Glaucoma: A Leading Cause of Blindness
Medical experts estimate that over three million Americans have glaucoma, but only half of those are aware that they have the disease. Glaucoma is dangerous. Awareness is our best defense. Widespread misconceptions about glaucoma contribute to unnecessary loss of vision. A majority of adults incorrectly believe that symptoms of pressure or pain in the eyes are associated with the early stages of this disease and that vision loss from glaucoma can be corrected. Both are mistaken beliefs that impede detection and treatment of glaucoma in its early stages when loss of sight can be prevented.
What is Glaucoma?
Glaucoma is a group of eye diseases that gradually steals sight without warning and often without symptoms. There are several forms of glaucoma (see Glossary of Glaucoma). Some are the result of other underlying ocular abnormalities. The two main types of glaucoma are primary open-angle glaucoma (POAG), and angle-closure glaucoma.
To best understand glaucoma, it is helpful to view the front of the eye as an elaborate drainage system. Fluid, known as aqueous humor, is produced within the eye and flows out through a drainage channel located in the front of the eye. This fluid exerts a force on the eye creating an intraocular pressure (IOP). In individuals with glaucoma, the drainage system is abnormal and the inflow exceeds the outflow. Pressure builds up within the eye causing an elevated intraocular pressure.
Glaucoma is a curious disease. Changes in the flow of fluid in the front of the eye can cause damage to the delicate tissues at the back of the eye. At particular risk is the optic nerve (see diagram), which is a bundle of more than one million nerve fibers. It connects the retina to the brain and is responsible for carrying the images we see to the brain. Overtime, when individual optic fibers die, the optic nerve loses substance. Ophthalmologists looking into the back of the eye recognize this loss of substance as cupping (illustrated on the front cover) because the nerve is excavated causing a cupped-out area that normally contains the nerve fibers. The visual loss that occurs as a result of optic nerve damage causes defects in the peripheral vision (side vision). Typically, the peripheral vision will slowly reduce, often without other symptoms, until only a narrowed tunnel of vision remains. Glaucoma may also displace the blood vessels that run through the optic nerve. Because the optic nerve is not capable of regeneration, the damage that occurs is irreversible and loss of vision is permanent. The damage can lead to blindness.
Primary open-angle glaucoma (POAG) is the most common form of the disease. It occurs when, overtime, the eye’s drainage channels become clogged. The inner eye pressure-also called intraocular pressure (IOP)—rises because the correct amount of fluid cannot drain out of the eye. At first, POAG produces no symptoms and no early warning signs. It causes no pain. Vision stays normal. If POAG is not diagnosed and treated, it can cause a gradual loss of vision. Without regular, complete eye examinations, a patient’s glaucoma may remain undetected for many years and cause significant, irretrievable, and permanent loss of vision. We do not yet understand the chain of events that results in POAG, thus we have no cure for this form of the disease. However, if caught early and treated appropriately, POAG usually responds well to medication.
Angle-closure glaucoma is also known as narrow angle glaucoma and is different from open-angle glaucoma in that the eye pressure usually rises suddenly. This pressure change occurs when the drainage channels get blocked or covered, such as debris blocking the drain in a sink. Angle-closure glaucoma generally causes acute and noticeable symptoms that include headaches, severe eye pain, nausea, redness of the eye, blurred vision, and halos around bright lights. If you have these symptoms, seek treatment immediately. This is a medical emergency. If your doctor is unavailable, go to the nearest hospital or clinic. Without treatment to improve the flow of fluid, the eye can become blind in a short period of time (1-2 days).
Gonioscopy, a simple test performed in the ophthalmologist’s office, determines if the angle is normal and wide or narrow and abnormal, and therefore, capable of closing. Treatment of angle- closure glaucoma involves laser surgery to remove a small portion of the outer edge of the iris. This relieves the blocked drainage channels. Usually, surgery is successful and long lasting. However, you should still receive regular examinations.
Glaucoma is diagnosed through a comprehensive eye examination that includes: medical history, visual acuity, measurement of intraocular pressure with a special device called a tonometer, and a dilated eye examination (ophthalmoscopy), which allows the ophthalmologists to examine the interior of the eye, especially the retina and optic nerve. If the eye pressure is above normal limits or if the optic nerve looks suspicious, the doctor may perform two additional tests. One is a visual field test (perimetry), which maps out the field of vision and determines any loss of peripheral vision. The other is pachymetry that measures the thickness of the cornea, thought to be a factor in diagnosing glaucoma. All of these tests are simple, fast, and pain-free.