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)footnote 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.

  • 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.footnote 2 In some cases of glaucoma there is
    damage to the optic nerve even though the eye pressure is never above
    normal.

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

ByHealthwise 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 ofMarch 3, 2017