Comparison of four tonometers in the measurement of intraocular pressure in healthy horses

BACKGROUND

Measurement of the intraocular pressure (IOP) is a useful diagnostic tool in equine ophthalmology. Handheld tonometers, such as Tonovet and Tonovet Plus (rebound), Tono-Pen AVIA Vet (applanation), and Kowa HA-2 (applanation using the Goldmann methodology) are used to obtain IOP measurements in veterinary medicine.

OBJECTIVES

To compare and evaluate the accuracy of four handheld tonometers in measuring IOP using different methodologies in healthy horses.

STUDY DESIGN

In vivo experiment and cross-sectional survey of healthy horses.

METHODS

Intraocular pressure was measured in 72 eyes of 36 horses. An in vivo study was conducted on sedated horses to compare the real IOP values obtained using manometry versus those obtained using tonometry, and a field study was conducted on unsedated healthy horses with normal eyes to measure the IOP values using different tonometers.

RESULTS

In the in vivo study, the mean IOP values using ocular manometry was 24.9 ± 4.0 mmHg (range, 20.0-30.0 mmHg). The mean IOP values using tonometry were: Tonovet, 25.7 ± 5.8 mmHg (range 19.5-33.0 mmHg); Tonovet Plus, 24.8 ± 7.1 mmHg (range 13.2-33.2 mmHg); Tono Pen AVIA Vet, 19.2 ± 4.7 mmHg (range 13.1-26.5 mmHg); and Kowa Ha-2, 24.1 ± 1.2 mmHg (range 22.8-25.8 mmHg). In the field study, the IOP values were: Tonovet, 30.7 ± 5.6 mmHg (range 21.7-38.0 mmHg); Tonovet Plus, 29.6 ± 6.7 mmHg (range 16.2-38.6 mmHg); Tono-Pen AVIA Vet, 27.3 ± 5.8 mmHg (range 14.6-37.1 mmHg); and Kowa HA-2, 23.4 ± 2.2 mmHg (range 20.2-28.7 mmHg).

MAIN LIMITATIONS

This study included only healthy horses and a limited number of animals in the in vivo study.

CONCLUSIONS

There was a strong correlation between the IOP values and manometry for all tonometers. IOP should be estimated using the same tonometer over time, and the bias of the tonometer used, such as overestimation (rebound tonometer) and underestimation (applanation tonometer), should be acknowledged. A normal reference value for each tonometer should be established in horses.

Repeatability, reproducibility, and agreement of three tonometers for measuring intraocular pressure in rabbits

The aim of this study was to evaluate repeatability, reproducibility, and agreement of three commonly used tonometers in animal research (TonoLab, TonoVet, and TonoPEN AVIA) in a cohort of 24 rabbits. Additionally, the impact of sedation on IOP was investigated in 21 New Zealand White rabbits with the TonoVet tonometer. Repeatability was determined using the coefficient of variation (CoV) for two observers. For the TonoLab (6.55%) and TonoVet (6.38%) the CoV was lower than for the TonoPEN AVIA (10.88%). The reproducibility was highest for the TonoVet (0.2 ± 3.3 mmHg), followed by the TonoLab (0 ± 12.89 mmHg) and lowest for the TonoPEN AVIA (− 1.48 ± 10.3 mmHg). The TonoLab and TonoVet showed the highest agreement (r = 0.85, R² = 0.73). After sedation, a significant IOP reduction (often > 25%) was observed. Our results show that among the three tonometers tested, the TonoVet tonometer is best for use in rabbits while the TonoLab should be avoided. The impact of sedation on IOP was substantial and should be taken into account during experimentation.

Comparison of two rebound tonometers in healthy horses

Objective

To obtain a reference range for evaluation of intraocular pressure (IOP) in horses using Tonovet Plus^®, to compare the IOP readings obtained with Tonovet^® and Tonovet Plus^®, and to evaluate the repeatability of readings.

Animals studied and Procedures

Intraocular pressure of 30 client‐owned horses (60 eyes) with no signs of illness or ocular disease was evaluated using Tonovet^® and Tonovet Plus^® rebound tonometers. Horses’ mean age was 10.7 (range 6‐17) years. Triplicate measurements were performed without using sedatives or local anesthetics, with minimal restraint.

Results

Calculated reference intervals (the CLSI robust method) were 14.4‐27.2 mmHg for Tonovet^® and 16.0‐26.1 mmHg for Tonovet Plus^®. Mean values (± standard deviation, SD [± coefficient of variation, CV]) obtained with Tonovet Plus^® (21.6 ± 2.45 mmHg [11.3%]) were on average 0.6 mmHg higher than with Tonovet^® (21.0 ± 3.14 mmHg [15.0%]), and a negligible statistical difference between the devices was found using the paired sample t test (P = .049). The correlation coefficient for the averaged triplicate measurements was 0.73. The average CV was 4.6% and 4.4% for Tonovet^® and Tonovet Plus^®, respectively.

Conclusions

The repeatability of measurements was very good with both devices. The readings between the two devices differed statistically significantly, but the correlation was considered good and the variation was numerically small, and thus, the difference was considered clinically irrelevant. When monitoring disease process or treatment response in an individual patient, repeated readings are best performed using a similar device to avoid false interpretation of results.

The prevalence of uveitis in a population of donkeys in the UK

BACKGROUND

Uveitis is reported to be a leading cause of blindness in horses. Little work has been undertaken to date to investigate whether donkeys are affected by a similar disease prevalence.

OBJECTIVES

To investigate the disease prevalence of uveitis in a population of donkeys in the UK.

STUDY DESIGN

Descriptive observational study.

METHODS

An ophthalmic examination was performed on each donkey, in a darkened stable. Each donkey underwent slit lamp biomicroscopy, and direct and/or indirect ophthalmoscopy. Fluorescein staining, STT1 and IOPs were measured when deemed clinically necessary. Pharmacological pupillary dilation was achieved using 1% tropicamide.

RESULTS

A total of 207 donkeys were examined: 139 males (67.1%) and 68 females (32.9%). Age range was 2-37 years (median: 17 years, interquartile range: 9-25 years). Three donkeys (1.5%) were blind in one eye, and one was monocular at the time of examination. Signs consistent with either previous or current uveitis were identified in eight eyes of six animals (2.9%). Clinical signs included are as follows: miosis (n = 1), corpora nigra atrophy (n = 6), anterior lens capsule pigment (n = 2), cataract (n = 8), posterior synechiae (n = 3), lens subluxation (n = 1), vitreal changes (n = 2), peripapillary scarring (n = 3) and phthisis bulbi (n = 1). Significant ocular pathology precluded fundic examination in three eyes. Three out of eight eyes (37.5%) were blinded by the pathology. The risk of uveitis was found to increase with age (OR 1.1, CI: 1.01-1.25) on univariable analysis (P = .046.).

MAIN LIMITATIONS

Fluorescein staining, tonometry and Schirmer tear test measurements were not performed on all donkeys.

CONCLUSIONS

The disease prevalence reported here is comparable to that reported for horses in the UK. Larger prospective studies are necessary to determine whether there is a similar trend in disease prevalence in the donkey population worldwide. In addition, further investigations should aim to establish whether uveitis in donkeys is similar to the ERU syndrome seen in horses.