Evaluation of two applanation tonometers in horses.

The research study evaluated and compared the performance of two applanation tonometers used for measuring intraocular pressure (IOP) in horses. The researchers requested their accuracy in vitro and in vivo and found that despite their inaccuracy in the lab (in vitro), a calculated correction could be made for more accurate estimations of IOP when used in actual subjects (in vivo).

Study Methodology

• The study involved the use of two different applanation tonometers, referred to as tonometer 1 (MacKay-Marg) and tonometer 2 (Tono-Pen).
• Comparisons of these tonometers were made both in vitro (in the lab) and in vivo (in live horses).
• The pressure readings from the two tonometers were compared against manometric measurements – the ‘gold standard’ of pressure measurement in an eye.
• Auriculopalpebral nerve blocks were administered in the horses to check if it altered the IOP measurement. An auriculopalpebral nerve block numbs the facial muscles controlling the eyelid and is often used during eye examinations in horses.

Key Findings

• In vitro, neither of the tonometers accurately recorded intraocular pressure (IOP) in comparison with manometric measurements. However, in both cases, a linear relationship was established that could be used to correct measurements.
• The actual IOP in horses could be calculated for tonometer 1 as ‘1.48 – 0.9 mm of Hg’ and for tonometer 2 as ‘1.38 + 2.3 mm of Hg’. The coefficients of determination (r2) were very high (0.99 for both equations), suggesting a strong linear relationship and indicating the formulae’s accuracy.
• In vivo, there were no significant differences between the IOP readings made by the two tonometers in clinically healthy horses. The use of an auriculopalpebral nerve block did not significantly change baseline IOP readings.
• The average IOP in healthy horses was found to be ‘23.5 +/- 6.10 mm of Hg’ and ‘23.3 +/- 6.89 mm of Hg’ for tonometers 1 and 2, respectively.

Implication of the Study

• This study provides valuable insights into the reliability of two commonly used tonometers for measuring IOP in horses.
• Depite their initial inaccuracy in lab testing, appropriate adjustment calculations allow these tools to produce reliable and accurate measures of eye pressure in live horses.
• The results can help veterinarians improve their diagnostic accuracy for ocular conditions in equine patients, particularly those related to IOP, such as glaucoma.