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Recognition and quantification of acute pain...

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01 June 2015, at 12:00am

Dr JO MURRELL discusses the use of pain scoring tools in various species and says that more widespread adoption of them in veterinary practice would bring significant benefits to animal welfare

ACCURATE recognition and quantification of pain is pivotal to effective pain management in cats and dogs.

Without this cornerstone of pain management there always remains the risk that pain will be inadequately treated, leading to suffering, or that analgesics will be administered that are not required, leading to the risk of drug-related side-effects. Therefore, frequent assessments of pain level combined with pain management are paramount in the peri-operative period.

Pain is a complex phenomenon with both an emotional and sensory component. Assessment of both of these elements is important in order to accurately gauge pain level in an individual patient.

The sensory component can be described as the component relating to pain location and intensity (i.e. where does it hurt and how much does it hurt?), whereas the emotional component can be more difficult to discern. It is the “affective” component of pain, for example the effect of pain on the animal’s psychological state (mood or demeanour).

It is now widely accepted that evaluating changes in behaviour in an individual patient can best elucidate both the sensory and emotional components of pain. This requires that the assessor has knowledge of the animal’s normal behaviour in the same environment (e.g. allowing for behavioural changes associated with anxiety when hospitalised).

Veterinary nurses are often best placed to assess pain in hospitalised patients because they have good knowledge of the animal and how it behaved in the veterinary environment before surgery or before the onset of trauma and spend more time with patients, allowing changes in behaviour in response to the administration of analgesic drugs to be detected.

One recent development in the recognition of pain in animals, mirroring developments in neonatal and paediatric pain recognition, has been the use of changes in facial expression to identify animals that are in pain.

Langford et al (2010) were the first group to code facial expressions in mice following induction of a visceral pain model and developed the Mouse Grimace Scale (MSG). Changes in five facial features associated with pain were identified as follows: orbital tightening; bulging of the nose and cheek; drawing back or flattening of the ears; and change in whisker position so that the whiskers were either backward against the face or forward as if standing on end.

Interestingly, lesioning of the rostral anterior insula, an area of the brain that is activated by pain in humans with a strong emotional component, attenuated the changes in facial expression in mice following induction of the same visceral pain model, suggesting that these changes in facial expression reflected pain-related negative affect.

Since publication of the MSG, other researchers have published scales for rats (Sotocinal, 2011), rabbits (Keating et al, 2012) and horses (Dalla Costa, 2014), defining similar facial action units to those identified in mice.

Recently, researchers at the University of Glasgow investigated changes in facial expression in cats in pain (Holden et al, 2014) and found that cats in pain tend to also show flattening of the ears and muzzle and cheek area.

However, observers showed a poor ability to discriminate between painfree and painful cats using facial images alone. This suggests that training is required in the use of facial expression to detect pain in cats. Further research is also needed to elucidate the effects of drug administration and concurrent anxiety on facial expression in all species.

The use of tools to facilitate the quantification of pain is recommended in both cats and dogs. Putting a “number” to pain level helps with decision-making about requirement for rescue analgesia and makes it easier to track changes in pain level over time.

However, recent data suggest that use of pain scoring tools has not been widely adopted by veterinarians in general practice (Hunt et al, 2014). Composite pain scoring tools are recommended because they take into account both the sensory and emotional components of pain and a number of these have been developed recently.

Probably the most widely-used pain assessment tool is the Glasgow Composite Pain Scale for dogs (Holton et al, 2001) (http://www.newmetrica.com/cmps/).

This tool is validated, easy to use, taking approximately five minutes to complete for an individual patient, and robust in a multi-user setting where it is likely that more than one person will assess pain in the same patient over time.

Importantly, an intervention level has been defined, whereby a score of ≥5/20 (if the dog is non-ambulatory) or ≥6/24 (if the dog is ambulatory) indicates that additional analgesia is required for an individual patient.

Disadvantage

The major disadvantage of the tool is that it discriminates poorly between dogs that are very sedated and dogs that are painful, therefore a “sense checker” is required when using the tool. If a dog scores highly but is very sedated, it is likely that additional analgesia is not required.

