A tonometry test measures the pressure
inside your eye, which is called intraocular pressure (IOP). This test is used
to check for
glaucoma, an eye disease that can cause blindness by
damaging the nerve in the back of the eye (optic nerve ).
Damage to the optic nerve may be caused by a buildup of fluid that does not
drain properly out of the eye.
Tonometry measures IOP by recording
the resistance of your
cornea to pressure (indentation). Eyedrops to numb the
surface of your eye are used with most of the following methods.
Applanation (Goldmann) tonometry. This type of tonometry uses a small probe to gently flatten
part of your cornea to measure eye pressure and a microscope called a slit lamp
to look at your eye. The pressure in your eye is measured by how much force is
needed to flatten your cornea. This type of tonometry is very accurate and is
often used to measure IOP after a simple screening test (such as air-puff
tonometry) finds an increased IOP.
Electronic indentation tonometry. Electronic tonometry is being used more often to
check for increased IOP. Although it is very accurate, electronic tonometry
results can be different than applanation tonometry. Your doctor gently places
the rounded tip of a tool that looks like a pen directly on your cornea. The
IOP reading shows on a small computer panel.
Noncontact tonometry (pneumotonometry). Noncontact (or
air-puff) tonometry does not touch your eye but uses a puff of air to flatten
your cornea. This type of tonometry is not the best way to measure intraocular
pressure. But it is often used as a simple way to check for high IOP and is the
easiest way to test children. This type of tonometry
does not use numbing eyedrops.
Why It Is Done
Tonometry may be done:
- As part of a regular eye examination to check
for increased intraocular pressure (IOP), which increases your risk of
- To check the treatment for glaucoma. Tonometry can be
used to see if medicine is keeping your IOP below a certain
target pressure set by your doctor.
How To Prepare
Tell your doctor if you or someone in
your family has glaucoma or risk factors for glaucoma.
If you wear
contact lenses, remove them before the test. Do not put your contacts back in
for 2 hours after the test. Bring your eyeglasses to wear after the test until
you can wear your contact lenses.
Loosen or remove any tight
clothing around your neck. Pressure on the veins in your neck can increase the
pressure inside your eyes. Stay relaxed.
How It Is Done
Tonometry takes only a few minutes to
Applanation (Goldmann) method
This type of
tonometry is done by an ophthalmologist or an optometrist. Your doctor will use
eyedrops to numb the surface of your eyes so that you will not feel the
tonometer during the test. A strip of paper containing a dye (fluorescein) will
be touched to your eye, or eyedrops containing the dye will be applied. The dye
makes it easier for your doctor to see your cornea.