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

To determine the analgesic effects of detomidine and morphine administered by cervical epidural catheter (CEC) on a model of thoracic limb pain. Prospective, randomized, blinded crossover study. Five adult horses. 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. 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. These results support further exploration of cervical epidural treatment with DM for thoracic limb analgesia. Further studies are warranted to optimize dosing and assess neurologic side effects of this drug combination.