What Kind of Eye Disease is Glaucoma?
Glaucoma is not a simple eye disease where the eye pressure is elevated and therefore you have glaucoma. Glaucoma is caused by a variety of etiologies and presents clinically in a variety of forms. In simple terms, glaucoma is a disease process involving the optic nerve inside the back portion of the eye.
The optic nerve is the size of a thin pencil and contains nearly 1.5 million nerve fibers. The optic nerve transfers the visual information that the retina has collected to the vision center in the occipital lobe in the back portion of the brain.
Glaucoma develops secondary to eye pressure or intraocular pressure inside the eye being higher than what the optic nerve can tolerate. This eye pressure leads to a slow death or damage to the optic nerve fibers, which results in a slow, progressive loss of vision. As the nerve fibers die, a person slowly loses vision, with only side vision in the beginning but can lead to blindness that may not be perceived by a person until significant damage has occurred.
There are approximately 3.5 million people in the United States with glaucoma. Almost half of the people with glaucoma do not know they have it. Glaucoma is the second leading cause of permanent blindness in people over age 65. Macular degeneration is the leading cause of permanent blindness in people over 65 in the United States. Worldwide, only cataracts cause more blindness than glaucoma.
Why Do People Get Glaucoma?
The eye normally slowly produces a fluid inside the eye called the aqueous humor. This fluid contains nutritional substances needed by different tissues inside the eye and helps maintain the normal shape and structure of the eye. The aqueous humor is produced by a structure called the ciliary body which is located behind the colored portion of the eye called the iris. The aqueous humor flows from behind the iris and through the pupil into the anterior chamber (the anterior chamber is the fluid space between the iris and the cornea which is the clear portion of the front of the eye).
The trabecular meshwork has the appearance of a cheesecloth. The aqueous humor has to filter its way through this cheesecloth membrane into a drainage channel called Schlemm’s canal which circles all the way around the eye. The fluid flows from Schlemm’s canal into venous canals which are located on the surface of the eye. The aqueous humor enters the veins and does not exit onto the surface of the eye. There is a steady state of fluid production inside the eye and flow from inside the eye, maintaining normal eye pressure.
The most common type of glaucoma occurs secondary to the trabecular meshwork gradually becoming obstructed with debris. This material makes it more difficult for the fluid to flow out of the anterior chamber. This obstruction raises intraocular pressure. This increased eye pressure over time causes damage to the optic nerve, resulting in blindness.
The theory of glaucoma damage is that the intraocular pressure limits or diminishes the blood flow in the optic nerve. This blood flow reduction and therefore diminished oxygen supply causes damage to the nerve. It is similar to blowing up a balloon. As more air goes into the balloon, it becomes tighter and there is an increased resistance to force more air into the balloon. Elevated eye pressure reduces the flow of blood into the eye and diminishes oxygen to the nerve.
There are many other factors and risks related to the development of glaucoma. Increased eye pressure is the major factor in glaucoma, but there are many others as well.
Who is Going to Develop Glaucoma?
The chance of developing glaucoma increases with age. Glaucoma begins to develop in most people after age sixty. There are exceptions to the rule. African-Americans are three times more likely to have glaucoma compared to Caucasians in the United States. Latins, Hispanics, American Indians, and Filipino Americans develop glaucoma at a much higher incidence.
African-Americans may have higher rates of glaucoma because studies have shown they have thinner corneas. In thin corneas, the eye pressure is actually measured lower than what is really inside the eye. A less common form of glaucoma, called normal pressure or low-pressure glaucoma, occurs more often in Filipino Americans.
There are several genetic factors that have been isolated in causing the development of glaucoma. Recent clinical studies have shown that consistent routine exercise may help reduce the risk of glaucoma.
What are the Risk Factors for Glaucoma?
The nickname for glaucoma is “thief in the night” or “silent thief of sight.” Most types of glaucoma do not cause any symptoms in the beginning. Symptoms do not occur until there has been significant damage to the optic nerve.
In the beginning, there is not any blurred vision, as that usually does not happen until the end stage of the disease. Loss of the central clear vision does not usually occur until the end. Over months and years of time, glaucoma slowly causes loss of side vision. This loss of vision is so insidious that most people are totally unaware of the loss. The brain is very astute and will compensate for the loss of side vision. You will start turning your head more or your brain will have you move your eyes more to compensate for this loss. These are all happening with you completely unaware.
Most of the time, people with more advanced glaucoma with a significant loss of their side vision are not aware of the loss until they are tested and are shown the loss. Unfortunately, there can be significant and irreversible vision loss from glaucoma before it is diagnosed.