Repeatability and reproducibility of intraocular pressure measurements in horses using rebound and applanation tonometry

Background: Intraocular pressure (IOP) assessment is a fundamental component of ophthalmic examinations in both veterinary and human medicine. Due to variations in IOP across species and potential differences among examiners, knowledge of normal IOP values obtained using different tonometers is essential. Aim: This study aimed to compare the repeatability and reproducibility of IOP values in horses obtained by three different examiners using rebound (Tonovet®) and applanation (Tonopen Avia®) tonometers. Methods: Thirty healthy criollo horses were examined. IOP was measured by three veterinarians with different levels of experience following routine ophthalmic evaluation. Statistical analyses included analysis of variance, Student’s t-test, and calculation of the intraclass correlation coefficient (ICC). Results: The mean IOP values were 26.16 ± 5.13 mm Hg with rebound tonometry and 22.55 ± 7.32 mm Hg with applanation tonometry, indicating a significant difference between the two devices. For the rebound measurements, the examiner-specific means were as follows: A = 25.27 ± 4.9 mm Hg, B = 25.80 ± 5.2 mm Hg, and C = 27.40 ± 5.1 mm Hg. For applanation tonometry, the mean values were: A = 20.03 ± 4.5 mm Hg, B = 19.67 ± 6.0 mm Hg, and C = 27.97 ± 10.3 mm Hg. Examiner C consistently obtained higher IOP values with the Tonopen Avia®. The ICC was 0.41 for rebound tonometry, indicating satisfactory reproducibility, and 0.18 for applanation tonometry, indicating poor reproducibility. Conclusion: Rebound tonometry (Tonovet®) yielded higher IOP values than applanation tonometry (Tonopen Avia®) but demonstrated superior reproducibility and was less affected by examiner variability. These findings demonstrate that Tonovet® is a more reliable tool for measuring IOP in horses under clinical and research conditions.