In Ophthalmology, what is Tonometry?

In ophthalmology, tonometry refers to measurements of intra-ocular pressure that can diagnose conditions like glaucoma. Intraocular pressure (IOP) is the pressure caused by fluids in the eye. High intraocular pressure can damage the nerves in the eyes and result in loss of sight. Since high intraocular pressure is often the greatest risk factor for illnesses like glaucoma, eye care professionals usually check IOP along with other examinations of the eyes during a check up.

The tool used in these examinations is called a tonometer, which can adequately measure IOP. It does so by measuring the cornea’s resistance to outside pressure. There are several ways to do these tests.

Some ophthalmologists perform a noncontact measurement. A puff of air is shot at the eye at close range and the corneal reaction is observed. This is a frequently used form of tonometry, but according to most physicians, is also the least accurate means to detect high IOP.

Another more accurate form is called applanation or Goldman tonometry. In this type of exam, the ophthalmologist uses a tiny probe that flattens a small portion of the cornea. IOP is determined by the weight or force needed to cause the cornea to indent.

Some ophthalmologists and optometrists are now switching to the more modern method of electronic indentation tonometry. This procedure does not exert pressure on the eye, but instead performs an electrical reading of eye pressure. It’s rather like the strips people place on foreheads to take the body’s temperature. If the results are questionable more than one type of tonometer may be used in an examination.

Another form is impression tonometry. A tiny plunger like instrument causes the cornea to indent, similar to the technique used in applanation. The amount of weight needed to indent the cornea determines the IOP-greater weight translates to higher pressure.

This procedure is generally painless and is part of a routine eye examination. Some people may feel slight discomfort or slight irritation at having their eyes touched, but usually the eyes are treated with a temporary anesthetic. This makes feeling any instruments touching the eyes, highly unlikely. Measurements are taken very quickly, only taking up a minute to two of a total examination.

In fact, since tonometry is generally painless, one should report eye discomfort to the eye doctor. If the eye remains painful for a day to two days this may indicate infection or a scratch on the cornea. This rarely occurs. Some people may note some pain with this procedure after having laser eye surgery. In these cases the pain is usually the result of the surgery and not the tonometry examination.