Diagnosis and Management of Glaucoma
How do the eyes see?
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 your retina to your brain, where these signals are interpreted as the images you see.
In the healthy eye, a clear fluid called aqueous (pronounced AY-kwee-us) humor circulates inside the front portion of your eye. To maintain a constant healthy eye pressure, your eye continually produces a small amount of aqueous humor while an equal amount of this fluid flows out of your 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.
The most common form of glaucoma is called open-angle glaucoma. It occurs when the trabecular meshwork of the eye gradually becomes less efficient at draining fluid. As this happens, your eye pressure, also called your intraocular pressure (IOP), rises. High eye pressure leads to damage of the optic nerve fibers. This damage to the optic nerve fibers can occur at different eye pressures among different patients. If you have glaucoma, Doctor Murphy will prescribe glaucoma eye drops to lower your eye pressure. This lower eye pressure will protect your optic nerve from further damage.
Typically, open-angle glaucoma has no symptoms in its early stages and your vision remains normal. As the optic nerve fibers become more damaged, blank spots begin to appear in your field of vision. You usually won’t notice these blank spots in your 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.
Only about half of the three million Americans who have glaucoma are even aware that they have the condition.
Early detection of glaucoma is the best way to avoid visual loss from glaucoma. That is why your complete eye examination will include measurement of your eye pressure and a careful examination of your optic nerve.
Optic Nerve-Fiber Layers and Damage in Glaucoma
Early in the disease process of glaucoma, individual nerve fibers in the eye’s optic nerve die. This nerve fiber death results in thinning of the nerve fiber layer. As glaucoma progresses, more nerve fibers are lost, which can result in visual-field loss or blank spots, and, ultimately, loss of vision. A visual-field test, which is performed in Doctor Murphy's office, will detect these visual-field losses or blank spots when they are small.
As a result of advances in technology, a new testing method has been found to measure the thickness of the nerve fiber layer. This method uses low-power laser light in a process called Optical Coherence Tomography (OCT). An OCT scan can provide a precise measurement of the nerve-fiber layer, which improves Doctor Murphy's ability to diagnose and monitor glaucoma. An OCT scan is done in the office, and takes only a few minutes.
Laser Surgery for Glaucoma
Argon Laser Trabeculoplasty (ALT) and Selective Laser Trabeculoplasty (SLT) are laser surgical procedures used for patients with open-angle glaucoma to help lower intraocular pressure (IOP). ALT and SLT are used to treat the trabecular meshwork — the mesh-like drainage canals surrounding the iris — that serves as the eye’s drainage system. The goal of treatment with ALT and SLT is to improve the flow of fluid out of the eye, helping to lower IOP. ALT and SLT are typically performed in an outpatient surgery center. The procedure usually takes about five to ten minutes. First, anesthetic eye drops are placed in your eye. The laser device looks similar to the examination microscope that Doctor Murphy uses to look at your eyes at each office visit. You will experience a flash of light with each laser application. Most people are comfortable and do not experience any significant pain, although some may feel a little pressure in their eye during the laser procedure.
It will take several weeks to determine how much your pressure will be lowered with ALT or SLT. You may require additional laser or glaucoma-drainage surgery to lower the IOP if it is not sufficiently lowered after the first laser treatment. In most cases, glaucoma eye drops are still necessary to control your eye pressure. However, the laser procedure may lessen the amount of glaucoma medication you need.
Other Types of Glaucoma
Eye pressure is expressed in millimeters of mercury (mm Hg), the same unit of measurement used in weather barometers.
Although normal eye pressure is considered a measurement less than 21 mm Hg, this can be misleading. Some people have a type of glaucoma called normal-tension, or low-tension glaucoma. Their eye pressure is consistently below 21 mm Hg, but optic nerve damage and visual field loss still occur. People with normal-tension glaucoma typically receive the same methods of treatment used for open-angle glaucoma.
Conversely, ocular hypertension is a condition where someone has higher eye pressure than normal, but does not have other signs of glaucoma, such as optic nerve damage or blank spots that show up in their peripheral (side) vision when tested. Someone with ocular hypertension is considered a “glaucoma suspect” because he or she may be at risk for developing glaucoma later. Just like people with glaucoma, people with ocular hypertension need to be closely monitored by Doctor Murphy to ensure they receive appropriate treatment.
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.
People at risk for closed-angle glaucoma should avoid over-the-counter decongestants and other medications where the packaging states not to use these products if you have glaucoma.
Glaucoma can often develop as the result of another eye condition or disease. This is known as secondary glaucoma. For example, someone who has a tumor or people undergoing long-term steroid therapy may develop secondary glaucoma. Other causes of secondary glaucoma include:
- Eye injury;
- Inflammation of the eye;
- Abnormal blood vessel formation from diabetes or retinal blood vessel blockage;
- Use of steroid-containing medications (pills, eyedrops, sprays);
- Pigment dispersion: tiny fragments or granules from the iris (the colored part of the eye) can circulate in the aqueous humor (the fluid within the front portion of the eye) and block the trabecular meshwork, the tiny drain for the eye’s aqueous humor.
Congenital glaucoma is a rare type of glaucoma that develops in infants and young children. It happens when the eye’s drainage system doesn’t develop fully or correctly before birth. Congenital glaucoma can be inherited.