Glaucoma: A General Overview of the Disease
What is Glaucoma?
Glaucoma is a group of eye disorders that can lead to progressive loss of vision, often without any symptoms. Visual loss results from damage to the optic nerve, which transmits signals from the eye to the brain much like an electrical cable. Once nerve fibers are damaged by glaucoma, the information they supply can no longer reach the brain. Peripheral vision is usually lost first. Because central vision is typically preserved until late in the disease and pain is usually absent, most people with glaucoma don’t realize that anything is wrong. Optic nerve damage and vision loss due to glaucoma cannot be reversed, making early diagnosis and initiation of treatment very important.
It is estimated that as much as one to two percent of the US population has open angle glaucoma, and that half of those affected have not been diagnosed. It was once believed that glaucoma is caused by high pressure within the eye, known as intraocular pressure, or IOP. While IOP plays a large role in glaucoma, it is now considered a major risk factor, though not necessarily the specific cause of the disease. The higher the pressure the more likely one is to get glaucoma. However, up to 50 percent of patients with glaucoma have normal intraocular pressures, indicating that other factors must be involved. Genetics, nutrition, blood flow, and toxins may all play a part, though details are currently not well understood. Extensive research is being conducted to identify these factors.
Types of Glaucoma
Glaucoma is usually broken down into two major categories: open angle glaucoma and closed angle glaucoma. To understand the difference, we must first know a bit about how eye pressure develops. The eye is essentially a hollow sphere filled with fluid. In the front section of the eye, this fluid is called the aqueous humor. Aqueous humor is produced by specialized cells behind the iris, the colored part of the eye. Fluid then travels through the pupil, the central opening in the iris, to enter the anterior chamber, which is the space in front of the iris and behind the cornea, the clear window on the front of the eye. The cornea and iris meet peripherally to form the drainage angle of the eye. Within this angle is a structure known as the trabecular meshwork, which is the sieve-like entrance to the drainage system of the eye. The aqueous humor passes through this mesh of tissue, exits the eye through a drainage canal, and is collected by veins to return to the bloodstream.
The physician examining your eye can see into the angle using a special mirrored lens. In open angle glaucoma, there is plenty of space between the iris and the cornea, and the trabecular meshwork is unobstructed.
Open angle glaucoma can be further divided into primary and secondary forms. In primary open angle glaucoma, despite the fact that the entrance to the drain is visibly open, there is resistance to the outflow of aqueous humor, as if the drain is clogged somewhere inside. The exact abnormality is not fully understood but is thought to be due to a defect in the trabecular meshwork. The vast majority of people with glaucoma have this form of the disease.
Secondary open angle glaucoma occurs when another ocular disease or condition causes increased resistance to the outflow of fluid. Two common types of secondary open angle glaucoma, pseudo-exfoliation glaucoma and pigmentary dispersion syndrome, are discussed in greater detail in separate articles.
In closed angle glaucoma, the entrance to the trabecular meshwork becomes blocked by the iris, which is positioned abnormally forward, toward the cornea. People with primary closed angle glaucoma often have a shorter than average eye in which all the internal structures, including the cornea and the iris, are closer together than normal. With age and the development of cataracts, the front of the eye becomes even more crowded, exacerbating this underlying issue.
This situation causes the drainage angle to become narrow and eventually to close, blocking the flow of aqueous humor from the eye. Unlike symptom-free open angle glaucoma, this condition, sometimes called “narrow angle glaucoma” or “angle closure glaucoma,” often presents suddenly, with pain, red eye, tearing, and colored haloes around lights. Vision can be lost quickly, and urgent treatment is required. The goal is to discover the patient with a narrow angle early and to provide treatment, thus preventing an attack of angle closure. As with open angle glaucoma, secondary closed angle glaucoma is caused by other diseases within the eye.
Risk Factors for Glaucoma
A number of risk factors for the development of glaucoma have been identified:
- Intraocular pressure, as already mentioned, is presently the most significant. Intraocular pressure normally runs between 9 and 21 millimeters of mercury. IOP above 21 is considered high, and with increasing pressure comes increasing risk of glaucoma. However, not all high pressure will lead to glaucoma. Furthermore, as mentioned earlier, glaucoma occurs frequently in the presence of normal levels of IOP. Therefore, intraocular pressure alone cannot be used to diagnose the disease, and must be evaluated as part of the entire clinical picture.
