Study Examines Post-Anesthetic Myelopathy in Horses

Researchers at Washington State University (WSU) recently completed a study aimed at better characterizing equine post-anesthetic myelopathy, a rare by deadly neurologic complication of general anesthesia.

"Post-anesthetic myelopathy (PAM) has been reported mostly in young horses anesthetized in dorsal recumbency," explained Stavros Yiannikouris, DVM, MS, equine surgery instructor at the WSU College of Veterinary Medicine. "Upon recovery or shortly thereafter, the horses were mentally alert but had a loss of voluntary movement in the hind limbs and tail."

Yiannikouris said in most cases veterinarians administered supportive care, but all horses died or were euthanized shortly after the onset of signs.

"Due to the rare occurrence of equine PAM, many anesthetists and surgeons have not encountered this complication," he said.

Thirty veterinary teaching hospitals and 20 private equine hospitals throughout the United States and Canada partook in a survey conducted by researchers to identify possible PAM occurrences. The research team asked a variety of questions about possible occurrences including, but not limited to, the patient's preoperative health status, duration of anesthesia, surgical position and procedure, and the amount of time from the onset of signs to death or euthanasia. The team also included cases found in a literature review.

The researchers identified and evaluated 18 new cases and 12 reported in literature, bringing the total number of cases evaluated to 30. Key findings from retrospective evaluation of the 30 occurrences included:

  • There was no specific breed association with PAM; however, the researchers found the "large-framed or heavily muscled breeds were commonly affected";
  • Twenty-two of the 30 affected horses were 2 years of age or younger;
  • There was no correlation between any specific anesthetic drug and PAM development;
  • All but one affected horse was positioned in dorsal recumbency (lying on his back);
  • Hind limb paralysis was the most commonly reported identifying factor of PAM;
  • Significant deficits, including hind limb paralysis, were identified on neurologic examination;
  • All horses retained their normal mentation after PAM development, and none of them tolerated their inability to move well, which the team noted made the animals difficult to manage;
  • None of the treatments administered (including supportive care, intravenous DMSO, corticosteroids, anti-inflammatories, sedatives, fluid therapy, and acupuncture, among others) had any effect on the horses' condition;
  • All horses died or were euthanized within 72 hours; and
  • The most common histopathologic finding on necropsy was poliomyelomalacia (softening of the gray matter of the central nervous system).

"All factors contributing to the development of equine PAM have not been identified," Yiannikouris said. "This report adds and consolidates valuable information on this condition for equine veterinarians. Due to the relative rarity of the condition, it is extremely difficult to identify specific factors that may predispose a horse to develop the condition."

Yiannikouris noted that more research is needed on PAM and that a better understanding of the condition could lead to prevention techniques and/or viable treatment options.

The study, "Development of equine post anaesthetic myelopathy: Thirty cases (1979-2010)," was published in the December 2011 issue of Equine Veterinary Education. The abstract can be viewed online.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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