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Tonometry

Intraocular pressure (IOP) measurement; Glaucoma test; Goldmann applanation tonometry (GAT)

Tonometry is a test to measure the pressure inside your eyes. The test is used to screen for glaucoma. It is also used to measure how well glaucoma treatment is working.

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How the Test is Performed

There are three main methods of measuring eye pressure.

The most accurate method measures the force needed to flatten an area of the cornea.

A second method uses a handheld device shaped like a pencil. You are given numbing eye drops to prevent any discomfort. The device touches the surface of the cornea and instantly records eye pressure.

The last method is the noncontact method (air puff). In this method, your chin rests on a device similar to a slit lamp.

How to Prepare for the Test

Remove contact lenses before the exam. The dye can permanently stain contact lenses.

Tell your provider if you have a history of corneal ulcers or eye infections, or a history of glaucoma in your family. Always tell your provider what medicines you are taking.

How the Test will Feel

If numbing eye drops were used, you should not have any pain. In the noncontact method, you may feel mild pressure on your eye from the air puff.

Why the Test is Performed

Tonometry is a test to measure the pressure inside your eyes. The test is used to screen for glaucoma and to measure how well glaucoma treatment is working.

People over age 40 years, particularly African Americans, have the highest risk for developing glaucoma. Regular eye exams can help detect glaucoma early. If it is detected early, glaucoma can be treated before too much damage is done.

The test may also be done before and after eye surgery.

Normal Results

A normal result means your eye pressure is within the normal range. The normal eye pressure range is 10 to 21 mm Hg.

The thickness of your cornea can affect measurements. Normal eyes with thick corneas have higher readings, and normal eyes with thin corneas have lower readings. A thin cornea with a high reading may be very abnormal (the actual eye pressure will be higher than shown on the tonometer).

A corneal thickness measurement (pachymetry) is needed to get a correct pressure measurement.

Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

Abnormal results may be due to:

Risks

If the applanation method is used, there is a small chance the cornea may be scratched (corneal abrasion). The scratch will normally heal within a few days.

Related Information

Glaucoma
Blindness and vision loss
Hyphema

References

Bowling B. Glaucoma. In: Bowling B, ed. Kanski's Clinical Ophthalmology. 8th ed. Philadelphia, PA: Elsevier; 2016:chap 10.

Knoop KJ, Dennis WR. Ophthalmologic procedures. In: Roberts JR, Custalow CB, Thomsen TW, eds. Roberts and Hedges' Clinical Procedures in Emergency Medicine and Acute Care. 7th ed. Philadelphia, PA: Elsevier; 2019:chap 62.

Lee D, Yung ES, Katz LJ. Clinical examination of glaucoma. In: Yanoff M, Duker JS, eds. Ophthalmology. 5th ed. Philadelphia, PA: Elsevier; 2019:chap 10.4.

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Review Date: 8/28/2018  

Reviewed By: Franklin W. Lusby, MD, ophthalmologist, Lusby Vision Institute, La Jolla, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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