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Glaucoma
Glaucoma
Medical Author: Lawrence J. Schwartz, M.D.
Medical Editor: Leslie J. Schoenfield, M.D., Ph.d.
What is glaucoma?
Glaucoma is usually, but not always, associated with elevated pressure in the eye ( intraocular pressure ). This pressure leads to damage to the eye (optic) nerve. Actually, glaucoma is now considered a disease of the optic nerve (optic neuropathy) that causes a loss of vision, usually in both eyes (bilateral). This loss often begins with a subtle decrease in side (peripheral field) vision. If the glaucoma is not diagnosed and treated, it may progress to loss of central vision and blindness.
How common is glaucoma?
Worldwide, glaucoma is the leading cause of irreversible blindness. In fact, as many as 6 million individuals are blind in both eyes from this disease. In the United States alone, according to one estimate, over 3 million people have glaucoma. As many as half of the individuals with glaucoma, however, may not know that they have the disease. The reason they are unaware is that glaucoma initially causes no symptoms, and the loss of vision on the side (periphery) is hardly noticeable.
What causes glaucoma?
Elevated pressure in the eye is the main factor leading to glaucomatous damage to the eye (optic) nerve. Glaucoma with normal intraocular pressure is discussed below in the section on the different types of glaucoma. The optic nerve, which is located in back of the eye, is the main seeing nerve for the eye. This nerve transmits the images we see back to the brain for interpretation. The eye is firm and round, like a basketball. Its tone and shape are maintained by a pressure within the eye (the intraocular pressure), which normally ranges between 8 and 22 mm (millimeters) of mercury. When the pressure is too low, the eye becomes softer, while a too high pressure causes the eye to become harder. It turns out that the optic nerve is the most susceptible part of the eye to high pressure because the delicate fibers in this nerve are easily damaged.
The front of the eye is filled with a clear fluid called the aqueous humor, which provides nourishment to the structures in the front of the eye. This fluid is produced constantly by the ciliary body , which surrounds the lens of the eye. The aqueous humor then flows through the pupil and leaves the eye through tiny channels, called the trabecular meshwork. These channels are located at what is called the filtering, or drainage angle of the eye. This angle is where the clear cornea , which covers the front of the eye, attaches to the base (root or periphery) of the iris , which is the colored part of the eye. The cornea covers the iris and the pupil, which are in front of the lens. The pupil is the small, round, black-appearing opening in the center of the iris. Light passes through the pupil, on through the lens, and to the retina at the back of the eye. Please see the figure, which is a diagram that shows the filtering angle of the eye.
Legend for figure: This diagram of the front part of the eye is in cross section to show the filtering, or drainage angle. This angle is between the cornea and the iris, which join each other right where the drainage channels (trabecular meshwork) are located. The arrow shows the flow of the aqueous fluid from the ciliary body, through the pupil, and into the drainage channels. This figure is recreated from "Understanding and Treating Glaucoma," a Human anatomy Board Book by Tim Peters and Company Inc., Gladstone N. J..
In most people, the filtering angles are wide open, although in some individuals, they can be narrow. For example, the usual filtering angle is about 45 degrees, whereas a narrow angle is about 25 degrees or less. After exiting through the trabecular meshwork in the filtering angle, the aqueous fluid then drains into tiny blood vessels (capillaries) into the main bloodstream. The aqueous humor should not be confused with tears, which are produced by a gland just outside of the eye.
This process of producing and removing the fluid from the eye is similar to that of a sink with the faucet always turned on, producing and draining the water. If the sink's drain becomes clogged, the water may overflow. If this sink were a closed system, as is the eye, and unable to overflow, the pressure in the sink would rise. Likewise, if the eye's trabecular meshwork becomes clogged or blocked, the intraocular pressure may become elevated. Also, if the sink's faucet is on too high, the water may overflow. Again, if this sink were a closed system, the pressure within the sink would increase. Likewise, if too much fluid is being produced within the eye, the intraocular pressure may become too high. In either event, since the eye is a closed system, if it cannot remove the increased fluid, the pressure builds up and nerve damage results.
What are the risk factors for glaucoma?
