Tonometry

Written by Colleen M. Story | Published on July 25, 2012
Medically Reviewed by George Krucik, MD

Glaucoma and Tonometry

Tonometry is an eye test that measures the pressure inside your eyes, to determine whether or not you may be at risk for glaucoma. Glaucoma is an eye disease in which fluid buildup within the eye can damage the optic nerve. Though often referred to as one disease, glaucoma is actually a group of diseases that all cause the same damage and are most likely to strike people over the age of 60 (NIH Senior Health).

In most cases of glaucoma, the fluid that normally bathes and nourishes the eye drains too slowly, causing a pressure buildup. Without treatment, that pressure can eventually harm the optic nerve, causing a loss in vision. Because these changes within the eye are often painless, they can progress for years unnoticed.

Since glaucoma can cause blindness if it’s not treated, a tonometry test is critical for detecting eye changes early. If the test comes back positive, your eye doctor will begin treatments, which can delay the progression of the disease.

What Is Tonometry?

Tonometry is an eye test that can detect changes in eye pressure long before you may be aware of them. The most common type of tonometry test is called "Goldmann applanation tonometry," which for decades has been considered the international gold standard for measuring intraocular pressure (IOP) (M Egli, 2011). There are other methods of testing for eye pressure, including alternative types of tonometry called "pneumotonometry," as well as the Tono-Pen.

Pneumotonometry is a procedure that involves contacting the eye with air pressure, using an instrument that resembles an air piston. The instrument blows a brief puff of air at the cornea, measuring the pressure in the eye. If it shows abnormal results, usually the doctor will perform other tests to confirm the diagnosis, as this test is usually considered less accurate than Goldmann applanation tonometry.

The Tono-Pen is shaped like a large marker. A hand-held device, it provides a digital readout of eye pressure. The doctor uses it to touch the eye and measure pressure. Though useful in many cases, the Tono-Pen is considered less accurate than the Goldmann tonometer in people with normal corneas (Augusto Azuara-Blanco, 1998).

Who Needs a Tonometry Test?

Your eye doctor may order the Goldman applanation tonometry test if he or she suspects you may be at risk for glaucoma. If the air puff test or other eye tests indicate a potential problem, he may ask you to go through the tonometry test to confirm or rule out glaucoma.

According to the Mayo Clinic, you may be at risk for glaucoma if you are African American, as individuals in this population are five times more likely to get this condition than Caucasians. They are subsequently more likely to experience permanent blindness as a result (Mayo). You may also be at risk if you:

  • are over 60 years old
  • have a family history of the disease
  • have diabetes or hypothyroidism
  • have other eye conditions or injuries
  • are nearsighted
  • have used corticosteroid medications for prolonged periods of time

You might also be tested if you’re experiencing symptoms like a gradual loss of peripheral vision, tunnel vision, severe eye pain, blurred vision, halos around lights, or reddening of the eye. All of these symptoms may be signs of glaucoma.

What Happens During a Tonometry Test?

Before the tonometry test, your eye doctor will put numbing drops in your eyes so that you don’t feel anything touching them. Once the eye is numb, the doctor may touch a tiny, thin strip of paper that contains orange dye to the surface of the eye to stain it. This helps increase the accuracy of the test.

The doctor then puts a machine called a "slit-lamp" in front of you, and you rest your chin and forehead in the supports. The lamp is then moved toward your eyes until just the tip of the tonometer "probe" touches the cornea. By flattening it just a bit, the instrument is able to detect the pressure in your eye. Your eye doctor will adjust the tension until he or she gets a proper reading.

Because your eyes are numb, you will feel no pain during the procedure. Tonometry is extremely safe. However, there is a very small risk that the cornea could be scratched when the tonometer touches the eye. Even if this happens, however, it will normally heal itself within a few days.

What Do the Test Results Mean?

A normal test result means that the pressure in your eye is within the normal range, and you do not have glaucoma or other pressure-related eye problems. According to the Glaucoma Research Foundation, the normal pressure range is 10 to 21 mmHg (GRS). The measurement "mmHg" means "millimeters of mercury," which is a scale used to record eye pressure.

If your test comes back with a pressure reading exceeding 20 mmHg, you may have glaucoma. This isn’t always the case, however. High pressure readings may also be present if you’ve had an eye injury, or if you have bleeding in the front of the eye caused by blood vessel problems, inflammation, or other issues.

Your doctor will discuss treatment options with you if he or she diagnoses you with glaucoma.

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