Glaucoma
Glaucoma is a condition whereby the optic nerve cells, which transmit information from the eye to the brain, become damaged. The eye is a fluid filled structure. The fluid filling the eye gives the eye its normal shape. Without the normal amount of fluid, the eye would collapse like a balloon without air. There are mechanisms in the body that act to regulate the normal fluid pressure. One structure produces the fluid continuously, while another simultaneously drains the old fluid. When these mechanisms break down, the end result is glaucoma. The best analogy for understanding this delicate balance is that of a bathtub with the water faucet constantly running and the drain open. As long as the faucet flows at the correct rate with the drain open, the water level in the tub stays the same. If either the faucet flow is too fast, or the drain becomes blocked, the water level in the tub rises. People who suffer from glaucoma have increased intra-ocular pressure as a direct result of one of these two basic mechanisms. The increased pressure in the eye acts like a boa constrictor to squeeze the optic nerve, which enters the globe from behind. This constriction on the sensitive nerve fibers pinches off the blood supply to the nerve, which causes a type of slow strangulation death of the optic nerve. The nerve dies from the outside inward, and because of the anatomical arrangement of the nerve fibers, peripheral vision is the first to go, followed by the central vision in the late stages of the disease. Since the pressure typically ebbs and flows, this nerve cell death occurs over an extended period of time. The death of the nerve cell fibers prevents the transmission of visual information from the retina in the eye to the brain. Since the majority of those afflicted are not symptomatic, glaucoma is known as “the silent thief of sight”. Often you will not be aware that you have glaucoma until it’s too late. Usually there are no symptoms until permanent visual field defects occur. Vision stays normal, and there is no pain. Although they see things clearly in front of them, they miss objects to the side and out of the corner of their eye. Without treatment, glaucoma symptoms progress and people with the condition may find that they suddenly have no side vision. It may seem as though they are looking through a tunnel. Over time, the remaining forward vision may decrease until the central vision is “snuffed out” in the end stages. Treatment of the disease revolves around maintaining and controlling the normal fluid pressure in the eye.
Glaucoma Tests
Glaucoma is a puzzle with many pieces, therefore multiple pieces of the puzzle must be obtained to make an accurate diagnosis. There are multiple versions of a device called a tonometer, which is used to measure the intra-ocular pressure. If the eye pressure readings are elevated, compared to statistical norms, your doctor may decide to do additional testing to gather more information, or collect additional pieces of the puzzle. There is a genetic component to the disease, so a positive family history for glaucoma is one piece of the overall canvas. The anatomical appearance of the optic nerve head as it inserts into the posterior pole of the globe, can give clues to whether the presentation is normal, or the result of glaucomatous pathological changes. As the nerve fibers become atrophic and die in the course of the disease, the central portion of the nerve head, called the cup, becomes enlarged and bean pots in the end stages. A digital fundus camera, used to take three dimensional photographs of the retina and optic nerve heads inside the eye, is an invaluable tool to monitor changes to the nerve over time, which may detect early pathological progression. Normal nerves do not change over time, whereas, the cup size in a glaucomatous nerve would gradually enlarge and or become excavated over time. A threshold visual field analyzer is used to map out the peripheral and central vision areas, to determine if there has been any damage or progressive changes to the optic nerve, that would result in visual field defects. A procedure called gonioscopy, which is basically a contact lens with mirrors attached, can be performed to view the anterior chamber angle. The structures in the angle act to drain and filter the fluid that fills the eye in constant continuum. Open angles indicate low risk, and narrow or highly pigmented angles add another piece of risk to the puzzle.
A corneal pachymeter is used to measure the corneal thickness. This information is valuable, because studies have determined that people with above average corneal thickness have higher than what is considered normal eye pressure readings. Conversely, people with thin corneas may show lower pressures, but have greater risk for developing glaucoma. Newer entries into glaucoma diagnosis include Heidelberg Retinal Tomography (HRT) and Ocular Coherence Tomography (OCT). These devices measure the thickness of the retinal nerve fiber layer and compare these measurements with a database of statistical normals. As a general rule, a thicker nerve fiber layer indicates healthy retina, and progressively thinning nerve fiber layers indicate pathological changes. Both of these devices are objective tests able to detect glaucomatous changes and/or patients at higher risk for developing glaucoma at a much earlier stage in the progression of the disease. The baseline measurements from these instruments can then be used to monitor changes to determine if the prescribed treatment is preventing further tissue damage, and therefore vision loss.
Glaucoma Treatment
Glaucoma treatment revolves around various drugs and surgical procedures designed to normalize the intraocular fluid pressure. The treatment would be tailored by your eye care provider to minimize your risk for vision loss from the time of your diagnosis going forward. The take home pearl here is, the condition is treatable and controllable under the careful eyes of a medical professional. Although there are no guarantees, the one absolute in the course of the disease is, if left untreated, patients with glaucoma will ultimately lose their sight. Thus, the importance of early diagnosis and treatment is paramount in the long term success of each patient.