The human eye is often compared to a camera. Both capture images of the world through a lens, and the eye's cornea and iris are akin to a camera's aperture, which provides focus. The optic nerve, then, is like a camera's USB cord. It conveys the images we see to the brain, which makes sense of them, just as a USB cord transmits images to a computer so we can sort and print our favorites. And while you don't necessarily need a USB cord to see the images on your camera, you most certainly need your optic nerve to see the world around you. Even if every other part of the eye works perfectly, damage to the optic nerve can cause blindness, because the brain never gets the message that things are being seen.
That damage to the optic nerve is the result of a condition called glaucoma, which affects an estimated 65 million people worldwide [source: Glaucoma Research Foundation]. It's the second leading cause of blindness, right after macular degeneration, and it can affect everyone from babies to senior citizens -- even the family dog. It's an especially frightening condition because there are no symptoms in most cases of glaucoma; the condition can rob you blind before you realize it. It's estimated that in the United States alone, more than 4 million Americans suffer from glaucoma -- but only half of them know it [source: Bakalar]. Currently, there's no cure.
Glaucoma is the result of a damaged optic nerve; that damage can be done in a few different ways. On the next page, we'll examine the most common type of glaucoma.
Primary Open-angle Glaucoma
The most common form of glaucoma is primary open-angle glaucoma. This condition occurs as a result of pressure on the eye, more formally known as intraocular pressure.
To understand how this pressure builds up, we need a little background on the workings of the eye. Our eyes have their own mini windshield-wiping system built in, and just like in automobiles, this system ensures we can see. At the front of the eye, clear fluid known as aqueous humor is released from the anterior chamber. The fluid travels to nearby eye tissue, nourishing it and enabling it to do its job. After feeding the eye tissue, the fluid exits the eye through a drain known as the trabecular meshwork. Never noticed that drain in your eyes? Well, it's only one-fiftieth of an inch wide, located at an angle where the cornea and iris meet [source: Glaucoma Foundation].
Sometimes though, that fluid can't get through the drain. Doctors don't know exactly why this happens; it may have something to do with the aging of cells in that area, a structural defect or a change in the immune system. With nowhere to go, the fluid builds up and raises intraocular pressure. We all need some eye pressure; that's what helps the eye keep its shape. How much is too much, though, varies from person to person. In the case of too much pressure, cells start to die at the weak point, which unfortunately is the all-important point where the optic nerve leaves the eye on its journey to the brain.
There are no symptoms to alert you to the fact that the optic nerve is being damaged by intraocular pressure. The first thing to go is peripheral vision, which can be an easy loss to overlook, because people just turn their heads. Once vision is lost, though, there's no way to restore it.
One common misconception about glaucoma is that the condition is merely the intraocular pressure, but a person doesn't actually have glaucoma until there's damage to the optic nerve. Not everyone with increased eye pressure will develop glaucoma, and in one instance of glaucoma, increased eye pressure isn't even present beforehand. Normal-tension glaucoma occurs without the buildup of intraocular pressure and is believed to be related to reduced blood flow in the area of the optic nerve. The lack of blood flow is what ends up doing the damage to the optic nerve.
There's one other instance in which the intraocular pressure doesn't cause damage to the optic nerve over time; rather, the intraocular pressure comes on fast and causes an emergency situation. Find out about this type of glaucoma on the next page.
Angle-closure Glaucoma and Other Types of Glaucoma
In most cases, people with glaucoma may not even know they have it, and the irreparable damage to the optic nerve happens over time. However, in angle-closure glaucoma (also known as narrow-angle glaucoma) the pressure in the eye builds so fast that debilitating symptoms come on quickly and require prompt treatment.
In primary open-angle glaucoma, which we discussed on the previous page, the aqueous humor that nourishes the eyes reaches the trabecular meshwork located at an angle where the cornea and iris meet, but it can't drain through the meshwork. As you may be able to tell from the name, the aqueous humor can't even reach that point in angle-closure glaucoma, because the angle is closed or blocked. When this happens, the aqueous humor causes a sudden increase in eye pressure that brings on symptoms like blurred vision, headaches and nausea.
In most people, the angle where the cornea and iris meet is at about 45 degrees [source Glaucoma Foundation]. In certain people, the angle is narrower, which puts the iris very close to that crucial drain of the trabecular meshwork. The angle can also close when the pupil dilates, because it puts the lens and iris in close contact and blocks the meshwork. That's why many cases of angle closure glaucoma come when a person is stressed or in a dark room -- both of those situations cause the pupils to dilate.
When these symptoms come on, a person should seek emergency treatment to prevent blindness. But before we go over glaucoma treatment, let's examine a few other less common types of glaucoma:
- Congenital glaucoma occurs when children are born with a defect in the angle where the trabecular meshwork is located, resulting in sensitivity in the eye, cloudy eyes and excessive tears.
