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Tonometry
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Tonometry is the test used to measure intraocular pressure, that is, the pressure inside the eye. It is done with a calibrated instrument, and the range considered normal runs from ten to twenty-one millimeters of mercury.
The method is called applanation, and the idea behind it is simple. Without putting anything into the eye, it measures how much force it takes to flatten a set area of the cornea, the clear window at the front of the eye. The relationship is direct: the higher the pressure inside, the more force it takes to flatten that surface. The surface of the eye is pressed in slightly, and the force it takes is measured.
It helps to know that the reading is an estimate, not the true pressure measured inside the eye. The true pressure of the eyeball can only be obtained by manometry, which involves placing a probe inside the eye.
In brief
Tonometry measures intraocular pressure with a calibrated instrument; the normal range runs from ten to twenty-one millimeters of mercury.
Applanation measures the force needed to flatten the cornea over a given area: the higher the intraocular pressure, the more force it takes.
The surface of the eye is pressed in slightly, and the force it takes is measured.
The result is an estimate: true pressure can only be obtained by manometry, which requires placing a probe inside the eye.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingTonometry, StatPearlsPublished
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