An air puff pressure reading for example detects only a certain percentage of those with glaucoma. In the center of the nerve is a small depression in the center of which is a hole carrying veins and arteries into and out of the eye. In glaucoma damage occurs at the weakest point, which is this depression, or cup. Nerve fiber damage occurs because of a relative inability to tolerate eye pressure by nerve fibers, either because the eye pressure is very high, the nerve circulation is very poor, or a combination of the above. By the time visual loss can be documented, considerable permanent loss of nerve fibers has occurred.
Larger cups are less resistant to eye pressure.
There is a genetic predisposition to glaucoma in many cases. With increasing damage the cup enlarges, destroying surrounding nerve fibers. The presenting symptoms are related to the rate at which these fibers become damaged, and the fact that the fibers immediately surrounding the cup represent distant side vision and are not easily noticed as damaged.