- In exfoliative glaucoma, material from other parts of the eye floats away and clogs the drain for the aqueous humor. Scientists believe that a defect in a single gene leads to this type of glaucoma [source: Wade].
- Similarly, pigmentary glaucoma occurs when pieces of pigment break off from the iris and block the meshwork drain.
- Neovascular glaucoma is linked to diabetes.
- Trauma-related glaucoma occurs after an eye injury.
How is glaucoma treated? Find out on the next page.
Glaucoma Diagnosis and Treatment
Those most at risk for glaucoma include people over the age of 60, blacks over the age of 40, those with a family history of the disease and folks who have conditions like high blood pressure and diabetes. These groups should have annual eye exams starting at age 40, while everyone else should strive to have an eye exam every two to four years [source: Bain]. An instrument known as a tonometer can measure intraocular pressure, while a pachymeter measures the thickness of the central cornea, which can help an optometrist determine the intraocular pressure that's reasonable for the patient.
However, as we mentioned, some forms of glaucoma present themselves without any advanced intraocular pressure, which is why it's important to have an eye exam that includes dilation of the pupils. When the pupils are dilated, the doctor can use an ophthalmoscope to look directly through the eye to the optic nerve to check for signs of damage.
With primary open-angle glaucoma, treatment usually begins with eye drops, which work either to curtail the production of fluid in the eye or to assist the meshwork in draining more fluid. The goal is to lower intraocular pressure, thus delaying or completely preventing the onset of glaucoma, but eye drops can't cure damage that is already done. These eye drops have the potential to react adversely to other medications, so, as always, let your doctor know what other medicines you might be taking.
Sometimes pills may be used if eye drops fail to halt the damage to the optic nerve, but more often, doctors will turn to laser treatment. Lasers can be used to create tiny openings in the trabecular meshwork that allow the aqueous humor to drain. This procedure will typically help for a few years, but further treatment may be required down the road. If you go to the emergency room with a case of angle-closure glaucoma (the emergency form of glaucoma that we mentioned on the previous page), doctors may recommend this therapy.
The last resort for many doctors is a surgery known as trabeculectomy, in which a new drainage system for the eye is created. During the surgery, doctors cut a new flap into the eye so the fluid can leave the eye. Surgery is how congenital glaucoma is treated and is sometimes an option for angle-closure glaucoma as well.
Because there's no cure for glaucoma, early diagnosis and treatment are key to protecting vision. Scientists are hard at work, though, on future diagnostic and treatment tools that might include a vaccine, optic nerve regeneration and smart contact lenses that monitor the condition.
Related HowStuffWorks Articles
- "About Glaucoma." Glaucoma Foundation. (April 13, 2009)http://www.glaucomafoundation.org/about_glaucoma.htm
- Bain, Julie. "As Glaucoma Treatment Advances, Vision is Saved." New York Times. Dec. 25, 2001. (April 13, 2009)http://www.nytimes.com/2001/12/25/health/as-glaucoma-treatment-advances-vision-is-saved.html?scp=2&sq=glaucoma&st=cse
- Bakalar, Nicholas. "Care of the Eyes Demands Diligence." New York Times. May 13, 2008. (April 13, 2009)http://query.nytimes.com/gst/fullpage.html?res=9C00E3DC143BF930A25756C0A96E9C8B63
- Barras, Colin. "Smart contact lens feels the pressure of glaucoma." New Scientist. July 9, 2008. (April 13, 2009)http://www.newscientist.com/article/dn14285-smart-contact-lens-feels-the-pressure-of-glaucoma.html
- "Glaucoma." Mayo Clinic. July 17, 2008. (April 13, 2009)http://www.mayoclinic.com/health/glaucoma/DS00283
- "Glaucoma Facts and Stats." Glaucoma Research Foundation. Jan. 12, 2009. (April 13, 2009)http://www.glaucoma.org/learn/glaucoma_facts.php
- Singer, Natasha. "Love the Long Eyelashes. Who's Your Doctor?" New York Times. Jan. 14, 2009. (April 13, 2009) http://query.nytimes.com/gst/fullpage.html?res=9C06EEDB133FF937A25752C0A96F9C8B63
- Wade, Nicholas. "Study Finds Genetic Key to a Kind of Glaucoma." New York Times. Aug. 10, 2007. (April 13, 2009)http://www.nytimes.com/2007/08/10/science/10eye.html?_r=1
- "What You Should Know." National Eye Institute, National Institutes of Health. (April 13, 2009)http://www.nei.nih.gov/glaucoma/content/english/know.asp