Glaucoma is a disease that damages the eye's optic nerve. The optic nerve is connected to the retina — a layer of light-sensitive tissue lining the back of the eye. The optic nerve is made up of many nerve fibers, like an electric cable is made up of many wires. The optic nerve sends signals from the retina to the brain, where these signals are interpreted as the images.
When damage to the optic nerve fibers occurs, blind spots develop which usually go undetected until the optic nerve is significantly damaged.
Glaucoma can cause blindness if it is left untreated. Only about half of the three million Americans who have glaucoma are even aware that they have the condition. When glaucoma develops, usually early symptoms are not detected and the disease progresses slowly which is why glaucoma is known as "The Silent Thief of Sight". Glaucoma can steal sight very gradually. Fortunately, early detection through regular eye exams and proactive treatment can help preserve vision.
What is Glaucoma?
In the healthy eye, a clear fluid called aqueous humor circulates inside the front portion of the eye. To maintain a constant healthy eye pressure, the eye continually produces a small amount of aqueous humor while an equal amount of this fluid flows out of the eye. The fluid flows out through a very tiny drain called the trabecular meshwork, a complex network of cells and tissue in an area called the drainage angle.
With glaucoma, the aqueous humor does not flow through the trabecular meshwork properly. Fluid pressure in the eye builds up and over time causes damage to the nerve fibers.
There are two main types of Glaucoma:
The most common form of glaucoma is called primary open-angle glaucoma. It occurs when the trabecular meshwork of the eye gradually becomes less efficient at draining fluid. As this happens, eye pressure, called intraocular pressure (IOP), rises. Raised eye pressure leads to damage of the optic nerve. Damage to the optic nerve can occur at different eye pressures among different patients.
Typically, open-angle glaucoma has no symptoms in its early stages and vision remains normal. As the optic nerve becomes more damaged, blank spots begin to appear in the field of vision. These blank spots in the vision are typically not noticed in day-to-day activities until the optic nerve is significantly damaged and these spots become large. If all of the optic nerve fibers die, blindness results.
Half of patients with glaucoma do not have high eye pressure when first examined. That is why it is essential that the optic nerve be examined by an ophthalmologist for proper diagnosis.
A less common form of glaucoma is closed-angle, or narrow-angle, glaucoma. Closed-angle glaucoma occurs when the drainage angle of the eye becomes blocked. Unlike open-angle glaucoma, eye pressure usually goes up very fast. The pressure rises because the iris — the colored part of the eye — partially or completely blocks off the drainage angle. People of Asian descent and those with hyperopia (farsightedness) tend to be more at risk for developing this form of glaucoma.
If the drainage angle becomes completely blocked, eye pressure rises quickly resulting in a closed-angle glaucoma attack. Symptoms of an attack include:
- Severe eye or brow pain
- Redness of the eye
- Decreased or blurred vision
- Seeing colored rainbows or halos
A closed-angle glaucoma attack is a medical emergency and must be treated immediately. Unfortunately, people at risk for developing closed-angle glaucoma often have few or no symptoms before the attack.
Who is at risk for Glaucoma?
Some people are at greater risk for developing glaucoma and should see their ophthalmologist on a regular basis, specifically for glaucoma testing. Risk factors for glaucoma include:
- Family history of glaucoma
- African or Hispanic ancestry
- Farsightedness or nearsightedness
- Elevated eye pressure
- Past eye injury
- Having a thinner central cornea (the clear, front part of the eye covering the pupil and colored iris)
- Not having eye examinations when they are recommended
- Low blood pressure
- Conditions that affect blood flow, such as migraines, diabetes, and low blood pressure
Early detection is key. People of any age with symptoms of or risk factors for glaucoma, such as those with diabetes, a family history of glaucoma, or those of African descent, should see an ophthalmologist for an eye exam and establish a schedule of routine follow-up exams.
Adults with no symptoms of or risk factors for eye disease should have a complete eye disease screening by age 40 — the time when early signs of disease and changes in vision may start to happen. Based on the results of the initial screening, your ophthalmologist will let you know how often to return for follow-up exams.
Adults 65 or older should have an eye exam every one to two years.
Ophthalmologists measurer eye pressure using tonometry. Testing eye pressure is an important part of a glaucoma evaluation. A high pressure reading is often the first sign of glaucoma. During this test, the eye is numbed with eyedrops and an instrument, called a tonometer, measures the eye pressure. The instrument measures how the cornea resists pressure. Normal eye pressure generally ranges between 10 and 21 mm Hg. However, people with normal-tension glaucoma can have damage to their optic nerve and visual field loss even though their eye pressure remains consistently lower than 21 mm Hg.
