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.

Comparison of two rebound tonometers in healthy horses

“…The mean readings of 21.0 ± 3.14 mmHg (CV ± 15.0%) obtained with Tonovet ® were only slightly lower than readings reported by Knollinger et al 6 of 22.1 ± 5.9 mmHg (CV ± 26.7%) in horses without sedation or auriculopalpebral (AP) block, and somewhat lower than readings reported by Holve et al 9 of 24.3 ± 3.09 mmHg (CV ± 12.7%) and Lewin et al 14 of 25.4 ± 7 mmHg (CV ± 27.6%) in horses without sedation but with AP block. The mean readings of 21.6 ± 2.45 mmHg (CV ± 11.3%) obtained with Tonovet Plus ® were very close to those reported by Knollinger et al 6 …”

“…Tonovet ® rebound tonometer (Icare Finland) has been shown to provide accurate results with low interobserver variability in both human and animal patients, 11‐13 while Tono‐Pen ® applanation tonometer is more prone to inaccuracy due to incorrect technique 12 . A recent study showed significantly lower inter‐user and intra‐user variation in unsedated horses and lower intra‐user variation in sedated horses with Tonovet ® than with Tono‐Pen ® 14 . In dogs and cats, Tono‐Pen ® has been found to provide accurate readings in the physiological range of IOP, but has a tendency to underestimate IOP in the high range and to overestimate IOP in the low range 12,15 .…”

Highlights of recent clinically relevant papers

“…This study by Andrew Lewin and colleagues in the USA aimed to determine whether applanation or rebound tonometry has the lowest inter‐ and intra‐user variation when measuring intraocular pressure (IOP) in horses.…”

Effect of multiple head positions on intraocular pressure in healthy, anesthetized horses during hoisting

“…In clinical practice, a more limited range from 15 to 30 mm Hg is generally accepted 6 . Estimation of IOP in horses is most commonly accomplished using handheld measurement devices that employ corneal applanation or rebound mechanisms 2,5,7 ; of these two modalities, rebound tonometry has demonstrated the least intra‐user variability in this species 8 …”