Intraocular Pressure (IOP)

A tonometry test measures the pressure inside the eye, called intraocular pressure (IOP). This test is commonly used to check for glaucoma, an eye disease that can cause blindness by damaging the optic nerve in the back of the eye. Damage to the optic nerve may be caused by a buildup of fluid that does not drain properly out of the eye. (The eye normally produces 4mm of fluid each day).

Tonometry measures IOP by recording the resistance of the cornea to pressure (indentation). Eye drops to numb the surface of the eye may be used.

Tonometry Methods

Applanation (Goldmann) Tonometry uses a small probe to gently flatten part of the cornea to measure eye pressure, and a microscope, called a slit lamp, to look at the eye. The pressure in the eye is measured by how much force is needed to flatten the 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 is being used more often to check for increased IOP. Although it is very accurate, electronic tonometry results can be different than applanation tonometry. A doctor places the rounded tip of a tool that looks like a pen directly on the cornea. The IOP reading shows on a small computer panel.

Noncontact tonometry (Pneumotonometry), or air-puff tonometry does not touch the eye, but uses a puff of air to flatten the 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. This type of tonometry does not require numbing eye drops.

Why it is Done?

Tonometry may be done:

    • As part of a regular eye examination to check for increased intraocular pressure (IOP), which increases the risk of glaucoma.
    • To check the treatment for glaucoma. Tonometry can be used to see if medicine is keeping IOP below a certain target pressure set by a doctor.
    • To check if medications to control inflammation associated with Dry Eye, such as steroid drops or ointments, are increasing IOP.


WebMD Eye Health Center
Retrieved October 5, 2015 from