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.
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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.