- Race is another significant factor. African Americans are five to six times more likely than Caucasians to be affected by the disease, which tends to be more severe and progress more quickly in this population. Latinos are also at greater risk than Caucasians. The reasons for these racial differences are not well understood, but are most likely genetic.
- Family history of glaucoma, particularly in a first degree relative, i.e. parent or sibling, increases one’s risk. The genetics of glaucoma, however, are usually not simple, and there is no way to predict which family members will be affected. At the time that this article was last updated, there have been at least 20 genes identified as causing or contributing to a form of glaucoma, with new genes being discovered fairly frequently. Within the next 5 to 10 years, genetic testing may become routinely available to aid in the diagnosis and prognosis of glaucoma.
- Age: Glaucoma is uncommon under 40, but risk increases with each decade of life.
- High blood pressure has been associated with increased intraocular pressure in some studies, but it remains unclear exactly how significant this association is clinically.
- Low blood pressure may be a risk factor for glaucoma progression, particularly in patients with low tension glaucoma, a form of open angle glaucoma in which the pressure is in the normal range, often quite low. The key seems to be the diastolic perfusion pressure- the pressure experienced by the tiny blood vessels of the optic nerve at the end of each heartbeat, measured as the diastolic blood pressure (the second number in your blood pressure measurement) minus the intraocular pressure. Diastolic perfusion pressures below 50 appear to be associated with increased risk of optic nerve damage and glaucoma. This is currently an area of very active study.
- Sleep-associated breathing disorders, such as sleep apnea, may also be a risk factor, particularly for low tension glaucoma, again due to abnormal blood flow to the optic nerves.
Other possible risk factors include diabetes and myopia (nearsightedness), however studies have provided inconsistent results as to the degree of association with these factors.
Diagnosing Glaucoma
Your physician will perform a complete medical history and eye examination in order to determine your risk factors and to search for signs of disease. The entire eye will be carefully evaluated, with special attention paid to the optic nerve, the structure damaged by glaucoma. The optic nerve is viewed through your dilated pupil using a special lens and an ophthalmic microscope. Gonioscopy may also be performed, in which a special mirrored lens in placed on your eye in order to directly examine the drainage angle and trabecular meshwork. The thickness of your cornea may also be measured, as an abnormally thick or thin cornea can affect the accuracy of intraocular pressure measurement. After your exam, if glaucoma is suspected, additional tests may be recommended, such as:
- Visual field testing is conducted to evaluate the function of the optic nerve. As glaucoma damage progresses vision is lost, typically beginning in the periphery and moving toward the center. Computerized visual field devices flash a series of lights in your central and peripheral vision in order to map out any areas of visual loss. Again, by repeating this test over a period of time, stability or progression of disease can be ascertained and treatment adjusted as necessary. Completion of this test can take anywhere between 15 and 30 minutes, depending upon the exact type of examination your doctor orders. Your eyes will not be dilated for a visual field.
- Ocular Coherence Tomography, or OCT, uses a laser to create a three-dimensional image of the optic nerve and surrounding retina, which is then analyzed by a computer. This allows your physician to better evaluate the structure of the fibers of the optic nerve, which are altered by glaucoma. Changes in the structure of the optic nerve occur early in glaucoma, often preceding any visual changes, and identification of these changes may be the best way to diagnose disease. By repeating this exam over time, the success of glaucoma treatment can be monitored. Performance of this test usually does not require dilation and takes about 10 minutes.
Unfortunately, the diagnosis is glaucoma is not always straightforward, even after all testing is performed. There is significant variation in the appearance of normal optic nerves, with many healthy nerves looking a bit suspicious for glaucoma. Because glaucoma is usually a very slowly progressive disease, it is not uncommon for patients to be watched carefully without treatment while the diagnosis of glaucoma is considered. These “glaucoma suspects” may be followed for many years before a final diagnosis is made and treatment is initiated. If suspicion is relatively low, watchful waiting avoids the potential risks and side effects of treatment.
Open Angle Glaucoma Treatment
Treatment for glaucoma is individualized for each patient’s specific condition. When required, three basic options are available, all of which serve to lower the intraocular pressure.
Initial therapy has traditionally been medications, applied topically as eye drops. Medications lower pressure by either decreasing the production of fluid within the eye or by opening the drainage system to allow more fluid out. There are numerous classes of medications available, each with its own set of side effects. Your doctor will suggest medications for you based upon their safety and efficacy, taking your general health and use of other medications into account.
