Repeatability and reproducibility of intraocular pressure measurements in horses using rebound and applanation tonometry.

Overview

• This study evaluated the consistency and reliability of intraocular pressure (IOP) measurements in horses using two different tonometry devices—rebound and applanation tonometers—performed by three examiners with varying experience levels.
• The goal was to determine which device provides more repeatable and reproducible IOP readings, important for accurate ophthalmic assessments in veterinary medicine.

Introduction and Background

• Intraocular pressure (IOP) is a key parameter measured during eye examinations to detect and monitor ocular health issues.
• Different tonometers exist to measure IOP, with rebound and applanation being two common types.
• Species differences and variability between examiners can influence IOP readings, so understanding reliable measurement methods is crucial, especially in horses, where normative values and measurement reliability are less established.

Objectives

• Compare repeatability (consistency of repeated measurements by the same examiner) and reproducibility (consistency across different examiners) of IOP measurements in horses.
• Use two tonometry techniques: rebound tonometry (Tonovet) and applanation tonometry (Tonopen Avia).
• Assess measurements by three veterinarians with different experience levels to evaluate examiner effect on measurement variability.

Methodology

• Subjects: 30 healthy Criollo horses were selected for the study to establish baseline data in a healthy population.
• Measurement: IOP was measured on each horse’s eyes using both devices by three different veterinarians, ensuring varied clinical experience.
• Statistical Analysis:
  • Analysis of variance (ANOVA) to detect significant differences in readings.
  • Student’s t-test for comparing mean IOP values between devices.
  • Intraclass correlation coefficient (ICC) to evaluate measurement repeatability and reproducibility among examiners.

Results

• Mean IOP values:
  • Rebound tonometry (Tonovet): 26.16 ± 5.13 mm Hg
  • Applanation tonometry (Tonopen Avia): 22.55 ± 7.32 mm Hg
• This difference in mean IOP values between devices was statistically significant, with rebound tonometry producing higher readings.
• Examiner-specific mean values for rebound tonometry:
  • Examiner A: 25.27 ± 4.9 mm Hg
  • Examiner B: 25.80 ± 5.2 mm Hg
  • Examiner C: 27.40 ± 5.1 mm Hg
• Examiner-specific mean values for applanation tonometry:
  • Examiner A: 20.03 ± 4.5 mm Hg
  • Examiner B: 19.67 ± 6.0 mm Hg
  • Examiner C: 27.97 ± 10.3 mm Hg (notably higher and more variable)
• Intraclass Correlation Coefficients (ICC):
  • Rebound tonometry: 0.41, indicating satisfactory reproducibility among examiners.
  • Applanation tonometry: 0.18, indicating poor reproducibility and more variability among examiners.
• Examiner C showed a tendency to record higher IOP values with applanation tonometry, indicating possible effect of examiner technique or experience on this device.

Discussion and Interpretation

• Rebound tonometry (Tonovet) provided higher IOP measurements than applanation tonometry (Tonopen Avia), which aligns with previous observations in veterinary ophthalmology.
• The rebound tonometer demonstrated better reproducibility and was less influenced by who performed the measurement, suggesting higher reliability for clinical and research use in horses.
• Applanation tonometry showed greater variability depending on the examiner, lowering its reliability, possibly due to its measurement technique being more sensitive to user skill.
• This finding implies that when consistent IOP readings are required in equine ophthalmic exams, rebound tonometry is preferred.

Conclusions

• Tonovet rebound tonometer offers more reliable and consistent IOP measurements in horses compared to Tonopen Avia applanation tonometer.
• Rebound tonometry is less affected by examiner variability, making it suitable for both clinical evaluations and research involving equine IOP measurement.
• Practitioners and researchers should consider device choice carefully to ensure reliable IOP monitoring in horses.