Non-invasive determination of intraocular pressure in the rat eye. Comparison of an electronic tonometer (TonoPen), and a rebound (impact probe) tonometer

Background: The reproducibility and accuracy of the induction/impact (I/I) probe device (rebound tonometer) and the TonoPen XL electronic tonometer were compared through the measurement of intraocular pressure (IOP) differences between contralateral rat eyes. Methods: IOP was measured in 18 female Wistar rats with variable, modest elevations of IOP in one eye. Mean IOP from five measurements on each of the 36 eyes was determined using the rebound tonometer, followed by the TonoPen XL. Following cannulation, true (manometrically determined) IOP was recorded with a calibrated pressure transducer. Differences between the operated and normal eye of each animal for each tonometric method of measurement were correlated with the true difference in IOP. Results: IOP differences between the operated and contralateral normal eye ranged from +5.9 to -1.7 mmHg (mean +1.7 mmHg) when measured by cannulation and from +7.2 to -1.4 mmHg (mean +2.4 mmHg) and +9.8 to -3.2 mmHg (mean +3.6 mmHg) when measured with the rebound tonometer and TonoPen XL respectively. Differences between eyes recorded by rebound tonometer (y) correlated with those determined by cannulation (x) in a linear fashion (y=0.8243x+1.0721; R 2 =0.6603). Correlation for the TonoPen XL was much weaker (y=0.8675x+2.1672; R 2 =0.2077). IOP measurements using the rebound tonometer did not differ significantly from true IOP, whereas TonoPen XL increasingly underestimated IOP with increasing IOP (9-20 mmHg). Conclusion: The rebound tonometer displayed greater accuracy and less variability than TonoPen XL in measuring the IOP of the rat eye (range 9-20 mmHg).