Despite significant advances in pain management for companion animals, pain is still undertreated. One of the reasons for this is the difficulty in recognizing and "measuring" pain in non-lingual species. 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".
Many attempts have been made to correlate objective measurements such as heart rate and blood pressure with pain. In cats, no study has found a consistently reliable objective measure, which is not surprising since these parameters can be affected by many factors other than pain.1,2 Cats and dogs suffer from "white coat" syndrome just as humans do; for example, the fear and the stress associated with a journey to a veterinary hospital will alter heart rate in most animals.
Mechanical nociceptive threshold testing has proved a useful technique for evaluating both primary (wound) and secondary (remote areas unrelated to the wound) hyperalgesia in dogs and cats suggesting that an assessment of wound tenderness should be incorporated into an overall assessment of post-operative pain.3 A painful animal may remain very still and quiet because they are painful and without interaction these animal will be overlooked.
Currently there is no gold standard for assessing pain in dogs and cats but several tools do exist. Any system that is used must be validated, reliable and sensitive. Without strictly defined criteria and use of 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 a SDS, NRS and VAS for assessing pain in dogs following surgery and reported significant observer variability, which could be as high as 36%, with all three scales.4
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 post-operatively if an animal is comfortable. Grooming is a normal feline behavior but licking excessively at a wound or incision can be an indicator of pain, so the two should be differentiated. The occurrence of new behaviors such as a previously friendly animal becoming aggressive, or a playful and friendly animal becoming reclusive should raise our suspicion that pain may not have been adequately addressed.
Pain Assessment Tools for Dogs
The Glasgow Composite Measures Pain Scale is a validated tool for use in dogs5 and the short-form version is user-friendly6. The short form can be downloaded in several different languages including Spanish at: www.newmetrica.com/. The categories for assessment include: vocalization, attention to the wound or painful area, posture and movement, response to palpation and overall demeanor.
Pain Assessment Tools for Cats
Two clinically useful tools are available. Brondani and colleagues have developed a multidimensional composite scale for use in cats following ovariohysterectomy.7 This tool along with many videos of assessing pain in cats is available at www.animalpain.com.br/en-us/. This scale has been translated and validated in Spanish, French and Italian. The scale in Spanish can be downloaded at http://www.scielo.cl/scielo.php?pid=S0301-732X2014000300020&script=sci_arttext. Another tool that is readily applicable in practice is the Glasgow Composite Measure Pain Scale for cats.8 The assessment domains include vocalization, posture, attention to the wound, response to people, response to palpation of the wound, overall demeanor and facial expressions. Pain face or grimace scales have been developed for rodents, horses, cattle (http://www.appliedanimalbehaviour.com/article/S0168-1591(15)00226-9/abstrac [VIN editor: Link not available 2/22/17]), rabbits. There is now information on facial expressions related to pain in cats.9 Ear position and tension around the muzzle are scored as part of the Glasgow CMPS; the updated scale has a maximum score of 20 and intervention is advised at ≥5. The tool can be downloaded at: www.newmetrica.com.
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.
Confounding factors should also be considered. It can be difficult to differentiate fear and stress and the personality of the cat may preclude interactive assessments. Ketamine can also lead to behaviors that may be misinterpreted as pain.10
Using Pain Assessment Tools in Practice
Each clinic should choose a scoring system that fits their specific needs, and this may require some trial and error. Whichever one is chosen should be user friendly, quick to complete and easily performed by all caretakers and it should be an integral part of the animal's evaluation. After temperature, pulse and respiration are checked, pain, which has been coined the "fourth vital sign," should also be assessed. A scale should include both non-interactive and interactive components (when possible) and rely heavily on changes in behavior.
The health status of the animal, extent of surgery/injuries, and anticipated duration of analgesic drugs determine the frequency of pain assessments. For example, if an animal is resting comfortably following the postoperative administration of an opioid, it may not need to be re-assessed for two to four hours. If the animal has a high pain score one should intervene and treat, then score the animal within a short time frame to assess the response to treatment. Animals should be allowed to sleep following analgesic therapy. Vital signs can often be checked without unduly disturbing a sleeping animal. In general, animals are not woken up to check their pain status; however, this does not mean they should not receive their scheduled analgesics.
Undisturbed observations, coupled with periodic interactive observations (e.g., palpation of the wound) are likely to provide more information than only occasionally observing the animal through the cage door. Routinely using a pain assessment tool enhances the care of patients in the perioperative period.
Suggested Reading
http://www.wsava.org/guidelines/global-pain-council-guidelines.
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. Lascelles BD, Cripps PJ, Jones A, Waterman-Pearson AE. Efficacy and kinetics of carprofen, administered preoperatively or postoperatively, for the prevention of pain in dogs undergoing ovariohysterectomy. Vet Surg. 1998;27(6):568–582.
4. Holton LL, Scott EM, Nolan AM, Reid J, Welsh E, Flaherty D. Comparison of three methods used for assessment of pain in dogs. J Am Vet Med Assoc. 1998;212(1):61–66.
5. Holton L, Reid J, Scott EM, Pawson P, Nolan A. Development of a behaviour-based scale to measure acute pain in dogs. Vet Rec. 2001;148(17):525–531.
6. Reid J, Nolan AM, Hughes JML, Lascelles D, Pawson P, Scott EM. Development of the short-form Glasgow Composite Measures Pain Scale (CMPS-SF) and derivation of an analgesic intervention score. Animal Welfare. 2007;16(S):97–104.
7. Brondani JT, Luna SP, Padovani CR. Refinement and initial validation of a multidimensional composite scale for use in assessing acute postoperative pain in cats. Am J Vet Res. 2011;72(2):174–183.
8. Calvo G, Holden E, Reid J, 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.
9. Holden E, Calvo G, Collins M, et al. Evaluation of facial expression in acute pain in cats. J Small Anim Pract. 2014;55(12):615–621.
10. Buisman M, Wagner MC, Hasiuk MM, Prebble M, Law L, Pang DS. Effects of ketamine and alfaxalone on application of a feline pain assessment scale. J Feline Med Surg. 2016;18(8):643–651.