Comparison of a rebound and an applanation tonometer for measuring intraocular pressure in normal rabbits

OBJECTIVE To compare intraocular pressure (IOP) measurements made on healthy adult rabbits without the effect of tranquilizers using the new applanation tonometer, Tono-Pen Avia(®), and the rebound tonometer Tonovet(®). METHODS Intraocular pressure was measured throughout the day (6:00, 9:00, 12:00, 15:00, and 18:00 h) in 38 adult New Zealand White rabbits (76 eyes). The animals were 20 males and 18 females, with a mean weight of 3.5 kg and an average age of 6 months. A complete ocular exam (including Schirmer tear test, fluorescein staining, slit-lamp biomicroscopy, and direct ophthalmoscopy) was performed on all animals at the beginning of the trial. Rebound tonometry was performed, and after 10 min, anesthetic drops were instilled and applanation tonometry was carried out. IOP values obtained using the two techniques were analyzed statistically. RESULTS The mean IOP was 9.51 ± 2.62 mmHg with Tonovet(®), and 15.44 ± 2.16 mmHg with the Tono-Pen Avia(®). Significant differences between measurements with the two tonometers were observed (P < 0.001). The linear regression equation describing the relationship between the two tonometers was y = 0.4923x + 10.754 (y = Tonovet(®) and x = Tono-Pen Avia(®)). High IOPs were recorded in the early measurements (6:00), but the average IOPs from both devices were statistically similar throughout the day (P = 0.086). The correlation coefficient was r(2) = 0.357. No significant difference in IOP regarding gender was observed. CONCLUSION The Tono-Pen Avia(®) recorded higher levels of IOP compared with the Tonovet(®). Early in the day, the IOP of rabbits was higher than later in the day, regardless of the tonometer used.