Eyes

18 May

Eyes
The eye is the organ of sight. Light rays enter the eye through the pupil (the cir- cular opening in the center of the iris). The cornea (the tough, transparent, dome- shaped covering of the front of the eyeball) and the lens (the transparent, internal optical component of the eye) focus the light rays on the retina (the light- sensitive membrane that lines the back of the eye) to form an image. The image is converted to electrical impulses that move along the optic nerves to the visual cortex (the area of the brain that is concerned with vision), where the image is perceived.

Although many vision problems are minor and may be easy to treat, others may cause serious complications that, if left untreated, can lead to vision loss or even blindness. Most vision problems are detected during a routine eye exami- nation. That is why it is important for you to have regular eye examinations, especially as you get older.

Glaucoma

There are two major types of glaucoma—chronic open-angle glaucoma and acute closed-angle glaucoma. Chronic open-angle glaucoma is the most com- mon. It usually develops gradually over a number of years. In chronic open- angle glaucoma, the normal pressure of the fluid in the eyes slowly rises. At the front of your eyes lies a small space known as the anterior chamber. A clear fluid called the aqueous humor flows in and out of the chamber to deliver nutrients to nearby tissues and remove wastes. In people with chronic open-angle glaucoma, the drainage angle (the channel through which fluid leaves the eyeball) does not function normally and the fluid does not drain properly. As the fluid builds up, the pressure inside the eyes increases, potentially causing irreversible damage to the optic nerve. Because it transmits visual images to the brain, damage to the optic nerve produces vision loss that can result in blindness.

In acute closed-angle glaucoma, the drainage angle becomes completely blocked and there is a sudden, very high increase in pressure inside the eyes that can quickly lead to blindness. In a less common type of glaucoma, called normal tension glaucoma, the optic nerve is damaged, even though pres- sure inside the eyes is within the normal range.

Warning Signs of Acute Glaucoma
C
hronic open-angle glaucoma, the most common  type  of  glaucoma,  develops gradually and usually produces no symptoms until blind spots begin to appear in peripheral (side) vision. Acute closed-angle glaucoma occurs  suddenly  and  causes  the  following symptoms:

•   blurred vision

•   severe eye pain

•   headaches

•   halos or rainbows around lights

•   nausea and vomiting

Acute closed-angle glaucoma is a medical emergency that requires immediate medical treatment. If you suddenly experience any of these symptoms, call your doctor immedi- ately or go directly to the nearest hospital emergency department. Do not delay. If not treated promptly, acute closed-angle glau- coma can cause blindness.

This type of glaucoma is poorly understood.

About 3 million people in the United States have glaucoma; it is the third most common cause of blindness in Americans. Anyone can develop the disorder, but some people are at higher risk than others, including people with a family history of glaucoma; people who are near- sighted, have diabetes, or are over age 60; and African Americans over age 40. In fact, glaucoma is five times more likely to occur in African Americans than in whites, and the disease causes blindness more often in African Americans.

Chronic open-angle glaucoma usually pro- duces no symptoms until the optic nerve has been damaged. Blind spots may gradually develop, especially in the peripheral (side) vision. Objects in the front of the field of vision still may be seen clearly, but those at the side may be missed. As the disease progresses, the person’s field of vision becomes increasingly narrow until total blindness occurs. Acute closed-angle glaucoma comes on suddenly and usually without warning (see box). Symptoms may include blurred vision, severe eye pain, headache, halos around lights, nausea, and vomiting. This type of glaucoma is a medical emergency that requires immediate medical treatment.

Because chronic open-angle glaucoma causes no symptoms, it is usually detected during a routine eye examination performed by an ophthalmologist, a physician who specializes in treating disorders of the eyes. The doctor will examine your eyes to determine if the disease has affected them. He or she also will test your peripheral vision to determine the extent and the stability or pro- gression of any damage caused by glaucoma. This is done by performing a visual field test, in which you cover one eye and look straight ahead while an object is shown in different areas of your field of vision. You will be instructed to give a response when you see the object. Your responses will be recorded and used to evaluate your field of vision.

In a computerized visual field test, you sit facing a screen, place your chin on a chin rest, and press a button each time you see a tiny flashing light. A computer printout of your responses allows your doctor to evaluate your field of vision. A similar test that does not use a computer can also be performed.

To measure the pressure inside your eyes, the doctor will perform a test called applanation tonometry. After he or she puts a drop of a local anesthetic on each cornea, the doctor puts a drop of an orange fluid called fluorescein in each eye. Then the doctor gently places an instrument called a tonometer against the cornea to measure pressure in each eyeball. The test is safe and painless. You will not feel the tonometer against your eyeball, but you will see a bright blue circle of light moving toward your eye. The test takes only a few seconds. (In a similar, though less accurate, procedure called air tonometry, a gentle puff of air is directed onto the cornea to measure the pressure inside your eyes. Anesthetic is not needed because the tonometer does not touch the cornea.) The doctor then places eyedrops in your eyes to dilate (widen) the pupils. Once the pupils are dilated, the doctor can examine the insides of your eyes with an ophthalmoscope (a handheld viewing instrument that projects a very bright light onto the back of the eye) to assess the condition of the optic nerves. After the examination your eyes will be sensitive to light for a while, so try to arrange in advance for some- one to drive you home.

Additional tests may include gonioscopy, in which the ophthalmologist anes- thetizes the eyes with eyedrops and places a special contact lens called a gonio- scope on the cornea to examine the drainage angle of each eye for any changes or signs of blockage. The doctor also may take special photographs (called disc photos) of the optic nerves at various intervals so that he or she can monitor any changes or damage due to progression of the disease.

Glaucoma usually cannot be cured because damage to the optic nerve is irre- versible. However, the disease can be controlled with medication and surgery. Medication prescribed to treat glaucoma is usually given as eyedrops. Some glaucoma medications reduce the pressure in the eyes by slowing the flow of

fluid into the eyes. Other medications help to improve fluid drainage. Although regular use of these medications will control the fluid pressure inside your eyes, the drugs may lose their effectiveness over time. Also, some medications may cause unwanted side effects. In such cases, the ophthalmologist may alter the dosage, change medication, or suggest other ways to solve the problem. You will need to take the medication for the rest of your life.

The goal of eye surgery in treating glaucoma is usually to make it easier for fluid to drain from the eye. For acute closed-angle glaucoma, the doctor per- forms a surgical procedure called an iridotomy. In this procedure, a laser (a highly concentrated, powerful beam of light) is used to make a small hole in the iris to relieve pressure inside the eyeball. The procedure is brief and is usually performed in the doctor’s office or in an outpatient facility. For chronic open- angle glaucoma that cannot be controlled with medication, the doctor performs laser surgery to change the structure of the drainage angle to allow for better fluid drainage. This type of surgery is also performed in the doctor’s office or in an outpatient facility. Over time, the effects of this laser surgery may wear off, and additional treatment may be needed. Conventional eye surgery in an operat- ing room may be performed for people whose glaucoma cannot be controlled with medication or laser surgery.

Early detection and treatment of glaucoma is the best way to control the dis- order. If you are at high risk for developing glaucoma and are 40 or older, be sure to have your eyes thoroughly examined by an ophthalmologist at least every 2 years.

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