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Tonometry
Table of Contents
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Test Overview
Why It Is Done
How To Prepare
How It Is Done
How It Feels
Risks
Results
What Affects the Test
What To Think About
Related Information
References
Credits
Test Overview
A tonometry test measures the pressure inside your eye, which is called intraocular
pressure (IOP). This test is used to check for glaucoma, an eye disease that can cause
blindness by damaging the nerve in the back of the eye (optic nerve). Damage to the
optic nerve may be caused by a buildup of fluid that does not drain properly out of the
eye.
Tonometry measures IOP by recording the resistance of your cornea to pressure
(indentation). Eyedrops to numb the surface of your eye are used with most of the
following methods.
Tonometry methods
• Applanation (Goldmann) tonometry. This type of tonometry uses a small probe
to gently flatten part of your cornea to measure eye pressure and a microscope
called a slit lamp to look at your eye. The pressure in your eye is measured by how
much force is needed to flatten your cornea. This type of tonometry is very accurate
and is often used to measure IOP after a simple screening test (such as air-puff
tonometry) finds an increased IOP.
• Electronic indentation tonometry. Electronic tonometry is being used more often
to check for increased IOP. Although it is very accurate, electronic tonometry
results can be different than applanation tonometry. Your doctor gently places the
rounded tip of a tool that looks like a pen directly on your cornea. The IOP reading
shows on a small computer panel.
• Noncontact tonometry (pneumotonometry). Noncontact (or air-puff) tonometry
does not touch your eye but uses a puff of air to flatten your cornea. This type of
tonometry is not the best way to measure intraocular pressure. But it is often used
as a simple way to check for high IOP and is the easiest way to test children. This
type of tonometry does not use numbing eyedrops.
Why It Is Done
Tonometry may be done:
• As part of a regular eye examination to check for increased intraocular pressure
(IOP), which increases your risk of glaucoma.
• To check the treatment for glaucoma. Tonometry can be used to see if medicine is
keeping your IOP below a certain target pressure set by your doctor.
How To Prepare
Tell your doctor if you or someone in your family has glaucoma or risk factors for
glaucoma.
If you wear contact lenses, remove them before the test. Do not put your contacts back in
for 2 hours after the test. Bring your eyeglasses to wear after the test until you can wear
your contact lenses.
Loosen or remove any tight clothing around your neck. Pressure on the veins in your
neck can increase the pressure inside your eyes. Stay relaxed.
How It Is Done
Tonometry takes only a few minutes to do.
Applanation (Goldmann) method
This type of tonometry is done by an ophthalmologist or an optometrist. Your doctor will
use eyedrops to numb the surface of your eyes so that you will not feel the tonometer
during the test. A strip of paper containing a dye (fluorescein) will be touched to your eye,
or eyedrops containing the dye will be applied. The dye makes it easier for your doctor to
see your cornea.
You will rest your chin on a padded support and stare straight into the microscope (slit
lamp). Your doctor sits in front of you and shines a bright light into your eye. Your doctor
gently touches the tonometer probe to your eye. Your doctor checks the tension dial on
the tonometer that measures the IOP of your eye.
Do not rub your eyes for 30 minutes until the numbing medicine has worn off.
Electronic indentation method
Electronic tonometry can be done by a technician, an optometrist, an ophthalmologist, or
a family medicine doctor. Your doctor will use eyedrops to numb the surface of your eyes
so that you will not feel the tonometer during the test.
You will stare straight ahead, or sometimes look down. Your doctor gently touches the
tonometer probe to your eye. Several readings will be taken on each eye. You will hear a
clicking sound each time a reading is obtained. After enough accurate readings have
been obtained, a beep will sound, and the averaged IOP measurement will appear on the
instrument's display panel.
Do not rub your eyes for 30 minutes until the numbing medicine has worn off.
Noncontact (or air-puff) method
This type of tonometry is done by an ophthalmologist or an optometrist. You do not need
drops to numb your eye for this method.
You will rest your chin on a padded support and stare straight into the machine. A brief
puff of air is blown at your eye. You will hear the puffing sound and feel a coolness or
mild pressure on your eye. The tonometer records the intraocular pressure (IOP) from
the change in the light reflected off the cornea as it is indented by the air puff. The test
may be done several times for each eye.