Two composite pain scoring tools have recently been validated for use in cats and are recommended for use in place of the Colorado Feline Acute Pain Scale, which is not validated and relatively insensitive to the detection of pain.

The Botucatu Multidimensional Composite Pain Scale (Brondani et al, 2011) can be freely downloaded from http://www.animalpain.com.br/en-us/avaliacao-da-dor-em-gatos.php and comprises a comprehensive series of questions relating to both the sensory and emotional component of pain. The maximum possible score is 30, with a defined intervention score ≥8 indicating that additional analgesia is necessary.

Scale’s complexity

The major limitation to the scale is its complexity, taking approximately 7-8 minutes to complete, which can seem off-putting, particularly when you are unfamiliar with the questions. Furthermore, the scale includes measurement of blood pressure, both before and after surgery, which can be problematic and time-consuming in some cats.

The author has used the scale without the inclusion of blood pressure and adjusted the intervention level to ≥7 to allow for the deletion of one item from the scale. Very recently, the University of Glasgow has published a feline equivalent of its Composite Pain Scale for dogs [Calvo et al, 2014, appendix 2 (Revised CMPF)]. The scale was developed using similar principles to the canine scale and is much simpler to use than the Botucatu scale, which will facilitate widespread adoption in feline practice.

It comprises six questions which, similar to the Glasgow pain scale for dogs, require you to evaluate the cat from outside of the cage, interact with the cat and assess responsiveness to stroking, and finally to quantify the response to gentle palpation of any wound or painful area.

Quantification of the response to gentle palpation is a means to detect primary and secondary mechanical hyperalgesia, which occurs with upregulation of the pain pathways and central sensitisation.

The maximum possible score for the Feline Glasgow Composite Pain Scale is 16, with a defined intervention level of ≥4 indicating that additional analgesia is needed.

Unlike the pain scale in dogs, the cat scale appears to be less influenced by concurrent sedation resulting from analgesic drug administration or systemic disease, allowing better discrimination between cats in pain from other causes of altered behaviour.

Over the last 10 years, knowledge about pain recognition and quantification in companion animals has significantly increased. Particularly important developments in this field have been the publication of composite pain scoring tools for cats and dogs that have undergone some validation and are easy and practical to use in practice. More widespread adoption of these pain scoring tools in veterinary practice would bring significant benefits to animal welfare.

References

Brondani, J., Luna, S. and Padovani, C. (2011) Refinement and initial validation of a multidimensional composite pain scale for use in assessing acute post-operative pain in cats. American Journal of Veterinary Research 72: 174-183.

Calvo, G., Holden, E., Reid, J. et al (2014) Development of a behaviour-based measurement tool with defined intervention level for assessing acute pain in cats. Journal of Small Animal Practice 55: 622-629.

Dalla Costa, E., Minero, M., Lebelt, D. et al (2014) Development of the horse grimace scale (HGS) as a pain assessment tool in horses undergoing routine castration. PLoS One 9: 1-10.

Holton, L., Reid, J., Scott, E. M. et al (2001). Development of a behaviour-based scale to measure acute pain in dogs. Veterinary Record 148: 525-531.

Hunt, J., Knowles, T., Lascelles, B. D. X. et al (2014) Prescription of perioperative analgesics by UK small animal veterinary surgeons in 2013. Proceedings of the Spring Meeting of the Association of Veterinary Anaesthetists: Nottingham, UK.

Keating, S. C. J., Thomas, A. A., Flecknell, P. A. et al (2012) Evaluation of EMLA cream for preventing pain during tattooing of rabbits: changes in physiological, behavioural and facial expression responses. PLoS One 7: e44437.

Langford, D. J., Bailey, A. L., Chanda, M. L. et al (2010) Coding of facial expressions of pain in the laboratory mouse. Nature Methods 7: 447-449.

Sotocinal, S. G., Sorge, R. E., Zaloum, A. et al (2011) The rat grimace scale: a partially automated method for quantifying pain in the laboratory rat via facial expressions. Molecular Pain 7: 55-126.