By Harvey B. Simon, M.D.
Harvard Medical School
Glaucoma is the second leading cause of blindness in the United States; about 3 million Americans have the disease, but only half know it.
Regular testing can detect glaucoma. And good treatment can protect vision.
That's why every man should understand the basic facts about glaucoma.
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What is Glaucoma?
Doctors used to think glaucoma was the result of increased fluid pressure — intraocular pressure (IOP) — in the eye. If your pressure was high, you had glaucoma. But this definition is no longer accurate.
Now we know it's a disease of the optic nerve caused by damage to some of its 1.2 million nerve fibers. The optic nerve carries electrical signals to the brain. The brain decodes them into images we call vision. In most cases, the optic nerve is damaged when the IOP at the front part of the eye rises. But about 15% of patients with glaucoma have normal IOPs. And many people have high IOPs without any nerve damage.
Doctors are not sure what actually damages the optic nerve in glaucoma. In many cases, the nerve may be unusually sensitive to eye pressure. Or the damage may be related to blood pressure, oxidative stress that can damage the optic nerve, and the delivery of oxygen and nutrients.
Anyone can develop glaucoma, but the major risk factors are:
- Age – The disease is rare before age 40, but becomes more common thereafter, especially beyond age 60.
- Family history
- Severe nearsightedness
- Previous eye trauma
- Being African American
Less well-established risk factors include migraine and heart disease. And even though it's no longer the defining characteristic of glaucoma, high eye pressure (elevated IOP) is still the strongest risk factor of all. It's also the only risk factor that you can modify with prescription eye drops or eye surgery to prevent loss of vision.
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The Types of Glaucoma
Glaucoma is actually several diseases.
- Open-angle glaucoma. This is the most common form. It accounts for about two-thirds of all cases. The angle is where the iris (pupil) and cornea (a protective cover for the eye) meet.
In open-angle glaucoma, the angle itself appears normal, but the fluid may still press too hard on the optic nerve. Over time, the nerve is damaged. Because the damage is gradual, most people don't notice the vision loss until the damage is severe and irreversible. Open-angle glaucoma is "the silent blinder."
- Angle-closure glaucoma. This is the second most common type. It develops when the angle narrows and normal fluid flow within the eye becomes blocked. In many cases, the disease is slow and painless, just like open-angle glaucoma.
But angle-closure glaucoma can also produce a sudden painful attack, usually when the pupil is wide open. Because the blockage occurs abruptly, IOP rises quickly. It can cause severe pain and nausea. The eye itself is red and cloudy, with the pupil fixed in a partially opened position. An acute attack of angle-closure glaucoma is a medical emergency that requires prompt treatment to save vision.
The less common forms of glaucoma include:
- Childhood glaucoma
- Glaucoma that results from other eye diseases, such as trauma, inflammation or tumors
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You can't rely on symptoms to alert you to open-angle glaucoma. The only symptom is visual loss, which begins gradually with your peripheral vision. So you won't know you have a problem until it's too late.
Routine eye check-ups are essential, ideally every year or two after age 40 or even sooner if glaucoma risk factors are present.
Eye doctors can diagnose glaucoma by:
- Measuring IOP using a tonometer. Pressure measurements are a necessary part of the complete eye exam. But they're not enough to screen for glaucoma.
- Examining the optic nerve through an ophthalmoscope. If the nerve is unusually indented, pale or asymmetric, glaucoma should be suspected even if the IOP is normal.
- Mapping the peripheral visual fields. This is the best way to screen for glaucoma, but it also takes the most time.
A diagnosis of glaucoma requires both an abnormal appearance of the optic nerve and a visual field defect in at least one eye.
There are additional considerations for angle-closure glaucoma. By checking the angle during your eye exam, your doctor can tell you if you are at risk for an acute attack. If you have a narrow angle, you should avoid medicines that could trigger an attack by causing the iris to dilate. These include drugs ranging from over-the-counter antihistamines to tricyclic antidepressants. Ask your doctor for a list; better yet, read drug labels and instructions carefully. And if your narrow angle is causing the optic nerve damage of glaucoma, the doctor may recommend laser surgery to allow good fluid flow.
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At present, the only way to treat glaucoma, even when eye pressure is normal, is to lower IOP.
Angle-closure glaucoma is a medical emergency that requires prompt hospital treatment to lower pressure. In addition to eye drops, intravenous medication may be required. Afterwards, surgery with a laser beam (laser iridotomy) can make a passage through the pupil, allowing fluid to flow freely. Both eyes should have this procedure.
With other forms of glaucoma, prescription eye drops help lower IOP. Some reduce the production of fluid, while others boost the outflow of fluid. A doctor who specializes in eye diseases (ophthalmologist) will decide which is best for you and will explain potential side effects.
If medicines don't bring your IOP to safe levels, surgery is another options. In many cases, the first choice is laser trabeculoplasty. A laser beam is used to enlarge the tracts that drain fluid out of the eye. Or surgery can make a new route to drain fluid from the eye.
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Looking Toward the Future
The new definition of glaucoma — as a disease of the optic nerve instead of a disease of elevated IOP — raises new questions:
- Will patients who have high pressures but normal nerves benefit from early pressure-lowering therapy?
- How does blood pressure affect the optic nerve?
- Is laser trabeculoplasty better than eye drops for the initial treatment of glaucoma — or is a traditional filtering operation the best approach?
- Can laser or filtering surgery help patients with advanced glaucoma?
For now, regular eye exams for men over 40 and younger men with risk factors can diagnose glaucoma before advanced nerve damage occurs. And carefully supervised therapy can protect your vision.
Harvey B. Simon, M.D. is an Associate Professor of Medicine at Harvard Medical School and a member of the Health Sciences Technology Faculty at Massachusetts Institute of Technology. He is the founding editor of the Harvard Men's Health Watch newsletter and author of six consumer health books, including The Harvard Medical School Guide to Men's Health (Simon and Schuster, 2002) and The No Sweat Exercise Plan, Lose Weight, Get Healthy and Live Longer (McGraw-Hill, 2006). Dr. Simon practices at the Massachusetts General Hospital; he received the London Prize for Excellence in Teaching from Harvard and MIT.