Tonometry measures intraocular pressure (IOP) to diagnose conditions like glaucoma. A tonometer is used to measure IOP by testing the cornea’s resistance to external pressure. There are several types of tonometry, including non-contact, applanation, electronic indentation, and impression tonometry. Tonometry is usually painless and part of a routine eye exam. Eye discomfort should be reported to a doctor.
In ophthalmology, tonometry refers to intraocular pressure measurements that can diagnose conditions such as glaucoma. Intraocular pressure (IOP) is the pressure caused by fluid in the eye. High intraocular pressure can damage the nerves in the eye and cause vision loss. Because high intraocular pressure is often the biggest risk factor for diseases like glaucoma, eye doctors usually check IOP along with other eye exams during a checkup.
The instrument used in these tests is called a tonometer, which can adequately measure IOP. It does this by measuring the cornea’s resistance to external pressure. There are several ways to run these tests.
Some ophthalmologists perform a non-contact 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 doctors, it is also the least accurate means of detecting elevated IOP.
Another more accurate form is called applanation or Goldman tonometry. In this type of exam, the eye doctor uses a tiny probe that flattens a small portion of the cornea. IOP is determined by the weight or force required to retract the cornea.
Some ophthalmologists and optometrists are now switching to the more modern method of electronic indentation tonometry. This procedure does not put pressure on the eye, but instead takes an electrical reading of eye pressure. It’s kind of like the strips people put on their foreheads to measure their body temperature. If results are questionable, more than one type of tonometer may be used in an exam.
Another form is impression tonometry. A tiny plunger-like instrument indents the cornea, similar to the technique used in applanation. The amount of weight required to indent the cornea determines that the greater weight of the IOP results in a higher pressure.
This procedure is usually painless and is part of a routine eye examination. Some people may experience slight discomfort or slight irritation when touching their eyes, but usually the eyes are treated with a temporary anesthetic. This makes it highly unlikely to feel the instruments touching your eyes. Measurements are taken very quickly, taking only a minute to two of a total exam.
In fact, because tonometry is usually painless, you should report eye discomfort to your eye doctor. If the eye remains painful for a day or two, this could indicate an infection or a scratch on the cornea. This rarely occurs. Some people may notice some pain with this procedure after laser eye surgery. In these cases the pain is usually the result of the surgery and not of the tonometry examination.
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