Another treatment option is a laser procedure, known as selective laser trabeculoplasty, or SLT. A variation of this procedure has been safely performed since the early 1980s. The treatment is performed in the physician’s office with the patient comfortably seated at a clinical microscope. Over a period of about five minutes, the laser light is directed at the trabecular meshwork within the eye, serving to “unclog” the eye’s drain and allow more fluid to exit. Postoperatively, one can expect mild discomfort, sensitivity to light, and slightly blurred vision for a few days. There is a 60 to 90 percent success rate of lowering intraocular pressure by approximately 15 to 30 percent. Success may be impacted by a number of factors, including current use of eye medications. The treatment effect lasts, on average, two to five years, and can later be repeated if necessary. A large, randomized clinical study published in 2019 (known as the LiGHT trial) found that SLT and medication are equally effective at lowering IOP in patients who have not been previously treated for glaucoma or high IOP. The patients who underwent initial SLT maintained well-controlled IOP for 3 years without drops. Given the extremely good safety profile and efficacy of the procedure, SLT is a good first-line option for those who do not want to take daily medication for glaucoma. Your doctor will determine if you are a good candidate for this procedure based upon your eyes’ anatomy.
The final treatment option, when medications and laser therapy have either failed or are no longer tolerated, is surgery. Traditional glaucoma “filtering surgery” serves to bypass the damaged, clogged drainage system of the eye. In effect, a new drain is surgically created, allowing the aqueous humor to escape the eye without having to pass through the clogged trabecular meshwork. The most commonly performed operation, called a trabeculectomy, often produces very low eye pressures, lower than can be achieved with medication or laser. Such low pressures are often required in advanced glaucoma to prevent further loss of vision. Occasionally, another procedure, known as a tube shunt or glaucoma drainage device, may be required. In this type of surgery, a tiny plastic tube is inserted into the eye to drain fluid. The type of glaucoma and the condition of your eye will influence your physician’s decision regarding which procedure to perform. While effective and safe, surgery entails more risks than other therapies and serious complications, although rare, can occur.
Newer surgical techniques seek to avoid bypassing the eye’s natural drainage canal in favor of trying to re-establish a more normal drainage pathway through the damaged trabecular meshwork. Known collectively as ‘micro-invasive glaucoma surgery,” or MIGS, numerous procedures have been and continue to be developed. These include tiny stents that can be placed through the trabecular meshwork and into the eye’s main drainage canal, and are typically implanted at the time of cataract surgery. Other options include dilating and cleaning out the eye’s drainage system or removing sections of the trabecular meshwork to gain more direct access to the drainage system. While effective at lowering intraocular pressure, these surgical techniques generally cannot achieve pressures as low as with the previously mentioned filtering surgeries. Therefore, decisions about which surgical procedure to perform must take into account many factors, including patient age, ocular anatomy, severity of disease, and level of post-operative IOP required.
Your physician will discuss these options with you and will recommend the treatment he or she believes to be most appropriate for your condition
Finally, we are often asked if there are any lifestyle or dietary issues that might affect glaucoma. A number of factors have been studied, and the simple answer is that there are no lifestyle modifications which have been found to be clearly beneficial to the long-term management of glaucoma. The following factors have been found to affect intraocular pressure:
- Caffeine intake has been associated with elevated IOP. Studies indicate that a single cup of coffee can elevate IOP by 1 to 4 millimeters of mercury for at least 90 minutes. There have been no studies to determine whether caffeine intake is detrimental to glaucoma management, however. We recommend moderation- a cup or two of coffee or soda each day are unlikely to cause any significant problems.
- Alcohol has been found to cause a dose-dependent decrease in IOP after ingestion. However, some studies suggest that regular alcohol consumption is associated with increased IOP. No studies have shown alcohol use to be a risk factor for the development of glaucoma. Again, moderation is recommended.
- Dietary issues remain unclear. Little research has been performed on the role of antioxidants in glaucoma, although one study failed to find a significant role for antioxidants such as carotenoids, vitamin C, and vitamin E in the development of glaucoma. In another study, dietary fat intake did not appear to affect risk of glaucoma, although types of fats such as omega-3 or omega-6 fatty acids may have some role in regulating IOP. Clearly, more study is needed in this area, and no dietary recommendations can be made in this regard.
