Perineural dexamethasone does not prolong bupivacaine sensory blockade in equine palmar digital nerve blocks: a randomized crossover trial.

Overview

• This study investigated whether adding dexamethasone sodium phosphate to bupivacaine improves the duration of sensory nerve block in horses during palmar digital nerve blocks.
• The researchers found that dexamethasone did not prolong, and possibly even shortened, the sensory blockade duration compared to bupivacaine alone in healthy horses.

Background and Purpose

• Perineural nerve blocks are commonly used to provide pain relief in horses, particularly in the distal limbs.
• Bupivacaine is a local anesthetic used to numb the palmar digital nerves, blocking sensation.
• Dexamethasone, a corticosteroid, has been reported in human and rodent studies to prolong the duration of nerve blocks when added to local anesthetics.
• The study aimed to test if perineural dexamethasone sodium phosphate (dexamethasone SP) can similarly extend the duration of sensory blockade produced by bupivacaine in equine palmar digital nerve blocks.

Study Design and Methods

• The study was a randomized, blinded, 4×4 crossover trial involving 8 healthy adult horses.
• Each horse received each of the following four treatments on the left forelimb palmar digital nerves, with at least 7 days washout between treatments to avoid carryover effects:
  • Bupivacaine alone
  • Bupivacaine combined with dexamethasone SP
  • Dexamethasone SP alone
  • Saline (control)
• Researchers measured the Mechanical Nociceptive Threshold (MNT), which is the pressure level causing a pain response, to evaluate sensory block intensity and duration.
• MNT was measured at baseline and at scheduled intervals for up to 12 hours post-injection.
• The “duration of sensory blockade” was defined as the time during which MNT remained at the maximal measurable value (3.5 kg/cm²), indicating full sensory block.
• Time-to-event statistical analyses (Kaplan-Meier curves and Gehan-Breslow-Wilcoxon tests) were used to compare durations across treatments, with significance set at α = 0.05.

Results

• Bupivacaine alone and bupivacaine plus dexamethasone SP both significantly increased MNT compared to saline or dexamethasone SP alone, indicating effective sensory block.
• Dexamethasone SP alone did not change MNT compared to saline, showing it has no anesthetic effect by itself.
• Contrary to expectations, adding dexamethasone SP to bupivacaine did not prolong the sensory block duration; in fact, the median blockade lasted 4.5 hours with bupivacaine alone but only 2.5 hours with the combination.
• No local or systemic adverse effects were observed with any treatments, indicating safety in this context.

Conclusions and Implications

• In horses, perineural dexamethasone SP does not enhance and may even reduce the duration of sensory blockade when combined with bupivacaine in palmar digital nerve blocks.
• These findings contrast with human and mouse studies, highlighting important interspecies differences in drug effects on nerve blocks.
• The results suggest that pharmacological interactions between corticosteroids and local anesthetics differ between species and require further investigation in equine models.
• Clinicians should be cautious in extrapolating findings from humans or rodents to horses regarding nerve block prolongation with dexamethasone additives.
• The study encourages exploring alternative analgesic protocols or adjuncts to improve block duration in equine practice.