- Glaucoma is an eye disease that is often associated with elevated intraocular pressure. Damage to the eye (optic) nerve can lead to loss of vision and even blindness.
- Glaucoma is the leading cause of irreversible blindness in the world.
- Glaucoma usually causes no symptoms early in its course. At this 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 develop 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 angle-closure glaucoma, which may be either a sudden (acute) condition or a chronic disease.
- Damage to the optic nerve and impairment of vision from glaucoma is irreversible.
- Several painless tests that determine the intraocular pressure, the optic nerve and drainage angle status, and visual fields are used to diagnose the presence of glaucoma and monitor its progression.
- Glaucoma is usually treated with eye drops, although laser treatment 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 are crucial to preserving sight in people with glaucoma.
Glaucoma is the term applied to a group of eye diseases that gradually lose vision by permanently damaging the optic nerve. This nerve transmits visual images to the brain. The leading cause of irreversible blindness, glaucoma often produces no symptoms until it is too late, and vision loss has begun.
Glaucoma is a disease of the major nerve of vision, called the optic nerve. The optic nerve receives light-generated nerve impulses from the retina. It transmits these to the brain, where we recognize those electrical signals as vision. Glaucoma is characterized by a particular pattern of progressive damage to the optic nerve that generally begins with a slight loss of side vision (peripheral vision). If glaucoma is not diagnosed and treated, it can progress to loss of central vision and blindness.
Glaucoma is usually, but not always, associated with elevated pressure in the eye (intraocular pressure). Generally, this elevated eye pressure leads to damage to the eye (optic) nerve. In some cases, glaucoma may occur in the presence of normal eye pressure. This form of glaucoma is believed to be caused by poor blood flow regulation to the optic nerve.
Worldwide, glaucoma is the second leading cause of irreversible blindness. As many as 6 million individuals are blind in both eyes from this disease. In the United States alone, according to one estimate, more than 3 million people have glaucoma. As many as half of these individuals with glaucoma may not know that they have the disease. They are unaware of the disease’s presence because glaucoma initially causes no symptoms. The subsequent loss of side vision (peripheral vision) is usually not recognized.
Causes of 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, located in the back of the eye, is the eye’s main visual nerve. This nerve transmits the images we see back to the brain for interpretation. The eye is firm and round, like a basketball. A pressure maintains its tone and shapes within the eye (the intraocular pressure), ranging between 8 millimeters (mm) and 22 mm of mercury. When the pressure is too low, the eye becomes softer, while elevated pressure causes the eye to become harder. The optic nerve is the most susceptible part of the eye to high pressure. The nerve’s delicate fibers are easily damaged either by direct pressure on the nerve or decreased blood flow.
The front of the eye is filled with a clear fluid called the aqueous humor, which provides nourishment to the front of the eye’s structures. This fluid is continuously produced 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 drainage channels called the trabecular meshwork. These channels are located at what is called the drainage angle of the eye. This angle is where the clear cornea, which covers the front of the eye, attaches to the iris’ base (root or periphery), 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 drainage angle of the eye.
In most people, the drainage angles are wide open, but they can be narrow in some individuals. For example, the usual angle is about 45 degrees, whereas a narrow-angle is about 25 degrees or less. After exiting through the trabecular meshwork in the drainage angle, the aqueous fluid then drains into tiny blood vessels (capillaries) into the main bloodstream. The aqueous humor should not be confused with tears produced by the lacrimal glands under the upper eyelid outside of the eyeball itself.
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 sink’s pressure 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 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 adequately remove the increased fluid, the pressure builds up, and optic-nerve damage may result.
Risk Factors of Glaucoma
Glaucoma is often called “the sneak thief of sight.” As already mentioned, in most cases, the intraocular pressure can build up and destroy sight without causing apparent symptoms. Thus, awareness and early detection of glaucoma are vital because they can usually be successfully treated when diagnosed early. While everyone is at risk for glaucoma, certain people are at a much higher risk and need to be checked more frequently by their eye doctor. The significant risk factors for glaucoma include the following:
- Age over 45 years
- Family history of glaucoma
- Black racial ancestry
- History of elevated intraocular pressure
- Decrease in corneal thickness and rigidity
- 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 drainage angles, which predispose them to be acute [sudden] attacks of angle-closure glaucoma.)
Different Types of Glaucoma
There are two main types of glaucoma. However, most can be classified as either open-angle glaucomas, which are usually conditions of extended duration (chronic). Or angle-closure (closed-angle) glaucomas, which include conditions occurring both suddenly (acute) and over a long period (chronic). Primary glaucoma is not a result of another eye disease. In contrast, secondary glaucoma is a result of other eye conditions, trauma, or medication. Glaucomas usually affects both eyes, but the disease can progress more rapidly in one eye than in the other. The involvement of just one eye occurs only when the glaucoma is brought on by factors such as a prior injury, inflammation, or the use of steroids only in that eye.