Glaucoma is often called "the sneak thief of sight." This is because, as already mentioned, in most cases, the intraocular pressure can build up and destroy sight without causing obvious symptoms. Thus, awareness and early detection of glaucoma are extremely important because this disease can be successfully treated when diagnosed early. While everyone is at risk for glaucoma, certain persons are at a much higher risk and need to be checked more frequently by their eye doctor. The major risk factors include: - Age over 45 years
- Family history of glaucoma
- Black racial ancestry
- Diabetes
- History of elevated intraocular pressure
- Nearsightedness (high degree of myopia), which is the inability to see distant objects clearly
- History of injury to the eye
- Use of cortisone (steroids), either in the eye or systemically (orally or injected)
- Farsightedness (hyperopia), which is seeing distant objects better than close ones (Farsighted people may have narrow filtering angles, which predispose them to acute (sudden) attacks of closed angle glaucoma.)
What are the different types of glaucoma?
There are many different types of glaucoma. Most, however, can be classified as either open angle glaucomas, which are conditions of long duration (chronic), or closed angle glaucomas, which are conditions occurring suddenly (acute). The glaucomas usually affect both eyes, but the disease can progress more rapidly in one eye than in the other. Involvement of just one eye occurs only when the glaucoma is brought on by factors such as a prior injury or the use of steroids in that eye.
Open angle glaucoma Chronic open angle glaucoma (COAG) is by far the most common type of glaucoma. Moreover, its frequency increases greatly with age. This increase occurs because the drainage mechanism gradually may become clogged with aging. As a consequence, the aqueous fluid does not drain from the eye properly. The pressure within the eye, therefore, builds up painlessly and without symptoms. Furthermore, as mentioned previously, since the resulting loss of vision starts on the side (peripherally), people are usually not aware of the problem until the loss encroaches on their central visual area.
Normal tension (pressure) glaucoma is a variant of open angle glaucoma that is being recognized more frequently than in the past. It is thought to be due to decreased blood flow to the optic nerve. This condition is characterized by progressive optic nerve damage and loss of peripheral vision (visual field) despite intraocular pressures in the normal range or even below normal. This type of glaucoma can be diagnosed by repeated examinations by the eye doctor to detect the nerve damage or the visual field loss. Currently, normal pressure glaucoma is receiving a lot of research attention because its cause and treatment are uncertain. Congenital (infantile) glaucoma is a relatively rare, inherited type of open angle glaucoma. In this condition, the drainage area is not properly developed. This results in increased pressure in the eye that can lead to the loss of vision from optic nerve damage and to an enlarged eye. The eye of a young child enlarges in response to increased intraocular pressure because it is more pliable than the eye of an adult. Early diagnosis and treatment with medicine and/or surgery are critical in these infants and children to preserve their sight.
Secondary glaucoma is another type of open angle glaucoma. It can result from an eye (ocular) injury, even one that occurred many years ago. Other causes of secondary glaucoma are inflammation in the iris of the eye (iritis), diabetes, cataracts , or, in steroid-susceptible individuals, the use of topical (drops) or systemic (oral or injected) steroids (cortisone). It can also be associated with a retinal detachment or retinal vein occlusion or blockage. (The retina is the layer that lines the inside of the back of the eye.) The treatments for the secondary glaucomas vary, depending on the cause.
Pigmentary glaucoma is a type of secondary glaucoma that is more common in younger men. In this condition, for reasons not yet understood, granules of pigment detach from the iris, which is the colored part of the eye. These granules then may block the trabecular meshwork, which, as noted above, is the drainage system of the eye. Finally, the blocked drainage system leads to elevated intraocular pressure, which results in damage to the optic nerve.
Closed angle glaucoma
Closed angle glaucoma is a relatively rare type of glaucoma. In this condition, the patient's intraocular pressure, which ordinarily is normal, can go up very suddenly (acutely). This sudden pressure increase occurs because the filtering angle becomes closed and blocks off the drainage channels. This type of glaucoma can occur when the pupil dilates (widens or enlarges). As a result, the peripheral edge of the iris can become bunched up against its corneal attachment, thereby causing the filtering angle to close. Thus, the problem in closed angle glaucoma is the difficulty with access of the eye fluid to the drainage system (trabecular meshwork). In contrast, remember that the problem in open angle glaucoma is clogging within the drainage system itself.
People with small eyes are predisposed to developing closed angle glaucoma because they tend to have narrow filtering angles. Small eyes are not obvious from their appearance, but they can be measured by an eye doctor. Thus, individuals who are farsighted or of Asian descent may have small eyes, narrow filtering angles, and an increased risk of developing closed angle glaucoma. Furthermore, this condition may be triggered by medications that can dilate the pupils. These agents can be found in certain eye drops, cold remedies, or patches used to prevent seasickness. This condition can also occur spontaneously in a darkened room or a movie theatre, wherein the pupil automatically dilates to let in more light. Sometimes, therefore, people with narrow angles are given eye drops to keep their pupils small. (See the section below on parasympathomimetic agents.)
