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

Objective: To compare the efficacy of low-volume (5-mL) locoregional retrobulbar anesthesia ("retrobulbar block") by use of 3 commercial local anesthetic formulations.

Animals: 8 healthy adult mares.

Methods: A block-randomized, masked, controlled design was used. A single ultrasound-guided retrobulbar block was performed with 2% lidocaine, 2% mepivacaine, or 0.5% bupivacaine (n = 5 eyes/group). Contralateral eyes served as untreated controls. End points performed at baseline and time intervals up to 24 hours postblock included the following: assessment of neurophthalmic reflexes/responses, intraocular pressure, and vertical pupil diameter measurement, corneal and periocular esthesiometry, and observation for adverse effects.

Results: Low-volume block did not result in increased intraocular pressure or other adverse effects at any time point in any treatment group. Statistically significant corneal anesthesia (P < .001) was observed 1 minute after block in all groups, persisting through 4 hours after lidocaine or mepivacaine block and through 24 hours after bupivacaine block. Clinically significant periocular anesthesia was not observed in any group. Significant vertical pupil diameter increase (P < .05) was observed for up to 4 hours after lidocaine or mepivacaine block and 6 hours after bupivacaine block.

Clinical relevance: Low-volume retrobulbar block with any of the 3 local anesthetic drugs evaluated was not associated with adverse effects. In terms of efficacy, mepivacaine block showed no clinical advantage over lidocaine block. However, bupivacaine block induced comparatively rapid and sustained corneal anesthesia. In comparison to published findings using a larger injection volume, low-volume retrobulbar block with lidocaine produced clinically comparable corneal anesthesia. However, periocular soft tissue anesthesia was not achieved with any local anesthetic drug at low volume.