Encyclopedia - Tonometer

A tonometer is an instrument used by eye care professionals to measure the intraocular pressure of a patient's eye. This pressure measurement is an important part of every eye examination because an increase in pressure may signal the onset of glaucoma, a sight-threatening disease. Pressure is measured in millimeters of mercury, and, as a general guideline, pressure above 21 millimeters is considered to be elevated pressure, though not all persons with that reading have glaucoma.

The most popular tonometry test is with the noncontact tonometer, often referred to as the "air-puff" test. The patient looks through a machine as it blows a gentle puff of air at the eye. This puff of air flattens the cornea slightly in order to give the pressure reading. The machine is gas pressurized and does not require direct contact with the eye.

Another form of tonometry, and one of the most accurate, is the contact tonometer, an instrument that looks like a pen. When using the contact tonometer, numbing eye-drops are administered, and then the tip of the tonometer touches the eye and measures the pressure.

An applanation tonometer measures the force required to flatten a small area of central cornea. A topical anesthetic and fluorescein dye are instilled before the measurement is taken. One type of applanation tonometer attaches to a slit lamp, while another is hand-held.

The Schiotz tonometer measures the amount the cornea is indented by a fixed weight that artificially raises the pressure. It is a simple, portable instrument, but is considered to be somewhat less accurate than other types of measuring devices.