Mathews et al (2014: 5) in the World Small Animal Veterinary Association (WSAVA) guideline described pain as the fourth vital sign, and proposed the following definition of pain: ‘a complex multi-dimensional experience involving sensory and affective (emotional) components. In other words, pain is not just about how it feels, but how it makes you feel, and it is those unpleasant feelings that cause the suffering we associate with pain.’ Based on this description, it is clear there are going to be inherent difficulties associated with a veterinary professional being able to quantify pain in animals as veterinary patients are unable to verbalise the location of their pain or indeed how it is making them feel. This problem is similar to that found in human medicine where the professionals are attempting to recognise, assess and quantify pain in (pre-lingual) newborn and infant children, whose ability to verbally communicate is limited or indeed absent (Bellieni, 2012). Bellieni (2012) highlighted that with these pre-lingual patients medical professionals observe and measure a number of physical and behavioural cues, and the responses they record feed into their overall judgement about levels of pain and whether the analgesic plan needs amending. These cues include:
Behavioural changes associated with pain in animals can be associated with or influenced by a number of different factors which veterinary professionals must be mindful of, including:
Using the Principle of Analogy is useful in veterinary patients when attempting to recognise and then assess pain; if the professional believes the disease or injury the animal has would be painful if they had the disease or injury themself, it is safe to assume the animal is in pain and analgesia is required. However, anthropomorphism and empathy, while no bad starting point for scoring pain in veterinary patients, may also adversely influence the recognition and interpretation of pain. Vocalisation for example, may lead to higher pain scores being awarded than more subtle indicators, even if the latter represent more severe pain, so veterinary professionals must be mindful of these principles when they are assessing their patients (Nuffield Council on Bioethics, 2005). Epstein et al (2015:69) stated that ‘it is now accepted that the most accurate method for evaluating pain in animals is not by physiological parameters but by observations of behaviour’. Both Mathews et al (2014) and Epstein et al (2015) emphasised that response to appropriate treatment is considered the gold standard in terms of measuring the presence and degree of pain.
Pain scoring: quantifying pain
Increased recognition of the significance of animal pain and the necessity to control it has led to veterinary professionals looking to human medicine for guidance in how to quantify pain, a notoriously difficult task due to the subjectivity of pain experiences. Attitudes towards pain have changed as people are more mindful and concerned about its effects, but still very little work has been done on the scoring and treatment of pain in veterinary species. Veterinary professionals are often responsible for disturbing their patient's natural state and behaviour, and are obliged, then, to return it to normal as promptly as possible.
Pain scoring in animals
The search for an objective pain scoring system for animals remains ongoing. Human medicine considers self-reporting by the patient to be the gold standard against which any other protocol is measured, and clearly this is not an option with animal patients. Problems arise because pain is an experience rather than purely a quantifiable physiological event. Veterinary professionals are attempting to find objective measures for something that is inherently subjective in nature, which will inevitably be flawed due to the previous experience and bias of the observer. The experience of pain is highly variable even with identical stimuli, and varies further between species. With no option of self-reporting, pain in animals can only ever be what the veterinary professional perceives it to be, rather than what the patient perceives it to be. Early attempts to quantify pain in animals were perhaps inevitably based on these protocols used in human medicine, mostly observer-based scales used in prelingual children. These scales tended to rely on subjective evaluation of behaviour without any real proven correlation with behavioural or physiological indicators of pain.
Any pain scoring system based on observation of behaviour relies on the observer being familiar with what is normal. This requires time spent with the patient, time talking to the owner of that patient and also good patient records so that a change of carer is not detrimental. Nevertheless, the accuracy of pain scoring will always be limited by observer bias. In addition to the factors already identified, it is inevitably influenced by personal experience for instance if the observer has undergone a similar procedure themselves, empathy with the patient will be increased, and a higher pain score will be given. The patient itself cannot be subject to the placebo effect but the observer certainly can, for instance if an analgesic has been given, and the observer is confident it will work, lower scores will subsequently be allocated. Humans also tend, when scoring anything, to avoid extremes, so the scores assigned will tend to be clustered in the mid range.
An ideal pain score
There are a few pain scoring systems validated for use in general veterinary practice at the moment, and attempts have been made to devise more pain scoring systems to increase and further improve the recognition of pain and form the basis of guidelines for treatment. The goal, of course, is to reduce the score to as near normal as possible with analgesic interventions.
A scale for assessing pain should, ideally have a means of detecting its presence or absence, as well as of evaluating its magnitude or severity, location and impact on quality of life. Such a scale would also have to be able to account for the differences between visceral, somatic and neuropathic pain, as well as species and age variations. Any intervals on the scale should be equal, and it should be borne in mind that the human ability to differentiate more than 10 to 20 levels of anything is questionable.
Any system that is devised needs to be reliable, sensitive and valid to be useful (Mathews et al, 2014):
It is additionally important that any scale is versatile enough to be readily adopted by all members of a practice team, or, indeed, profession. The systems that have been tried have so far relied on correlating different factors such as expected levels of pain, the type of surgery, physiological parameters and spontaneous behaviour on handling, manipulation and interaction. Clearly whatever the system being used, patients should be reassessed regularly, ideally as far as possible by the same person to minimise observer variation.
