Purpose: The purpose of this study was to compare the accuracy of 2 portable tonometers, Tonopen and iCare, in extreme corneal edema, while also accounting for the location of pressure measurement over a wide range of intraocular pressure (IOP).
Methods: Utilizing an experimental laboratory investigation, IOP measurements were obtained with iCare and Tonopen against an internal digital manometer in 2 enucleated human cadaveric eyes with edematous corneas. Pachymetry was used to measure central corneal thickness in each eye. Each eye was then mounted on an upright-positioned mannequin, and a 30-G needle connected to a digital manometer was introduced into the anterior chamber at the limbus. Pressures were maintained at 0, 10, 20, 30, 40, and 50 mmHg using the digital manometer. Three measurements were taken with the tonometers at the central cornea, limbal cornea, and sclera, for a total of 9 measurements per device per eye at each manometric pressure reading.
Results: Comparing the mean absolute error of Tonopen versus iCare measurements at the central cornea for each manometric pressure reading, both devices underestimated the control at pressures >10 mmHg, with mean error increasing as IOP increased [mean absolute error (MAE) ranged from 1.67 to 13.33 with Tonopen and 3.67 to 21.00 with iCare]. Tonopen more accurately estimated pressure with an average MAE of 5.72 at the central cornea, compared with MAE of 10.83 at the central cornea for iCare (P<0.03). Comparing MAE between the devices at the limbus, there was no significant difference between the 2 (P=0.91), with average MAE for the Tonopen 5.47 and average MAE for the iCare 5.22. There was no significant difference between average MAE for Tonopen measurements at the central cornea versus the limbus (P=0.89), while the iCare was more accurate at the limbus with average MAE of 5.22 than the central cornea with average MAE of 10.83 (P<0.03). When measuring at the sclera, there was no statistically significant difference between average MAE for Tonopen (18.03) versus iCare (10.39) (P=0.24).
Discussion: Tonopen was more accurate than iCare when measuring at the central cornea, but consistently underestimated the control. MAE for both devices increased linearly with increasing true IOP. With limbus measurements in an edematous cornea, Tonopen and iCare performed similarly. The iCare performed better at the limbus than at the central cornea. Scleral measurements for both Tonopen and iCare were poor. In this experimental model, Tonopen performed superiorly to iCare with central cornea measurements in a grossly edematous cornea. Both devices underestimated true IOP.