TYPES OF GLAUCOMA
PRIMARY OPEN-ANGLE GLAUCOMA (POAG)
This is the most common form of glaucoma in the world.
The term refers to the specific way in which the iris meets the cornea, forming an angle that is wide open. There are typically no symptoms associated with early POAG. The pressure in the eye slowly rises and the cornea adapts without swelling. Because it is painless, patients often do not realize they are slowly losing vision until the later stages of the disease. By the time vision is impaired, the damage is irreversible.
In POAG, there is no visible abnormality of the trabecular meshwork. It is believed that something is wrong with the ability of the cells in the trabecular meshwork to carry out their normal function, or there may be fewer cells present, as a natural result of aging, inflammation or damage. Some believe it is due to a structural defect of the eye’s drainage system. Others believe there is an enzymatic problem. Genetic factors are also known to contribute. These theories, and others, are currently being studied at research centers across the world.
Elevated intraocular pressure (IOP) is the most important risk factor for glaucoma. Eye pressure is measured in millimeters of mercury (mm Hg).The average IOP in a normal population is 14 -16 millimeters of mercury. But pressure up to 21 mm Hg may be within normal range. A pressure of 2 2 is considered suspicious and possibly abnormal. However, not all patients with elevated IOP develop glaucoma-related eye damage. Conversely, some patients will develop glaucoma with normal pressures. What causes one person to develop damage while another does not is another topic of active research.
POAG is a chronic, progressive disease. Once a sufficient number of optic nerve cells are destroyed, blind spots begin to form in the field of vision. These blind spots usually develop first in the peripheral field of vision, the outer sides of the field of vision. In later stages, central vision is affected. Once visual loss occurs, it is irreversible because to date optic nerve cells cannot be restored. So it’s crucial that your eye doctor detect glaucoma in its earliest stages – before any visual damage occurs. The treatment for POAG is to lower the IOP, initially by medication. Keeping the IOP under control is the key to preventing loss of vision from glaucoma.
Normal-tension glaucoma, also known as low-tension glaucoma, is characterized by progressive optic nerve damage and visual field loss with IOP levels that are usually considered to be within the normal range (10 -2 1 mm Hg). It should be noted that the level of IOP often does not correlate with the degree of optic nerve damage or visual field abnormality.
Normal-tension glaucoma is being increasingly diagnosed, and may account for as many as one-third of all cases of open-angle glaucoma in the western world where detection of glaucoma occurs relatively early. It is thought to be related, in part, to poor blood flow to the optic nerve, which leads to death of the cells which carry impulses from the retina to the brain. In addition, these eyes appear to be susceptible to pressure-related damage even in the high normal range, and therefore a pressure lower than normal is often necessary to prevent further visual loss. Studies suggest that sleep apnea and low blood pressure at night might be additional risk factors for normal tension glaucoma. Research is ongoing in the field of optic nerve blood flow and its role in glaucoma.
Angle-closure glaucoma affects nearly half a million people in the United States. There is a tendency for this disease to be inherited, and several members of a family will often be afflicted. It is most common in people of Asian descent and people who are far- sighted. However, people of any race can be affected. Worldwide, this is the most common type of glaucoma because it is so common among persons of Asian descent.
As mentioned earlier, the trabecular meshwork, which functions as the eye’s drain, is situated in the angle formed where the cornea meets the iris. In most people, this angle is about 4 5 degrees. In primary angle- closure glaucoma, the angle is smaller than normal. The narrower the angle, the closer the iris is to the trabecular meshwork. The ability of aqueous humor to pass between the iris and the lens on its way to the anterior chamber decreases, causing fluid and pressure to build up behind the iris, which further narrows the angle. If the pressure becomes sufficiently high, the iris is forced against the trabecular meshwork, blocking drainage, similar to putting a stopper over the drain of a sink. When this space becomes completely blocked, an angle- closure glaucoma attack (acute glaucoma) results.
ACUTE ANGLE-CLOSURE ATTACK
Unlike POAG, where the IOP increases slowly, in acute angle- closure, it increases suddenly. This rapid rise can occur within a matter of hours and become very painful. Symptoms of acute angle- closure glaucoma may include headaches, eye pain, nausea, vomiting, halos around lights at night, and very blurred vision.
An acute attack is a medical emergency. If treatment is delayed, damage to the optic nerve may occur quickly and cause permanent vision loss. Scarring of the trabecular meshwork may also occur and result in chronic glaucoma which is much more difficult to control. Cataracts may also develop.
Many of these sudden attacks occur in darkened rooms, such as movie theaters, or in other darkened environments which cause the pupil to dilate, or increase in size. When this happens, there is maximum contact between the eye’s lens and the iris, further narrowing the angle. The pupil also dilates when one is excited or anxious, so these attacks can occur during periods of stress. Medications that dilate the pupil (for example, anti- depressants, cold medications, antihistamines, and some medications to treat nausea) can also lead to an attack.
An acute attack may be stopped with a combination of drops which constrict the pupil and help reduce the eye’s fluid production. Soon after the IOP has dropped to a safe level, your ophthalmologist will perform a laser iridotomy to make a small opening in the iris allowing the fluid to flow more freely. Since it is common for both eyes to suffer from narrowed angles, operating on the unaffected eye is done as a preventive measure.
