In many countries pet cats outnumber dogs, yet our understanding and treatment of pain in this species have lagged behind those for dogs. One of the main reasons for under-treatment of pain results from the difficulty in recognizing and assessing pain and, until recently, lack of validated pain-scoring tools. Other reasons include a lack of species-specific data on analgesic agents, fear of adverse drug effects, and the lack of products with market authorization for cats.
To treat pain we must first look for it, recognize it, and quantify it in some way so we can assess the efficacy of our interventions. Pain is a complex multidimensional experience with both sensory and psychological components. The sensory-discriminative component is "how it feels" (type, source, and intensity of pain) and the affective-emotional component is "how does it make the animal feel?"
In humans who can self-report, pain is what the patient says it is, but in neonates, cognitively impaired people, and animals, pain is what the observer says it is. As animal caregivers, we make "proxy" assessments on the patient's behalf and this puts an extra burden on us, to "get it right," otherwise they may not receive the analgesia they need and deserve.
In cats, the correlation between easily measured physiologic (objective) variables such as heart rate, respiratory rate, and blood pressure, and pain is poor.1,2 This is not surprising since these variables can be affected by many factors other than pain. Cats suffer from "white coat" syndrome, just as humans do; for example, fear and the stress of a journey to a veterinary hospital will alter heart rate, blood pressure, and respiratory rate in most cats.3 Pressure platform gait analysis can be successfully used in cats and this may provide an objective method of assessing pain after limb procedures, at least in research settings.4 Mechanical nociceptive threshold testing with devices such as palpometers has proved a useful technique for objectively evaluating primary hyperalgesia in cats.5 Changes in wound sensitivity correlate with observational pain scores in cats suggesting that an assessment of wound tenderness should be incorporated into an overall assessment whenever possible.
Currently, there is no gold standard for assessing pain in cats, but several good tools do exist. Any system that is used must be valid, reliable, and sensitive. Without strictly defined criteria and use of well-trained and experienced observers, many scoring systems are highly variable. Basic pain scales include simple descriptive scales (SDS), numerical rating scales (NRS), and visual analogue scales (VAS). Holton and others compared the use of SDS, NRS, and VAS for assessing pain in dogs following surgery and reported significant interobserver variability (as high as 36%) with all three scales; the same is likely true in cats.
It is now accepted that quantitative measurements of behavior are the most reliable methods for assessing pain in animals and that if the methodology used to develop and validate these systems is rigorous they can be objective with minimal observer bias. Multidimensional systems are particularly important when self-reporting is not possible. However they must incorporate components that have been proven as sensitive and specific indicators of pain in the species being studied. Knowledge of the normal behavior for the individual being evaluated is important and deviations from normal behavior may suggest pain, anxiety, fear, or some combination of stressors. Normal behaviors should be maintained following surgery if an appropriate analgesic plan has been used. Loss of normal behaviors, for example a playful and friendly cat becoming reclusive, should raise our suspicion that pain may not have been adequately addressed. The development of new behaviors, such as a previously friendly animal becoming aggressive, should also raise our level of suspicion that the cat may be painful.
In cats, the following categories are important to assess: posture, comfort, activity, attitude, interaction with people (e.g., response to stroking), response to palpation of the wound, appetite, vocalization, and miscellaneous behaviors such as facial expression, tail activity, chewing and licking at the wound, and flexing and extending the hind limbs.
Waran and others6 constructed detailed ethograms of cats before and after abdominal surgery (ovariohysterectomy) and reported that a hunched, crouched, or "tucked-up" posture was rarely recorded before surgery, but was frequently observed after surgery. In addition, these postures were seen more frequently in cats that only received preoperative analgesics compared to those that received pre- and postoperative analgesics.
Brondani and colleagues have developed a multidimensional composite scale for use in cats following ovariohysterectomy with specific scores for each behavior within a category; the maximum score is 30 and the authors recommend intervention (rescue) if the cat scores ≥ 8. These investigators also emphasize that it is preferable to record preoperative (baseline) data for comparison. The scale is available for downloading and examples (videos) of the behaviors used for assessment with this tool can be found at: www.animalpain.com.br/en-us/. You can also test your skills and see how you score a cat compared to an expert at: www.animalpain.com.br/en-us/avalie-sua-habilidade.php
A simple one-page tool that is readily applicable in practice is the Glasgow Composite Measure Pain Scale for cats.7 This scale has a maximum score of 16 and intervention is advised at ≥ 4. The major assessment domains used in this tool are vocalization, posture, attention to the wound, response to stroking, response to palpation of the wound or painful area, and overall demeanor (e.g., happy and content, disinterested/quiet, anxious/fearful, dull or depressed/grumpy).
Attention to the wound is important to assess; grooming is a normal behavior in cats and they spend a considerable time each day doing this; however, licking excessively at a wound or incision can be an indicator of pain, so the two should be differentiated. In general, most cats dislike any restrictive dressings or bandages and may roll around, pay excessive attention to, or try to remove these. These behaviors could indicate pain or dislike of the bandage, so it is important to differentiate between these two by performing a careful assessment.
Another area of active research is the interpretation of facial expressions as indicators of pain. Grimace and "pain face" scales have been developed for newborn babies, infants, mice, rats, rabbits, and horses, and preliminary work with cats has been published.8 Ear position and areas around the mouth/muzzle are important to look at and have excellent discrimination properties allowing observers to correctly differentiate pain-free and painful cats in 98% of cases;8 further studies are underway to develop this tool.
There have been significant advances in our ability to measure and quantify pain in cats and behavior-based, validated composite tools are available. These are quick and easy to use and should be a routine part of perioperative care, just as measuring temperature, pulse, and respiration are.
References
1. Smith JD, Allen SW, Quandt JE, Tackett RL. Indicators of postoperative pain in cats and correlation with clinical criteria. Am J Vet Res. 1996;57(11):1674–1678.
2. Cambridge AJ, Tobias KM, Newberry RC, Sarkar DK. Subjective and objective measurements of postoperative pain in cats. J Am Vet Med Assoc. 2000;217(5):685–690.
3. Quimby JM, Smith ML, Lunn KF. Evaluation of the effects of hospital visit stress on physiologic parameters in the cat. J Feline Med Surg. 2011;13(10):733–737.
4. Romans CW, Gordon WJ, Robinson DA, Evans R, Conzemius MG. Effect of postoperative analgesic protocol on limb function following onychectomy in cats. J Am Vet Med Assoc. 2005;227(1):89–93.
5. Slingsby LS, Jones A, Waterman-Pearson AE. Use of a new finger-mounted device to compare mechanical nociceptive thresholds in cats given pethidine or no medication after castration. Res Vet Sci. 2001;70(3):243–246.
6. Waran N, Best L, Williams VM, Salinsky J, Dale A, Clarke N. A preliminary study of behaviour-based indicators of pain in cats. Anim Welf. 2007;16(S):105–108.
7. Calvo G, Holden E, Reid J, Scott EM, Firth A, Bell A, et al. Development of a behaviour-based measurement tool with defined intervention level for assessing acute pain in cats. J Small Anim Pract. 2014;55(12):622–629.
8. Holden E, Calvo G, Collins M, Bell A, Reid J, Scott EM, et al. Evaluation of facial expression in acute pain in cats. J Small Anim Pract. 2014;55(12):615–621.