The difference in analgesic effects between epidural neostigmine, neostigmine with morphine, and morphine alone was studied in dogs following renewed interest in human medicine after finding that neostigmine may potentiate some analgesics if given epidurally. Forty healthy female dogs underwent standard ovariohysterectomy. All dogs received a preoperative epidural, and results were compared with those in dogs that received a placebo epidural. The authors used a visual analog scale (VAS) that reflected visual assessment of pain (eg, vital signs, pressing on the incision, coaxing the dog), as well as a numeric descriptive scale that contained 11 variables. This strategy was meant to control for individual thresholds for pain, as well as to try to control for the subjective nature of pain assessment. Areas for comparison included a postoperative assessment of pain, including the need for additional analgesic agents and how long it took for the patient's appetite to return to normal. Investigators found that epidural administration of neostigmine resulted in a moderate degree of analgesia, as indicated by the significant reduction in use of supplemental analgesia during the early postoperative period. However, neostigmine alone was inferior to morphine or morphine combined with neostigmine. In fact, there was little difference between the effects of epidural morphine and morphine combined with neostigmine. The authors acknowledge that pain after spaying may not be sufficient to allow detection of a possible benefit that may have been achieved by adding neostigmine to morphine.

COMMENTARY: Since only experienced and specially trained clinicians can administer epidural anesthesia, it is unlikely that it would become a practical means of analgesia in the common clinical setting. Perhaps we could extrapolate these findings to more invasive surgeries involving the pelvic limbs or abdomen. However, the authors state that results may not reflect what is possible with neostigmine if a different and more painful procedure were performed. Therefore, it is not possible to predict from this study whether a combination neostigmine epidural would be useful in a practical setting.

Postoperative analgesic effects of epidural administration of neostigmine alone or in combination with morphine in ovariohysterectomized dogs. Marucio RL, Luna SP, Neto FJ, et al. VET RES 69:854-860, 2008.