Tonometry is the measurement of pressure within the eye (called intraocular pressure) to determine if glaucoma is present. Intraocular pressure (IOP) is the pressure exerted against the outer layers by the eyeball. Tonometry is performed using an instrument called the tonometer.

What Does Tonometry Reveal?

Tonometry measures intraocular pressure to diagnose glaucoma. Glaucoma is caused by the buildup of fluid within the eye. Abnormally high pressure can damage the optic nerve, leading to vision loss.

Tonometry is also useful for identifying low IOP, which may occur with anterior uveitis (inflammation within the eye) or following intraocular surgery. Low IOP is also associated with dehydration.

Normal values vary between species of animals, and sometimes vary between breeds and individuals. Normal values are also affected by the technique used to measure them, so values are usually given as a range.

Normal ranges are as follows:
Cat: 15-30 mmHg

Values that differ between eyes are considered significant if the difference is > 8 mmHg.

How is Tonometry Performed?

There are two methods of measuring IOP in animals: indentation tonometry and applanation tonometry.

With indentation tonometry, the measuring instrument applies a rod of a certain weight to the surface of the cornea. The distance that the rod indents the cornea is inversely related to the pressure within the eye. In other words, the softer the eye, the more the rod indents the cornea. The harder the eye and higher the IOP, the less indentation occurs.

The primary tool used for indentation tonometry is the Schiotz tonometer. When using the Schiotz tonometer, the distance the rod moves is reflected in the movement of a small metal tone arm across a number scale on the top of the instrument. This number is then converted to mmHg through the use of two conversion scales specifically established for the dog and the cat. In order to get an accurate reading, the animal's nose is pointed toward the ceiling so the Shiotz can utilize the effects of gravity to indent the cornea.

With applanation tonometry, the force required to flatten a given area of cornea is subjected to a mathematical calculation. This calculation states that the force required to flatten a certain area of a sphere is equal to the pressure within that sphere.

In human medicine, an air jet is used to test for glaucoma – that's the brief blast of air against the eyeball most people are familiar with. In veterinary medicine, electronic and pneumatic versions are more commonly used to apply force to the cornea.

Is Tonometry Painful?

Tonometry is not a painful procedure. The human method of tonometry, using an air jet tonometer, is not often performed because it is more uncomfortable for pets, as it is for humans.

Does Tonometry Require Sedation?

Sedation is not required. A local anesthetic eye drop, such as proparacaine, is used to numb the eyeball. This allows the veterinarian to examine the eye without alarming the patient and for the pet not to feel the minor presence of the tonometry .