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Health Encylopedia

Definition Tonometry is a procedure that measures the pressure inside the eyes. The test is used to screen for glaucoma , which is a disease in which the pressure inside the eyes increases to the point that it impairs vision and if left untreated, may cause blindness .
Alternative Names Intraocular pressure measurement (IOP)
How the test is performed There are several methods of testing for glaucoma . In the Schiotz method, you will be lying on a table. Loosen any tight clothing around your neck (tightness may increase the pressure inside the eyes). You will be asked to look down as the upper eyelid is raised and a drop of anesthetic is placed in the eye. You then need to look at a spot on the ceiling. A tonometer (a small, smooth instrument) is lowered onto the surface of the eye for a few seconds. The procedure is then repeated for the other eye. This method is rarely performed today, but in some circumstances is useful. The applanation method measures the force required to flatten a certain area of the cornea. A fine strip of paper that is stained with an orange colored dye (fluorescein) is touched to the side of the eye. The dye stains the front part of the eye to help with the examination. The dye rinses out of the eye 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 the support that keeps your head steady. The lamp is moved forward until the tonometer touches the cornea. The light is usually a blue circle and there is no discomfort associated with the test. The health care provider looks through the eyepiece on the lamp and adjusts the tension on the tonometer. A slightly different method of applanation uses a portable object similar to pencil. Again the person is 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 aligned 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. Children: The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
  • preschooler test or procedure preparation
  • (3 to 6 years)
  • schoolage test or procedure preparation
  • (6 to 12 years)
  • adolescent test or procedure preparation
  • (12 to 18 years)
  • How the test will feel There should be no pain with the Schiotz and 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 either the Schiotz or 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