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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 .
Glaucoma
Glaucoma is a group of eye conditions that can damage the optic nerve. This nerve sends the images you see to your brain. Most often, optic nerve da...
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.
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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.
Slit-lamp
The slit-lamp examination looks at structures that are at the front of the eye.
ImageRead Article Now Book Mark Article - 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.
Corneal ulcers or eye infections
The cornea is the clear tissue at the front of the eye. A corneal ulcer is an open sore in the outer layer of the cornea. It is often caused by inf...
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)
Hyphema
Hyphema is blood in the front area (anterior chamber) of the eye. The blood collects behind the cornea and in front of the iris.
ImageRead Article Now Book Mark Article - 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.
Corneal abrasion
Corneal injury is a wound to the part of the eye known as the cornea. The cornea is the crystal clear (transparent) tissue that covers the front of ...
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.
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Eye - illustration
The eye is the organ of sight, a nearly spherical hollow globe filled with fluids (humors). The outer layer or tunic (sclera, or white, and cornea) is fibrous and protective. The middle tunic layer (choroid, ciliary body and the iris) is vascular. The innermost layer (the retina) is nervous or sensory. The fluids in the eye are divided by the lens into the vitreous humor (behind the lens) and the aqueous humor (in front of the lens). The lens itself is flexible and suspended by ligaments which allow it to change shape to focus light on the retina, which is composed of sensory neurons.
Eye
illustration
-
Eye - illustration
The eye is the organ of sight, a nearly spherical hollow globe filled with fluids (humors). The outer layer or tunic (sclera, or white, and cornea) is fibrous and protective. The middle tunic layer (choroid, ciliary body and the iris) is vascular. The innermost layer (the retina) is nervous or sensory. The fluids in the eye are divided by the lens into the vitreous humor (behind the lens) and the aqueous humor (in front of the lens). The lens itself is flexible and suspended by ligaments which allow it to change shape to focus light on the retina, which is composed of sensory neurons.
Eye
illustration
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.
