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; Applanation

How the test is performed

There are several methods of testing for glaucoma.

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

  • The surface of the eye is numbed with eye drops. A fine strip of paper stained with orange dye is touched to the side of the eye. The dye stains the front of the eye to help with the examination.
  • The slit-lamp is placed in front of you, and you 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.
  • The health care provider looks through the eyepiece on the lamp and the machine gives a pressure reading. There is no discomfort with the test.

A slightly different method uses a handheld device similar in shape to a pencil. Again, 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 eye doctor 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 examination. The dye can permanently stain contact lenses.

Tell your health care provider if you have corneal ulcers and eye infections or a history of glaucoma in your family. Always tell your doctor or nurse 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.

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 Values

A normal result means your eye pressure is within the normal range. The normal eye pressure range is 10 - 21 mmHg.

How thick your cornea is 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).

Currently, 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
  • Inflammation in the eye
  • Injury to the eye or head

What the risks are

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

References

Stamper RL, Punjabi O, Tanaka G. Intraocular Pressure: Measurement, Regulation, and Flow Relationships. In: Tasman W, Jaeger EA, eds. Foundations of Clinical Ophthalmology. 2012 ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2012: vol 2, chap 7.

Review Date:9/18/2012

Reviewed by:David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc., and Franklin W. Lusby, MD, Ophthalmologist, Lusby Vision Institute, La Jolla, California.

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