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B. D. Currie, H. Bagga, A. P. Tanna; Effects of Instrument Orientation on the Accuracy of IOP Measurements in Eye Bank Eyes: Manometric Evaluation of the Pneumatonometer and Tono-Pen. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1050.
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The purpose of this study was to manometrically determine the effects of probe orientation on the accuracy of intraocular pressure measurements taken by the Model 30 Classic Pneumatonometer and the Tono-Pen XL.
Six enucleated human eyes were obtained from the Illinois Eye Bank less than 24 hours post-mortem. IOP was maintained at 10, 20, and 30 mmHg via insertion into the vitreous of a 25G needle connected to a liquid column manometer filled with DPBS. After changing each pressure setpoint, the IOP was allowed to equilibrate for 4 minutes. At each setpoint, the eyes were positioned to mimic a sitting, supine, and prone patient. The Model 30 Classic Pneumatonometer (Reichert Ophthalmic Instruments) was used to take measurements in the sitting and supine orientations, and the Tono-Pen XL (ROI) was used to take measurements in the sitting, supine, and prone orientations. With each instrument at each probe orientation for every eye, 3 measurements were taken and their mean was calculated. Paired t-tests were used to compare these means for each of the six eyes (n=6).
At the setpoints of 10 mmHg and 20 mmHg the pneumatonometer overestimated the IOP in both the sitting orientation (12.9 mmHg, p=0.005; and 21.8 mmHg, p=0.02, respectively) and the supine orientation (12.9 mmHg, p=0.0003; and 22.4 mmHg, p=0.03, respectively). At the 30 mmHg setpoint the pneumatonometer accurately measured the IOP in both the sitting and supine orientations.The Tono-Pen accurately measured the IOP in all orientations at all setpoints except the following: overestimated sitting at 10 mmHg (11.3 mmHg, p=0.03); overestimated prone at 10 mmHg (11.5 mmHg, p=0.015); underestimated prone at 30 mmHg (27.8 mmHg, p=0.001).At all pressure setpoints, probe orientation had no significant effect on IOP measurement with either instrument (Pneumatonometer: sitting=supine; Tono-Pen: sitting=supine=prone).
The Tono-Pen fairly accurately measured the manometric IOP at all three probe orientations, especially at 20 mmHg. The pneumatonometer consistently overestimated the IOP at both its probe orientations at lower setpoints, but accurately measured IOP at 30 mmHg. Since there was no significant effect of probe orientation on IOP measurements for either instrument, this study suggests that they can be used in any of the tested configurations without correction for probe orientation.
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