Prospective Comparative Analysis of 4 Different Intraocular Pressure Measurement Techniques and Their Effects on Pressure Readings

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To the Editor:
We read with interest the article by Berk et al.1 Authors have shown that intraocular pressure (IOP) measurement technique can significantly affect the pressure results. This is clinically important information, because precise measurement of IOP in glaucoma patients is critical for diagnosis and efficiency of glaucoma treatment. This experiment is very interesting and necessary; however in our opinion, some aspects of this study need to be discussed.
First, the order of measurements should be analyzed. Authors conducted a prospective comparative analysis of 4 IOP measurement techniques in a cohort of 40 healthy patients in the following order: (1) Goldmann applanation tonometry (GAT) without fluorescein, (2) GAT with fluorescein paper strip, (3) GAT with fluorescein droplet, and (4) Tono-Pen Avia (TPA).
This sequence of measurements in our opinion might be wrong. Although all 4 methods of IOP measurement need the topical anesthesia and the tips of the instruments touch the corneal surface, there is an obvious difference between GAT and TPA. The GAT is based on measuring the IOP with 3.06-mm prism, whereas TPA fits the cornea with the small area of its silicon tip. Therefore, the IOP measurements with TPA after GAT may produce false results. We suggest that TPA measurement should be performed first, before GAT measurements. Another suggestion is making the IOP measurements in different (mixed), but randomized order with a suitable time interval.2 The IOP in each follow GAT measurements was higher and higher. Probably this is because of irritation of the eye after medicines used. During prolonging measurement the eyes are even more irritating, the production of tears increases, the rings observed by an ophthalmologist are even slightly thicker.3
Second, the effect of repeated applanation tonometry on the accuracy of IOP measurements should be taken into account. In the study by Gaton et al4 3 GAT measurements with fluorescein strips were repeated with a few seconds pause, which allowed for a 1-time fluorescein application. Berk and colleagues demonstrated statistically significant IOP increase during the subsequent GAT measurements in healthy eyes. However, they used fluorescein several times, which may be the reason for higher IOP results by irritation, stinging, and burning effect after fluorescein application.3 Another possibility of IOP increase is a simple chemical reaction between proparcain hydrochloride and fluorescein natrium. Mixture of them, gave sodium chloride, and with the excess of fluorescein, the anesthetic compounds are less water-soluble, so less active. It could develop the inadequate local anesthesia and finally higher measurement results.
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