Analgesia after onychectomy in cats
Roughly 14 million cats are declawed each year in the United States.1 Although many receive medications for postoperative pain, defining the most effective protocol is limited by the inability to compare analgesic regimens. Interpreting behavior in cats to assess pain has been notoriously difficult.2-9 And interpreting objective physiological parameters has not been consistent because of confounding factors such as stress or drug effects.2,3,7-13 Additionally, veterinarians perform several different onychectomy techniques at very different skill levels. And because of the ethical considerations that surround this procedure, providing both the owner and the cat with effective perioperative and postoperative analgesic options is critical. In this article, we offer guidance on what drug or combination of drugs to use, concentrating mainly on postoperative analgesia.
MEASURING THE EFFECT OF ANALGESICS
Historically, pain studies in animals have used visual analog scales, numerical rating scales, and physiological indices to evaluate the efficacy of drugs and procedures at reducing pain.2,9,11,12 These methods have shown great diversity in results.2,10,12-14 Regardless of the method, a subjective evaluation is based on interpreting an animal's behavior. Behavioral characteristics such as vocalization, depression, or aggression that may be associated with animal pain can be difficult to differentiate from drug effects, anxiety induced from the anesthetic or surgical experience, or an animal's nature.3,15,16 Additionally, animals exhausted from surgery or obtunded may be incapable of displaying painful responses, and some animals do not display painful behavior until the severity reaches a high threshold.15 Behavior changes due to pain in cats may even be more subtle than those in dogs, so pain in cats is likely underestimated.2,3,17 Furthermore, both forelimbs are usually operated on in most studies involving onychectomy in cats, requiring the cats to use their paws despite pain. This forced usage may further hide painful behavior. In an effort to reduce the variability of pain scoring, grading scales may be assigned that use strict definitions for mild, moderate, and severe pain. However, these scores are still subjective.Physiological indices are arguably better than subjective measures for pain evaluation. Heart rate, blood pressure, pupil size, and blood catecholamine, glucose, β-endorphin, and cortisol concentrations have all been used for pain evaluation.2,3,7-13 Unfortunately, they do not correlate well with subjective pain evaluations and are affected by environmental stresses unrelated to pain.2,7-13 Cortisol concentrations, although highly variable, may be the most useful of these physiological indicators.3,11 However, in one study in cats undergoing onychectomy, the control group, which did not receive surgery but was bandaged, did not have significantly different blood cortisol concentrations from the surgery group.3
More recently, pressure platform gait analysis, an objective, reliable, noninvasive method of measurement, has been used to quantify postoperative pain in cats after onychectomy.18 Precise and accurate measurement of limb function after unilateral onychectomy allows objective conclusions to be made regarding which methods provide the most effective pain management.18 This methodology is similar to force platform gait analysis, which is the gold standard for measuring limb function in dogs.19-22 Most clinicians would agree that after elective surgery on a limb, a reasonable estimation of pain is the willingness of the patient to use the affected leg.
INFLUENCE OF ONYCHECTOMY TECHNIQUES ON PAIN
Various surgical techniques have been used to declaw cats, including the scalpel, guillotine, and laser.1,23,24 One study that compared the laser technique with the scalpel technique found that although the cats declawed with the laser had significantly lower pain scores within the first 24 hours, the difference was not clinically significant.23 These pain scores were assessed purely by subjective means.23 Another study that evaluated cortisol concentrations as a determinant of pain found that in the first 24 hours, cats in the scalpel group had increased blood and urine cortisol concentrations; however, complication rates for the laser group were higher than those for the scalpel group.25 This finding conflicts with the results of previous studies and most empirical claims.1,24,25