The effects of subconjunctival bupivacaine, lidocaine, and mepivacaine on corneal sensitivity in healthy horses.

The research article investigates how three local anesthetic drugs – bupivacaine, lidocaine, and mepivacaine – reduce the sensitivity of the corneas when injected into the conjunctiva, or whites of the eyes, of healthy horses. The study concluded that all three medicines were effective and safe, potentially useful for managing pain in equine corneal procedures.

Experimental Design

• The research experiment involved eight horses and used a randomised, masked, crossover study design. This approach ensures blinding to avoid any potential bias, and each horse received each treatment, but in a different order and with a two-week interval between trials to allow any effects from the previous trial to wear off.
• Each horse received an injection of either 0.2 mL of bupivacaine (0.5%), lidocaine (2%), mepivacaine (2%), or a saline solution, into the conjunctiva of one randomly selected eye. The opposite eye served as a control.
• Researchers used a Cochet-Bonnet esthesiometer to measure the corneal touch threshold (CTT) or corneal sensitivity in both eyes before and after sedation, and at 10-minute intervals post injection until the sensitivity returned to its baseline level.

Results of the Experiment

• The study found that all three anesthetics were more effective than the saline solution in decreasing CTT, and mepivacaine was more effective than lidocaine. The longest period of reduced CTT was observed with mepivacaine (138.8 minutes), followed by bupivacaine (105.0 minutes), and lidocaine (103.8 minutes).
• The mean minimum CTT was lowest with mepivacaine (0.73 cm), followed by lidocaine (1.42 cm), bupivacaine (1.67 cm), and each of these was significantly lower than the saline control (4.73 cm).
• Importantly, the researchers observed no signs of corneal toxicity with any of the treatments administered, indicative of the safety of these drugs for this application.

Conclusions

• The study concluded that subconjunctival injections of the three local anesthetics – bupivacaine, lidocaine, or mepivacaine – were effective in safely reducing corneal sensitivity in the eyes of healthy horses for a period of 1.5 to 2 hours.
• This discovery suggests their potential utility in providing perioperative pain relief during equine corneal procedures.