How Tonometry Eye Pressure Tests Work

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Tonometry is a test that measures intraocular eye pressure (IOP), or the pressure inside your eyes. It is used to screen you for risk of glaucoma. This is a condition that occurs when high pressure damages nerve fibers in the back of your eye, causing vision loss and sometimes blindness.

This test is an important part of a comprehensive eye examination. It is also used to assess the effectiveness of glaucoma treatment. Your healthcare provider may use one of several types of tonometry to measure the pressure inside your eyes.

This article looks at the different kinds of tonometry tests, what they're used for, and who should get them.

Intraocular pressure testing for glaucoma

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Why You Need a Tonometry Test

Glaucoma is a serious eye disease that causes damage to the optic nerve, which is in the back of your eye. The fibers that make up the optic nerve are very delicate. High pressure can cause irreparable damage to them, so tonometry is done to make sure your pressure is within the normal range.

This test is not perfect, as factors like corneal thickness can throw off the readings. Still, a result that is above normal warrants further investigation, since it indicates an increased risk of developing glaucoma. The higher the pressure, the greater the risk.

Glaucoma is most common in older adults, but it can happen at any age. There are several different types. Open-angle, the most common type, may take many years to develop. It does not cause pain and most people don't have symptoms until the later stages. Left untreated, it can lead to blindness.

Since loss of vision is usually the first sign of glaucoma, it's important to be diagnosed in the early stages. This is why it is important to get a tonometry test even if you have good vision.

When you receive an early diagnosis, your healthcare provider can prescribe medications to halt the progression of the disease before it causes vision loss.

It is also possible to have low eye pressure, or hypotony. This can sometimes happen after eye surgery, but it can also happen because of long-term eye inflammation. It can lead to problems with vision.

Who Should Be Tested?

Healthy adults with good vision should have a complete eye exam, including a tonometry test, once in their 20s, twice in their 30s, and at age 40. Your ophthalmologist will recommend a schedule for future exams based on your results.

Some conditions can put you at risk for eye disease. If you have any of these risk factors, you will need to schedule earlier and more frequent exams:

Your eye care provider may also perform a tonometry test if you have any of the following symptoms:

Tonometry can also be used to diagnose angle-closure glaucoma, a type of glaucoma that comes on suddenly. Symptoms can include:

  • Severe eye pain
  • Headache
  • Blurry vision
  • Seeing rainbow auras around lights
  • Loss of vision
  • Nausea and vomiting
  • Blurry vision
  • Red eyes

You may also need regular tonometry tests after you've been diagnosed with glaucoma. These will help your healthcare provider monitor how well your treatment is working.

Types of Tonometry Tests

An eye care provider uses a tonometer for this test. This instrument calculates how resistant your cornea—the clear, outer layer of the eye—is to an attempt to flatten it. This is done in different ways depending on the kind of tonometry test used.

Some are more accurate than others, but each has distinct advantages.

Goldmann Tonometry

The Goldmann applanation tonometer is the most common tonometry test.

If you wear contact lenses, you will need to remove them. Your healthcare provider will put anesthetic eye drops and a small amount of dye into your eyes. After a few minutes, you will be asked to position yourself in front of a slit lamp. This is an instrument that allows the provider to shine light into your eye and examine its structures.

You will need to place your chin on a chin rest and your forehead up against a bar. Leaning forward helps you get into the proper position.

Staying as still as possible, you will be asked to focus straight on something in the distance. Try to stay relaxed and don't hold your breath. The practitioner then holds up your upper eyelid and swings a blue light in front of your eye, which illuminates the dye.

A small probe attached to the slit lamp is moved close enough to your eye to just touch the cornea. It gently indents the cornea and the tonometer measures the force necessary to flatten the cornea.

This is then repeated on the other eye. When done, your healthcare provider records the readings and compares them to prior test results.

Non-Contact Tonometry

With non-contact tonometry (NCT), a gentle puff of air flattens the cornea. This is why it is also called the "air puff" test. Many prefer this test because it does not involve touching the eye with a probe. Studies have shown that this test is accurate even if the patient is wearing contact lenses.

For this test, you will sit at a machine and position yourself so that your eyes are properly aligned. Making sure your chin and forehead are in the designated rests helps, but the technician will check and adjust you further if needed.

The technician will ask you to look straight ahead at a light. There will be a quick burst of air. Since blinking invalidates the results, the reading may have to be taken more than once. The test is then repeated on the other eye.

While some studies show that NCT tonometry is not as accurate as Goldmann tonometry, it is still a good option for children or adults who are sensitive or have a hard time remaining very still.

Electronic Tonometry

An electronic tonometer is a handheld, mobile device that looks like a writing pen. It can be gently applied to your cornea like the probe used in Goldmann tonometry, but is quicker to use. You will need to remove your contact lenses before this test (if applicable).

Typically, this test needs to be repeated a few times in order to produce an accurate measurement. Overall, it's not as reliable or accurate as Goldmann tonometry.

Schiotz Tonometry

A Schiotz tonometer is an analog device that depresses the eye with a small metal plunger. The device calculates eye pressure by measuring the depth of the indentation on the cornea.

For this test, you will need to remove your contact lenses, if you wear them. You will lie flat and look at a spot straight above you. The technician will put a drop of anesthetic in each eye and pull back one eyelid. When you are relaxed, they will gently lower the probe onto your cornea.

Modern eye care practitioners don't use this type of tonometry as much as the other kinds. It is sometimes used in more remote settings, however.

Summary

Tonometry measures the pressure in the eye. Higher pressure in your eyes could put you at risk of developing glaucoma.

You may undergo a tonometry test as part of a complete eye exam. The test is most important when you are 40 or older or you have risk factors like diabetes and high blood pressure.

There are several different types of tonometry tests. Most involve touching the eye with a probe. If you are sensitive, a test that measures eye pressure with a puff of air is also an option.

A Word From Verywell

Tonometry can be done by either an optometrist (who specialized in vision care) or an ophthalmologist (who specializes in eye diseases). If your readings are high, however, you will need to see an ophthalmologist for further testing.

A visual field test, imaging studies, and an examination of your optic nerve can help determine whether or not you have glaucoma. 

High eye pressure alone doesn't mean you have glaucoma. It just makes you a suspect for the condition, and further testing can confirm (or discount) the diagnosis.

Frequently Asked Questions

  • How long does it take to do a tonometry test?

    Tonometry tests typically only take a minute or two. With contact tests, the probe will only touch your eye for a few seconds. The results of the test should be available right away. Your healthcare provider should discuss them with you before your appointment is complete.

  • Is the puff test for glaucoma accurate?

    Non-contact tonometry is sometimes called a "puff test." Many patients prefer it because the machine doesn't physically contact the cornea. The puff test is considered accurate, but it can sometimes overestimate the amount of pressure in the eye.

  • What is an OCT glaucoma test?

    An optical coherence tomography (OCT) test is a non-invasive test that can help diagnose glaucoma and a number of other eye conditions, including:

    During this test, a machine uses light waves to create images of your retina.

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9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading
  • Carlson NB, Kurtz D, Hines C. Clinical procedures for ocular examination. New York: McGraw-Hill; 2015:289-300.