Tonometry
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).
Tonometry measures IOP by recording the resistance of your cornea to pressure (indentation). Eye drops to numb the surface of your eye are used with most of the following methods.
Tonometry Methods
Applanation 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 : Noncontact (or air-puff) tonometry does not touch your eye but uses a puff of air to flatten your cornea to measure intraocular pressure. 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 eye drops.
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 Glaucoma.
• 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
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 eye glasses 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 do.
Noncontact (or Air-Puff) Method
This type of tonometry is done by an ophthalmologist or an optometrist. You do not need drops to numb your eye for this method.
You will rest your chin on a padded support and stare straight into the machine. A brief puff of air is blown at your eye. You will hear the puffing sound and feel a coolness or mild pressure on your eye. The tonometer records the intraocular pressure (IOP) from the change in the light reflected off the cornea as it is indented by the air puff. The test may be done several times for each eye
With the air-puff (noncontact) method, there is no risk of scratches or infection, since nothing but air touches your eyes.
Do not rub your eyes for 30 minutes until the numbing medicine has worn off.
Results
A tonometry test measures the pressure inside your eye, which is called intraocular pressure (IOP). This test is used to check for Glaucoma.
Normal eye pressure is different for each person and is usually higher just after you wake up. IOP changes more in people who have glaucoma. Women usually have a higher IOP than men, and IOP normally gets higher as you get older.
| Intraocular pressure (IOP) | |
|---|---|
| Normal: | 10-21 millimeters of mercury (mm Hg) |
| Abnormal: | Higher than 21 mm Hg |
High values
• A high IOP may mean that you have glaucoma or that you are at high risk for developing glaucoma. People who have ongoing pressures above 27 mm Hg usually develop glaucoma unless the pressure is lowered with medicines.
• People who have an ongoing IOP higher than 21 mm Hg but do not have optic nerve damage have a condition called ocular hypertension. These people may be at risk for developing glaucoma over time.
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