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Tonometry is a test of intraocular pressure (IOP). Intraocular pressure depends on the degree of balance between the production of aqueous fluid (intraocular fluid) and its outflow from the eyeball. In healthy individuals, a range of 10-21 mm Hg is commonly considered normal values for intraocular pressure. Intraocular pressure fluctuates throughout the day (it is highest in the morning). Its average daily fluctuations do not exceed 5 mm Hg. Tonometry is part of a routine eye examination.

Tonometry performed before deciding on laser vision correction is designed to determine whether the procedure poses any risk to the patient. Modern non-contact tonometers measure not only corneal thickness and intraocular pressure, but also the biomechanical properties of the cornea when an air pulse is applied to it.

Intraocular pressure testing methods

Applanation (Goldmann) tonometry involves the patient sitting in front of a slit lamp, anesthetizing the surface of the eye, and staining the tear film with a solution of fluorescein, which begins to glow (exhibit fluorescence) when exposed to cobalt-blue light. With the tip of the prism, the top of the cornea is pressed.

Depression (impression) tonometry involves the doctor applying pressure to the patient’s cornea with a Schiøtz tonometer while the patient is lying down or has his head tilted back. Corneal resistance depends on the pressure in the eyeball. Measurement errors are due to an increased corneal stiffness coefficient, which happens in the elderly (false high pressure), or to increased corneal elasticity, which, in turn, happens in patients with significant myopia or patients with Graves-Basedow disease (false low pressure).

Non-contact tonometry ( air-puff type) involves directing a blast of compressed air at the patient’s eyeball, which flattens the cornea. The test does not require anesthesia, the force of the blast can be startling and unpleasant, but does not cause pain. Measurement with a handheld thermometer Tono-Pen, in which a special transducer flattens the cornea. Its advantage is that it can be used even in subjects with contact lenses in place.

Applanation (Goldmann) tonometry involves the patient sitting in front of a slit lamp, anesthetizing the surface of the eye, and staining the tear film with a solution of fluorescein, which begins to glow (exhibit fluorescence) when exposed to cobalt-blue light. With the tip of the prism, the top of the cornea is pressed.

Depression (impression) tonometry involves the doctor applying pressure to the patient’s cornea with a Schiøtz tonometer while the patient is lying down or has his head tilted back. Corneal resistance depends on the pressure in the eyeball. Measurement errors are due to an increased corneal stiffness coefficient, which happens in the elderly (false high pressure), or to increased corneal elasticity, which, in turn, happens in patients with significant myopia or patients with Graves-Basedow disease (false low pressure).

Non-contact tonometry ( air-puff type) involves directing a blast of compressed air at the patient’s eyeball, which flattens the cornea. The test does not require anesthesia, the force of the blast can be startling and unpleasant, but does not cause pain. Measurement with a handheld thermometer Tono-Pen, in which a special transducer flattens the cornea. Its advantage is that it can be used even in subjects with contact lenses in place.

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