What Is Tonometry?

Medically Reviewed by Whitney Seltman, OD on May 30, 2023
3 min read

Tonometry is a quick and simple test that checks the pressure inside your eyes. The results can help your doctor see if you’re at risk for glaucoma.

Glaucoma is a disease in which the nerve of the eye (the optic nerve) is gradually damaged over time, resulting in a loss of vision. This disease is often associated with increased eye pressure. Identifying this elevated eye pressure and trying to lower it is the only known way to prevent blindness from glaucoma. Early screening can help protect your eyesight and slow down vision loss.

Your eyes are filled with different fluids that keep them healthy. New fluid is constantly being made and old fluid drained out. But if this drainage system gets plugged, the fluids build up. That causes the pressure inside your eyes to rise.

Sometimes the pressure is caused by an eye injury or trauma. Once your eye heals, everything may go back to normal. But some people have a drainage system that doesn’t work like it should.

Over time, high pressure inside your eye can damage your optic nerve, which sends images from your eyes to your brain. Left untreated, it can cause glaucoma.

Since the disease usually does not have symptoms, regular eye exams are a good idea. Your eye doctor will check your eye pressure by doing a tonometry test. They can also track any changes in pressure over time.

Anyone can get glaucoma, but your odds are higher if you:

  • Are over 40
  • Have a family member with glaucoma
  • Are African, Hispanic or Asian
  • Have had an eye injury
  • Are far-sighted or near-sighted
  • Have been told your corneas are thin in the middle
  • Have diabetes
  • Get migraines
  • Have circulation (blood flow) problems

Your doctor can check your eye pressure in a couple of different ways:

  • Tonometer/Applanation. After special numbing drops are put in your eye, your doctor gently holds this pencil-shaped device against the outside of your eyeball. The reading tells how well your cornea pushes back.
  • “Puff of air.” Your doctor could also use an instrument that blows a small puff of air at your eye while you look into a light. This also measures the pressure inside your eyeball.

Both these methods are painless and only last a few seconds. If your doctor does the “puff of air” test, you may feel a small amount of pressure against your eye.

Your doctor will share the results with you right away.

Eye pressure is different from person to person. Normally, it lies somewhere between 12-22 mmHg (“millimeters of Mercury”). Most people diagnosed with glaucoma have an eye pressure above 20 mmHg.

If your eye pressure is high but your optic nerve looks normal, you may have what’s called “ocular hypertension.” You might not have any symptoms, but this could lead to glaucoma over time.

Some people with ocular hypertension never end up with glaucoma. Others develop it even though their eye pressure falls within a normal range. Because of this, tonometry is only part of a complete eye exam. These results, along with other vision tests, help your doctor get a better idea of your eye health. They’ll also talk to you about your health history and any symptoms you’re having.

If the test shows you have high pressure in your eyes, your doctor may keep a close watch on it by having you come in for regular testing. Or, they could decide to lower the pressure by prescribing eye drops that you’ll take every day. They’ll help protect your optic nerve from future damage and, in the long run, could save your sight.