A sudden glaucoma attack may be associated with severe eye pain and headache, a red (inflamed) eye, nausea, vomiting, and blurry vision. In addition, the high intraocular pressure leads to corneal swelling (edema), which causes the patient to see haloes around lights. Sometimes, acute glaucoma is treated with oral carbonic anhydrase inhibitors. (See the section below on these medications.) An attack of acute glaucoma, however, is usually relieved by eye surgery. In this operation, the doctor makes a small hole in the iris with a laser (laser iridotomy) to allow the fluid to resume draining into its normal outflow channels.
How is glaucoma diagnosed?
An eye doctor (ophthalmologist) can usually detect those individuals who are at risk for glaucoma (because of, for example, a narrow filtering angle or increased intraocular pressure) before nerve damage occurs. The doctor also can diagnose patients who already have glaucoma by observing their nerve damage or visual field loss. The following tests, all of which are painless, may be part of this evaluation. - Tonometry determines the pressure in the eye by measuring the tone or firmness of its surface. Several types of tonometers are available for this test, the most common being the applanation tonometer. After the eye has been numbed with anesthetic eye drops, the tonometer's sensor is placed against the front surface of the eye. The firmer the tone of the surface of the eye, the higher the pressure reading.
- Gonioscopy is done by numbing the eye with anesthetic drops and placing a special type of thick contact lens with mirrors inside on the eye. The mirrors enable the doctor to view the interior of the eye from different directions. The purpose of this test is to examine the filtering angle and drainage area of the eye. In this procedure, the doctor can determine whether the angle is open or narrow. As indicated earlier, individuals with narrow angles have an increased risk for a sudden closure of the angle, which can cause an acute glaucomatous attack. Gonioscopy can also determine if anything, such as abnormal blood vessels, might be blocking the drainage of the aqueous fluid out of the eye.
- Ophthalmoscopy is an examination in which the doctor uses a hand-held device to look directly through the pupil (the opening in the colored iris) into the eye. This procedure is done to examine the optic nerve (seen as the optic disc) at the back of the eye. Damage to the optic nerve, called cupping of the disc, can be detected in this way. Cupping, which is an indentation of the optic disc, can be caused by increased intraocular pressure. Additionally, a pale color of the nerve can suggest damage to the nerve from poor blood flow or increased intraocular pressure. Special cameras can be used to take photographs of the optic nerve to compare changes over time.
- Visual Field testing actually maps the visual fields to detect any early (or late) signs of glaucomatous damage to the optic nerve. This test can be done by having the patient look straight ahead and count the fingers shown by the examiner from the side. More typically, however, visual fields are measured by a computerized assessment. For this procedure, one eye is covered and the patient places his or her chin in a type of bowl. Then, when the patient sees lights of various intensities and at different locations, he or she pushes a button. This process produces a computerized map of the visual field.
Other, more sophisticated tests may also be employed. All of these tests need to be repeated at intervals to assess the progress of the disease and the effect of the treatment.
How often should someone be checked (screened) for glaucoma?
The American Academy of Ophthalmology's recommended intervals for eye exams are: - Age 20-29: Individuals of African descent or with a family history of glaucoma should have an eye examination every three to five years. Others should have an eye exam at least once during this period.
- Age 30 -39: Individuals of African descent or with a family history of glaucoma should have an eye examination every two to four years. Others should have an eye exam at least twice during this period.
- Age 40-64: Every two to four years.
- Age 65 or older: Every one to two years.
These routine screening eye examinations are mandatory because, as already stated, glaucoma usually causes no symptoms (asymptomatic) in its early stages. What's more, in order to preserve vision, glaucoma must be diagnosed early. Also, patients with glaucoma need to be aware that it is a lifelong disease. Moreover, to keep their vision, they must be compliant with their scheduled visits to the eye doctor and with their medication regimens.
How is glaucoma treated?
General approach
Although nerve damage and visual loss from glaucoma cannot usually be reversed, it is a disease that can generally be controlled. That is, treatment can make the intraocular pressure normal and, therefore, prevent or retard further nerve damage and visual loss. Treatment may involve the use of eye drops, pills (rarely), laser, or surgery.
