Tonometry is a procedure that measures the pressure inside the eyes. The test is used to screen for glaucoma, a disease in which pressure inside the eyes increases to the point that it impairs vision. If left untreated, glaucoma may cause blindness.

There are several methods of testing for glaucoma.

The applanation method measures the force required to flatten a certain area of the cornea. A fine strip of paper stained with orange dye (fluorescein) is touched to the side of the eye. The dye stains the front of the eye to help with the examination, then rinses out with tears. An anesthetic drop is also placed in the eye.

The slit-lamp is placed in front of you and you rest your chin and forehead on a support that keeps your head steady. The lamp is moved forward until the tonometer touches the cornea. The light is usually a blue circle. The health care provider looks through the eyepiece on the lamp and adjusts the tension on the tonometer. There is no discomfort associated with the test.

A slightly different method of applanation uses a portable object similar to pencil. Again, you are given anesthetic eye drops to prevent any discomfort. The device touches the outside of the eye and an instant digital measurement is recorded.

The last method is the noncontact method (air puff). In this method, your chin is resting on a padded stand. You stare straight into the examining instrument. The examiner will shine a bright light into your eye to properly align the instrument. A brief puff of air is blown at your eye. The instrument calculates pressure from the change in the light reflected off the corneas as the air puff is blown.

Remove contact lenses before the examination. The dye can permanently stain contact lenses.

Inform the health care provider if you have corneal ulcers and infections, an eye infection, if you are taking any drugs, or if you have a history of glaucoma in your family.

There should be no pain with the applanation method because of the anesthetic. In the noncontact method, you may feel mild pressure on your eye.

People over 40 years old, especially African Americans, are at the highest risk for developing glaucoma. If glaucoma is detected early, it can usually be treated. However, it may go unrecognized for years, because there usually are no symptoms.