A tonometry test measures the pressure inside your eye, which is called intraocular pressure (IOP). This test is used to check for glaucoma, an eye disease that can cause blindness by damaging the nerve in the back of the eye (optic nerve). Damage to the optic nerve may be caused by a buildup of fluid that does not drain properly out of the eye. See a picture of the eye.

Tonometry measures IOP by recording the resistance of your cornea to pressure (indentation). Eyedrops to numb the surface of your eye are used with most of the following methods.

Tonometry methods

• Applanation (Goldmann) tonometry. This type of tonometry uses a small probe to gently flatten part of your cornea to measure eye pressure and a microscope called a slit lamp to look at your eye. The pressure in your eye is measured by how much force is needed to flatten your cornea. This type of tonometry is very accurate and is often used to measure IOP after a simple screening test (such as air-puff tonometry) finds an increased IOP.
• Electronic indentation tonometry. Electronic tonometry is being used more often to check for increased IOP. Although it is very accurate, electronic tonometry results can be different than applanation tonometry. Your doctor gently places the rounded tip of a tool that looks like a pen directly on your cornea. The IOP reading shows on a small computer panel.
• Noncontact tonometry (pneumotonometry). Noncontact (or air-puff) tonometry does not touch your eye but uses a puff of air to flatten your cornea. This type of tonometry is not the best way to measure intraocular pressure. But it is often used as a simple way to check for high IOP and is the easiest way to test children. It may also be used for people who have had laser-assisted in-situ keratomileusis (LASIK) surgery. Noncontact tonometry does not use numbing eyedrops.
• Indentation (Schiotz) tonometry. This type of tonometry uses a plunger to gently push on your cornea. The pressure in your eye is measured by how much weight is needed to flatten your cornea. This test is not as accurate as applanation tonometry and is not used very much by ophthalmologists and optometrists. However, other doctors, such as family medicine doctors or urgent care doctors, may still commonly use this test.

Why It Is Done

Tonometry may be done:

• As part of a regular eye examination to check for increased intraocular pressure (IOP), which increases your risk of glaucoma.
• To check the treatment for glaucoma. Tonometry can be used to see if medicine is keeping your IOP below a certain target pressure set by your doctor.

How To Prepare

Tell your doctor if you or someone in your family has glaucoma or risk factors for glaucoma.