High eye pressure¹¹ Higher risk of glaucoma among African-Americans; American Academy of Ophthalmology. Available at http://www.glaucoma.org/learn/africanamerican.php. Accessed November 12, 2009.

In some ways, your eye is like a sink where the faucet is always running. In the eye, there is a fluid that is constantly circulating. It is called aqueous humor, and it contains vital nutrients to nourish the eye. It flows between the iris and the lens.

The “drain” of your eye is tissue called the trabecular meshwork. In a sink, if the drain becomes clogged, the water will eventually overflow. But when the “drain” in the eye becomes clogged, the fluid that is inside the eye cannot overflow. So it remains in the eye and begins to back up. This causes pressure to build up within the eye. The pressure is called intraocular pressure (IOP).

When glaucoma occurs.
Glaucoma damage is most often the result of the aqueous fluid passing too slowly through the trabecular meshwork drainage system, increasing the pressure inside the eye. This extra force presses on the optic nerve, a bundle of 1.2 million nerve fibers in the back of the eye, that transmits images from your eye to your brain. The pressure against the optic nerve causes damage to the nerve fibers.

Unless high eye pressure is controlled, it can damage the optic nerve and, over time, cause vision loss. That's why early detection and diagnosis is critical to controlling IOP and to maintaining vision.

When is eye pressure considered high?
While IOP differs with each person, the average IOP in a normal population is 14-16 millimeters of mercury (mm Hg). Even pressures up to 20 mm Hg may be within normal range. But a pressure of 21 or higher is considered suspicious and possibly abnormal.

Not everyone with high eye pressure develops glaucoma
Everyone's optic nerve is different. An IOP measurement deemed normal in some eyes might be considered high in others. It depends on the level of pressure that your optic nerve can tolerate without being damaged. This level is different for each person. In fact, that's one of the questions researchers continue to explore: What causes one person to develop damage from high eye pressure while another doesn't?

Talk to your doctor to find the best options for keeping your IOP under control.

A more helpful measurement than IOP alone.
Abnormal fluctuation in IOP is a more helpful measurement than just high IOP. Normal IOP can fluctuate up to 5 mm Hg over a 24-hour daily cycle. This occurs in about 30 percent of the population. But in the most common form of glaucoma, primary open-angle glaucoma (POAG), this fluctuation is more pronounced and occurs in about 90 percent of patients. So several IOP measurements should be taken throughout the day to arrive at the most accurate diagnosis. An IOP discrepancy of 5 mm Hg or higher between the two eyes also should be viewed with suspicion.

IOP “after hours.”
IOP is generally measured during doctors' hours. But high IOP is a 24-hour condition. What happens to it during the night? People used to think that IOP was lower at this time²² Review of Ophthalmology; Nocturnal IOP Matters in Glaucoma Management, Donald L. Budenz, MD, MPH, Vol. No: 15:06Issue: 6/1/2008, Copyright© 2000 - 2009 Jobson Medical Information LLC. Accessed through http://www.revophth.com/index.asp?page=1_13861.htm , but we now know that eye pressure actually goes up at night, when patients lie down³³ EuroTimes, European Society of Cataract and Refractive Surgeons (ESCRS): Nocturnal IOP; Arthur J. Sit. Accessed through http://www.escrs.org/PUBLICATIONS/EUROTIMES/07mar/ NocturnalIOP.pd .  Because IOP must be managed around the clock, medication should be taken as your doctor directs.

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