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Tonometry

Definition

Tonometry is a test to measure the pressure inside your eyes. The test is used to screen for glaucoma.

Alternative Names

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

How the Test is Performed

There are many methods of testing for glaucoma.

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

  • The surface of the eye is numbed with eye drops. A fine strip of paper stained with orange dye is held to the side of the eye. The dye stains the front of the eye to help with the exam.
  • The slit-lamp is placed in front of you. You will rest your chin and forehead on a support that keeps your head steady. The lamp is moved forward until the tip of the tonometer just touches the cornea.
  • Blue light is used so that the orange dye will glow green. The health care provider looks through the eyepiece on the slit-lamp and adjusts a dial on the machine to give the pressure reading.
  • There is no discomfort with the test.

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

The last method is the noncontact method (air puff). In this method, your chin rests on a padded stand.

  • You stare straight into the examining device. The provider shines a light into your eye to properly line up the instrument, and then delivers a brief puff of air at your eye.
  • The machine measures eye pressure by looking at how the light reflections change as the air hits the eye.

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.

People over age 40, especially 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:

  • Glaucoma
  • Hyphema (blood in the front chamber of the eye)
  • Inflammation in the eye
  • Injury to the eye or head

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.

References

Ali M, Romos-Esteban JC, Katz J. Clinical examination of glaucoma. In: Yanoff M, Duker JS, eds. Ophthalmology. 4th ed. Philadelphia, PA: Elsevier Saunders; 2014:chap 10.4.

Stamper RL, Punjabi O, Tanaka G. Intraocular pressure: measurement, regulation, and flow relationships. In: Tasman W, Jaeger EA, eds. Duane's Foundations of Clinical Ophthalmology. Philadelphia, PA: Lippincott Williams & Wilkins; 2013:vol 2, chap 7.

Vasudevan DA, Fowler GC. Tonometry. In: Pfenninger JL, Fowler GC, eds. Pfenninger and Fowler's Procedures for Primary Care. 3rd ed. Philadelphia, PA: Elsevier Mosby; 2011:chap 68.

Review Date:8/20/2016
Reviewed By:Franklin W. Lusby, MD, ophthalmologist, Lusby Vision Institute, La Jolla, CA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.

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