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.
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 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.
Tonometry is a test to measure the fluid pressure inside your eyes. This test is used to screen for glaucoma. It is also used to measure how well glaucoma treatment is working. Applanation tonometry uses a slit lamp with supports for your forehead and chin and a small cone that gently touches your cornea after it has been made numb with eye drops. This cone is used to measure the force (pressure) needed to temporarily flatten a part of your cornea.
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.
- 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. Sometimes the dye is in the numbing drops.
- You will rest your chin and forehead on the support of a slit lamp so that your head is steady. You will be asked to keep your eyes open and to look straight ahead. 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 second method uses a handheld device shaped like a pen. 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.
- You stare straight into the examining device. When you are at the correct distance from the device, a tiny beam of light reflects off of your cornea onto a detector.
- When the test is performed, a puff of air will slightly flatten the cornea; how much it flattens depends on the eye pressure.
- This causes the tiny beam of light to move to a different spot on the detector. The instrument calculates eye pressure by looking at how far the beam of light moved.
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 for a brief moment 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 provider 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
Too much pressure is almost never a good thing. When it's bearing down on you from work or family responsibilities, pressure can stress you out. But when pressure is building in your eye from a disease called glaucoma, it can cause permanent blindness if it's not treated. Glaucoma involves the clear fluid in the front part of your eye, which is called the aqueous humor. Your eye constantly makes this fluid, which then drains out through a chamber in the front of the eye. When you have glaucoma, the fluid becomes blocked so it can't drain out of your eye. As the fluid builds up, it causes the pressure to rise. That pressure eventually damages the optic nerve, the important nerve which sends images to your brain and allows you to see. There are four different types of glaucoma. The most common type is called open-angle glaucoma. Although no one knows for sure what causes it, open-angle glaucoma tends to run in families. In people with this form, the pressure rises slowly over time. Another type of glaucoma appears in babies at birth, it's called congenital glaucoma. Certain drugs and eye diseases can cause yet another form of the disease, called secondary glaucoma. But probably the most serious form of the disease is closed-angle glaucoma. It occurs when the angle becomes suddenly blocked, causing pressure in the eye to rise sharply. This is an emergency situation. Without treatment, you can lose sight very quickly. Because most people don't have any symptoms of glaucoma until they've already lost sight, the best way to diagnose it is by having regular eye exams. The eye doctor will dilate, or widen your pupil to get a better view of your eye. Your doctor may also do a test called tonometry to check your eye pressure, and take a photo or laser image of your optic nerve to make sure it's healthy. The main treatment for glaucoma is eye drops to reduce the pressure inside your eyes. If drops can't control your pressure, or you have closed-angle glaucoma and your pressure rises very quickly, you'll probably need surgery, or laser therapy to open up a new drainage channel in your eye. Your best defense against glaucoma is a good offense. See your eye doctor for a complete eye exam before you turn 40, or even sooner if you have a family history of glaucoma. That way, your doctor can spot the disease before it can steal your sight.
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
Chen S, Lee BD, Al-Aswad LA. Clinical examination of glaucoma. In: Yanoff M, Duker JS, eds. Ophthalmology. 6th ed. Philadelphia, PA: Elsevier; 2023:chap 10.4.
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.
Salmon JF. Glaucoma. In: Salmon JF, ed. Kanski's Clinical Ophthalmology. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 11.
Version Info
Last reviewed on: 8/22/2022
Reviewed by: Franklin W. Lusby, MD, Ophthalmologist, Lusby Vision Institute, La Jolla, CA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