Open- Angle glaucoma:
Primary open-angle glaucoma (POAG) is by far the most common type of glaucoma. Moreover, its frequency increases significantly with age, and it is a chronic, not acute, disease. This increase occurs because the drainage mechanism gradually may become clogged secondary to aging, even though the drainage angle is open. 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, since the resulting loss of vision starts on the side (peripherally), people are usually unaware of the problem until the loss encroaches near or into their central visual area. This type of glaucoma is primary because its cause cannot be attributed to any noticeable structural changes within the eye.
Normal-tension (pressure) glaucoma or low tension glaucoma are variants of primary chronic open-angle glaucoma that are being recognized more frequently than in the past. This type of glaucoma 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 below average. This type of glaucoma can be diagnosed by the eye doctor’s repeated examinations to detect the nerve damage or visual field loss.
Congenital glaucoma (infantile glaucoma) is a relatively rare, inherited type of open-angle glaucoma. In this condition, the drainage area is not adequately developed before birth. This results in increased pressure in the eye that can lead to the loss of vision from optic nerve damage and an enlarged eye. The eye of a young child enlarges in response to increased intraocular pressure because it is more pliable than an adult’s eye. Early diagnosis and treatment with medication and or surgery are critical in these infants and children to preserve their sight.
Secondary open-angle 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 secondary open-angle 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 a critical element in the drainage system of the eye. Finally, the blocked drainage system leads to elevated intraocular pressure, which results in damage to the optic nerve.
Exfoliative glaucoma (pseudoexfoliation or PXE) is another type of glaucoma that can occur with either open or closed angles. Deposits of flaky material characterize this type of glaucoma on the lens’s front surface (anterior capsule) and in the eye’s angle. The accumulation of this material in the angle is believed to block the eye’s drainage system and raise the eye pressure. While this type of glaucoma can occur in any population, it is more prevalent in older people and Scandinavian descent. It has recently been shown to be often associated with hearing loss in older people.
Angle-closure glaucoma (closed-angle glaucoma) is a less common form of glaucoma in the Western world but is extremely common in Asia. Angle-closure glaucoma may be acute or chronic. The common element in both is that a portion of or the entire drainage angle becomes anatomically closed. The aqueous fluid within the eye cannot reach all or part of the trabecular meshwork. In acute angle-closure glaucoma, the patient’s intraocular pressure, which is ordinarily normal, can suddenly go up very suddenly (acutely). This sudden pressure increase occurs because the drainage angle becomes closed and blocks off all the drainage channels. This type of glaucoma can occur when the pupil dilates (widens or enlarges). As a result, the iris’s peripheral edge can become bunched up against its corneal attachment, thereby causing the drainage angle to close. Thus, angle-closure glaucoma is the difficulty with access to 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. In chronic open-angle glaucoma, portions of the drainage angle remain closed over a long time and damage the drainage system. As more and more areas become closed, the pressure within the eye rises, often over months or years.
People with small eyes are predisposed to developing angle-closure glaucoma because they tend to have narrow drainage angles. Little eyes are not apparent from their appearance, but an eye doctor can measure the eye’s size. Thus, farsighted individuals of Asian descent may have smaller eyes, narrow drainage angles, and an increased risk of developing angle-closure glaucoma. Furthermore, this condition may be acutely triggered by medications that can dilate the pupils. These agents can be found in individual eye drops, cold remedies, citalopram (Celexa), other selective serotonin reuptake inhibitors, topiramate (Topamax), or patches used to prevent seasickness. This condition can also occur spontaneously in a darkened room or a movie theater when 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.)
An acute angle-closure glaucoma attack may be associated with severe eye pain and headache, a red (inflamed) eye, nausea, vomiting, and blurry vision. Also, 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.
Symptoms and Signs of Glaucoma
Patients with open-angle glaucoma and chronic angle-closure glaucoma, in general, have no symptoms early in the course of the disease. Visual field loss (side vision loss) is not a symptom until late in the disease course. Rarely patients with fluctuating intraocular pressure levels may have haziness of vision and see haloes around lights, especially in the morning.
On the other hand, acute angle-closure symptoms are often extremely dramatic, with the rapid onset of severe eye pain, headache, nausea and vomiting, and visual blurring. Occasionally, nausea and vomiting exceed the ocular symptoms to the extent that an ocular cause is not considered when attempting to diagnose.
The eyes of patients with open-angle glaucoma or chronic angle-closure glaucoma may appear normal in the mirror or to family or friends. Some patients get slightly red eyes from the regular use of eye drops. On examining the patient, the ophthalmologist may find elevated intraocular pressure, optic nerve abnormalities, or visual field loss in addition to other less common signs.
The eyes of patients with acute angle-closure glaucoma will appear red, and the pupil of the eye may be large and nonreactive to light. The cornea may appear cloudy to the naked eye. The ophthalmologist will typically find decreased visual acuity, corneal swelling, highly elevated intraocular pressure, and a closed drainage angle.