Glaucoma is a group of diseases characterized by damage to the optic nerve. Most often, it is due to pressure in the eye being too high. It is one of the leading causes of blindness in the world.
What is the optic nerve?
The optic nerve is like a fiber-optic cable that transmits visual signals from our eyes to our brain. In our brain, these signals are processed to create an image. There is one optic nerve for each eye.
Diagram showing the location where the optic nerve leaves the eye. This region, called the optic disc, is where visible damage occurs in glaucoma.
What causes glaucoma?
For the eye to maintain its shape, there must be a certain amount of pressure inside the eye (similar to the air pressure in a balloon). This is referred to as intraocular pressure, or IOP.
Intraocular pressure is maintained by the fluid inside the eye. In the front part of the eye, this fluid is constantly being produced and drained. If the production increases and/or the drainage decreases, the IOP will rise.
If the IOP is too high, damage to the optic nerve can result, leading to vision problems. This is referred to as glaucoma.
How is fluid in the eye produced and drained?
In the front of the eye, the fluid is referred to as aqueous humour. The aqueous humour is made in the ciliary body, and circulates forward (through the pupil) and across the iris to drain into a meshwork called the trabecular meshwork.
The trabecular meshwork is located near where the cornea and iris meet (at an “angle”). Fluid drains through this meshwork, into a series of canals, and into the bloodstream.
Diagram showing the flow of aqueous humour in the eye. The fluid is produced in the ciliary body, flows through the pupil and drains into the trabecular meshwork. Too much aqueous humour will increase pressure in the eye.
What are the different types of glaucoma?
There are several different types of glaucoma. Some important types are:
Primary open-angle glaucoma: This is the most common type of glaucoma. In this form of glaucoma the drainage system of the eye appears normal, however there are microscopic abnormalities. Consequently, drainage through the trabecular meshwork is decreased and pressure increases. Acute angle-closure glaucoma: In this type of glaucoma, the iris and cornea are very close together and even come into contact with each other; this blocks the entrance to the eye’s drainage system and impairs the drainage of fluid. Intraocular pressure can rise rapidly and cause severe pain, nausea, red eye and vision problems. This is termed acute angle-closure glaucoma and is an emergency. However, this angle closure process may also occur slowly and silently; thereby raising the intraocular pressure and mimicking primary open angle glaucoma. Congenital glaucoma: Congenital (“born with”) glaucoma occurs in children that are born with a defect in the drainage system of the eye. The drainage of fluid is impaired, again leading to high pressure.
Secondary glaucomas: The term “secondary” means that the glaucoma is caused by another factor. Secondary glaucoma may be caused by another eye disease, eye injury, eye surgery, inflammation in the eye, or certain drugs.
Can I have glaucoma even though my intraocular pressure is normal?
Yes, not all patients with glaucoma necessarily have high intraocular pressure. In people with “normal tension (pressure) glaucoma” the intraocular pressure is normal but the optic nerve undergoes the same degeneration seen in high pressure glaucoma.
What are risk factors for glaucoma?
There are many risk factors for the development of glaucoma. These include: • High intraocular pressure • Age: especially over the age of 60 • Race: African-Ameicans and Inuit are at increased risk of glaucoma • Sex: women are more likely to have glaucoma than men • Family history of glaucoma • Nearsightedness (myopia) • Decreased corneal thickness • Certain diseases: diabetes, high blood pressure and certain eye diseases increase the risk of glaucoma
• Nearsightedness (myopia)
What are the symptoms of glaucoma?
In the most common form of glaucoma – primary open angle glaucoma – there are no symptoms until the disease is very advanced and vision loss becomes noticeable. By this time there is usually significant damage to the optic nerve. This is why screening for glaucoma is so important.
When glaucoma does start to produce symptoms, it usually causes loss of vision, especially in the periphery. In other words, central vision is relatively normal. As the disease progresses, central vision may be lost.
In acute angle-closure glaucoma, the symptoms are sudden and consist of severe eye pain, nausea, red eye and vision loss. As mentioned before, this type of glaucoma is an emergency.
How is glaucoma screened?
It is important that all people be screened for glaucoma, especially if they are considered “high risk” (see risk factors section). Optic nerve damage can be detected by an optometrist or ophthalmologist long before symptoms develop, and treatment can prevent further damage and visual loss from occurring.
Screening for glaucoma involves measurements of intraocular pressure and a complete eye examination including examination of the optic nerve and retina.
How is glaucoma diagnosed?
Glaucoma is typically diagnosed by measurements of intraocular pressure and a complete eye examination including examination of the optic nerve and retina.. Doctors can also monitor the disease by performing a visual field test.
How is glaucoma treated?
Unfortunately, damage to the optic nerve is irreversible. This means that once the vision is lost, it cannot be regained.
Treatment of glaucoma is focused on preventing further damage to the optic nerve by decreasing high pressure in the eye. Intraocular pressure can be reduced by:
• Drugs: Drugs (in the form of eyedrops) are often the first-line treatment for glaucoma or high intraocular pressure. There are many options available.
• Laser surgery: an ophthalmologist can use a laser directed at the drainage system of the eye (trabecular meshwork) to increase drainage of fluid through the system and decrease intraocular pressure.
• Trabeculectomy: a trabeculectomy involves creating a canal to drain fluid from the eye. To read more about a trabeculectomy, click here.
• Implants: an ophthalmologist can also insert implants into the eye to help fluid drain.
The choice of treatment depends on the type of glaucoma, the wishes of the patient, the stage of the disease and the recommendations of the ophthalmologist. Usually, surgery is not performed unless eye drops are ineffective.
Is it true that marijuana decreases intraocular pressure?
Yes. Smoking marijuana does decrease intraocular pressure. However, this is not promoted as a treatment for glaucoma because eye drops are more effective, are cheaper, and have fewer side-effects.
