|
WHAT IS GLAUCOMA?
This term is used to describe progressive damage to the optic nerve - the connection between the eye and the brain. It is specific for damage occurring in the eye. The causes for the loss of nerve tissue are manifold. That means there are different diseases that result in glaucoma and one person's glaucoma might be very different from the next person's.
The optic nerve is formed by the fibers of the ganglion cells, which are located in the retina, the film which lines the back of the eye and receives the light. One can think of them as the fiberoptic cables which connect the eye to the brain. Except these are living structures and as such depend on the flow of substances to and from the brain. Although the information is conducted by a weak electrical current, without some feedback from the brain in the form of chemicals (even tiny body parts have their dependencies) the ganglion cells will die. If damage occurs, it most commonly does so to the nerve fibers at the point where they are leaving the eye. This can be thought of as a narrow juncture through which the fibers have to squeeze to get through the coat of the eye. The part of the passage which is visible when looking into the eye is called the optic disc. Once damage has occurred to a nerve fiber, the bidirectional flow is interrupted and the originating ganglion cell dies. Once lost, it cannot be re-created.
The portion of the optic nerve that is within the eye is especially susceptible in four different ways. It can be damaged by pressure within the eye, which one can conceptualize as squeezing the nerve. Secondly, a compromise of the blood supply of the nerve will result in malnutrition of the nerve and loss of function. Thirdly, the cells which supply the nerve fibers may be genetically programmed to cease function at a certain point. Lastly, as cells die, they shed toxic substances which can poison their neighbours. TOP
 As the structures supporting the nerve vary in their strength from patient to patient, different levels of pressure create problems in different patients. Further, a nerve that is already badly damaged becomes more susceptible to pressure. Hence, there is no absolute upper limit of normal pressure that applies to all patients. The pressure within the eye is measured in millimetres of mercury. The official normal is defined to include 95% of those people who have normal eyes and no glaucoma. This means that there are 5% of the population with pressures above or below the normal range who still are normal and not at risk of any disease. The normal numbers, again only including 95% of normal, range from 10-21 with a median of 15 mm of mercury. Hence, at least 2.5% of the population who are normal have a pressure of greater than 21 by definition alone. Actually, this number is even higher as the distribution of normal people is not even and is slanted towards the higher end. As the incidence of glaucoma is about 1% in the population, there are far more normal people with high eye pressures than people with glaucoma. The difficult part is to separate those people who need treatment from those who don't. Once the pressure rises to 30 mm of mercury or higher the eye is at increased risk of a stroke in addition to glaucoma and often treatment is elected regardless.
One needs to remember, however, that pressure is a risk factor and does in itself not equate with the presence of glaucoma, and that a pressure within the normal range can damage susceptible eyes. The same holds true for the blood supply, although it has proven very difficult to measure it very accurately. Certainly, diseases like diabetes and hypertension can damage the blood vessels and create a setting in which the nerve is very easily damaged. Certain patients suffer from spasms of the blood vessels which might deprive the nerve temporarily of blood supply. Others experience a low blood pressure during sleep which could damage the nerve.
Genetics are certainly a factor. A family history of glaucoma carries a 10-50% risk of developing the disease during one's life. However, different genes are affected and usually the problem involves several genes as well as interactions with the environment. Hence, one genetic defect can cause two totally different types of glaucoma. At this point only a handful of genes associated with glaucoma are known covering about 1% of the patients with glaucoma.
It is now clear that the dying ganglion cells shed substances, especially an amino acid called glutamate, that are toxic to their neighbours. This may well set a cycle in motion whereby one dead cell triggers a circle of death around itself. TOP
Disclaimer To go forward, please acknowledge the following statement: The material provided on this site is designed for information and educational purposes only. The materials are not intended to be a self diagnostic and/or self treatment tool. We encourage you to use this information as a tool for discussing your situation with your health practitioner. |
|
|
