The Induction/Impact Tonometer: a New Instrument to Measure Intraocular Pressure in the Rat ANTTI I. KONTIOLA a ,DAVID GOLDBLUM b ,THOM MITTAG bc AND JOHN DANIAS b * a Department of Ophthalmology, Helsinki University Central Hospital, Helsinki, Finland, b Department of Ophthalmology, Mount Sinai School of Medicine, New York, NY, U.S.A. and c Department of Pharmacology, Mount Sinai School of Medicine, New York, NY, U.S.A. Received Erlangen 26 February 2001 and accepted in revised form 3 August 2001) 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 ®rst 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 258 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. # 2001 Academic Press Keywords: tonometry; IOP; rat; measurement; method; impact; rebound. 1. Introduction Attempts to determine intraocular pressure IOP) in the rat eye non-invasively have used the techniques of pneumotonometry Shareef et al., 1995), electronic Mackay±Marg tonometry i.e. Tonopen) Moore et al., 1993, 1995), and applanation tonometry Kurata et al., 1996; Cohan and Bohr, 2001). Of these methods, measurement with the Tonopen is the one most widely used because in contrast to applanation tonometry, it can be applied without modi®cations to the clinically used instrument Moore et al., 1993; Mermoud et al., 1994). However, rat IOP measure- ments by Tonopen depend on the pressure and direction with which the probe is applied to the eye Mermoud et al., 1994). Consequently the measure- ments have a large inter-observer variability requiring calibration of the instrument for the speci®c individual performing the measurements and a high degree of consistency by that individual Moore et al., 1993). Recently, mouse strains with elevated IOP havebeen identi®ed Sheldon et al., 1995; John et al., 1997, 1998). However, no method for non-invasive measurement of IOP in the mouse is currently available because none of the probes applied to the rat eye are suitable for the much smaller mouse eye. Therefore, alternative IOP measurement concepts were explored to develop a non-invasive method for IOP measurement in mice. As a ®rst step towards this goal a different principle was evaluated for IOP measurement. The recently introduced induction/ impact I/I) tonometer which is based on the rebound principle, i.e. bouncing a magnetized probe onto the eye and using its motion parameters to determine IOP Kontiola, 1997) was adapted to the rat eye. This was done by lowering the mass of the probe, appropriately adjusting the electronic circuitry and adding to the basic design of the clinical I/I tonometer Kontiola, 2000) a separate coil for moving the probe. The purpose of this study is to evaluate the accuracy of the I/I tonometer and the principle of rebound tonometry for measuring IOP in the rat. 2. Materials and Methods All experiments were performed according to the ARVO guidelines for animal research. Exp.EyeRes. 2001) 73, 781±785 doi:10.1006/exer.2001.1088, available online at http://www.idealibrary.com on 0014-4835/01/12078105 $35.00/0 # 2001 Academic Press * Address correspondence to: John Danias, Department of Ophthalmology, Box 1183, Mount Sinai School of Medicine, 1 Gustave L Levy Place, New York, NY 10029, U.S.A. E-mail: Danias@msvax.mssm.edu