Comparison of cervical epidural morphine with intravenous morphine administration on antinociception in adult horses using thermal threshold testing

Objective To compare the antinociceptive effects of morphine administered via cervical epidural catheter to intravenously administered morphine using a thermal threshold (TT) testing model in healthy adult horses. Study design Prospective, randomized, blinded experimental study. Animals A total of six university-owned adult horses. Methods Horses were instrumented with a cervical (C1 -C2) epidural catheter and TT testing device with probes at withers and thoracic limb coronary bands. All horses underwent three TT testing cycles including cervical epidural morphine administration (treatment EpiM; 0.1 mg kg(-1)), systemic morphine administration (treatment SystM; 0.1 mg kg(-1)) and no morphine administration (treatment Control). Baseline TT was established prior to treatments, and TT was tested at 15, 30, 60, 90, 120, 150, 180, 240, 300, 360, 420, 480, 600 and 720 minutes following treatment. Horses underwent a 5 day washout period between treatments and the order of treatment was randomized. Differences between treatments were analyzed with repeated measures ANOVA. Results Systemic and epidural morphine administration resulted in significantly higher TT values compared with baseline and control treatment. The duration of effect was significantly longer in treatment EpiM (10-12 hours) than in treatment SystM (1.5-2.0 hours). Horses in treatment EpiM had significantly higher TT values at time points 180-600 minutes (withers) and 300-600 minutes (coronary band) than horses in treatment SystM. Conclusions and clinical relevance Cervical epidural administration of morphine provided antinociceptive effects as measured by increased TT for 10-12 hours compared with 1.5-2.0 hours for intravenously administered morphine. No complications or adverse effects were noticed following epidural placement of a C1 -C2 catheter and administration of morphine. The use of a cervical epidural catheter can be considered for analgesia administration in treatment of thoracic limb and cervical pain in the horse.