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S. Fan, T.V. Johnson, C.B. Toris; Measurement of Intraocular Pressure in Mice by TonoLab Tonometry . Invest. Ophthalmol. Vis. Sci. 2006;47(13):215.
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To determine the efficacy of the TonoLab (Tiolat Oy, Helsinki) induction–based rebound tonometer for intraocular pressure (IOP) measurement in mice.
Seven CD–1 mice were anesthetized (100 mg/kg ketamine + 5 mg/kg xylazine) and their eyes were cannulated with a glass microneedle through the cornea and into the anterior chamber. The microneedle was connected in series to PE–tubing, a pressure transducer and a water column. Manometric data were collected using Powerlab software and stored on a computer. The IOP was changed by raising or lowering the water in the column. At each water level, IOP was measured simultaneously by the TonoLab and pressure transducer. Measurements were performed in a masked fashion with one investigator setting the IOP and a masked investigator taking the TonoLab reading. The values from the two methods were compared by regression analysis. In a separate group of mice (n=7), TonoLab measurements were taken every two minutes for 38 minutes starting with the time the animals stopped moving after administration of anesthetics (time zero). On a separate day, timolol maleate (10µL of 0.05%) was applied to one eye and an hour later, tonometric IOPs were collected every 2 minutes starting with time zero (n=6). IOPs in timolol–treated eyes were compared to before treatment, and to contralateral untreated eyes using two–tailed paired t–tests.
IOP readings by TonoLab tonometry correlated well with the readings by direct manometric measurement for pressures between 8 and 45 mmHg (y=0.98x – 0.32, R2 = 0.94, p<0.001). The TonoLab was not able to discriminate between IOPs below 8mmHg. IOPs were significantly (p<0.001) reduced from 17.4±2.5 mmHg at time zero to 8.0±1.1 mmHg 14 minutes later. By TonoLab measurement, eyes treated with timolol had significantly lower IOPs (13.7±1.7 mmHg) compared to before treatment (previous day, 16.8±1.7 mmHg, p=0.03) or to contralateral untreated eyes (same day, 17.2±2.5 mmHg, p=0.005) when the measurement was taken at time zero. IOPs taken 4 or more minutes later were not different when comparing contralateral eyes or measurements times.
TonoLab induction–based rebound tonometry can be used to obtain accurate IOP measurements in mice. Ketamine and xylazine anesthesia can greatly affect IOP in these animals.
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