A tonometer is a tool used to check the pressure exerted by the fluid inside a person's eyes in terms of millimeters of mercury (mmHg). This is done to make sure the eyes and optic nerves are healthy. There are several different types, including those that touch the eyeball directly, those that only touch the eyelid, and those that don't touch the eye at all. Though most are usually very accurate, some things can cause inaccurate readings.
The eyes are filled with fluid, which exerts pressure on the optic nerves and the outside of the eyeball. This is called intraocular pressure (IOP), and measures between 10 mmHg and 21 mmHg in most healthy humans. Having a too high IOP is a very common sign of glaucoma, but it can also be a symptom of an inflamed iris or retinal detachment. A too low IOP can be a sign that fluid is leaking from the eye or that the eye is not producing enough fluid to keep up with normal drainage. This can increase a person's risk for cataracts and retinal detachment, and often leads to a decrease in vision. Optometrists often use tonometers to screen for these conditions and monitor those with known eye problems, particularly glaucoma.
Many tonometers measure IOP by pressing or bouncing a device against the cornea, which is the front part of the eyeball that covers the iris, the pupil, and a small chamber containing fluid. Though these are very commonly used, some people don't like them because they usually require the use of numbing drops in the eyes. Common types of corneal contact tonometers include the following:
- Goldmann: This is considered the industry standard for tonometry, and works by touching the end of the device to the cornea to measure IOP. This process is called applantation. Perkins and Maklakov tonometers can also be used to do this.
- PASCAL Dynamic Contour Tonometer (DCT): The device works by placing a small, pressure-sensitized concave onto the cornea.
- Tono-Pen/Accu-Pen: This type comes in a pen shape and works by means of electronic indentation tonometry, measuring IOP with an electronic transducer.
- Icare: This measures IOP by bouncing a small probe against the cornea. The recoil creates an induction current, which can be used to measure IOP. This method is called rebound tonometry.
- Schiötz: A device that works by means of impression tonometry, a process in which the optometrist measures the depth of the impression a small plunger makes on the cornea.
There are also devices that measure IOP through the eyelid, as opposed to actually touching the cornea. The most common type is the Diaton tonometer, which works by bouncing a rod off of the eyelid, then measuring the resulting rebound. Some people prefer this method because it usually doesn't involve anesthetic drops.
Some tools work without touching the eye at all. This is known as non-contact or "air puff" tonometry, since most non-contact versions work by shooting a small puff of air at the cornea, and then measuring the force needed to flatten it. Unlike most corneal contact tools, air puff devices do not usually require eye drops, and the results are available within seconds. Another type is an Ocular Response Analyzer (ORA), which uses two puffs of air to measures the difference between the pressure on the cornea as it's going inward and then as it returns to its normal shape.
The accuracy of a tonometer reading can be affected by several factors. People tend to have slight differences in the thickness and hardness of their corneas, so a person with a particularly hard cornea might have an abnormally high IOP reading but still be healthy. Other factors, like illness, eye inflammation, caffeine consumption, or exercise can also influence a person's IOP. Eye doctors may have a hard time getting a measurement if the person moves around during the procedure, which is why air puff, Icare, or Diaton models are usually used for children, people who are uncomfortable with items touching the eye, and those who prefer not to use eye drops.