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When the optic nerve is damaged or injured, the transmission of visual information and, hence, the visual image is impaired. Fortunately, ophthalmologists can usually detect glaucomatous optic nerve damage before there is noticeable visual loss by examining the back of the eye. Sometimes, damage to the nerve can be observed before there is a change in vision by specialized tests of vision.
The Optic Nerve ExaminationFor these reasons, examination of the optic nerve and its ability to transmit the visual message is an essential portion of the examination for glaucoma. By determining whether or not the optic nerve has been damaged and whether the damage is getting worse, your eye doctor can diagnose glaucoma or determine whether it is worse. In most patients, the optic nerve can be readily examined. It can be seen directly inside the eye with an instrument called the ophthalmoscope. Although many different diseases affect the optic nerve, the damage from glaucoma has a characteristic appearance that permits your ophthalmologist to recognize whether glaucoma is present. The optic nerve exits through the back of the eye. The nerve is made up of fibers which originate in nerve cells located in the retina, the light-sensitive film coating the inside of the eye. When looking into the eye, the optic nerve is seen end on (optic nerve head), and the nerve fibers are seen to fan out onto the retina (Figure 1). In the normal state, the optic nerve head looks much like a doughnut, with the outer ring consisting of the nerve tissue. The hole (called the optic cup) is the space which remains after the nerve fibers turn to fan out into the retina. In glaucoma, the nerve fibers are damaged and erode away, leaving a larger cup (or hole of the doughnut). Especially when the degree of enlargement is different between the two eyes, the physician can diagnose early glaucoma from this appearance alone (Figure 2).
New methods of detecting damage to the optic nerve appearance using computers are being developed, but standard examination with conventional instruments can often detect even early damage. Newer methods that may permit even earlier detection also are being developed to examine and measure the nerve fibers as they spread into the retina. Visual Field TestingAnother important tool used to examine and measure the optic nerve is the visual field test. This measures how well the optic nerve functions in carrying visual information to the brain. Most visual field tests measure the ability of the patient to recognize light at each area of the retina. Many new tests of optic nerve function are also being developed, including lights of different colors, flickering lights, and special targets (such as rings). Testing of the visual field and examination of the optic nerve are the most important determinants of whether or not there is glaucoma. If glaucoma is present, they also allow your eye doctor to determine whether it is stable or becoming progressively worse. Most patients dislike having their visual field tested. The test is often long, tiring, and boring. It is not uncommon to feel that you have performed badly. Most modern visual field machines keep testing each spot measured until the patient misses, in order to determine the dimmest light detectable. Thus, it is necessary to have many misses to complete the test. Missed points, however, may be perfectly normal and should not cause one to be upset. What Causes Glaucoma?In many cases, glaucoma optic nerve damage occurs from high eye pressure. Nerve damage can usually be stopped or slowed by lowering the eye pressure. Most glaucoma treatment, with medicines, laser, or conventional surgery, is designed only to lower the eye pressure. Some eyes with glaucoma optic nerve damage continue to deteriorate despite having the lowest possible eye pressures. It is not known why this happens. Intensive research around the world is now directed at understanding the cause of the damage in these patients and to develop new treatments to preserve the optic nerve. Many different eye disorders cause high eye pressure. After measuring the eye pressure, your ophthalmologist attempts to determine the cause of the elevation. Nearly always, some form of “clogging” or blockage of the drainage of internal fluid within the eye (aqueous humor) causes increased eye pressure. Since the eye continually produces this fluid, obstruction of the drainage causes the eye pressure to increase. Almost any eye disorder associated with aging, inflammation, bleeding, injury, tumor or even birth defects can raise the eye pressure. However, in most cases of glaucoma, the eye has no specific abnormality and is said to have primary open-angle glaucoma. In other cases, the eye may be unusually small or exhibit other minor shape abnormalities that cause closed-angle glaucoma. In closed-angle glaucoma, the drainage system is totally blocked instead of just being clogged. At least fifty different mechanisms have been described that can raise the eye pressure, but all produce similar damage of the optic nerve. All methods of treatment are designed to lower the eye pressure to a level that will prevent further optic nerve damage. “How’s My Optic Nerve, Doctor?”Ongoing assessment of glaucoma depends upon regular examination of the optic nerve and visual field in addition to the eye pressure. The well informed glaucoma patient asks not “What is my eye pressure, doctor?,” but rather, “How is my optic nerve?” | Home
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