Introduction

Despite advances in pain management for companion animals, pain is still undertreated. One of the reasons for this is the difficulty in “measuring” pain in populations that cannot “self-report.” To treat pain, we must look for it, recognize it and quantify it in some way so that the efficacy of our interventions can be assessed. Pain is a complex multidimensional experience with both sensory and psychological components. Pain is a conscious emotion and is always unpleasant. 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.

Acute Pain Assessment

There have been attempts to correlate objective measurements (e.g., heart rate and blood pressure) with pain. In cats, no study has found a consistently reliable objective measure, which is not surprising.^1,2 Animals suffer from “white coat” syndrome just as humans do. The fear and the stress of a journey to a veterinary hospital will alter heart rate, respiratory rate and blood pressure.³ Mechanical nociceptive threshold testing is a useful technique for evaluating both primary and secondary hyperalgesia and can differentiate between treated and control groups of animals.⁴ This suggests that testing wound tenderness should be incorporated into the assessment of post-operative pain. Observation from a distance and interacting with the animal are both essential components of an evaluation. A painful dog or cat may remain very still and quiet because moving hurts; without interaction these patients are overlooked. Any system that is used must be valid, reliable and sensitive. Without strictly defined criteria and instructions to follow during assessment, many scoring systems are highly variable. Basic pain scales include simple descriptive scales, numerical rating scales and visual analogue scales; these are unidimensional scales as they do not assess all the complex aspects of pain. Holton and others compared these scales for assessing pain in dogs following surgery and reported significant variability between observers, which was as high as 36%.⁵ The quantitative measurement of behavior, posture and facial expressions is the most reliable method for assessing pain in animals. If the methodology used to develop and validate these instruments is rigorous, they minimize observer bias and improve inter-observer agreement. Composite systems are particularly important when self-reporting is not possible. However, these systems must utilize components that have been proven as sensitive and specific indicators of pain in the species being studied. Normal behaviors should be maintained post-operatively if a patient is comfortable. Grooming is a normal behavior but licking excessively at a wound or incision can be an indicator of pain, so the two should be differentiated.

Acute Pain Assessment Tools for Cats

Brondani and colleagues developed a multidimensional composite scale based on observing cats that underwent ovariohysterectomy.^6,7 This tool along with explanatory videos is available at www.animalpain.com.br/en-us.

The Glasgow Composite Measures Pain Scale-Feline (rCMPS-Feline) includes facial expressions and was developed using cats undergoing different types of surgery or with medically related pain.^8-10 It can be downloaded at: www.newmetrica.com/acute-pain-measurement.

The major assessment domains in cats include:

• Vocalization
• Posture
• Attention to the wound
• Response to people
• Response to palpation of the wound or painful area
• Facial expressions
• Overall demeanor

The facial expressions of pain in cats include changes in ear position, half-shut or “squinty” eyes, tension in the muzzle which alters whisker position and the position of the head. A feline Grimace Scale is currently under development.¹¹

Acute Pain Assessment Tools for Dogs

Firth and Haldane developed the Melbourne Pain Scale, which incorporates physiologic data (heart and respiratory rates) and behaviors (response to palpation, activity, mental status, posture, and vocalization).¹²

The Glasgow Composite Measures Pain Scale is another tested tool for use in dogs and the short-form (CMPSSF) is user-friendly and quick to perform.¹³ This version can be downloaded in several different languages at: www.newmetrica.com/cmps (VIN editor: URL was inacessible as of 9-23-2020). There are 6 categories for assessment: vocalization, attention to the wound or painful area, how easily they rise and walk, response to palpation, overall demeanor and posture. Even if a dog cannot get up (e.g., after spinal cord injury), the tool can be used and is valid. If all 6 categories are scored the maximum score that can be achieved is 24 and an intervention level of 6/24 is suggested; if mobility cannot be assessed the maximum score is 20 with 5/20 recommended for intervention.

The Colorado State University Canine Acute Pain scale is widely used and is derived from the Melbourne Pan Scale, the Glasgow scale and a numeric rating scale. It requires both observation and “hands-on” evaluation to complete the assessment. It provides drawings of dogs that fit into its 5 categories (0, 1, 2, 3, 4), which enhances its utility. It has not been fully validated but has proven to be a useful teaching tool for veterinary students.¹⁴

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 team members, and it should be an integral part of the animal’s evaluation.

The severity of surgery or trauma, the patient’s response to analgesic therapy and the expected duration of action of the analgesic drug(s) administered will help to determine the frequency of evaluations. For example, if an animal is resting comfortably following administration of an opioid, it may not need to be re-assessed for two to four hours. Animals should be allowed to sleep following analgesic therapy. Vital signs can often be checked without disturbing a sleeping patient. 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 are likely to provide more information than the occasional observation of the animal through the cage door. Routinely using a pain assessment tool enhances the care of patients in the perioperative period.

References

1.  Smith JD, Allen SW, Quandt JE. Changes in cortisol concentration in response to stress and postoperative pain in client-owned cats and correlation with objective clinical variables. Am J Vet Res. 1999;60(4):432–6.

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):685–90.

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):733–7.

4.  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):243–6.

5.  Holton LL, Scott EM, Nolan AM, Reid J, Welsh E, Flaherty Comparison of three methods used for assessment of pain in dogs. J Am Vet Med Assoc. 1998;212(1):61–6.

6.  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):174–83.

7.  Brondani JT, Mama KR, Luna SP, Wright BD, Niyom S, Ambrosio J, et al. Validation of the English version of the UNESP-Botucatu multidimensional composite pain scale for assessing postoperative pain in cats. BMC Vet Res. 2013;9:143.

8.  Reid J, Scott EM, Calvo G, Nolan AM. Definitive Glasgow acute pain scale for cats: validation and intervention level. Vet Rec. 2017;180(18):449–53.

9.  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):622–9.

10.  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):615–21.

11.  Evangelista MC, Watanabe R, O›Toole E, Pang DSJ, Steagall PV, editors. Facial Expressions of pain in cats: development of the Feline Grimace Scale. Association of Veterinary Anaesthetists Spring Meeting; 2018 March 12–13th 2018; Grenada.

12.  Firth AM, Haldane SL. Development of a scale to evaluate postoperative pain in dogs. J Am Vet Med Assoc. 1999;214(5):651–9.

13.  Reid J, Nolan A, Hughes J, Lascelles D, Pawson P, Scott. Development of the short-form Glasgow Composite Measure Pain Scale (CMPS-SF) and derivation of an analgesic intervention score. Anim Welf. 2007;16(S1):97–104(8).

14.  Mich PM, Hellyer PW, Kogan L, Schoenfeld-Tacher R. Effects of a pilot training program on veterinary students’ pain knowledge, attitude, and assessment skills. J Vet Med Assoc. 2010;37(4):358–68.