Introduction to Glaucoma
Glaucoma results from nerve damage to the optic nerve by high eye pressure. The optic nerve connects the brain to the eye, and it is very sensitive to changes in eye pressure, also known as intraocular pressure (IOP). Most people consider ideal blood pressure to be around 120/80 mmHg, but eye pressure is highly individualized, and different people may develop glaucoma at different IOPs. At first, glaucoma damages peripheral (or side) vision, but most people would not detect this early peripheral visual loss. At more severe stages, peripheral vision loss spreads toward the center of vision, causing the so-called “tunnel vision.” At this severe stage, some patients may still be able to see 20/20 centrally and not realize the severity of their peripheral vision loss. Eventually, even the little bit of central vision can be lost if glaucoma is not treated. For these reasons, glaucoma is sometimes called “The Silent Thief of Sight.”
Vision Testing in the Clinic
Pachymetry – This test measures the thickness of the cornea, the transparent, front-most part of the eye. IOP is directly related to the thickness of the cornea. Thicker corneas tend to have falsely high IOPs, and thinner corneas tend to have falsely low IOPs. Each person’s IOP must be correlated with her corneal thickness.
Visual Field test – As discussed above, peripheral vision is damaged first by glaucoma. The visual field test maps out the eye’s central and peripheral vision, and is used to track one’s visual field over time. It is done with one eye shut at a time, with the eye fixated at a central target, and takes about 4-7 minutes per eye depending on a person’s response time.
Gonioscopy – This is a test using a special contact lens on a person’s eye that has been anesthetized with eyedrops. Gonioscopy allows for viewing of the internal angle between the cornea and the iris (the colored part of the eye). This angle inside the eye is important in distinguishing between the different types of glaucoma, since medical treatment may vary depending on the type of glaucoma.
Optical Coherence Tomography (OCT) – The OCT uses light to digitally scan the optic nerve and measures its tissue thickness. This test is analogous to an ultrasound test, except that instead of using sound waves, it uses light rays to visualize the optic nerve. In glaucoma, the tissues of the optic nerve are compressed by high IOP and gradually become thinner. The OCT can be used to digitally track the optic nerve thickness and health over time.
Appearance of glaucoma on OCT: note the middle section towards the top of the photo titled “Neuro-retinal rim thickness.” The solid black line represents the right eye; the dotted line represents the left eye. When the line dips into the red zone, it means that the optic nerve tissue has been compressed, damaged, and “thinned out” from high eye pressure. Note that in this particular patient, the right optic nerve has sustained more glaucomatous damage.
Since the primary cause of glaucoma is high IOP, the treatment is to lower IOP to minimize optic nerve damage by the following methods:
Eyedrops are the first line treatment to lower IOP. Many medications are commercially available by prescription, and each has their unique side effect profile. Talk to your physician about the best medication for your glaucoma treatment.
If eyedrops fail to adequately lower IOP, a laser procedure called trabeculoplasty can be performed to enhance the eye’s drainage system, thereby lowering the IOP. This procedure can be done in the office without going to the operating room, and only requires topical eyedrops to anesthetize the eye before starting the procedure.
If both eyedrops and laser trabeculoplasty fail to adequately lower one’s IOP, then surgical intervention is recommended to create a permanent diversion of fluid from inside the eye to its surrounding tissues to lower IOP. This procedure must be performed in the operating room.
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