The induction/impact tonometer: a new instrument to measure intraocular pressure in the rat

Non-invasive intraocular pressure (IOP) measurement in rats can be performed with a variety of methods, none of which seems appropriate for scaling down for the mouse eye. In an attempt to develop such a method for non-invasive IOP measurement in mice, an alternative concept, that of rebound tonometry, was explored using an induction/impact (I/I) tonometer probe. IOP measurement using the rebound concept is based on bouncing a probe onto the eye and detecting its motion. Motion parameters of the probe, which vary according to eye pressure, are used to calculate the IOP. As a first step towards this goal a prototype I/I tonometer was evaluated for IOP measurement of the rat eye. Two similar instruments were constructed and tested for their ability to measure accurately and reliably rat IOP by comparing the measurements against the manometric (true) IOP as determined by cannulation ex vivo. Good correlation between the true IOP and the I/I measurements (R(2) = 0.95) was detected for IOP between 7.4 and 56 mmHg. Although individual eyes tested showed some variability in the relationship of the measured IOP with the true IOP, this variability was minimal. Starting probe-cornea distance between 3 and 5 mm, and angle of impact up to 25 degrees relative to the visual axis at the corneal apex, did not affect the reproducibility of the I/I tonometer. Comparison of I/I tonometer measurements to direct manometric determination of IOP by in vivo cannulation of eyes in anesthetized normal female Wistar rats correlated reasonably well (R(2) = 0.67) with manometrically determined IOP within the narrow range of normal rat IOPs (10-16.5 mmHg), underestimating the true (manometric) IOP by an average of 11.6%. The I/I tonometer is a reliable and accurate instrument for non-invasive IOP measurement in rat eyes that can potentially be adapted for IOP measurement in mice.