Glaucoma is a condition in which the eye pressure, also known as intra-ocular pressure (IOP), is too high for a given eye and thus causes damage to the optic nerve of that eye. (Note: each individual has a different pressure which causes damage to the optic nerve) Damage to the optic nerve results in progressive loss of peripheral or side vision. Central or straight ahead vision is preserved until the end of the disease (tunnel vision). Pressure is measured in mm of Hg (mercury). Normal pressure is between 10 - 21 mm Hg with 14 being the average. Prior to 1978, glaucoma was defined as a disease in which the pressure was above 21 mm Hg in an eye.
Today we know that this is not correct. Only 10-20% of the patients having an eye pressure between 21-24 mm Hg go on to develop loss of the peripheral vision or loss of side vision over a ten year period of time. Fifty percent loose peripheral vision if their pressure is consistently between 25-27 mm Hg, and 90% loose vision if the pressure is 30 mm Hg. The natural history of patients that have non-treated glaucoma is slow progression. Glaucoma is usually a slow disease.
Untreated glaucoma takes on an average of 15 years to progress from early damage to blindness with an IOP of 21 to 25 mmHg, 7 years with 25 to 30 mmHg, and 3 years with a pressure more than 30 mmHg. Thus, most eye doctors will treat a pressure over 30 with out signs of damage. We tend to treat a little earlier.
Like blood pressure the eye pressure (intra-ocular pressure) will vary from day to day and time of day, usually higher in the morning and lower in the early evening. Therefore, it is important to measure the pressure at different times of the day. Patients without glaucoma may vary 4 mm Hg while patients with glaucoma tend to vary more. Unless the pressure is very high, one reading is meaningless. In addition, it takes years for the pressure to damage the eyes, thus, time is on our side. One should use the time to make an appropriate decision without panicking.
Everybody's eye produces a fluid like water in it's middle chamber. This fluid then flows around inside the eye to the front chamber.
Then, from the front chamber the fluid leaves the eye by entering a drainage meshwork, like the drainpipe of a sink or bath. From this drainage system the fluid enters the bloodstream.
In the common type of glaucoma this drainage system can block. The fluid gets trapped in the eye, and the pressure inside the eye goes up like a tyre being blown up to much.
This pressure or fluid then presses on the nerve at the back of the eye. If the pressure is high or continues for a long time, usually years, the nerve at the back of the eye may become damaged, and eventually the sight may be affected. This pressure effect is shown by the red arrow in the diagram below.
Visual fields tests measure side or peripheral vision. Glaucoma causes loss of peripheral or side vision before central vision. It is not until late into the disease is that central or visual acuity is effected. The problem is that defects in visual fields do not show up until glaucoma is relatively advanced (over 50% of the nerve fibers must be lost before visual fields changes). Once visual fields changes are noted it is very sensitive to progression. Even with perfect control of eye pressure, a very, few patients will continue to loose fields. This occurs only in very advanced glaucoma. Previously, the best method for monitoring early glaucoma was careful evaluation of the optic nerve. As long as the nerve doesn't change, there is no progression. Newer tests use laser scanning (HRII) to create a three dimensional picture of the optic nerve. The scanning lasers are accurate and quantitative than the doctor just observing the nerve. These tests may replace visual field testing in detecting early glaucoma in the future. Late glaucoma is best followed with visual fields testing.