An overview of pain

The International Association for the Study of Pain defines pain as ‘an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage’. Pain is a potent negative affective state that focuses an animal’s attention and biases its behavior.

One of the problems with assessing pain in animals is that pain can only be measured indirectly; while humans can self-assess their levels of pain and verbally report pain scores, the subjective experiences of animals are particularly difficult to assess. An animal in pain will withdraw from the source of the insult if it can be identified, protect the area affected both through immobilization and active defensive aggression and may communicate the pain to others through changes in facial expression, body postures and vocalizations.

By contrast, health and happiness are identifiable by an open and relaxed posture, facial expressions of contentment and production of chemicals that are associated with pleasure, such as endorphins. The ability to recognize and to respond to painful stimuli has evolved to protect individuals against tissue damage and provides information to safeguard against dangerous or threatening stimuli in the future. Pain may be associated with suffering at many different levels, depending on both the circumstance and the cognitive ability of the animal concerned. At its simplest it may be a temporary negative state, which guides the animal’s withdrawal from a noxious stimulus.

A variety of animals may be able to anticipate pain and generate feelings of anxiety when faced with a predictably painful stimulus and will take avoidance action as appropriate. This will cause the activation of the hypothalamic–pituitary axis and behaviorally might include threatening behavior or attempts at escape (fight or flight response). The intensity of the response is usually directly related to the intensity of the perceived threat. It is important to realize that the perceived threat arises from a combination of factors (e.g. previous experience, sensitivity to pain, emotional state) without a single cause; and as a result of the accumulation of several risk factors within the three levels of behavior assessment discussed in the previous section. Therefore, simply approaching the animal may not seem threatening from the person’s perspective, but very threatening from the animal’s perspective. It is also thought by some that certain species such as horses and dogs may be capable of a pain phobia; this involves the generation of an ungraded and extreme reaction in response to even the most low grade sign of any pain.

While pain phobias may exist, they should be distinguished from extreme responses that have been conditioned and allodynia. This is an exaggerated pain response to normally innocuous stimuli, and although mechanisms are unknown, allodynia probably arises in the structures of the limbic system of the brain, such as the amygdala and periaqueductal grey, which are associated with the processing of emotions.

Animals showing an extreme response for whatever reason are potentially very dangerous and require specialist intervention in consultation with a veterinary behaviorist. An even higher cognitive level of response to pain is pain empathy, i.e. responding to the pain of others and many owners may report that their pets are capable of this, although it remains to be demonstrated scientifically.

Pain is also often classified according to its temporal pattern and this is associated with different psychological impacts and behavioural tendencies, which might be apparent in a range of species. In humans, individual painful episodes may be referred to as per-acute pain episodes and are behaviorally characterized by vocalization and withdrawal of the painful area. Acute pain refers to episodes that last up to about 3 weeks and are associated with fear and anxiety, reduced activity and care-soliciting behaviour. Subacute pain lasts for between 3 and 12 weeks and is characterized by oscillating bouts of activity and inactivity, signs of frustration (including irritability) and the development of coping strategies associated with longer term adaptation to the pain. Early signs of depression may also become apparent at this time.

Beyond this, the pain may be considered chronic and depression, together with other passive coping strategies, is more likely. Often subacute episodes may occur against a background of chronic pain in individuals with longstanding musculoskeletal lesions, and in the horse this may present as periodic bucking set against a ‘loss of spirit’. While the changes over time may partly reflect natural adaptive developmental changes to an unresolved lesion, it is important to recognise that learning will also occur as a consequence of the responses made over time and affect the response that is shown.

Reference: Animal Physiotherapy: Assessment, Treatment and Rehabilitation of Animals

                        Edited by Catherine M. McGowan, Lesley Goff, Narelle Stubbs