Postoperative pain assessment in cats: How do I know my treatment is working? (Proceedings)

In the USA pet cats outnumber dogs yet our understanding and treatment of pain in this species has lagged behind that for dogs. Veterinarians consider surgical procedures in dogs and cats to be equally painful, but treat cats less often. This under-treatment of pain results from the difficulty in recognizing and assessing pain, but also the lack of species-specific data on analgesic agents, fear of side-effects and the limited number of licensed products for cats.

Pain assessment

To treat pain we must 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 affective (emotional) components. Because animals, and some sub-populations of humans are non-verbal, the IASP1 added the following important caveat to its definition of pain: "The inability to communicate in no way negates the possibility that an individual is experiencing pain and is in need of appropriate pain relieving treatment". Pain is subjective and no one can "feel" another person's pain. Even after the same surgical procedure, humans do not experience the same quality and intensity of pain so how can we determine with any degree of certainty what an animal feels? Put simply, in humans, pain is what the patient says it is and in animals it is what we say it is. There is no gold standard for assessing pain in animals at this time. Many different scoring methods that include physiologic and behavioural variables have been published, but few are validated. Assessment systems must take into account different types and sources of pain, for example abdominal versus musculoskeletal pain. As more studies focus on species-specific pain behaviors our ability to recognize and treat pain in animals will improve but at present the assessment of pain in animals is subjective and inaccurate. However, ignoring pain simply because we have difficulty measuring it condemns our patients to undue suffering.

Acute pain assessment tools

Several studies have focused on assessing acute post-operative pain in cats, but no system has been validated or rigorously tested. In cats, the correlation between easily measured physiologic (objective) variables such as heart rate, respiratory rate, and blood pressure, has been disappointing. No study found a consistently reliable objective measure, which is not surprising since these parameters can be affected by many factors other than pain; for example fear. Plasma cortisol and β-endorphins are components of the "stress response" to anaesthesia and surgery and much effort has been expended trying to correlate these hormones with pain in laboratory and clinical trials. Plasma cortisol is unreliable in cats as a direct indicator of pain. Pressure platform gait analysis can be successfully used in cats and this may provide an objective method of assessing pain after procedures such as onychectomy, at least in research settings.

Mechanical nociceptive threshold testing with devices such as palpometers has proved a useful technique for evaluating both primary (wound) and secondary (remote areas) hyperalgesia in cats. Changes in wound sensitivity have correlated with visual analogue scoring in cats suggesting that wound tenderness should be incorporated into an overall assessment protocol.

Scoring systems

All scoring systems that depend on human observers must by definition be subjective to some degree and leave room for error, which could be either under-, or over-assessment of the animal's pain. 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; one scoring system may show an analgesic agent to be effective yet another will show that same analgesic to be ineffective. These differences are inevitable if a system is insensitive and results in large inter-observer variability. The most basic pain scales are the simple descriptive scales. These usually have four or five descriptors to choose from such as no pain, mild, moderate, severe or very severe pain. Although simple to use these scales are extremely subjective and do not detect small changes in pain. Numerical rating scales are similar to simple descriptive scales but with numbers assigned for ease of tabulation and analyses; for example no pain would be assigned the number 0, and very severe pain the number 5. This system implies equal difference or weighting between each category, which is not the case. A further development of these systems is a categorized numerical rating system where certain behaviours are chosen and assigned a value.

In an attempt to improve on discontinuous scales, the visual analogue scale (VAS) has been widely used in veterinary medicine. This tool consists of a continuous line, anchored at either end with a description of the limits of the scale, for example no pain at one end and severe pain at the other end. The observer places a mark on the line that they think correlates to the animal under observation and this is later translated into a number by measuring the distance of that mark from zero. These scales can be improved by adding a descriptor that says "worst possible pain for this procedure", because the worst pain associated with a castration is likely to be different from the worst pain after a thoracotomy. Holton and others compared the use of a simple descriptive, numerical rating and visual analogue scales for assessing pain in dogs following surgery. They showed significant observer variability, which could be as high as 36%, with all three scales.

The use of a dynamic and interactive visual analogue scale (DIVAS) as an extension of the classic VAS system. With this system, the animals are first observed undisturbed and from a distance and then approached, handled and encouraged to walk or move around. Finally the surgical incision and surrounding area is palpated and a final overall assessment of pain is made. This approach overcomes some of the deficiencies of purely observational systems; for example a cat may lie very still and quiet because a wound is painful and this would go undetected without interaction with the animal. The DIVAS system has been used to assess postoperative pain in cats [13, 14] and when performed by one individual unaware of treatments it detected differences between different analgesics and between treated and untreated cats.

In humans, multidimensional systems such as the McGill pain questionnaire that account for intensity but also sensory and affective (emotional) qualities of pain have provided a more comprehensive assessment of the patient's pain. Multidimensional systems are particularly important when self-reporting is not possible but must incorporate components that are proven to be sensitive and specific to pain (for example facial expressions in infants) in the species being studied.

It is now accepted that quantitative measurements of behaviour 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. Knowledge of the normal behaviour for the individual being evaluated is essential and deviations from normal behaviour suggest pain, anxiety, or some combination of stressors. Currently there is no published "library" of validated pain behaviours in cats.

Suggestions for creating an acute pain scoring system

Each practice 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 and readily used by all caretakers and should be an integral part of the animal's postoperative 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 and rely heavily on changes in behavior.

How often should animals be assessed?

The health status of the animal, extent of surgery/injuries, and anticipated duration of analgesic drugs determine the frequency and interval of evaluations. In general, evaluations should be made hourly for the first four to six hours after surgery provided the animal has recovered from anesthesia, has stable vital signs, and is resting comfortably.

Patient response to analgesic therapy, and expected duration of analgesic drug(s) administered, will help to determine frequency of evaluations. For example, if a cat is resting comfortably following the postoperative administration of buprenorphine, 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 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.

Continuous, undisturbed observations, coupled with periodic interactive observations (open the cage, palpate wound, etc) are likely to provide more information than occasionally observing the animal through the cage door. Unfortunately, continuous observations are not practical for most clinical situations. In general, the more frequent the observations, the more likely that subtle signs of pain will be detected.

Behaviors suggestive of acute pain in cats

Cats that adopt a hunched posture, sit quietly and seek no attention, or resent being handled are likely experiencing pain. A cat sitting quietly in the back of the cage after surgery may be painful; however, pain would not be recognized if the caregiver expects to see more active signs of pain such as pacing, agitation, or vocalizing. In one of the few studies where detailed behavioral ethograms have been constructed, a hunched or tucked up posture appears to be correlated with acute pain in cats following abdominal surgery[16]. This has been corroborated by preliminary work at Glasgow University (Robertson, unpublished data). In addition a head down posture, with eye-lids half closed and eyes held in a slanted position may correlate with pain. One of the many commonly reported problems following onychectomy is excessive licking and chewing of the feet[17]. An important component of pain assessment is palpation of the wound. Cats will often shake their paws and try to bite at their feet if bandages after declaw surgery. In general many cats dislike (shake, bite, "freak out") any restrictive dressings including the tape used to secure intravenous catheters. These behaviors could indicate pain or dislike of the bandage so it is important to differentiate between these two.

International Association for the Study of Pain

References

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