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Veterinary Forum February 2007 (Vol 24, No 2)

Clinical Report: "Update on pain management"

by Gayle Trotter, Sophia Yin, DVM, MS (Animal Science), Nicole Ehrhart

    While recent advances in pain medication have greatly improved our treatment of pain for such common conditions as canine osteoarthritis, not all painful conditions are receiving the treatment they should, says Jamie Gaynor, DVM, MS, DACVA, director of the Animal Anesthesia and Pain Management Center in Colorado Springs, Colo.: "Pain for routine surgical procedures is still highly under-treated."

    Dogs DO feel pain

    One reason veterinarians may limit analgesic use in surgeries is that many can be performed quickly or are relatively noninvasive; therefore, a veterinarian might assume the procedure must not be causing much pain. However, a landmark study1 in the 1980s by Clifford Woolf, a neurophysiologist now at the Harvard Medical School, demonstrated that when nociceptors repeatedly fire and bombard the spinal cord with signals, the central nervous system becomes sensitized to pain signals. Consequently, the animal becomes more sensitive to pain. This phenomenon is termed "wind-up." Woolf also found that preemptive use of analgesics before surgery could prevent wind-up from occurring.

    A similar finding in dogs was reported by B. Duncan X. Lascelles in 1998. In this study,2 the short-acting opioid meperidine (also referred to as pethidine) was administered to a group of dogs undergoing ovariohysterectomy. The analgesic was given preoperatively, postoperatively, or not at all. After surgery, the researchers tested for pain by placing pressure over the incision area or on the back leg and measuring the dog's behavioral response. In the short term, no difference in pain response was noted, but at 18 and 24 hours after surgery, researchers found that the animals receiving meperidine before surgery reacted less to the applied pressure.

    They can hide pain well

    Despite these findings, veterinarians often fail to recognize pain. But for good reason, says Gaynor. "Animals often don't show clear behavioral signs of pain. Once an animal is showing signs of pain, the pain is likely severe enough that the animal can no longer hide the signs." Gaynor adds, "We know animals experience pain because we have documented evidence showing that if they are treated with pain relievers, they will revert to more normal behavior sooner."

    Indeed, in a study3 published in 1997 in Applied Animal Behaviour Science, researchers evaluated the behavior of dogs that received oxymorphone before ovariohysterectomy and dogs that did not receive the analgesic and compared them with control dogs that underwent anesthesia but no surgery. The researchers considered the time the dogs devoted to a number of behaviors, including resting, body position, grooming, licking of the incision site, panting, greeting behavior toward a handler who interacted with them, and willingness to escape when the cage was opened.

    According to the study, dogs that underwent surgery spent more time sleeping, less time grooming, and more time licking their surgical site. In addition, during handler interactions, these dogs vocalized and wagged their tails less and tried to escape fewer times when the cage door was open. Taken together, these findings suggest that these dogs were experiencing more pain than the dogs that received oxymorphone before surgery. Dogs receiving oxymorphone returned to normal greeting behavior more quickly and were more likely to leave their cage.

    Because these behavioral signs are subtle, they can be easily missed. Peter J. Pascoe, BVSc, DACVA, DECVA, professor in the department of surgical and radiological sciences, University of California, Davis, School of Veterinary Medicine, suggests that, "Veterinarians need to know what baseline behaviors are in the animal in order to judge how they are responding to surgery. In some cases, owners may be better at judging this than their veterinarians."

    Plan to use analgesics

    Both Pascoe and Gaynor agree that analgesics should be administered before routine surgeries, such as spays and ­castrations, and advise following a 3- to 4-day plan that includes an opioid preoperatively and an NSAID, such as carprofen or meloxicam, with or without opioids during and following surgery. And, Pascoe says, "NSAIDs [COX-2 inhibitors] can be given preoperatively if there is little chance of developing hypotension during surgery."

    What about pain management in other minor surgeries? Pascoe and Gaynor again agree that analgesia is warranted for any surgery that might be considered painful in humans.

    1. Woolf CJ: Evidence for a central component of post-injury pain hypersensitivity. Nature 1985;306:686-688.

    2. Lascelles BDX, Cripps PJ, Jones A, Waterman-Pearson AE: Post-operative central hypersensitivity and pain: The pre-emptive value of pethidine for ovariohysterectomy. Pain 1997;73:461-471.

    3. Hardie EM, Hansen BD, Carroll GS: Behavior after ovariohysterectomy in the dog: What's normal? Appl Anim Behav Sci 1997; 51(1/2):111-128.

    References »

    NEXT: Dental Dilemma: "Treating Severe Malocclusion"


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