Rebound tonometry in conscious, conditioned mice avoids the acute and profound effects of anesthesia on intraocular pressure

Aims: The aims of this study were to evaluate the accuracy, repeatability, and safety of multiple intraocular pressure (IOP) measurements by a commercially available rebound tonometer in conscious, conditioned mice, and to characterize the acute and profound effects of anesthesia on IOP in mice. Methods: To test the accuracy of the tonometer, IOPs of CD-1 mice under ketamine/xylazine anesthesia were experimentally set and monitored with a water manometer/transducer system following transcorneal cannulation while simultaneously performing tonometry. The long- and short-term repeatability of the tonometer was tested in conscious, restrained mice, as measurements were taken once-daily in the afternoon for 4 consecutive days. On day 5, IOPs were measured in the same mice once every 4 min for 32 min. On 2 separate days, mice were administered ketamine/xylazine or 2,2,2-tribromoethanol anesthesia, in a crossover design, and IOPs were measured once every 2 min for 32 min. Rebound tonometry was performed in conscious mice before and 1 hour after 1 drop of timolol maleate (10 μL of 0.5%) application to 1 eye. Results: IOP measurements by rebound tonometry correlated well with manometry for pressures between 8 and 38 mmHg (y = 0.98x - 0.32, R² = 0.94; P < 0.001). The average tonometric IOP was invariant over 4 days (range, 11.7-13.2 mmHg). IOPs dropped significantly (P ≤ 0.05) within 6 min (ketamine/xylazine) or 10 min (2,2,2-tribromoethanol) postadministration of anesthesia but not with conscious restraint. Timolol significantly (P < 0.001) lowered IOP from 12.8 ± 0.3 (mean ± standard error of the mean) to 10.1 ± 0.6 mmHg, as measured by the tonometer. Conclusions: Rebound tonometry can be used to obtain accurate IOP measurements in conscious, restrained mice while avoiding the rapid and profound ocular hypotensive effects of general anesthesia. Small changes in IOP with an aqueous-flow suppressant are readily detectable with conscious restraint that may be missed with chemical restraint.