Gonioscopy allows an ophthalmologist to get a clear look at the drainage angle to determine the type of glaucoma a patient may have. However, by using a mirrored lens, the doctor can examine the drainage angle to determine if a patient has open-angle glaucoma (where the drainage angle is not working efficiently enough), closed-angle glaucoma (where the drainage angle is at least partially blocked), or a dangerously narrow angle (where the iris is so close to the eye's drain that the iris could block it).
An ophthalmoscope magnifies the interior of the eye to inspect the optic nerve for signs of damage. The pupils are dilated (widened) with eyedrops to allow a better view of the optic nerve. A normal optic nerve is made up of more than one million tiny nerve fibers. As glaucoma damages the optic nerve, it causes the death of some of these nerve fibers. As a result, the appearance of the optic nerve changes. This is referred to as cupping. As the cupping increases, blank spots begin to develop in the field of vision.
Visual field test
The visual field test checks for blank spots in the vision. The test is performed using a bowl-shaped instrument called a perimeter. When taking the test, a patch is temporarily placed on one eye so only eye is tested at a time. Random points of light flash around the bowl-shaped perimeter, and the patient presses a button when he or she sees a light. Visual field testing is usually performed every 6 to 12 months to monitor for change.
Because the thickness of the cornea can affect eye pressure readings, pachymetry is used to measure corneal thickness. A probe called a pachymeter is gently placed on the cornea to measure its thickness.
How glaucoma is treated depends on the specific type of glaucoma, the severity of the disease, and how it responds to treatment.
Medicated eyedrops are the most common way to treat glaucoma. These medications lower eye pressure in one of two ways — either by slowing the production of aqueous humor or by improving the flow through the drainage angle. These eyedrops must be taken every day.
ALT and SLT Laser Treatment
To treat open-angle glaucoma, a surgery called laser trabeculoplasty is often used. There are two types of trabeculoplasty surgery: argon laser trabeculoplasty (ALT) and selective laser trabeculoplasty (SLT).
During ALT surgery, a laser makes tiny, evenly spaced burns in the trabecular meshwork. The laser does not create new drainage holes, but rather stimulates the drain to function more efficiently.
With SLT, a laser is used at different frequencies, allowing it to work at very low levels. SLT treats specific cells and leaves the mesh-like drainage canals surrounding the iris intact. SLT may be an alternative for those who have been treated unsuccessfully with traditional laser surgery or with pressure-lowering drops.
Even if laser trabeculoplasty is successful, most patients continue taking glaucoma medications after surgery. For many, this surgery is not a permanent solution. Nearly half who receive this surgery develop increased eye pressure again within five years. Many people who have had a successful laser trabeculoplasty have a repeat treatment.
Laser iridotomy is recommended for treating people with closed-angle glaucoma and those with very narrow drainage angles. A laser creates a small hole about the size of a pinhead through the top part of the iris to improve the flow of aqueous fluid to the drainage angle.
In trabeculectomy, a small flap is made in the sclera (the outer white coating of the eye). A filtration bleb, or reservoir, is created under the conjunctiva — the thin, filmy membrane that covers the white part of the eye. Once created, the bleb looks like a bump or blister on the white part of the eye above the iris, but the upper eyelid usually covers it. The aqueous humor can now drain through the flap made in the sclera and collect in the bleb, where the fluid will be absorbed into blood vessels around the eye.
Eye pressure is effectively controlled in three out of four people who have trabeculectomy. Although regular follow-up visits with your doctor are still necessary, many patients no longer need to use eyedrops. If the new drainage channel closes or too much fluid begins to drain from the eye, additional surgery may be needed.
Aqueous Shunt Surgery
If trabeculectomy cannot be performed, aqueous shunt surgery is usually successful in lowering eye pressure. An aqueous shunt is a small plastic tube or valve connected on one end to a reservoir (a roundish or oval plate). The shunt is an artificial drainage device and is implanted in the eye through a tiny incision. The shunt redirects aqueous humor to an area beneath the conjunctiva (the thin membrane that covers the inside of your eyelids and the white part of the eye). The fluid is then absorbed into the blood vessels. When healed, the reservoir is not easily seen.