With routine examinations using a technique called gonioscopy, patients with narrow angles can be warned of early symptoms so that they can seek immediate treatment.
Pigmentary glaucoma is a type of inherited open-angle glaucoma which develops more frequently in men than women. White people are more susceptible than other races and it most often begins in individuals in their 2 0 s and 3 0 s. This is the only type of glaucoma that may actually dissipate as we age. Nearsighted patients are more typically afflicted and the anatomy of the eye appears to play a key role.
Myopic (nearsighted) eyes have a concave-shaped iris which creates an usually wide angle. This causes the pigment layer of the iris to rub on the lens, causing the iris pigment to shed into the aqueous humor and onto neighboring structures, such as the trabecular meshwork. When pigment is released into the anterior chamber, the condition is called pigment dispersion syndrome. Most patients with pigment dispersion will not develop pigmentary glaucoma.
However, the pigment may plug the pores of the trabecular meshwork, causing it to clog, and thereby increasing the IOP. If the IOP is high and the optic nerve is damaged, then pigmentary glaucoma is diagnosed.
Medical therapy and laser trabeculoplasty are often effective in lowering the pressure in these patients. Laser iridotomy is currently being used in some centers to change the configuration of the iris and slow the release of pigment. This preventative step will change the anatomy of the iris but has not yet been shown to be effective in treating pigmentary glaucoma.
Exfoliation syndrome (XFS) is an age-related systemic disease characterized by the production and progressive accumulation of a whitish material in many ocular tissues and is the most common identifiable cause of open-angle glaucoma worldwide. XFS is a cause of open-angle glaucoma, angle-closure glaucoma, and cataract. It is accompanied by an increase in serious complications at the time of cataract extraction. This exfoliation material is rubbed off the lens by movement of the iris and at the same time, pigment and exfoliation material clog the trabecular meshwork, leading to IOP elevation, sometimes to very high levels.
About 2 5 percent of persons with XFS develop elevated IOP and one- third of these develop glaucoma. However, if you have XFS syndrome, your chances of developing glaucoma are about six times higher than if you don’t.
Exfoliation glaucoma behaves more aggressively than open- angle glaucoma and can be more difficult to control.
The exfoliation material often appears in one eye long before the other. If you have glaucoma in one eye only, this is most likely the cause. It can be detected before the glaucoma develops, so you can be more carefully observed and minimize your chances of vision loss.
An increasing list of associations with cardiovascular and cerebrovascular diseases makes XFS a condition of general medical importance. Recently described associations include stroke, cardiovascular dysfunction, Alzheimer’s disease, and hearing loss. The recently discovered genetic abnormalities in the lysyl oxidase gene, which is responsible for the formation and maintenance of elastic tissue, might turn out to explain these other links.
A blow to the eye, chemical burn, or penetrating injury may all lead to the development of glaucoma, either acute or chronic. This can be due to a mechanical disruption or physical change within the eye’s drainage system. It is therefore crucial for anyone who has suffered eye trauma to have their eyes examined at regular intervals throughout their life.
Several different drugs have the potential to cause the elevation of IOP. Steroid-induced glaucoma is a form of open-angle glaucoma that usually is associated with topical (eye drops and ointments) or periocular (injection into, near or beyond the eyeball) steroid use, but may develop with systemic (oral, inhaled, intravenous, injected) corticosteroid usage or exposure.
This type of glaucoma resembles POAG, but is of a more sudden onset. IOP elevations usually occur within a few weeks of beginning steroid therapy. In the majority of cases, the IOP lowers spontaneously within a few weeks to months upon stopping the steroid use. The effects of steroids on IOP depend on whether the patient has glaucoma. Individuals with POAG are far more susceptible to steroid-related elevations in IOP than individuals without glaucoma. In steroid induced glaucoma, the IOP increase is usually short term, but the longer the exposure, the greater the chance that the elevation will continue. The bottom line: steroids should be used cautiously and patients should consult their ophthalmologists about their usage and should have their eyes examined and IOP measured regularly.
Childhood glaucoma is an unusual eye disease and significant cause of childhood blindness. It is caused by a disease-related abnormal increase in IOP. The multiple potential causes fall into one of two categories and may be
primary or secondary to some other disease process. Primary congenital glaucoma results from abnormal development of the ocular drainage system. It occurs in about 1 out of 10,000 births in the United States and is the most common form of glaucoma in infants. Secondary glaucomas result from disorders of the body oreyeandmayormaynotbe genetic. Both types may be associated with other medical diseases. Ten percent of primary congenital glaucomas are present at birth, and 8 0 percent are diagnosed during the first year of life. The pediatrician or family first notices eye signs of glaucoma, including clouding and/or enlargement of the cornea. The elevated IOP can cause the eyeball itself to enlarge and injury to the cornea. Important early symptoms of glaucoma in infants and children are poor vision, light sensitivity, tearing, and blinking.
Pediatric glaucoma is treated differently than adult glaucoma. Most patients require surgery and this is typically performed early. Approximately 8 0 -9 0 percent of babies who receive prompt surgical treatment and ongoing care will do well. When childhood glaucoma is not recognized and treated promptly more permanent visual loss will result.
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