Evaluation of the applanation tonometer Tono-Pen Avia® Vet™ for the measurement of the canine and feline intraocular pressure

Abstract

Using tonometers, intraocular pressure (IOP) is merely measured indirectly leading to measurement inaccuracies. All available technical devices are designed for humans, hence a calibration via comparison to manometric measurements is necessary for their use in veterinary medicine. In this study, the applanation tonometer Tono-Pen Avia® Vet™ (TPA) was to be calibrated for use in dogs and cats resulting in the calculation of a correction factor. Additional objectives were the determination of reference values for TPA-derived IOP and the evaluation of possible influence of patient characteristics on measurements results. For the manometric study, 10 enucleated eyes from cats and dogs were subject to IOP measurement with a manometer and the TPA covering the range of 5-60 mmHg. The mean percentage of difference between manometer and TPA was used to calculate correction factors. Subsequently, the TPA was employed to measure the IOP in healthy animals without signs of ocular disease for the determination of reference values. The effect of the patient characteristics age, body weight, and gender on the IOP measurement results was examined using statistical mixed models. With rising intraocular pressure, the TPA underestimated IOP to an increasing degree. Multiplication of the TPA-derived IOP with the factor of 1.5 for dogs and cats resulted in values well in accordance with the manometrically-derived, actual IOP. In the second part of the study, a total of 94 dogs (188 eyes) and 64 cats (128 eyes) were examined. Reference values for the IOP measured with the TPA are 9-18 mmHg for dogs and 9-20 mmHg for cats. For both species, age exhibited a significant influence (dog, p = 0.001 and cat, p = 0.008) on the IOP measurement results in that the IOP-values displayed a decrease with increasing age. The TPA underestimated the actual IOP with increasing intraocular pressure. The calculated correction factor of 1.5 is suitable for the conversion of tonometric IOP results into actual pressure values. The TPA is suitable for the measurement of the IOP in dogs and cats.