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.
Purpose
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.
Main Types
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.
Mitigating Factors
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.