Various types of are used to measure the inflow and outflow of aqueous in the eye. The pressure created by the amount of fluid in the eye is known as intraocular pressure (IOP). The tonometer measures this pressure to determine if it is in within normal limits. If it is high, the patient may be prone to glaucoma.
Most doctors use the tonometer to measure IOP. The applanation tonometer's IOP reading is very reliable because it flattens an area of the cornea instead of buckling the eye's surface around a plunger. In fact, the IOP reading from an applanation tonometer is very close to that of the undisturbed eye.
This diagnostic procedure requires that the patient's cornea be anesthetized. During this tonometry procedure the patient is seated in the exam chair. The patient focuses on a distant object while the instrument is moved until it flattens the corneal surface. When the corneal surface has a constant flattening, the IOP is read.
One of the most commonly used methods to screen for high IOP is the puff tonomenter. This diagnostic test is a form of applanation tonometery in which the instrument does not come in contact with the eye, but instead shoots out a puff of air. This puff flattens the cornea and reads the IOP. This test does not require corneal anesthesia as only a jet of air touches the eye. Because this instrument does not touch the eye, anyone in the doctor's office can do this initial exam. The patient simply sits in front of the puff tonometer and fixates on a red light while the operator focuses the machine. A quick puff of air is shot at the eye and provides an initial reading.
The accuracy of the IOP readings taken by this method is relatively low, but can provide a base-line reading for the doctor to review.
When deemed necessary, electronic tonography is used to test IOP. The IOP is measured and recorded over a 4-minute period, providing a rough calculation of aqueous outflow.