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Pain assessment in companion small animals (Proceedings)
The challenge of pain assessment in animals lies in the very concept of pain itself.
The challenge of pain assessment in animals lies in the very concept of pain itself. The International Association for the Study of Pain defines pain as "an unpleasant sensory or emotional experience associated with actual or potential tissue damage". Given that definition, how is one to interpret what an animal might determine is unpleasant? Or how is a human observer to determine the extent of a non-verbal animal's emotional response to anticipated or actual tissue injury? Pain is a perception and the experience of pain in any individual animal or human being has no definitive physical dimensions and is subject to extremely complex sensory and cortical processing. There is a long road (upon which things happen fast!) between the event of tissue injury and the outcome of pain perception by the animal, and along this road many factors determine the animal's perception of pain, not the least of which are peripheral and spinal cord sensory modulation, cerebral cortical processing, learned behavior, and anxiety. All of these factors may play into the animal's perception and response to pain. As veterinarians, we impose upon our patients our own human interpretation of animal behaviors that we may suppose suggest pain. We have no way of determining the animal's actual perception of the degree of pain that it is experiencing at that moment: we simply intuit behavior and overlay that with our own experience of pain in ourselves or with our prior experience to "measure" that animal's pain.
If you are in the audience listening to this lecture on assessment of pain in animals, then I am already preaching to the choir. In the United States and Canada, and no doubt Europe is way ahead of those two countries, there have been great strides made in the last decade in the field of veterinary pain management. We still have a long way to go. Hellyer et al at Colorado State University published a study 10 years ago, in which various personnel in a veterinary teaching hospital were surveyed about their attitudes for treatment of pain in companion animals (dogs and cats). The authors found that as veterinary training advanced, individuals were less likely to consider aggressive approaches to pain management. For example, faculty and residents were less likely to administer analgesics to female dogs after routine ovariohysterectomy than were 2nd and 3rd year veterinary students. A decade later one would hope that these attitudes have changed a bit, but the pattern is probably still similar. A Canadian survey examined analgesic administration retrospectively and found that 66% of dogs and nearly 100% of cats did not receive analgesics after major surgical procedures. This study is also now almost a decade old. Some of the reasons for withholding pain medication include lack of knowledge about the potential side effects of analgesics, for example, fear of opioid–induced respiratory depression, fear of opioids or other analgesics "masking" physiologic parameters that may indicate a patient's deterioration. The trend, however, in the last ten years has been to place more emphasis on the subject of pain management and analgesics within the veterinary school curricula. This fact, in addition to growing public awareness and concern over the ethical rationale for treating animal pain, has resulted in a significantly higher percentage of veterinarians who graduated in the last decade using effective analgesics at appropriate doses and intervals as compared to veterinarians who graduated prior to 1995.
A relatively anthropomorphic approach to pain assessment in animals seems appropriate when one considers that peripheral and spinal cord sensory systems are almost identical between humans and non-human mammals. Pain perception, however, occurs in the prefrontal cortex and this area is anatomically smaller in non-human mammals. This does not necessarily mean that pain perception is different in animals, simply that we do not know how they interpret or perceive peripheral pain. A sensible approach is to start by assuming that if an observed disease state or tissue injury or surgical procedure would be painful to you, as a veterinarian, then it likely is painful to the animal to a similar degree. This approach must be taken in the context of extensive and thorough observation of the animal, its response to analgesic treatment, and the expected time course of the waxing and waning of the expected pain.
A systematic approach to pain assessment can be useful in guiding behavioral observation and in charting the "scored" pain over time and the response to treatment. One approach, taken from human medicine, is the visual analogue scale (VAS), wherein the observer (or in the case of human medicine, the patient themselves) draws a line somewhere between 0 (no pain) to 100 (the most terrible pain imaginable). In veterinary medicine, the (VAS) is really nothing more than scoring what you instinctively feel about the observed patient's pain, and, because it is instinctive, is subject to inter-individual variation and bias as well as individual differences in how much pain they "expect" that animal to feel at a given time point after the painful event. Another scoring system is the numerical rating system (NRS) where pain is scored on a 3 point scale: 1 = minimal/mild pain, 2 = moderate pain, 3 = severe pain. The NRS does not offer much more than the VAS and is subject to the same individual variation and bias. An expanded NRS may be more useful in veterinary medicine. Some recent research studies have taken "key" behaviors typical for the species under study and scored them on a 3-4 point scale to come up with a total NRS score summated from subscores in 10-15 different behavioral categories. This approach seems to be useful in that it encourages the observer to categorize various behaviors and to spend time evaluating them, rather than just making a "whole animal" observation. The following table has been modified from K.A. Matthews (2000) as a partial list of behavioral and physical characteristics that may be associated with pain in dogs and cats:
At the University of Wisconsin we utilize a modified version of the University of Melbourne Pain Scale (UMPS), in clinical trials that to assess efficacy of analgesic treatments, and in routine postoperative assessment of our canine patients. An example of this pain scale is shown below:
Note that in this scoring system, a maximum possible score is 20. A non-painful normal dog may score somewhere between 3-4, depending on their individual temperament, anxiety level, response to being caged, etc. In the vocalization category, we do not include alert barking as a positive score, for example, certain dog breeds that bark continuously in response to cage confinement. We also do baseline pain scoring on each patient prior to surgery so that we can compare post-operative behavior with how the animal behaved prior to surgery, whether it was painful at that time or not.
Note also that in this scoring system, we have not included any physiologic parameters, such as heart rate, blood pressure, respiratory rate, temperature, etc. This is because it is well established that these parameters do not correlate with pain in people and likely also in animals, particularly when one considers the confounding effects of anesthesia, surgical blood loss, ambient temperature, temperament.
Assessment of pain in cats is even more challenging, as this species tends to hide their pain and becomes quiet and easier to "ignore" when they're painful. A similar approach to pain scoring can be employed in cats, utilizing modified numerical rating scales to assess behaviors that may indicate pain. At Colorado State University, a Feline Acute Pain© scale has been developed for use in cats after surgery or acute trauma. This scale allows for 4 categories of pain, ranging from 0 = no pain to 4 = severe pain. Psychological and behavioral behavior, response to palpation, and body tension are the 3 parameters that are assessed and then each of those parameters is scored between 0 – 4. For example, under psychological and behavioral, if the cat is a. content and quiet when unattended, or b. comfortable when resting, or c. interested in or curious about surroundings, then that category gets a 0 for pain. In the same category, if the cat is a. constantly yowling, growling or hissing while unattended or b. bites or chews at wound but is unlikely to move when left alone, that category would receive a 3 for pain.
In summary, pain assessment in non-verbal companion animals is challenging and requires time, patience, keen observation skills, and experience. In many instances, the owner or caretaker of the animal may be the best able to assess its pain, so if the animal appears "normal" while in the veterinary clinic, the veterinarian should listen to the owner's evaluation of the animal's pain when it is in its normal environment. The fact that so many different pain scales and assessment tools are emerging is encouraging, because it means that this "art" is in a developing and exciting stage. It also means that more clinicians will incorporate pain scoring into routine patient assessment, leading to more thoughtful and perhaps more frequent use of analgesics.