressure, compared to an eye that has a thin or very flexible wall. A rubber balloon, for example, is very easy to indent even if the pressure inside is the same. But you can't indent a steel ball, purely because the wall is harder. The same holds true for the eye.
Eyes with thinner corneas tend to be less stiff, so when we press on the eye, it feels like there is not much pressure. We have found that people with normal tension glaucoma have thinner corneas than average, which could mean that we underestimate true intraocular pressures.
While we are not entirely sure of the best use for the new application at this point, the device helps us better examine the optic nerve.
Basically, all of these technologies perform quantitative measurements of the optic nerve. If you look at the optic nerves in 100 different people, they will all look very different. We look at the optic nerve to assess whether it has been damaged, but we don't know what happened in the past. The hallmark of glaucoma is that it is progressive, so the optic nerve should show changes over time. But if we see someone the very first time, we don't know what the optic nerve looked like ten years ago. These technologies quantify the appearance of the optic nerve and provide objective measurements that help down the road.
Post: When would you measure corneal thickness in your patients?
WuDunn: Certainly, anyone with or suspected of having glaucoma should have that pachymeter exam done once in their life. If their eyes appear healthy without optic nerve problems, there is no reason to do the test. But if they have a suspicious-appearing optic nerve and a pressure of 18, it's worthwhile to measure the thickness of the cornea. If thin, they might have or be at greater risk for developing glaucoma.
Post: Why is it so important to view the optic nerve?
WuDunn: The optic nerve is basically the cable that connects the eye to the brain. If you think of the eye as a video camera, the optic nerve is the cable that connects the video camera to the monitor. If you cut that cable, there is no image on the brain, so obviously it is a very important structure.
The optic nerve goes from the brain to the eye, then splays out onto the back surface of the eye, which is called the nerve fiber layer. So it starts as a cable, then splits up into its individual fiber-optics, if you will, then spreads out into the back of the eye.
By measuring the thickness of the nerve fiber layer at the back of the eye, we can more accurately estimate how much nerve damage has occurred. If damaged, the layer would be thinner. We are finding that the tool is very useful, because as the nerve fiber layer gets thinner, we know that the disease is progressing in that particular patient.
Post: Would a strong family history suggest the need to monitor a person more closely?
WuDunn: If you have a congenital history of glaucoma of any sort, it raises my index of suspicion that you are at higher risk for glaucoma.
Post: Can family members calculate their risk if they know they have three or four relatives, for example, with glaucoma?
WuDunn: For glaucoma in general, if you have a sibling with glaucoma, your risk is two- or three-fold higher than the normal population. If you have a parent, it is 1 1/2 times the risk. There are no statistics specific for normal tension glaucoma.
People most at risk for normal tension glaucoma are those with what we call vaso-spastic phenomena, such as migraine headaches or Raynaud's phenomenon, where individuals get cold hands or feet.
Post: If a person undergoes exams at many different centers, ophthalmologists wouldn't have a patient's history to follow changes. Is it important to follow up with one clinician?
WuDunn: Absolutely. It is very critical for someone who is either suspected to have or who has glaucoma to maintain follow-up with the same doctor. Records help monitor progression. Glaucoma is a condition of change, so we diagnose and treat the disease based on whether it is getting worse, and you can't tell that from seeing a patient one time.
Post: If a patient goes to a mall for a glaucoma test and pressure readings are normal, could they be getting a false sense of security?
WuDunn: That is the tricky part about normal tension glaucoma. An adequately trained person needs to look at the optic nerve to make the diagnosis. If you only measure pressure and it is normal, you may miss the diagnosis.
To order a video of the Post's interview with Dr. WuDunn, send $29.95 (includes s&h) to Saturday Evening Post, Dept. 91004, P.O. Box 567, Indianapolis, IN 46206.
EyeCare America's Glaucoma EyeCare Program provides free early screening for glaucoma. To see if you qualify for a free exam, call the toll-free help line at 1-800-391-EYES (3937).
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|Publication:||Saturday Evening Post|
|Date:||Sep 1, 2004|
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