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Andrea Wilson, Guorong Li, William Daniel Stamer; Optimization of novel intraocular pressure measurement system for mice. Invest. Ophthalmol. Vis. Sci. 2022;63(7):1286 – F0101.
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Mice are widely used to study intraocular pressure (IOP) regulation due to the similarities of their conventional outflow pathway to humans. Elevated IOP is the primary risk factor for glaucoma—a leading cause of irreversible blindness worldwide, but measuring IOP in mice is challenging. The aim of this study is to optimize conditions for reliably measuring IOP using a newly designed system for mice.
Modeled after a system designed by Dr. Simon John, our system includes an AmScope stereo microscope, custom-designed mouse platform that rotates in two planes, TonoLab rebound tonometer with a mounting clamp, foot pedal trigger, isoflurane anesthesia system, and monitors for heart rate (HR) and oxygen saturation percentage (O2S). For experiments, the same group of C57/B6 mice were used to evaluate two methods of anesthesia: Isoflurane (iso, 2% induction, 1% nosecone) vs. ketamine/xylazine (ket/xy, ~70mg/7mg/kg). Experiments were performed on different days, repeated 9 times for iso and 5 for ket/xy. IOP was measured with and without a microscope for tonometer positioning. IOP, HR, and O2S were recorded at 0 and 5 min after induction of sleep.
For ket/xy vs. iso, IOP was not significantly different (16.19±1.9 vs. 15.32±3.7 mmHg, p=0.46), but HR and O2S were significantly lower (HR: 328.57±37.2 vs. 560.35±62.6 per min, p<0.001; O2S: 81.68±8.6 vs. 75±7.4, p=0.01). After 5 min, IOP dropped 2.5 mmHg with ket/xy (16.19±1.9 vs 13.68±1.9 mmHg, p<0.001) but only 0.5 mmHg with iso (15.32±3.7 vs. 14.80±2.3 mmHg, p=0.68). HR decreased for both iso and ket/xy but was not significantly different at 5 min. IOP readings with vs. without a microscope did not show a significant difference for iso at 0 and 5 min (0 min: 15.32±3.7 vs. 14.73±1.9 mmHg, p=0.61; 5 min: 14.80±2.3 vs. 14.72±2.3 mmHg, p=0.93) or ket/xy (0 min: 16.19±1.9 vs. 16.33±1.3 mmHg, p=0.81; 5 min: 13.68±1.9 vs. 13.65±2.6 mmHg, p=0.97). There was a significant difference between IOPs in iso vs. ket/xy datasets at 0 min without a microscope (14.73± 1.9 vs. 16.33±1.3 mmHg, p=0.004).
With our newly designed IOP system, similar IOP readings were obtained using iso or ket/xy, and IOP drops more rapidly with ket/xy than iso. HR is not a determining factor for IOP readings. The use of a microscope for tonometer positioning and consistent time between induction of sleep and IOP collection are the most important parameters for reliable IOP measurements.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.
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