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

Objective: To determine the analgesic effects of detomidine and morphine administered by cervical epidural catheter (CEC) on a model of thoracic limb pain.

Study design: Prospective, randomized, blinded crossover study.

Animals: Five adult horses.

Methods: Cervical epidural catheters were placed under ultrasound guidance, followed by induction of lipopolysaccharide (LPS)-mediated carpal synovitis. Horses received either saline or detomidine/morphine (DM; 10 μg/kg, 0.1 mg/kg, respectively) via CEC 4 h after synovitis. After a 7 day washout, synovitis was induced in the contralateral carpus, and the alternate treatment was administered. Evaluations occurred prior to and at 4, 6, 8, 12, 24, 36, and 48 h after synovitis, including physical, lameness, and neurologic exams, pain scoring, mechanical threshold testing, and joint circumference measurement. Mixed-model linear regression was used to assess the effects of time, treatment, and their interaction, with horse as a random effect.

Results: All DM-treated horses became sedate, and four of five developed hypermetria and ataxia. Four of five DM-treated horses also showed at least a 50% decrease in lameness by 2 h after treatment. Rectal temperature, heart rate, and respiratory rate were elevated in the DM group in comparison with the saline group (p < .027). No differences were observed in mechanical nociceptive thresholds or pain scores between groups.

Conclusion: These results support further exploration of cervical epidural treatment with DM for thoracic limb analgesia.

Clinical significance: Further studies are warranted to optimize dosing and assess neurologic side effects of this drug combination.