What is ocular tonometry and when should it be done?

Ocular tonometry is a noninvasive, nonpainful examination performed during routine eye examinations to assess the eye's internal pressure or ocular tone. 

Intraocular pressure is measured in millimeters of mercury (mmHg) and can be influenced by corneal thickness: a thin cornea reports lower values, while a thick cornea reports higher values. Cortisone therapy, trauma or ophthalmic surgery can also change eye tone.

Values that are too high can cause irreversible damage to the optic nerve; therefore, in such cases, diagnostic testing is essential. Monitoring eye pressure is also very important for the diagnosis of possible diseases, such as glaucoma. 

This is an examination accessible to everyone, including children and pregnant women. It is especially indicated for people suffering from glaucoma, as the eye pressure need not exceed the threshold value of 21 mmHg.

There are several instruments for measuring intraocular pressure, the most widely used of which are: 

• Blow tonometer: blow tonometry involves no contact; the instrument is brought close to the patient's eye, and once the aim is focused, a puff of air is produced that reaches the cornea and is picked up by a photocell. This procedure takes a few seconds, about 15 per eye.
• Goldmann applanation tonometer: Goldmann applanation tonometry requires contact (albeit a very slight force). Anesthetic is applied before starting the examination to avoid discomfort to the patient during the measurement. The cone is, then, brought close to the eye until it makes contact: the pressure is calculated through the eye's resistance to the instrument. This procedure also takes a few seconds, about 20 in total.

Goldmann applanation tonometry is considered the gold standard internationally; however, it has been questioned because a correlation between intraocular pressure and central corneal thickness has been demonstrated. A thin cornea exposes, in fact, to underestimation of pressure, while a thick cornea to overestimation of results.

With the blow tonometer, on the other hand, the patient's corneal thickness is also taken into account: this allows the pressure value to be corrected according to the actual eye data and the results to be more accurate. 

Normal intraocular pressure values are between 10 and 21 mmHg (millimeters of mercury). For patients with borderline pressure values, a tonometric curve is recommended, which is a diagnostic test that consists of measuring eye tone throughout the day to monitor its progress. 

In patients with glaucoma or borderline pressure, the tonometric curve provides insight into whether the pressure remains within normal limits throughout the day. Usually, at least 3 measurements are taken: 

• the first - at around 9 am;
• the second - at 12 am;
• the last - in the afternoon.