Although sedation can help make a lameness exam go more smoothly, a new study from Germany suggests that it might affect how the horse moves, making diagnosis more difficult.
Xylazine or other mild sedatives are sometimes administered to horses who might otherwise make lameness examinations challenging, explains Matthias Rettig, DVM, of the Free University of Berlin. “Some horses are uncooperative during a lameness exam and kick, which can be difficult and dangerous,” he says. But sedation can make a horse sluggish, so Rettig and his fellow researchers set out to determine whether it affects the biomechanical parameters that veterinarians use to diagnose and pinpoint lameness.
For the study, researchers attached inertial sensors to 44 horses who were randomly split into two groups. Baseline lameness examinations were performed on all of the horses, then half were given a low dose of the xylazine. The remaining horses were given an injection of saline to serve as controls. The researchers repeated the lameness exams 20 minutes and 60 minutes after the injections. During all exams, the inertial sensors collected data on limb, head and pelvic movements.
The data revealed that sedation had no significant effect on pelvic movement in horses with hind-limb lameness. Nor, initially, did it influence the movement of horses with forelimb lameness. However, in a few of those horses, head movement decreased 60 minutes after sedation.
The reason for the delayed effect in a short-acting sedative is unclear, says Rettig, but he offers two theories: “One reason could be that it calms the horses down and they get more used to the trotting-up-and-down procedure during the exam, which results in a reduced head movement,” he says. “Also, the analgesic effect of xylazine has to be considered, meaning the sedating effect has been eliminated in an hour but it could be that there is some pain-reducing component.”
While the change in head movement among sedated horses with forelimb lameness was not statistically significant, Rettig says it’s worth keeping in mind: “A lameness exam ideally continues until the region of pain is detected. If you start at the hoof and work your way up, it is possible that the entire procedure can take far more than an hour.”
Rettig says he will adjust his own lameness exam protocols based on these findings: “For the hind limb, where performing a nerve0 block is far more difficult than in a front limb, I feel confident using xylazine to make it easier in difficult horses. The risk for veterinarians, technicians, clients and the patient is, in my opinion, not worth ‘playing the hero’ and fighting the horse. Sedation makes the situation easier and more relaxed for everyone involved---especially for the horse. But the results of this study will also make me look at the horse for a longer period of time to see if the lameness changes [on its own] before sedating and nerve blocking.”
This article first appeared in EQUUS issue #458, November 2015