How to measure the value of intraocular pressure as accurately as possible?
In our office, we measure intraocular pressure in two ways:
- Classical aplanation tonometry according to Goldman is still the gold standard in the diagnosis of glaucoma, the method is contact and requires patient preparation.
The patient is prepared by instilling anesthetic drops 1-2 times, then the tear film is stained with fluorescein, and then by applanation attached to the biomicroscope, the cornea is applauded with the tip of the device, the pressure is equalized inside and outside the eye and the value is read on a dot in mmH or digitally. - Non-contact tonometer with pachymetry, does not require patient preparation (without instillation of anesthetic and fluorescein drops).
Instead of applanation with an instrument, an air column is used, for a second the patient may feel discomfort due to the directed air towards the cornea, but it is only a few seconds. The newer generation devices that we use in our office have the so-called “Smart puff” and the amount of air immediately after the first wave decreases if the patient has a lower pressure or increases if it is higher. After three measurements, the mean value was automatically obtained. - Pachymertia as part of a non-contact tonometer.
Pachymetry is a method of measuring the central thickness of the cornea. The cornea is a concave lens at its cross section. It is centrally thinner and peripherally thicker, its central thickness is about 545 microns. The device performs three measurements, and then we get the mean value. After three non-contact IOP measurements, the device automatically correlates corneal thickness and pressure value and outputs the corrected CIOP value, which is extremely important for obtaining a real pressure value for that eye.
Information on corneal thickness is necessary for precise and realistic measurement of intraocular pressure as well as as part of preoperative preparation in patients who are preparing for laser diopter removal.
The thicker cornea provides greater resistance when measuring intraocular pressure. We have this situation with different types of corneal edema. It exists in patients with contact lens wearers or in farsighted people, ie in patients with metabolic diseases such as diabetes, so the corrected value will be less than measured.
We have a thinned cornea in short-sighted people or people in whom the cornea is modeled as part of laser diopter removal. In thinner corneas, the real corrected value is higher than the measured one.
The intraocular pressure must be corrected against the value of the central thickness of the cornea. Only in that way is it precisely measured for the eye on which the measurement was performed. Variations can be up to +/- 7 mmHg so that the corrected values can really surprise us and significantly affect the diagnosis of glaucoma.
