Comparative efficacy of low-volume retrobulbar anesthesia using three commercial local anesthetics in adult horses.

The researchers in this study carried out a comparison between three commercial local anesthetic formulations for efficiency in low-volume retrobulbar anesthesia in adult horses. The study found that the three anesthetics had no associated adverse effects. In terms of effectiveness, bupivacaine induced rapid and long-lasting corneal anesthesia compared to lidocaine and mepivacaine.

Methodology

• The research involved eight healthy adult mares.
• A block-randomized, masked, controlled design was used in the research execution.
• A single ultrasound was used to guide retrobulbar block and was conducted using 2% lidocaine, 2% mepivacaine, or 0.5% bupivacaine. Each type of anesthesia was used on five eyes per group.
• The untreated eye in each horse served as the control.
• The researchers performed assessment of neurophthalmic reflexes/responses, intraocular pressure, and vertical pupil diameter measurement at baseline and specific time intervals up to 24 hours postblock. They also conducted corneal and periocular esthesiometry alongside observing any adverse effects the anesthesia could have brought.

Key Findings

• There was no increased intraocular pressure or any other adverse side effects noted in the horses in any treatment group.
• All groups showed a statistically significant corneal anesthesia just 1 minute after block, this persisted through 4 hours for lidocaine or mepivacaine block and through 24 hours for bupivacaine block.
• In terms of efficacy, mepivacaine offered no clinical advantage over lidocaine. However, bupivacaine was noted to induce comparatively rapid and sustained corneal anesthesia.
• No group showed clinically significant periocular anesthesia.
• There was an observed increase in the vertical pupil diameter for up to 4 hours after lidocaine or mepivacaine block, and 6 hours after bupivacaine block.

Conclusions

• The study concluded that low-volume retrobulbar block with any of the three local anesthetic drugs evaluated was not associated with adverse effects.
• Mepivacaine block showed no clinical advantage over lidocaine block, however, bupivacaine block induced comparatively rapid and sustained corneal anesthesia.
• In comparison to previous studies that used larger injection volumes, the low-volume retrobulbar block with lidocaine produced clinically comparable corneal anesthesia.
• However, the study also found that no local anesthetic drug at low volume achieved periocular soft tissue anesthesia.