What Causes Glaucoma - Glaucoma: Eye Care

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What causes glaucoma?

Glaucoma is actually a group of eye diseases that cause vision loss through damage to the optic nerve. Doctors used to think that high pressure within the eye, called intraocular pressure (IOP), was the primary cause of this damage, but we now know that other factors besides pressure must also be involved, because some susceptible people with "normal" IOP can experience vision loss from glaucoma.

Normally, the aqueous humor — the liquid that fills the area behind the iris — circulates through the pupil into the front compartment of the eye, delivering nourishment to the lens and the cells lining the cornea. It then passes through a circular, sieve-like system of tissues called the trabecular meshwork and drains out of the eye through Schlemm's canal; from there, it is absorbed into surrounding blood vessels. The process works continuously as more aqueous humor is produced and excess fluid is eliminated through the trabecular meshwork to keep a healthy balance of pressure in the eye (see Figure 6).

Figure 6: Glaucoma In a healthy person, the ciliary body continuously produces aqueous humor, a clear liquid that circulates from the posterior chamber into the anterior chamber of the eye and helps maintain its shape and pressure. The fluid (see arrows) bathes and nourishes the interior of the eye, then drains through the trabecular meshwork into small blood vessels. If this sieve-like meshwork is blocked, aqueous humor accumulates and pressure increases.

In glaucoma, this drainage system breaks down, slowing or blocking the outflow of fluid. The fluid backs up in the eye, much like water in a clogged sink, and internal pressure rises. This, in turn, puts stress on the optic nerve, which provides the eye's wiring to the brain. If the pressure continues unabated, nerve fibers that carry the optical messages begin to die off, and vision starts to fade. Loss of vision may also result from the obstruction of tiny blood vessels that feed the retina and optic nerve. Nerve fibers on the outer edge are typically affected first, so vision loss begins with peripheral vision and gradually closes in until the cells supplying central vision are lost. The damage that occurs in glaucoma is irreversible.

While pressure plays an important role in most glaucoma cases, there seem to be other mechanisms causing cell death in nerve fibers. Scientists are now investigating ways to prevent this cell death through several different strategies that are collectively known as neuroprotection. However, these studies are preliminary, and drugs to lower intraocular pressure remain the mainstay of treatment for glaucoma (see "Medications for glaucoma").

Last updated:	June 19, 2007