Comparison of two non–invasive tonometers for the measurement of Intraocular Pressure (IOP) in the Mouse

Abstract

Abstract: : Purpose:To evaluate and compare the validity, the reproducibility and the ease of use of two prototype tonometers specifically developed for IOP measurement in the mouse. Methods:A duplicate of the prototype impact–rebound tonometer (Danias et al IOVS Mar 2003; 44: 1138 – 1141) and a prototype murine applanation tonometer utilizing a fiberoptic pressure sensor (Ahmed et al ARVO 2003) were compared. Enucleated eyes (n=12) from C57/BL6 mice were used for this purpose. The anterior chamber was cannulated with a 30G needle and the IOP was manometrically adjusted in increments of 5 cm of H₂0 (open stopcock method). Five measurements were taken at each pressure level. A calibration curve was generated for each individual eye along with a master calibration curve taking all the eyes into account. Two operators (one experienced and one inexperienced) were measuring the IOP. Results:Both tonometers yielded valid and almost equivalent calibration curves with individual R² 0.9878 (exponential) and 0.9902 (linear) respectively. 1.The impact rebound tonometer was superior with respect to intraobserver variability with a Coefficient of Variation ranging between 1% and 12% compared to the 1% to 33% of the applanation tonometer. 2.The impact rebound tonometer was superior with respect to interobserver variability. The R² for the full model (total of 330 IOP determinations) was 0.928 for the impact–rebound tonometer and 0.885 for the fiberoptic applanation tonometer. 3. Despite overall higher variability the applanation tonometer performed better in pressures higher than 40 cm H₂0 due to its linear behavior. The impact rebound tonometer manifested a smaller range of 95% CI for IOP <35 cm H₂0 and equivalent for IOP = 40cm H₂0 (= 29.41 mmHg). Conclusions:Noninvasive tonometry in the mouse using both methods is feasible. The learning curve was brief for both instruments with both operators achieving similar reproducibility after only a few measurements. Repeated determinations might be required to yield valid results.

Keywords: intraocular pressure • anterior chamber • ganglion cells