The tonometry test measures the pressure inside the eyeballs. This is called intraocular pressure, abbreviated as IOP.
Tonometry helps to diagnose glaucoma. It is often used as a screening test to detect glaucoma or to follow treatment in someone who has glaucoma. Tonometry is usually done as part of a complete eye exam.
There are 3 methods to measure the pressure inside the eyes. These procedures are all performed in the office of the eye care provider. One method uses a puff of air, and is a non-contact method. The person sits in a chair with the chin resting on a pad. The individual looks straight into the examining instrument. A bright light shines on the front of the eye as a short puff of air is blown at it. The instrument measures the amount of air deflected by the eye, which gives an estimate of the pressure.
Another method is called the Schiotz method. A drop of anesthetic is placed on the eye. When the eye is numb, the person looks up at the ceiling. A tonometer is lowered until it touches the surface of the eye. The machine measures the intraocular pressure.
Another method is the applanation method. A drop of anesthetic is placed on the eye. A small amount of fluorescent dye is placed on the cornea. The cornea is then lit with a slit lamp. The front of the eye is touched by the tonometer, and the intraocular pressure is measured. Applanation tonometry is considered the "gold standard" for office-based intraocular pressure measurement.
Specific instructions are available from the eye care professional. Usually, no preparation is required.
The person's intraocular pressure is compared with the normal range available on standard tables. Elevated intraocular pressure, greater than 21 mm HG, may, but does not necessarily, indicate that the individual has or is going to develop glaucoma. Further testing is indicated to evaluate the risk.