Tonometry

(This test is part of the initial diagnostic work-up)

Tonometry is the measurement of intraocular pressure (IOP). Before special instruments were invented, pressure was assessed by palpation: the ophthalmologist gently presses on the eye through the closed eyelids with the fingers and can roughly estimate the level of IOP. Today this method is used only in urgent situations, when it is necessary to quickly assess intraocular pressure and no measuring devices are available.

All methods and instruments for measuring intraocular pressure can be divided into two groups:

• contact – when the device comes into direct contact with the surface of the cornea;

• non-contact – when there is no direct contact with the eye.

The recognized international gold standard is the measurement of IOP using the Goldmann tonometer. A special prism (weight) is placed on the eye and exerts pressure on the cornea, flattening it; the force required for this is measured on a special calibrated scale. Before the measurement, fluorescein dye is instilled into the eye; by the fluorescein pattern on the prism, the ophthalmologist can judge whether the instrument is correctly positioned on the eye. This examination is performed at the slit lamp. Since this method is contact-based and the prism acts directly on the cornea, anesthetic drops are instilled into the eye beforehand.

Modern non-contact tonometers measure IOP using a jet of air that flattens the cornea, while the speed at which the cornea returns to its original shape is recorded by a special optical sensor. The device then converts this measurement into millimeters of mercury (mmHg). Because there is no direct contact with the eye in this method, no anesthetic drops are required and there is no potential risk of infection entering the eye, as may occur with contact methods. However, this test is less accurate than Goldmann tonometry, especially in patients with high IOP.

The latest models of non-contact tonometers used in our clinic simultaneously measure corneal thickness and apply a correction based on this parameter, making the readings more accurate. In addition, to determine the “true” pressure we use special tonometers that employ analysis of the elastic properties of the cornea.

In our clinic, contact instruments for measuring IOP are used to confirm non-contact results in cases where there is doubt; methods based on the analysis of the cornea’s elastic properties can also be applied. The ORA (Reichert) is an automatic device that implements a fundamentally new approach to measuring ocular tension (IOP). The analyzer of the eye’s biomechanical properties makes it possible to assess the viscoelastic properties of the cornea and its thickness.