Glaucoma is a type of progressive eye damage in which optic nerve cells are damaged by excess fluid pressure in the eyeball. It is one of the leading cause of blindness in Canada. Typically, glaucoma occurs gradually, and may not be noticed until it has progressed quite far. Vision loss from glaucoma cannot be reversed, so routine eye exams to prevent it are important.
Glaucoma often runs in families. There are several types of the disease and each is caused by a different disease process that tends to affect different racial groups. Overall, people of African origin are more likely than Caucasians to get glaucoma.
Other risk factors for developing glaucoma include being over 40 years old, having high intraocular pressure (IOP; pressure inside the eye), myopia (nearsightedness), diabetes, high blood pressure, and a history of an eye injury.
The front of the eye is a D-shaped chamber in which the lens and iris (the coloured part) make up the straight wall, while the cornea (the surface of the eye) forms the curved wall. A gland behind the upper eyelid fills this chamber with a clear liquid (aqueous humour) that supplies the front of the eye with oxygen and nutrients and keeps it inflated. A steady supply of liquid is produced, and it drains out through a mesh of tiny holes behind the lower eyelid, called the trabecular meshwork.
In glaucoma, the liquid is produced normally but the trabecular meshwork can't drain it due to clogging or some other reason. Liquid pressure builds up in the eye, pressing on the optic nerve (the nerve that links the eye to the brain). The nerve cells are then slowly strangled of blood, eventually dying. The outer nerves fail first, so vision loss tends to start at the edges, progressing to "tunnel vision" and blindness.
Many people don't notice this at first, and there's usually no pain, so glaucoma can be quite advanced before it's detected. The US Glaucoma Foundation estimates that only 50% people with glaucoma are aware of the disease.
Types of glaucoma are classified by what's stopping the liquid from draining:
Primary open-angle glaucoma (POAG) is the most common form of glaucoma in North America, affecting about 1 in 100 people. People of African descent are especially susceptible. It normally strikes after the age of 50. The trabecular meshwork looks fine on examination but doesn't drain properly. Some researchers believe this is because aging makes the cells less efficient. Others suspect a drainage problem under the eye, or another defect. In any case, pressure builds up and the optic nerve starts to fail. Blind spots eventually appear in the peripheral vision, and later in the central "seeing" area. This damage can't be repaired. POAG gets steadily worse over time if it isn't treated.
Angle-closure glaucoma is most common in people of Asian or Inuit descent, and in women and farsighted people. It can be chronic or intermittent. Since the trabecular meshwork is located in the angle of the D between the iris and the cornea, anything that swells the iris or pushes it forward can close that angle, blocking the meshwork. Certain diseases, such as diabetes or uveitis (inflammatory condition of the eye), can cause the iris to be pushed forward. Aging also tends to thicken the lens and iris, blocking liquid flow between the front and rear chambers of the eye. Pressure builds in the rear chamber (the main part of the eyeball), pushing the iris forward and closing the trabecular meshwork.
In normal tension glaucoma, the pressure in the eye is in the normal range, yet the optic nerve is still damaged. It's believed that poor blood flow to the optic nerve makes the nerves more vulnerable. Pressures just slightly above average can therefore damage the eye.
Pigmentary glaucoma mostly affects younger nearsighted people. The iris, which is concave in myopia, rubs against the pigment layer that holds the eye's colour. Flakes of pigment rub off and clog the trabecular meshwork, raising pressure. Exfoliation syndrome, most common in Caucasians aged 50 and older, is similar. Flaky white material appears on the lens before falling off to clog the meshwork. Exfoliation syndrome doesn't necessarily result in glaucoma, but it makes it six times more likely.
Traumatic glaucoma is the result of eye injury, often appearing years after the event.
Glaucoma in children is present at birth or develops during the first few years of life. The damage may be caused by the inability of the liquid to drain due to blockages, or it may be due to an underlying medical condition.
Symptoms and Complications
The biggest problem with glaucoma is the lack of symptoms, since you can't feel the pressure in the eye. Only a few people get headaches, red eye, or blurred vision. If you don't get your eyes tested regularly, the first noticeable symptom could be permanent "holes" in your vision or “tunnel vision” in advanced stages. Lack of treatment of glaucoma often leads to blindness.
If you do get severe eye pain, redness, nausea and vomiting, and blurred vision, it may be a symptom of an angle-closure attack. This is an acute complication in which the iris swells or moves forward to totally block the trabecular meshwork. If you are prone to angle-closure glaucoma, you're most likely to get an acute attack in the dark, because the pupil opens up in low light, making the angle smaller. In a mild attack, you may see haloes around objects and slight blurring, but there's no pain. Extreme pain and a red, swollen eye signal a medical emergency - people can go blind rapidly if it isn't treated.
Making the Diagnosis
The first thing an eye doctor will want to check is the eye's internal pressure. An air-puff tonometer can measure this without touching the eye. The doctor will also look into the eye through the transparent pupil with an ophthalmoscope. This is essential because normal tension glaucoma is missed by the eye pressure test.
Early damage from glaucoma can be detected with a visual field test. You'll be asked to spot flashing lights around the rim of your field of vision. A computer is often used to perform this test.
Treatment and Prevention
Medications are available that can reduce eye pressure and prevent damage. Most are given as eye drops. Beta-blockers (e.g., betaxolol*, levobunolol, timolol), alpha2-agonists (e.g., brimonidine), and carbonic anhydrase inhibitors (e.g., dorzolamide, brinzolamide) slow the production of eye fluid (aqueous humour), while prostaglandin analogs (e.g., latanoprost, travoprost, bimatoprost) and miotics (medications that contract the pupil, such as pilocarpine) improve drainage. Medications have to be used indefinitely, so many people with glaucoma end up opting for surgery or laser treatment.
The most common operation is laser trabeculoplasty, a painless 15-minute outpatient procedure. The laser is unfocused and harmless when it passes through the surface of the eye, but it concentrates its energy on the trabecular meshwork, shrinking it and reopening the holes. Many patients are able to stop using their glaucoma medications after this operation. There are other surgical procedures available if this doesn't work.
With early treatment, vision loss can be minimized or prevented. Because glaucoma isn't obvious, it's vital to get your eyes checked regularly, especially if you have any of these risk factors:
- family history of glaucoma
- African descent
- myopia (nearsightedness)
- previous eye injury
- high blood pressure
- long-term use of prednisone, cortisone, or other steroids
If you are between the ages of 40 and 64 years and have no glaucoma risk factors, you should have your eyes checked every 2 years. From the age of 65 years onwards, you should have them checked every year. But if you have any of these risk factors, you should go as often as your eye care professional recommends.
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