In an advanced glaucoma, the visual field in the peripheral retina is decreased or lost, leaving only the vision in the central retina (macular area) which is still intact. It is called the “tunnel vision.” The increase in eye pressure occurs when too much aqueous fluid enters the eye and/but not enough of the aqueous fluid is leaving the eye. Eye pressure can be measured using “tonometry” test.
Normally, fluid enters the eye by seeping out of the blood vessels, it occurs in the ciliary body. This fluid if flowing way past the crystalline lens, through the pupil (the central opening in the iris), into the irido-corneal angle, the anatomical angle formed where the iris and the cornea come together. And the fluid passes through the trabecular meshwork in the angle and leaves the eye, via the canal of Schlemm.
If the rate of aqueous fluid entering the eye is too much, or if the trabecular meshwork “drain” gets clogged (for example, with debris or cells) that the fluid is not able to leave the eye quickly enough, the pressure is increasing in what is known as “open angle glaucoma.” It usually occurs naturally with the increasing age.
Open angle glaucoma, which is a chronic and painless condition. It can happen when the posterior portion of the iris, surrounding the pupil, adheres to the anterior surface of the lens (creating a “pupillary block”). This could deter/prevent intraocular fluid from passing through the pupil into the anterior chamber.
However, if the angle between an iris and the cornea is too narrow, or is even closed, then the fluid backs up because it cannot flow out of the eye correctly/normally. This will increase the intraocular pressure which is known as “closed angle glaucoma.” Usually, there is an acute, and sudden painful onset. It might be accompanied by the appearance of rainbow-colored rings around the white lights.
An internal pressure which is more than what the eye can tolerate can deform the lamina cribrosa, the small cartilaginous section of the sclera in the back of the eye through which the optic nerve passes. A deformed lamina cribrosa seems to “pinch” nerve fibers passing though it, it will finally causes axon death. Untreated glaucoma will result in optic atrophy and blindness.
Eye pressure is measured using a “tonometer” (and the test is called “tonometry”), and the standard tonometer generally is considered to be the “Goldmann tonometer.” The normal range of intraocular pressure (IOP) is 10 mm Hg to 21 mm Hg, with an average of about 16 mm Hg. Typically, eyes with intraocular pressure measurements of 21 mm Hg or higher, using a Goldmann tonometer, are considered to be “ocular hypertensive” and it is a possibility of glaucoma.
Although glaucoma is associated with the increase of IOP, the amount of pressure which is causing glaucoma varies from person to person and eye to eye. Many people with glaucoma actually have IOP’s in the normal range (“low tension” glaucoma), and it indicates that their lamina cribrosas are too weak to withstand even the normal amount of pressure. Conversely, some people with IOP’s which is considered to be high have actually no evidence of glaucomatous damage whatsoever.
Glaucomatous changes in the optic disk (optic nerve head) usually can be detected in the long run (over time). If the optic cup within the optic disk increases in size over a period of months or years, if notching is observed anywhere around the nerve head rim, and/or if an asymmetry is observed between the optic cups of the two eyes, then that person can be seen as a “glaucoma suspect.” In glaucoma, optic nerve rim atrophy and/or notching, with a corresponding visual field decrease, usually will occur.
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