Tonometry

What is tonometry?

Tonometry refers to a type of eye test that measures pressure inside your eye (intraocular pressure). It’s one of the essential glaucoma tests. There are a few different methods and ways to do this test, all of which are quick and painless.

When is tonometry used?

Eye care specialists, especially ophthalmologists and optometrists, use tonometry to screen for and diagnose glaucoma. It’s a common part of routine eye exams and more specific exams when you have possible eye injuries or experience certain eye symptoms.

Eye specialists may also use tonometry to monitor your eye pressure if you’re taking certain medications. Monitoring your eye pressure makes sure that those medications don’t cause high intraocular pressure (ocular hypertension) as a side effect.

If you have glaucoma, regular tonometry determines treatment. Tracking your eye pressure is a key way to make sure treatment is effective, so frequent tonometry tests — including with devices you can use at home to take readings on yourself — are common.

How does tonometry work?

Tonometry measures the pressure of your eye’s anterior chamber. The anterior chamber is a fluid-filled space just behind your cornea at the front of your eye. Pressure from the aqueous humor fluid inside that chamber helps your eyes hold their globe-like shape. The unit of measurement for this is millimeters of mercury (mmHg), the same unit used for blood pressure tests.

Your eye care specialist can use a few different tonometry methods. Those include:

• Applanation tonometry. “Applanation” means “flattening,” and devices that use it have a small, disk-shaped extension that rests against your eye surface. The devices measure how much pressure it takes to make your eye surface start to flatten. This method is the most accurate. It’s common for eye specialists to use this method after other methods return unusual or concerning readings.
• Continuous monitoring. This method uses a sensor you wear on your eye like a contact lens. Researchers are investigating the wearable sensor and another, similar method that uses a surgically implanted sensor.
• Dynamic contour tonometry. These devices use a small, sensor-tipped extension that touches your eye (but doesn’t make an indention).
• Electronic indentation tonometry. Devices that use this method have a small probe that takes a measurement when it presses against the surface of your eye enough to make a small indention.
• Non-contact tonometry. Devices using this method push air at your cornea. The device then measures tiny, split-second changes in the shape of your cornea as the air bounces off its surface. Air puff tonometers do this with a small puff of air, while ocular response analyzers use a stream of air.
• Rebound tonometry. Devices using this method have a tiny, plastic ball that moves toward your eye and stops when it touches the surface. The device determines your intraocular pressure when the ball makes gentle, painless contact. Some devices that use this method are ones your eye specialist can prescribe for you to take and use at home.

How should I prepare for a tonometry test?

You shouldn’t need to prepare for a tonometry test. One exception is to make sure you don’t wear a tight collar doing the test (either by wearing another shirt or loosening the collar if possible). Pressure around your neck from your clothes can increase your intraocular pressure readings.

What should I expect during a tonometry test?

What you can expect from tonometry tests can vary depending on the method. If you’re having applanation tonometry, your provider will add eye drops containing an anesthetic and a dye called fluorescein to your eyes. But non-contact tonometry and most other methods don’t need either of these to work.

Some of the most common methods for the tonometry test only take a few seconds. Many of these faster methods work best with calculating an average of multiple readings, so don’t be surprised or think you’ve done something wrong if your eye specialist wants to retake the readings a few times. Other methods (like applanation tonometry) take up to a few minutes. Your eye specialist will tell you more about what to expect during the test.

What should I expect after the test?

Your provider can tell you the reading right after they take it.

If you received anesthetic eye drops, don’t let anything touch your eyes until the anesthetic wears off. Anesthetic drops keep you from feeling pain, so it’s easier to injure your eyes while they’re numb.

Is tonometry painful?

Tonometry shouldn’t hurt, even if the method used involves touching your eyeball in some way. If you have pain during the test, tell your eye care specialist. You can also ask them more about what to expect regarding that pain, including how long it should last and what you can do about it.

What is a normal tonometry range?

A normal reading for most people is 10 mmHg to 21 mmHg. If your results are outside the normal range, there are a few likely next steps.

If your results are too high

If simpler tests show high pressure, your eye specialist will probably recommend using applanation tonometry to verify the readings. If applanation tonometry confirms that your pressure is high, your provider may monitor closely or offer treatment options. They’ll want to schedule regular follow-up visits.

Your eye specialist may recommend that you test your eye pressure at home. Be sure to ask about what to do if you get readings that are outside the normal range. You may need a follow-up visit with your specialist if you get slightly higher-than-normal pressures. But, if you get much higher readings, you may need emergency medical care.

Eye pressure that’s high because of angle-closure glaucoma is a medical emergency that needs immediate treatment. Without quick treatment, angle-closure glaucoma can quickly cause eye damage and permanent vision loss.

Other next steps can vary depending on your specific situation and needs. Your eye specialist can tell you more about what you can expect for your specific case.

If your results are too low

Low intraocular pressure is also a cause for concern since it can lead to eye damage and vision loss. Low intraocular pressure is usually anything under 5 mmHg or 6 mmHg. If the pressure in your eye is too low, your eye specialist will talk to you about treatment options and follow-up visits to monitor your eye health.

Can I drive after an eye pressure test?

You might be able to drive right after an eye pressure test. It depends on a few factors. One key factor is if your eye exam involved dilating your eyes (during routine eye exams, dilation happens after the tonometry test). Dilating your eyes will cause light sensitivity, and you may have trouble focusing and reading things up close. These effects usually last for a few or several hours.

Eye specialists also commonly keep disposable shades you can wear after your exam (and these shades can commonly fit on top of eyeglasses if you wear them). These shades can help reduce brightness and glare so you’re more comfortable and can see a little better. And many people feel they can still drive after eye dilation. But if you have any concerns about whether you can drive after an eye exam, you may want to consider having transportation to and from your eye specialist visit.