Intraocular pressure (IOP) is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk of glaucoma. Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg). Intraocular pressure is measured with a tonometer as part of a comprehensive eye examination.
The eye is a closed ball filled with clear jelly (vitreous humor) in the back behind the lens and clear fluid (aqueous humor) in the front, between the iris and the cornea. Aqueous humor is created just behind the iris and is in continuous circulation throughout the front part of the eye before it drains out just in front of the iris where it meets the cornea. This fluid helps keep the eye "inflated" just like air inside a balloon.
We can measure pressure of the eye just like you can gauge how full a balloon is by poking the balloon with your finger. The less air or pressure in the balloon, the easier it will be to poke and conversely, the more air or pressure in the balloon, the stiffer the balloon will be and the harder it will be to poke it. We measure pressure in the eye the same way; by gently touching the cornea with special instruments to see how hard it is to "poke." The unit of measurement is millimeters of mercury, or mmHg.
Pressure in the human eye, known as intraocular pressure, varies throughout the day with "normal" pressure being anywhere between 10 and 21 mmHg. Glaucoma is an eye disease in which the optic nerve is damaged by the pressure inside the eye. In most cases, the damaging pressure is greater than 21 mmHg, but there are some patients who can have damage at lower pressures (known as normotensive glaucoma). Conversely, there are some people who can tolerate higher-than-normal pressures without having optic nerve damage. These people are said to have "ocular hypertension". Eye pressure can vary with the thickness of the cornea—this is also measured when trying to interpret the significance of an eye pressure—reading in any one patient.
In addition to high pressure, risk factors for glaucoma include African or Hispanic ancestry, having diabetes and having a family member with glaucoma. Glaucoma does not typically cause pain and many cases of glaucoma go undiagnosed. Checking eye pressure is an important part of a comprehensive eye exam and one good reason to get regular check-ups by your ophthalmologist or eye care provider.