Surgery

Anesthetic fails epidural test in horses

By Adam Marcus

A regional anesthetic commonly used in humans and small animals failed an important cost-benefit test in horses, producing minimal pain control at the expense of potentially serious side effects, new research shows.

The study, conducted by equine veterinarians at Mississippi State University, found that epidural injections of liposomal bupivacaine did not lead to clinically measurable analgesia but did produce ataxia in most animals in the trial. Some horses were unable to walk for 24 hours or longer, according to the researchers, who published their findings in a recent issue of Veterinary Surgery, the official journal of the American College of Veterinary Surgeons.

“A large animal that becomes recumbent will remain recumbent for a while, and that’s not a good thing,” said Christopher Stokes, DVM, a resident and master’s student at the Mississippi State College of Veterinary Medicine, who led the study.

Dr. Stokes said he was inspired to conduct the trial by Cláudio Correa Natalini, MV, MS, PhD, DACVAA, a Mississippi State veterinarian and expert in equine anesthesia who died in 2023 during the early stages of the project. “Dr. Natalini was the world’s expert on equine epidurals, and I wanted to learn how to place one,” Dr. Stokes said.

Liposomal bupivacaine is a formulation of local anesthesia in which the active ingredient is encapsulated in a fatty coating that gradually dissolves, prolonging the duration of numbness. The drug has become popular in human surgery and is even used in procedures involving small animals.

For the latest study, Dr. Stokes and his colleagues administered the agent (10 mL; 1.3%) via caudal epidurals to 10 healthy horses. Over the next 72 hours, they monitored the animals for pain sensitivity using pressure algometry and conducted physical exams every 4 hours, including assessments of rectal temperature, heart and respiratory rate, and mobility.

None of the physical metrics differed pre- and post-epidural, nor did the horses’ pain threshold appear to change after receiving the anesthetic. But the epidural was associated with potentially serious ataxia in most of the animals. Two of the horses were recumbent for prolonged periods, 3 required catheters to facilitate urination, and 6 needed manual removal of manure.

Dr. Stokes called the results surprising based on the data from humans and small animals. “It’s not what we thought we were going to see. Maybe there’s something different about the horse’s epidural space or receptors,” he said.

Robin L. Fontenot, DVM, MS, DACVS, of the Department of Pathobiology and Population Medicine at the Mississippi State College of Veterinary Medicine, and a co-author on the study, said another issue with the anesthetic was its variable effects. “It was all over the place. Some horses had massive problems, and some appeared to have no effect at all. That inconsistency has been described in other studies, if you read between the lines” on duration of effectiveness and other measures.

Although the use of algometry is “debatable” as a stand-in for the strength of an anesthetic/analgesic, according to Dr. Stokes, the researchers said even the relatively crude assessment provides valuable information. “If we couldn’t see a difference in pain response with a pressure sensor, we couldn’t make an incision in a horse without them feeling it,”
Dr. Fontenot said. MeV

FOR MORE INFORMATION
Stokes C, Natalini CC, Wills R, et al. Liposomal bupivacaine caudal epidural in horses results in adverse effects without analgesia. Vet Surg. 2026;55(1):218-224. doi:10.1111/vsu.70055
https://onlinelibrary.wiley.com/doi/epdf/10.1111/vsu.70055