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.
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.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
Having a sore on your eye or an eye infection. This increases your risk of an eye injury during the test.
Being extremely nearsighted, having an irregularly shaped cornea, or having had major eye surgery in the past.
Blinking or squeezing your eyes shut during the test.
Having had laser refractive surgery (such as LASIK).
What To Think About
Tonometry tests may be done over months or years to check for glaucoma. Also, because intraocular pressure (IOP) can change at different times of the day, tonometry is not the only test done to check for glaucoma. If the IOP is high, more tests, such as ophthalmoscopy, gonioscopy, and visual field testing, may be done.
Pachymetry uses ultrasound to measure the thickness of the cornea. The thickness of the cornea can affect IOP measurement. Pachymetry is often done during a tonometry test. It can help your doctor know your chance for developing glaucoma.
Normal IOP is different from person to person. About 40% to 50% of people who have optic nerve damage caused by glaucoma have normal IOP.2, 3 In some cases of glaucoma there is damage to the optic nerve even though the eye pressure is never above normal.