How It Feels
Tonometry should not cause any eye pain. Your doctor will use eyedrops to numb the
surface of your eyes so that you will not feel the tonometer during the test. You may have
a scratchy feeling on your cornea. This usually goes away in 24 hours.
Some people become anxious when the tonometer needs to be touched to the eye. In
air-puff tonometry, only a puff of air touches the eye.
Risks
There is a very slight risk that your cornea may be scratched during the methods that
involve touching a tonometer to your eye. Rubbing your eyes before the numbing
eyedrops wear off increases the risk of scratching the cornea. If tonometry causes a
scratch on the cornea, your eye may be uncomfortable until the scratch heals, which
normally takes about a day.
There is also a very small risk of an eye infection or an allergic reaction to the eyedrops
used to numb your eyes.
With the air-puff (noncontact) method, there is no risk of scratches or infection, since
nothing but air touches your eyes. But this method is not the best way to measure
intraocular pressure.
You should not have any eye pain or vision problems after tonometry. Call your doctor if
you feel any eye pain during the test or for 48 hours after the test.
Results
A tonometry test measures the pressure inside your eye, which is called intraocular
pressure (IOP). This test is used to check for glaucoma.
Normal eye pressure is different for each person and is usually higher just after you wake
up. IOP changes more in people who have glaucoma. Women usually have a higher IOP
than men, and IOP normally gets higher as you get older.
Intraocular pressure (IOP)1
Normal:
10–21 millimeters of mercury (mm Hg)
Abnormal: Higher than 21 mm Hg
High values
• A high IOP may mean that you have glaucoma or that you are at high risk for
developing glaucoma. People who have ongoing pressures above 27 mm Hg
usually develop glaucoma unless the pressure is lowered with medicines.
• People who have an ongoing IOP higher than 21 mm Hg but do not have optic
nerve damage have a condition called ocular hypertension. These people may be
at risk for developing glaucoma over time.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful
include:
• Having a sore on your eye or an eye infection. This increases your risk of an eye
injury during the test.
• Being extremely nearsighted, having an irregularly shaped cornea, or having had
major eye surgery in the past.
• Blinking or squeezing your eyes shut during the test.
• Having had laser refractive surgery (such as LASIK).
What To Think About
• Tonometry tests may be done over months or years to check for glaucoma. Also,
because intraocular pressure (IOP) can change at different times of the day,
tonometry is not the only test done to check for glaucoma. If the IOP is high, more
tests, such as ophthalmoscopy, gonioscopy, and visual field testing, may be done.
◦ Ophthalmoscopy
◦ Gonioscopy
◦ Vision Testing
• Pachymetry uses ultrasound to measure the thickness of the cornea. The thickness
of the cornea can affect IOP measurement. Pachymetry is often done during a
tonometry test. It can help your doctor know your chance for developing glaucoma.
• Normal IOP is different from person to person. About 40% of people who have optic
nerve damage caused by glaucoma have normal IOP.2 In some cases of glaucoma
there is damage to the optic nerve even though the eye pressure is never above
normal.
Related Information
• Physical Exam for Glaucoma
• Signs of Damage Caused by Glaucoma
References
Citations
1. Chang DF (2011). Ophthalmologic examination. In P Riordan-Eva, ET
Cunningham, eds., Vaughan and Asbury's General Ophthalmology, 18th
ed., pp. 27–57. New York: McGraw-Hill.
2. Dielemans I, et al. (1994). The prevalence of primary open-angle
glaucoma in a population-based study in the Netherlands: The Rotterdam
Study. Ophthalmology, 101(11): 1851–1855. DOI: http://dx.doi.org/10.1016/
S0161-6420(94)31090-6. Accessed June 14, 2016.
Other Works Consulted
• Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic
Procedures, 5th ed. St. Louis: Saunders.
Credits
By Healthwise Staff
Primary Medical Reviewer Adam Husney, MD - Family Medicine
E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer Christopher J. Rudnisky, MD, MPH, FRCSC - Ophthalmology
Current as of March 3, 2017
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