Discrepancies in Eye Pressure Evaluation Methods

The study of intraocular pressure (IOP) is essential when performing routine ophthalmological studies, especially for patients with glaucoma. In these cases, IOP is the only risk factor that doctors can treat. This is why it is important to obtain the IOP value, a fundamental element for the diagnosis and treatment of glaucoma.

Goldman applanation tonometry is the reference test par excellence to evaluate IOP. However, there are factors that affect its accuracy. Among them are those related to the morphology of the eye, such as the central corneal thickness (ECC) or the curvature of the cornea and those related to the biomechanical properties of the cornea.

ECC has been shown to alter IOP measurements using TAG, underestimating it in thin corneas and overestimating it in thicker corneas. Different formulas have been tested to improve the precision of the TAG, adjusting it according to the ECC, but so far none provides reliable and accurate results.

To solve the problem, new tonometers have been developed that avoid alterations due to the thickness of the cornea and also its biomechanical properties.

Dynamic contour tonometry (DCT Swiss Microtechnology, Switzerland), rebound tonometry (RC, Icare, Finland), ocular response analyzer (ORAM REichert, USA) and Corvis ST (CST, Oculus, Germany) are devices that measure IOP in different ways and that have been evaluated and compared with the GAD in healthy people in the present study.

Although studies have already been carried out on the subject, for the first time a comparison of the five devices was obtained in untreated eyes and an analysis of the differences related to the morphological parameters of the cornea in a large population.

Only the right eye of 146 patients was included in the study. Each participant underwent corneal analysis with a Scheimflug camera and IOP evaluation with each of the five devices.

Differences were sought between the IOP values ​​obtained with each of the instruments and then the correlations between these discrepancies and morphological characteristics such as mean keratometry and thickness of the central cornea were studied.

Intraocular pressure measurements in healthy participants, using ocular response analyzer, Goldman tonometer, dynamic contour tonometer, rebound tonometer and Corvis tonometer.

To prevent the progression of glaucoma, doctors need to be able to reduce IOP, for this it is important to obtain an accurate and reliable estimate.

We know that the gold standard test today is Goldman applanation tonometry, but this does not always provide an accurate measurement of IOP, but newer instruments have also not shown consistent accuracy according to published studies.

The tonometers tested in the present study evaluate IOP in different ways, three are contact tonometers (TAG, TCD and TR) while two do not require contact (ARO and CST).

As previously reported by Schneider et al, TCD is suitable for measuring the IOP of cooperative patients with sufficient bilateral fixation. ARO and CST, being non-contact tonometers, are less invasive for patients and can be used in the office where it is better to avoid contact due to the risk of infections. 

TR works through very light and well-tolerated contact with the cornea, however, results may be affected by the tear film more than other instruments studied.

All the tonometers studied as an alternative to the TAG gave significantly higher IOP values, probably due to the operating principles. It is not possible to determine which is the most reliable since they must be compared with real IOP measurements obtained by intraocular probe.

The differences observed between ARO and CST are not statistically significant, although in a previous study of the same group the difference had been 1.25 mmHg. This may be due to the larger number of participants in this study, none of the previous participants were in this study, and the CST software has been changing over time.

It is important to clarify that the refractive effect of the participants in this study was mostly myopic, this must be taken into account when comparing with other studies.