Surgery

Study Highlights Risk for Rare Muscle Breakdown After Anesthesia

By Adam Marcus

A case series of Warmblood horses that experienced a life-threatening reaction to anesthesia has experts cautioning equine veterinarians to be alert to the possibility of the complication.

The 7 animals, 6 of which died, had been recovering from surgery with general anesthesia when they developed rhabdomyolysis, a breakdown of muscle tissue that is then released into the bloodstream. None of the horses had a prior history of the condition, which triggers a surge in serum creatine kinase (CK) and lactate that can be used to diagnose the disorder.

“Although this is a rare complication of anesthesia, I think it is important that veterinarians are aware of it and that they assess serum CK and lactate in horses with difficult anesthetic recoveries,” said Stephanie Valberg, DVM, PhD, DACVIM, DACVSMR, the founder of the Valberg Neuromuscular Diagnostic Laboratory in Williamston, Mich., and senior author of the study.

Dr. Valberg and her colleagues published their findings in a recent issue of the Journal of the American Veterinary Medical Association.

Rhabdomyolysis after surgery—also called postanesthetic myopathy (PAM)—is believed to occur in roughly 1% to 2% of anesthetized horses, leading to death in as many as 20% of cases.

Localized PAM has been shown to occur in horses placed in a lateral recumbency during surgery, preventing the animal from bearing weight on the affected leg. Horses with systemic PAM, which affects a wide range of muscles, experience severe pain and rigidity and the inability to stand. They also show signs of tissue breakdown and spikes in muscle enzymes in their blood.

The complication has been associated with a variety of factors believed to increase the risk for ischemia, including positioning during surgery, lower mean arterial pressure (<65 mm Hg) during the procedure and higher body weight.

Previous studies have looked at the risk for PAM in draft animals, but the new research is the first case series focused on PAM in Warmbloods, according to the authors.

As Dr. Valberg’s group reported, the 7 horses showed no signs of being at risk for rhabdomyolysis prior to undergoing their operations, and 5 had been performing routine work in the days leading up to surgery. The procedures involved exploratory surgery, arthroscopy and other non-urgent indications.

In all cases, the horses experienced difficulty standing during the recovery period; 3 horses were euthanized because of recumbency. In the remaining 4 horses, PAM was diagnosed between 5 and 11 days after surgery, with the horses exhibiting signs of pain, stiffness, sweating and trouble standing. Three of the 4 affected horses did not survive.

“The remarkable feature of our study was the near complete depletion of muscle glycogen and the increase in lactic acid associated with rhabdomyolysis, often days after anesthesia in many of the horses,” Dr. Valberg said. “This suggests the horses may have undergone a hypermetabolic event associated with the rhabdomyolysis.”

The duration of anesthesia ranged from 60 to 200 minutes (data for 1 horse was unavailable) with no apparent complications, and anesthesia time did not appear to be the cause of rhabdomyolysis, the researchers reported. Similarly, the type of inhalational or IV agent did not seem to matter.

“From our study, the anesthesia was done with high standards and did not seem to be the cause of the horse’s poor recovery,” said Dr. Valberg, who also is an emerita professor and former Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University, in East Lansing, adding that her work focuses on the muscle pathology and not the role of anesthetics.

While PAM can be fatal, the drug dantrolene—which blocks the release of calcium from muscle tissue, a key part of the pathogenic process in the condition—can reduce the risk for death in some cases if initiated early after the appearance of PAM. The horse that survived began receiving dantrolene on the ninth day after surgery, which appeared to significantly reduce serum CK over the next 5 days. The drug was continued until day 22 after the procedure.

“The dose of dantrolene is important. When given at 6 mg/kg prior to anesthesia, it can raise serum potassium and suppress myocardial contractility during anesthesia,” Dr. Valberg noted.

She added: “Although we didn’t have enough data to emphatically state this, my personal experience is that if serum CK is elevated, 2 mg/kg of dantrolene is the best means to try to halt ongoing rhabdomyolysis—and the earlier it is started the better. The likelihood of successful treatment depends on the severity of the rhabdomyolysis and how early treatment can be instituted, but it is hard to put a number on the likelihood.” MeV

For more information:
Hepworth-Warren KL, Goldsmith D, Tsoi M, et al. Postanesthetic rhabdomyolysis in 7 warmblood horses. J Am Vet Med Assoc . 2024;263(2):248-258.
https://avmajournals.avma.org/view/journals/javma/263/2/javma.24.08.0522.xml