In the United States, eye drops are usually used first in treating most types of open angle glaucoma. In contrast, in Europe, laser or surgery is sometimes the first choice, at least more often than it is in the United States. One or more types of eye drops may have to be taken up to several times a day to lower intraocular pressure. These drops work either by reducing the production of the aqueous fluid (shutting the faucet) or by increasing the drainage of the fluid out of the eye. Each type of therapy has its benefits and potential complications.
Medications (eye drops)
Beta-adrenergic antagonists act against, or block, adrenalin-like substances. These drops work in the treatment of glaucoma by reducing the production of the aqueous humor. For years, they have been the gold standard (to which other agents are compared) for treating glaucoma. These medications include timolol (Timoptic), levobunolol (Betagan), carteolol (Ocupress), and metipranolol (Optipranolol).
Used once or twice daily, these drops are very effective. However, side effects, such as the worsening of asthma or emphysema , bradycardia (slow heart rate), low blood pressure , fatigue, and impotence prohibit their use in some patients. Betaxolol (Betoptic) is a beta-adrenergic antagonist that is more selective in working just on the eye and, therefore, carries less risk of heart (cardiac) or lung (pulmonary) side effects than other drops of this type. Prostaglandin analogs are similar in chemical structure to the body's prostaglandins. Prostaglandins are hormone-like substances that are involved in a wide range of functions throughout the body. These drops work in glaucoma by increasing the outflow (drainage) of fluid from the eye.
The prostaglandin analog, latanoprost (Xalatan), has replaced timolol as the most commonly prescribed drops for glaucoma. Latanoprost can be used just once a day. However, to preserve its stability, this drug needs to be kept refrigerated prior to opening. This medication has fewer systemic (involving the rest of the body) side effects than timolol, but it can change the color of the iris as well as thicken and darken the eyelashes. Sometimes, it can also cause inflammation in the eye.
The FDA has recently approved another prostaglandin analog, unoprostone (Rescula). In addition to increasing the outflow of the aqueous fluid from the eye, these drops work by increasing the blood flow to the optic nerve. Moreover, unoprostone does not need to be refrigerated and has few side effects. However, it is best used twice daily and may not be as effective as the timolol-like drugs mentioned above. Adrenergic agonists are a type of drops that act like adrenalin. They work in glaucoma by both reducing the production of fluid by the eye and increasing its outflow (drainage). The most popular adrenergic agonist is brimonidine (Alphagan). This agent seems to protect the optic nerve from damage beyond just lowering the intraocular pressure. Brimonidine is as effective as timolol with fewer side effects. However, there is up to a 12% risk of local (eye) allergic reactions. Other members of this class of drops include epinephrine , dipivefrin (Propine), and apraclonidine (Iopidine).
Carbonic anhydrase inhibitors work in glaucoma by reducing the production of fluid in the eye. An eye drop form of this type of medication, dorzolamide (Trusopt), is used 2 or 3 times daily and is often used in combination (as an adjunct) with other types of drops.
Carbonic anhydrase inhibitors are otherwise used as pills (systemically) to remove fluid from the body in patients with swelling (edema) that is caused by fluid retention. Oral forms of these medications include acetazolomide (Diamox), metazolamide (Neptazine) and dichlorphenamide (Daranide). They are the only oral medications used to treat glaucoma. Their use in this condition, however, is limited due to their systemic (throughout the body) side effects, including reduction of body potassium, kidney stones, numbness in the arms and legs, fatigue, and nausea. In fact, the oral form of carbonic anhydrase inhibitors is used today in glaucoma only for the treatment of acute glaucoma. Parasympathomimetic agents, which are also called miotics because they narrow (constrict) the pupils, act by opposing adrenalin-like substances. They work in glaucoma by increasing the aqueous outflow from the eye. These drops include pilocarpine , demecarium (Humorsol), echothiophate (Phospholine iodide) and isoflurophate (Floropryl).
The parasympathomimetics have been used for many years to treat glaucoma, but are currently out of favor because they need to be used 3 to 4 times a day and produce side effects in the eye. These side effects include a small pupil, blurred vision, an aching brow, and an increased risk of retinal detachment. Currently, these medications are used in glaucoma only to keep the pupil small in patients with a narrow angle who want to try to avoid undergoing a laser iridotomy. (See the section above on closed angle glaucoma.) Several new classes of glaucoma drops are currently under development or awaiting FDA approval.