- Exercise has generally been found to be associated with reduced IOP. Acutely, exercise lowers IOP in the immediate post-exercise period, the degree of lowering directly related to work intensity. Furthermore, it seems generally accepted that physical fitness due to a consistent exercise regimen results in lower baseline intraocular pressure. Again, however, the effects of exercise and physical fitness on glaucoma progression are not well studied. It appears sensible to maintain a modest level of physical conditioning.
- Cigarette smoking has been found in a number of studies to have minimal effect on IOP, possibly causing a modest elevation, and no clear association with the development of glaucoma. However, considering the host of other health concerns associated with smoking, including increased risk of cataract and macular degeneration, we strongly advise our patients to avoid smoking and other tobacco product use.
- Marijuana is not currently considered a viable, good treatment for glaucoma. While certain compounds (mainly THC) in marijuana do lower IOP, the effect is relatively small and very short-lived, lasting on 2-4 hours. Furthermore, other components of marijuana have recently been found to either raise IOP or to blunt the IOP-lowering effect of THC, complicating the picture.
- Other activities, such as weight lifting, yoga, and playing high-resistance wind instruments such as the trumpet, cause transient and significant elevations in intraocular pressure. To our knowledge, however, there have been no studies indicating increased risk of developing glaucoma or causing glaucoma progression among people who perform these activities.
Again, based on the above, no specific recommendations can be made regarding lifestyle alterations in the management of glaucoma. However, given the other known health implications of many of these factors, we generally recommend that our patients maintain some level of age and health-appropriate exercise, avoid smoking, avoid excessive alcohol and caffeine intake, and eat a balanced diet high in fruits and vegetables as suggested by the USDA.
Narrow Angle/ Angle Closure Glaucoma Treatment
Narrow angles and angle closure glaucoma, while often treated with some of the same medications and surgical procedures as open angle glaucoma, have one notable difference. In these conditions, the problem of a narrow angle caused by the iris and cornea being too close together is often exacerbated by a situation known as pupillary block. Remember that aqueous humor, the fluid in the front of the eye, is produced behind the iris and must pass through the pupil before gaining access into the trabecular meshwork. Due to resistance of flow at the pupil, there is actually a slightly higher pressure behind the iris than in front, and this tends to bow the iris forward, contributing to the already narrow angle, and further blocking outflow of fluid from the eye.
This “pupillary block” is commonly the cause of acute angle closure in predisposed individuals. Treatment is aimed at alleviating this condition. In younger patients, a laser is used to create a small opening (or openings) in the periphery of the iris. Known as laser peripheral iridotomy, or LPI, this procedure creates a lower resistance pathway for aqueous to pass from behind to in front of the iris, preventing pupillary block and significantly reducing the risk of acute closure of the angle. Iridotomy is effective and safe, with low risk of significant complication. The same procedure is required emergently to treat an actual attack of angle closure, however, in that situation is much more difficult to perform and may be too late to prevent permanent damage to the eye. Therefore, if your physician finds narrow angles, laser iridotomy may be recommended on a prophylactic basis.
For older patients found to have narrow angles, the treatment of choice is cataract surgery. A cataract is simply a thickening and clouding of the natural lens of the eye, a condition that begins to develop in nearly everyone after about age 50. Removing the thickened lens significantly increases the space within the front section of the eye, deepening the angle and eliminating pupillary block. This essentially cures most cases of narrow angles. Several studies, including a large, randomized clinical trial published in 2016 (the EAGLE trial), have found that removal of the natural lens, even when minimal cataract is present, is the most effective treatment for this condition.
The decision of whether to perform a laser iridotomy versus lens extraction/cataract surgery is based upon several factors which will be reviewed by your physician.
Future Directions
Much research is ongoing in the field of glaucoma. Genetic testing to help identify those at risk for the disease shows promise. Newer, better surgical techniques are constantly being evaluated to identify procedures that are more effective and safer. Much interest now focuses on medications which may provide what is known as neuroprotection. This term describes the protection of the optic nerve from damage by a number of factors, including poor blood supply, toxins, and inadequate nutrition. Neuroprotective medications are considered quite important, as we know that intraocular pressure is not the only influence on glaucoma progression, yet it is presently the only treatable aspect of the disease. Availability of these drugs will eventually be a major step forward in the treatment of the disease, however results from human trials are not yet conclusive.
If, after reading this information, you still have questions or concerns about glaucoma, please contact us to schedule an appointment for a complete evaluation with one of our doctors.
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