Correlation of Tonometric and Direct Measurements of Intraocular Pressure in the Porcine Eye

Abstract

Abstract: : Purpose: To compare Tono–Pen, Schiotz, and Perkins tonometer assessment of intraocular pressure (IOP) with the direct measurement of anterior chamber and intravitreal pressures in the porcine eye. Methods: Balanced saline solution (BSS) was infused (0 to 70 mmHg) into the anterior chamber of the porcine eye in vitro and in vivo. Direct invasive anterior chamber and intravitreal pressures were measured using a calibrated pressure transducer connected to an amplifier and data acquisition system. IOP was measured with the Tono–Pen, Schiotz, and Perkins tonometers. Results: In enucleated eyes, both anterior chamber and intravitreal pressures were identical and increased in a similar manner with increasing infusion pressure. However, these ocular pressures did not increase proportionally until reaching 30 mmHg infusion pressure. When infusion pressure was increased in stepwise manner from 15 to 70 mmHg, the IOPs measured by all 3 tonometers were downward deviated from the anterior chamber pressure by about 10 to 20 mmHg, at 30 and 70 mmHg infusion pressure, respectively. In the live pig under anesthesia, the anterior chamber and intravitreal pressures, via direct measurement, were 19±3 and 20±3 mmHg, respectively. Tono–Pen, Schiotz, and Perkins tonometer measurements were 8±2, 9±2, and 9±2 mmHg, respectively. Both anterior chamber and intravitreal pressures were consistently higher than the BSS infusion pressure until the BSS infusion pressure reached approximately 70 mmHg. The three different tonometers significantly underestimated the anterior chamber pressure by an average of 12 mmHg in an infusion pressure range of 0–60 mmHg. Based on the experimental data, linear regression equations were constructed to correct the deviation of IOP measurements by tonometers. Conclusions: The Tono–Pen, Schiotz, and Perkins tonometers underestimate the IOP in the porcine eye. Linear regression equations can be used to correct the IOP as measured by the above tonometers. This new fundamental information would be useful in the estimation of true IOP from tonometric measurements when direct measurement of IOP is not applicable or accessible in the porcine eye.

Keywords: intraocular pressure • anterior chamber • vitreous