Assessing pain in animals

Pain assessment involves the integration of measurements of behaviour and physiology together with knowledge of the bi-directional mechanisms that control pain. Morton and Griffiths (1985) proposed a framework for the recognition of pain, distress and discomfort based on a combined assessment of appearance, food and water intake, behaviour, cardiovascular functioning, digestive system activity and neurological/musculoskeletal signs. This provides a useful framework, but, the correlation between physiological measures such as heart rate, respiratory rate and pupil dilation versus subjective pain scores may be poor and there is a need for greater validation of pain scales.

Given the enormous range of individual factors that can affect pain perception in a given context discussed above, it should be apparent that it is difficult to accurately assess the pain of an individual without a thorough history, including baseline assessments of behaviour and temperament. In addition, given the differences that inevitably exist between assessors, it is also important that assessment is repeated by the same assessor on all possible occasions, in order to reduce this possible source of error.

Laboratory methods to assess pain in domesticated animals might be thought of as being more objective and are increasingly sophisticated. Clinical assessment generally relies on evaluating a range of behavioural signs of pain, and these may be integrated into subjective scoring systems. Verbal rating scales involve qualitative description of behaviour observed, and simple quantitative scales involve subjectively rating pain as No Pain, Mild, Moderate or Severe. These assessment protocols have been criticised not only for the large variation between different observers, but also for their lack of sensitivity.

Numeric scales rating pain between 0 and 10, and visual analogue scales marking pain on a ruler on which 0 = No Pain Present and 100 = Worst Pain Imaginable, are generally considered to provide better sensitivity and reliability. However, the validity of these systems may be questioned owing to a lack of transparency regarding pain parameters considered by observers, and these are weighted in the final score. Some observers may reliably weight vocalisations heavily because of ease of measurement and anthropomorphism, but these vocalisations may not correlate well with pain experiences since dogs occasionally vocalise while under anesthesia when pain is presumably prevented.

Although the science of valid pain assessment in animals is in its infancy, this does not negate the responsibility of those that work with animals in pain to institute and apply pain assessment criteria within their practice. Given current knowledge, the physiotherapist should at the very least use some form of pain scale that both the owner and the physiotherapist can complete and keep a behavioural diary of therapy sessions to monitor pain responses. Should there be any doubt that a certain condition is painful, it is good practice to assume that what would be painful for a person is painful for that animal (IRAC 1985). Further information on the recognition and assessment of animal pain is hosted by the University of Edinburgh at: www.vet.ed.ac.uk/animalpain/, and readers may wish to refer to this for further detail of some of the principles that have been discussed in this chapter.

Source of pain	Behavioural response
General responses	Lethargy Reduction in grooming Depression Reduced feeding, drinking Protection of painful site Vocalization (dog: whining, growling; equine: groaning) Aggression Hanging tail Ear position (equine: pinned ears) Facial expression (canine: furrowed brow; equine: clenched jaw, wrinkled muzzle) Restlessness/weight shifting between all limbs
Limb	Avoidance or reduction in weight bearing Abnormal gait Head bobbing during locomotion Rubbing, licking wound site Weight shifting away from painful limb
Abdominal/ Spinal/ Visceral pain	Tucked up posture Glancing or nosing abdomen Abnormal stance, stretching of hind limbs Restlessness Sweating Trembling
Head pain	Headshaking and facial rubbing Head shyness Grimacing Signs often exacerbated by exercise Intranasal pain Snorting and sneezing Turning of the upper lip Intra-oral pain Reduced appetite and/or dropping of food being chewed Teeth grinding

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

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