Tonometry. Larry Gatz

Definition: A tool used by optometrists, ophthalmologists and emergency room doctors to measure the pressure inside the eye, also referred to as intraocular pressure. A tonometer measures the production of aqueous humor, the liquid found inside the eye, and the rate at which it drains in to the tissue surrounding the cornea.

For the type of tonometer that actually touches the eye, numbing drops are used.

For the air-puff tonometer, though, no numbing drops are needed.

Also Known As: Tonometry

There are several different types of tonometers available to doctors:

  • Goldmann tonometer
  • Non-contact tonometer (NCT)
  • Electronic tonometer
  • Dynamic contour tonometer
  • Schiotz tonometer
  • Rebound tonometer

All of these types of tonometers measure eye pressure. Doctor's use certain types of tonometers based on their experience, field of expertise or they may believe one may be more accurate or more reliable than another. Although the science behind tonometry is always developing, it is thought that Goldmann tonometry is the most accurate and is considered the standard of care when performing tonometry. Tonometers are calibrated to measure pressure in millimeters of mercury and is recorded as "mm Hg."

Why do doctors measure eye pressure?

Eye pressure or intraocular pressure is a vital statistic for the eye. Eye pressure is an important measure in determining the health of the eye.

Doctors are interested in measuring eye pressure frequently as eye pressure can vary slightly depending on the time of day it is measured. Tonometry is most important as a test for glaucoma. Elevated eye pressure raises your risk for developing glaucoma. If the the eye pressure is high, or fluctuates from very low to very high frequently, damage can occur to the optic nerve, the nerve cable that connects the eye to the brain.

Long-term elevated eye pressure can cause optic nerve cupping, which is a sign of loss of the nerve fiber layer. The nerve fiber layer is responsible for capturing light impulses and converting them into a signal that is carried to the brain for vision. If there is enough nerve fiber layer destruction, vision loss can occur. Vision loss typically occurs in your peripheral vision first and then can advance to central vision. If your eye pressure is slightly elevated, it does not necessarily mean you have glaucoma. Normal eye pressure is usually considered below 20 mm Hg. 

Can eye pressure be too low?

Eye pressure can be too low, but this is rare. Very low eye pressure is usually only found in people that have just had eye surgery or experience eye trauma. 

Is there anything that affects the accuracy of a tonometer measurement?

Tonometry is difficult for many people. Most tonometers come very close to the eye and some of us have a very strong blink reflex, sensitivity or protective quality that make it difficult to get an accurate tonometry reading.

Some tonometers are easier to perform on certain individuals. Blinking or squeezing the eyelids right before the test can cause the measurement to be inaccurate.

Another important factor that can influence tonometry accuracy is the the thickness of your cornea. the cornea is the clear, dome-like structure on the front part of the eye. When tonometers were first developed, it was thought that most individuals had roughly the same corneal thickness. However, as our instrument quality improved and because refractive surgery such as LASIK became popular, it was discovered that corneal thickness varies in individuals and this can affect a tonometer measurement. A thicker cornea gives rise to a greater probability of eye pressure being overestimated and a thinner cornea of the pressure reading to be underestimated. Newer tonometers are being developed to take corneal thickness into consideration when calculating the measurement.

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