Tonometry is a measure of the pressure inside the eye, which is called intraocular pressure (IOP). This test is used to help detect glaucoma, a group of eye diseases that can cause blindness by damaging the nerve in the back of the eye (optic nerve). Damage to the optic nerve may be caused in part by fluid that fails to drain properly out of the eye.

Tonometry indirectly measures IOP by determining the resistance of the cornea to indentation. The most commonly used devices to measure IOP are:

• Noncontact tonometer (pneumotonometry). The noncontact (or air-puff) tonometer uses a puff of air to flatten the cornea. This type of tonometer is the least accurate way to measure intraocular pressure. It is often used as a simple way to screen for high IOP.
• Applanation (Goldmann) tonometer. This type of tonometer uses a special probe to flatten part of the cornea to measure eye pressure and a microscope called a slit lamp to examine the eye. The pressure within the eye is determined by how much weight is needed to flatten the cornea. This type of tonometer is very accurate and is often used to measure IOP after a simple screening test (such as air-puff tonometry) detects increased IOP.
• Indentation (Schiotz) tonometer. This type of tonometer uses a plunger to indent the cornea. After placing a drop of anesthesia on the eye to numb its surface, the doctor carefully places the instrument directly on the cornea. The pressure within the eye is determined by measuring how much the cornea is indented by a given weight. This test is less accurate than applanation tonometry and is less commonly used today by ophthalmologists and optometrists. However, other health professionals (such as family practice doctors or urgent care doctors) may still commonly use this test.
• Electronic indentation tonometer. Electronic tonometers are being used more frequently to detect increased IOP. Electronic tonometry may be very accurate, but can be more variable than applanation tonometry. After placing a drop of anesthesia on the eye to numb its surface, the doctor carefully places the rounded tip of a small penlike instrument directly on the cornea. The IOP reading is then digitally displayed on a liquid crystal display (LCD) panel.

Tonometry
Results

Normal eye pressure (intraocular pressure, or IOP) varies for each individual and is usually higher just after a person wakes up. IOP varies more in people who have glaucoma. Women usually have a higher IOP than men, and IOP usually increases as a person gets older.

Greater than normal values may mean

• A high intraocular pressure (IOP) may indicate that you have glaucoma or that you are at increased risk for developing glaucoma. People who have intraocular pressures consistently above 27 mm Hg usually develop glaucoma unless the pressure is lowered.
• Some people have an IOP consistently higher than 21 mm Hg but do not have optic nerve damage. This condition is called ocular hypertension. These people are at risk for developing glaucoma over time.