Tono-pen and Schiotz tonometer measurements in gas-filled eyes

Abstract

Purpose: This study aims to assess the difference between Tono-Pen and Schiotz tonometer measurements in gas-filled eyes and to create a nomogram and equation which can be used to estimate actual intraocular pressure in order to provide a safe IOP level at the end of the surgery.

Methods: Twenty-two eyes that underwent pars plana vitrectomy were included in the study. Perioperative Tono-Pen and Schiotz tonometer measurements were performed when the eyes were filled with air in the setting of certain vitrectomy infusion pressure levels. Measurements were performed when the eyes were filled with fluid to test the accuracy of the systems. The mean value of the Tono-Pen and Schiotz readings in air-filled eyes corresponding to certain actual intraocular pressure levels were analyzed to create nomograms.

Results: Both Tono-Pen and Schiotz tonometers underestimated the actual intraocular pressure set on the screen of the vitrectomy system in the air-filled eyes. The Tono-Pen deviation was 4.5mmHg at a level of 15mmHg actual intraocular pressure, and 16.9mmHg at a level of 55mmHg actual intraocular pressure. The Schiotz tonometer deviation was 10mmHg at a level of 15mmHg actual intraocular pressure, and 8.9mmHg at a level of 55mmHg actual intraocular pressure. All the mean differences between tonometer readings and actual intraocular pressure were statistically significant. (P<0.001) CONCLUSION: To achieve an adequate and safe tamponade at an actual IOP range of 20 - 25mmHg, one should adjust the IOP with Schiotz readings to a level of 9-12mmHg, or Tono-Pen readings to 12-18mmHg.

Keywords: Gas tamponade; Intraocular pressure; Pression intraoculaire; Schiotz tonometer; Tamponnade au gaz; Tono-Pen tonometer; Tonomètre Tono-Pen; Tonomètre de Schiötz; Vitrectomie; Vitrectomy.