Tonometry

Alternative names
Intraocular pressure measurement (IOP)

Definition
Tonometry is a procedure that measures the pressure inside the eyes. The test is used to screen for glaucoma, a disease in which pressure inside the eyes increases to the point that it impairs vision and if left untreated, may cause Blindness.

How the test is performed

There are several methods of testing for glaucoma.

The applanation method measures the force required to flatten a certain area of the cornea. A fine strip of paper stained with orange dye (fluorescein) is touched to the side of the eye. The dye stains the front of the eye to help with the examination, then rinses out with tears. An anesthetic drop is also placed in the eye.

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 tonometer touches the cornea. The light is usually a blue circle. The health care provider looks through the eyepiece on the lamp and adjusts the tension on the tonometer. There is no discomfort associated with the test.

A slightly different method of applanation uses a portable object similar to pencil. Again, you are given anesthetic eye drops to prevent any discomfort. The device touches the outside of the eye and an instant digital measurement is recorded.

The last method is the noncontact method (air puff). In this method, your chin is resting on a padded stand. You will be asked to stare straight into the examining instrument. The examiner will shine a bright light into your eye to properly align the instrument. A brief puff of air is blown at your eye. The instrument calculates pressure from the change in the light reflected off the corneas as the air puff is blown.

How to prepare for the test

Remove contact lenses before the examination. The dye can permanently stain contact lenses.

Inform the health care provider if you have corneal ulcers and infections, an eye infection, if you are taking any drugs, or if you have a history of glaucoma in your family.

For children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

How the test will feel
There should be no pain with the applanation method because of the anesthetic. In the noncontact method, you may feel mild pressure on your eye.

Why the test is performed
People over 40 years old, especially African Americans, are at the highest risk for developing glaucoma. If glaucoma is detected early, it can usually be treated, but it may go unrecognized for years because there usually are no symptoms.

Normal Values
The eye pressure is within the normal range.

What abnormal results mean
Glaucoma may be detected.

Additional conditions under which the test may be performed:

     
  • hyphema  
  • trauma to the eye or head  
  • before and after eye surgery

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.

Special considerations

Blacks are at least twice as likely as whites to develop glaucoma, and about five times more likely to suffer vision loss as a result of the disease.

Johns Hopkins patient information

Last revised: December 3, 2012
by Gevorg A. Poghosian, Ph.D.

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