Laser or surgery
Argon laser trabeculoplasty does not cure glaucoma, but is often done instead of increasing the number of different eye drops. In some cases, it is used as the initial or primary therapy for open angle glaucoma. This procedure is a quick, painless, and relatively safe method of lowering the intraocular pressure. With the eye numbed by anesthetic drops, the laser treatment is applied through a mirrored contact lens to the angle of the eye. Microscopic laser burns are made for the purpose of causing scarring in the area. The result is that the scar tissue firms up the adjacent filtering tissue to allow fluid to better exit the drainage channels.
Laser trabeculoplasty is often done in two sessions, weeks or months apart. Unfortunately, the improved drainage as a result of the treatment may last only about two years, by which time the drainage channels tend to clog again. Because this laser treatment cannot then be repeated, the options for the patient at that time are to resume the eye drops or proceed to surgery. Glaucoma surgery (trabeculectomy), which is delicate microsurgery, can actually cure glaucoma. In this operation, a small piece of the clogged trabecular meshwork is removed to create an opening and a new filtering pathway is made for the fluid to exit the eye. For the new pathway, a small filtering bleb is created from conjunctival tissue. (The conjunctiva is the clear covering over the white of the eye.) The filtering bleb is a cyst-like raised area that is placed at the top part of the eye under the upper lid.
The new drainage system allows fluid to leave the eye, enter the bleb, and then pass into the capillary blood circulation. The surgeon sometimes creates other types of drainage systems. While glaucoma surgery is often effective, complications, such as infection or bleeding, are possible. Accordingly, surgery is usually reserved for cases that cannot otherwise be controlled.
What is in the future for glaucoma?
New eye drops will continue to become available for the treatment of glaucoma. Some drops will be new classes of agents. Other drops will combine some already existing agents into one bottle to achieve an additive effect and to make it easier and cheaper for patients to take their medication.
Many researchers are investigating the therapeutic role of neuroprotection of the optic nerve, especially in patients who seem to be having progressive nerve damage and visual field loss, despite relatively normal intraocular pressures. Animal models have shown that certain chemical mediators can reduce injury or death of nerve cells. Proving such a benefit for the human optic nerve, however, is more difficult because, for one thing, biopsy or tissue specimens are not readily available. Nevertheless, if any of these mediators in eye drops can be shown to protect the human optic nerve from glaucomatous damage, this would be a wonderful advance in preventing blindness.
The National Eye Institute (NEI) is currently sponsoring a large national clinical trial, called the Collaborative Initial Glaucoma Treatment Study (CIGTS). The goal of this study is to determine the benefits and risks of early laser treatment or surgery versus the use of eye drops in the initial treatment of glaucoma. We are looking forward to the results of this trial. In other studies, new surgical methods are being evaluated to lower the intraocular pressure more safely without significant risk of damage to the eye or loss of vision.
Finally, increased efforts to enhance public awareness of glaucoma, national free screenings for those individuals at risk, and earlier diagnosis and treatment are our best hopes to eliminate the ravages of glaucoma. - Glaucoma is a disease that is often associated with elevated intraocular pressure, in which damage to the eye (optic) nerve can lead to loss of vision and even blindness.
- Glaucoma is the leading cause of blindness in the world.
- Glaucoma usually causes no symptoms early in its course, at which time it can only be diagnosed by regular eye examinations (screenings with the frequency of examination based on age and the presence of other risk factors.
- Intraocular pressure increases when either too much fluid is produced in the eye or the drainage or outflow channels (trabecular meshwork) of the eye become blocked.
- While anyone can get glaucoma, some people are at greater risk.
- The two main types of glaucoma are open angle glaucoma, which has several variants and is a long duration (chronic) condition, and closed angle glaucoma, which is a sudden (acute) condition.
- Damage to the optic nerve and impairment of vision from glaucoma are irreversible.
- Several painless tests that determine the intraocular pressure, the status of the optic nerve and drainage angle, and visual fields are used to diagnose glaucoma.
- Glaucoma is usually treated with eye drops, although lasers and surgery can also be used. Most cases can be controlled well with these treatments, thereby preventing further loss of vision.
- Much research into the causes and treatment of glaucoma is being carried out throughout the world.
- Early diagnosis and treatment is the key to preserving sight in people with glaucoma.
Last updated on 05/15/2008
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