Cervical epidural catheter for administration of detomidine and morphine in a model of carpal synovitis in the horse.

Cervical epidural administration of detomidine and morphine was evaluated for pain relief in horses with experimentally induced carpal joint inflammation, showing promising reduction in lameness but also causing sedation and neurological side effects.

Study Objective

• To assess the analgesic effects of detomidine and morphine delivered via cervical epidural catheter (CEC) in a horse model of thoracic limb pain caused by carpal synovitis.

Methods

• Design: Prospective, randomized, blinded crossover study.
• Subjects: Five adult horses.
• Procedure:
  • Placement of cervical epidural catheters under ultrasound guidance.
  • Induction of carpal synovitis using lipopolysaccharide (LPS) in one carpus.
  • Treatment administration 4 hours after synovitis induction via CEC:
    • Either saline (control) or detomidine/morphine (DM) at doses of 10 μg/kg and 0.1 mg/kg respectively.
  • After a 7-day washout period, the contralateral carpus was treated similarly but with the alternate drug.
• Assessment timings: Before synovitis induction and at 4, 6, 8, 12, 24, 36, and 48 hours post-induction.
• Evaluations included:
  • Physical examinations.
  • Lameness scoring.
  • Neurologic examinations.
  • Pain scoring.
  • Mechanical nociceptive threshold testing.
  • Joint circumference measurements.
• Statistical analysis: Mixed-model linear regression accounting for time, treatment, and their interaction; individual horses treated as random effects.

Results

• All horses receiving detomidine and morphine (DM) developed sedation.
• Four out of five DM-treated horses showed neurological signs including hypermetria (exaggerated movement) and ataxia (loss of coordination).
• Lameness:
  • Four out of five DM-treated horses demonstrated at least a 50% reduction in lameness within 2 hours post-treatment.
• Vital signs differences between groups:
  • Rectal temperature, heart rate, and respiratory rate were significantly higher in the DM group compared to saline controls (p < 0.027).
• No significant differences were observed between DM and saline groups regarding mechanical nociceptive thresholds or subjective pain scores.

Conclusions

• Cervical epidural administration of detomidine and morphine shows promise for providing analgesia in horses with thoracic limb pain from synovitis, as evidenced by reduced lameness.
• However, this treatment also caused notable sedation and neurologic side effects such as ataxia and hypermetria.
• The lack of differences in mechanical threshold and pain scores suggests complexities in pain assessment or drug effect mechanisms.
• Further research is necessary to:
  • Optimize dose regimens to maximize analgesia while reducing adverse neurological effects.
  • Better characterize the neurological side effects and overall safety profile of this drug combination when administered via cervical epidural catheter.
  • Explore the mechanisms underlying the observed physiological and behavioral changes.