Inter-user and intra-user variation of two tonometers in horses

Abstract

Background: It is currently unknown which of the two devices most commonly used in equine ophthalmology for intraocular pressure (IOP) estimation demonstrates the lowest inter-user and intra-user variation.

Objectives: To assess the inter-user and intra-user variation of two tonometers in sedated and unsedated horses.

Study design: Randomised masked cross-over trial.

Methods: Four examiners used the rebound (ICare^® TonoVet) and applanation (TonoPen^® ) tonometers to measure the intraocular pressure (IOP) in triplicate in 10 normal horses before and after sedation with xylazine. For inter-user variation, coefficient of variation (CV) values were calculated from the mean of each examiner for each condition combination. For intra-user variation, CV values were calculated from the individual measurements of each examiner for each condition combination. CV values were also assessed in relation to other variables using ANOVA.

Results: The rebound tonometer was found to have lower inter-user (15.4% vs 21.7%, P = .01) and intra-user (9.1% vs 16.1%, P < .0001) variation in unsedated horses and lower intra-user (8.4% vs 14.7%, P < .0001) variation in sedated horses than the applanation tonometer. Both instruments had similar inter-user variation in sedated horses. For the rebound tonometer, sedation did not affect inter-user or intra-user variation, but for the applanation tonometer inter-user variation was lowest while horses were sedated (16.0% vs 21.7%, P = .03). No other variable assessed was found to have an effect on IOP.

Main limitations: No animals with ocular disease were included in this study.

Conclusions: The rebound tonometer may be the preferred instrument to minimise intra-user and inter-user variation for IOP measurement in unsedated horses. The rebound tonometer is also likely to be the preferred instrument to minimise intra-user variation in sedated horses. If the applanation tonometer is used to perform IOP measurement in horses, it is recommended that this is performed while horses are sedated to minimise inter-user variation for this instrument.

Keywords: TonoPen; TonoVet; horse; tonometer; variability; variation.