Types of scoring systems/scales
One (uni)-dimensional pain tools
| Score | Description |
|---|---|
| 0 | No pain, no overt signs of discomfort and no resentment to firm pressure |
| 1 | Some pain, no overt signs of discomfort but resentment to firm pressure |
| 2 | Moderate pain, some overt signs of discomfort which are made worse by firm pressure |
| 3 | Severe pain, obvious signs of persistent discomfort which are made worse by firm pressure |
| Observation | Score | Criteria |
|---|---|---|
| Vocalisation | 0 | No vocalisation |
| 1 | Vocalising, responds to calm voice and stroking | |
| 2 | Vocalising, does not respond to calm voice and stroking | |
| Movement | 0 | None |
| 1 | Frequent position changes | |
| 2 | Thrashing | |
| Agitation | 0 | Asleep or calm |
| 1 | Mild agitation | |
| 2 | Moderate agitation | |
| 3 | Severe agitation |
Olesen et al (2012) identified that a limitation with onedimensional pain tools like the VAS and NRS is that they only assess a limited part of the pain experienced by an individual, and suggested that to fully appreciate the complexity of the experience, results from several assessment tools are typically required. Mathews et al (2014) added that the potential for inter-observer variability associated with the use of these tools can limit the reliability, however when used consistently they can be a useful and effective part of any pain assessment protocol. Mathews et al (2014: 9) would recommend the NRS scale in clinical practice due to ‘its enhanced sensitivity over the SDS and increased reliability over the VAS'.
Multi-dimensional/composite pain tools
‘Multi-dimensional pain measurement tools can be used to assess a wider pain experience’ (Olesen et al 2012: 744). The main limitation associated with these, highlighted by Olesen et al (2012) was the time required to complete them when compared with a one-dimensional tool.
| Variable | Criteria | Score |
|---|---|---|
| Heart rate | 0–10% greater than pre-op value | 0 |
| 11–30% greater than pre-op value | 1 | |
| 31–50% greater than pre-op value | 2 | |
| >50% greater than pre-op value | 3 | |
| Respiratory rate | Normal | 0 |
| Mild abdominal assistance | 1 | |
| Marked abdominal assistance | 2 | |
| Vocalisation | No crying | 0 |
| Crying, responsive to calm voice | 1 | |
| Crying, does not respond to calm voice | 2 | |
| Agitation | Asleep or calm | 0 |
| Mild agitation | 1 | |
| Moderate agitation | 2 | |
| Severe agitation | 3 | |
| Response to manipulation | No response | 0 |
| Minimal response, tries to move away | 1 | |
| Turns head towards site, slight vocalisation | 2 | |
| Turns head with intention to bite, howls |
This does provide a quite sensitive, reliable and valid means of assessing pain in patients, and several academic institutions have given their names to various versions of such a scale. Perhaps the most well known is the University of Melbourne Pain Scale (UMPS), a VRS incorporating physiological and behavioural variables divided into six categories and combining to give a maximum possible score of 27. The Colorado State University (CSU) have produced a number of different pain scoring systems for dogs and cats that combine aspects of the NRS and composite behavioural observations (Mathews et al, 2014), and include:
The University of Glasgow Veterinary School also has an acute pain scale based on the McGill Pain Questionnaire as used in humans — the Glasgow Composite Measure Pain Scale (GCMPS) and its short form (GCMPS-SF) (Crompton, 2010; Mathews et al, 2014). The GCMPS scale incorporates six behavioural categories, each with four to six descriptors, and again a composite score is given. The words used in the descriptors were suggested by veterinary practitioners. The GCMPS-SF of the scale was produced to make it more user-friendly in practice, and an acute feline scale has recently been tested and proved valid (Calvo et al, 2014). As validity should be paramount to the choice of a scale for use in practice, the fact that both of these Glasgow acute scales have been validated in research should prompt practitioners to choose them for implementation in practice for acute pain assessment in dogs and cats.
Another validated multidimensional pain scale for the assessment of acute pain in cats is the UNESP-Botucatu Multidimensional Composite Pain Scale (MCPS). This is a scale used to assess postoperative pain levels in cats by assessing pain expression, psychomotor changes and physiological variables, which adds up to a score out of 30. The scale was originally devised in Brazilian Portugese, but and English version has subsequently been validated (Brondani et al, 2013). Printable versions of this pain scale are available from the Animal Pain website: http://www.animalpain.com.br/en-us/avaliacao-da-dor-em-gatos.php, where there are explicit directions with regards to the use of the pain scale, and some very useful videos relating to each score criterion to ensure a standardised interpretation of each criterion by users.
Conclusion
The use of pain assessment tools/scales is essential in the optimal management of painful veterinary patients, however their inability to verbalise their pain or discomfort poses additional challenges to veterinary professionals. It is vital, whichever assessment tool or score is used, that patients are reassessed regularly and their response to appropriate treatment documented accurately. The outcome of observations and interactions with patients when these tools are being utilised must be considered alongside the veterinary professional's knowledge of any disease condition the patient has, and its current or historical clinical status to make an informed decision about the overall pain status. There are multiple tools/scales available for use in veterinary practice, and the decision about which to adopt is individual to each practice; it has to be one all staff understand and are able to apply in a standardised manner for it to be useful in managing a patient's pain.