Method for the noninvasive measurement of intraocular pressure in mice

PURPOSE. To evaluate the applicability of rebound tonometry for measurement of IOP in the mouse eye. METHODS. An induction-impact (I/I) tonometer, which operates on the rebound principle, was scaled down and adapted to determine IOP of the mouse eye. IOP measurement using this concept is based on contacting the eye with a probe and detecting the motion as the probe collides with the eye and bounces back. The motion parameters of the probe vary according to eye pressure and are used to calculate IOP. A prototype instrument was constructed for measurement of IOP in mouse eyes, and its ability to accurately and reliably measure IOP was tested by comparing the measurements against the manometric (true) IOP determined in cannulated mouse eyes ex vivo. The I/I tonometer was also used to measure IOP in vivo in anesthetized adult C57BL/6 mice. RESULTS. A strong correlation between the true IOP and the I/I measurements (R-2 = 0.95) was found for IOPs in the range of 3.7 to 44.1 mm Hg in cannulated mouse eyes. Repeat determinations in individual eyes showed a low degree of variability in the relationship of the measured IOP with the true IOP. In anesthetized mice, mean IOP +/- SD as determined by rebound tonometry was 9.8 +/- 3.9 mm Hg when the animals were anesthetized with ketamine alone and 7.6 +/- 1.9 mm Hg when a mixture of ketamine, acepromazine, and xylazine was used. Contralateral eyes differed by 0.9 +/- 2.5 and 0.1 +/- 2.7 mm Hg, respectively, for the two anesthetic regimens. CONCLUSIONS. The I/I tonometer can be used for noninvasive, in vivo IOP measurement in mouse eyes. The availability of an easy-to-use, reliable tonometer for IOP measurements in mice will allow more extensive use of the mouse as a model for glaucoma.