Colorectal distention in the horse: visceral sensitivity, rectal compliance and effect of i.v. xylazine or intrarectal lidocaine.
Authors: Sanchez L C, Merritt A M
Journal: Equine veterinary journal
Summary
# Colorectal Distention as a Non-invasive Model for Equine Visceral Sensitivity Researchers developed and validated a non-invasive protocol for measuring visceral pain perception in horses using controlled colorectal distention (CRD)—a technique well-established in human and small animal research but previously unavailable for equine work. Eight horses underwent baseline testing with a balloon catheter system delivering stepwise pressure increases to the rectum, whilst six additional horses received either intravenous xylazine or intrarectal lidocaine to evaluate analgesic effects. The procedure proved well tolerated across all subjects, with mean baseline distention threshold at 14.17 mmHg; notably, xylazine significantly elevated pain thresholds compared to baseline and lidocaine treatment, whilst intrarectal lidocaine markedly increased rectal compliance without raising pressure thresholds. This non-invasive approach offers practitioners and researchers a quantifiable tool for assessing visceral nociception and neuropathic responses, with direct clinical applications: the enhanced rectal compliance observed following lidocaine administration provides mechanistic explanation for its established utility during rectal examinations, and the differential effects of xylazine versus lidocaine may inform analgesic selection for horses with conditions involving visceral pain or those requiring rectal manipulation.
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Practical Takeaways
- •Intrarectal lidocaine is useful before difficult rectal examinations as it increases tissue compliance, making the procedure easier to perform
- •Xylazine increases visceral pain thresholds in horses, which may benefit colicky animals but should be considered when assessing pain responses
- •CRD offers practitioners a noninvasive research tool to evaluate equine visceral sensitivity without requiring surgical cannulation
Key Findings
- •Colorectal distention (CRD) is a well-tolerated, noninvasive method for visceral sensitivity testing in conscious horses with mean baseline threshold pressure of 14.17 mmHg
- •Intravenous xylazine significantly increased CRD threshold pressure compared to baseline or intrarectal lidocaine (P < 0.05)
- •Intrarectal lidocaine increased rectal compliance relative to baseline or xylazine treatment
- •The increased rectal compliance from lidocaine may explain its clinical benefit in facilitating rectal examination in horses