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Pain scales and scoring in clinical settings: part 2
Author : Rebecca Robinson
Categories : Companion animal, Vets
Date : March 7, 2016
A shift has been made in the attitudes towards animal pain. It is accepted animals feel pain,
so there is a requirement for analgesia provision for livestock, companion and laboratory
animals undergoing painful procedures.
Table 1. Physiological effects of pain.
Working within the veterinary profession, we have a moral obligation to ensure animals under our
care receive appropriate analgesia. Additionally, untreated pain can result in significant delays in a
return to normality with multiple physiological effects.
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Management of acute, postoperative pain in companion animals was discussed in VT46.07.
However, to successfully manage pain in non-verbal species, confidence in assessment is
required, in terms of its absolute presence or absence and its severity. This allows an appropriate
level of analgesia to be promptly administered and the required dosing frequency to be judged.
Using standardised pain scales allows a repeatable measure that is consistent between observers.
Additionally, if the pain score is recorded over time, it allows an objective evaluation of a patient’s
response to management.
Difficulties of assessing animal pain
Pain is considered to be a subjective experience; therefore, what might be painful for one individual
might not be for another.
In verbal humans this problem is overcome by asking the individual to self-report. This becomes
difficult when dealing with non-verbal beings, including neonatal humans or veterinary species. It
falls to the carer to recognise and interpret signs of pain, which requires a certain level of skill and
knowledge.
Table 1 illustrates the numerous physiological changes associated with pain. However, these
changes are not specific to pain, so we cannot rely solely on such variables.
In a laboratory setting it is possible to use neurophysiological techniques (Murrell and Johnson,
2006) and measurement of plasma hormone concentrations (such as cortisol or acute phase
proteins), with varying degrees of success – depending on the method employed, type of pain and
species.
In clinical practice, pain assessment primarily involves recognition of behavioural alterations. This
includes loss of normal behaviours and development of new ones (Table 2). It is, therefore, critical
any assessor is aware of normal behaviour for the specific species (Figure 1). This is particularly
important when assessing prey species, such as rodents that do not often show overt signs
associated with pain, unlike the vocalisation or aggression by dogs.
Numerous factors may contribute to our poor pain recognition in these species, including how long
humans have co-evolved with the species and whether overt signs of pain are likely to increase
assistance from social group members or the risk of predation.
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Figure 1a. Behavioural indicators are an important aspect of pain assessment, but one must be
aware of the normal behaviour for species. This cat is recumbent, tense and unwilling to move.
Note the facial expression with half-closed or “squinting” eyes. All of these are indicators of pain in
cats.
Figure 1b. Behavioural indicators are an important aspect of pain assessment, but one must be
aware of the normal behaviour for species. This cat looks relaxed with its feet curled under the
body and the head up. The cat looks alert and the eyes are held wide open.
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Pain assessment in non-verbal species can be complicated by external factors. The presence of
environmental or situational stressors can modulate the patient’s response to pain. Administration
of sedative drugs as part of a perianaesthetic protocol may mask behavioural changes associated
with pain. In humans, the pain response can be amplified by anxiety, causing hyperalgesia (Colloca
and Benedetti, 2007).
A similar scenario may possibly occur in veterinary species. Observer-specific factors, including
age, gender, clinical experience, previous personal experience and time since graduation, can
influence the overall pain assessment. For example, when laboratory mice are assessed by male
as opposed to female observers, the male scent causes a stress response, resulting in stressinduced analgesia and fewer pain-induced behavioural changes (Sorge et al, 2014). Such
phenomena will ultimately influence both the type and frequency of analgesia provision.
Types of pain scales
A number of different pain scales have been developed for use, most adapted from those for
human medicine. A key difference between human and veterinary pain scales is in veterinary
medicine all are based on observer interpretation of the intensity of the animal’s pain.
For maximal patient benefit, it is crucial only one pain scale is used throughout treatment. This
allows assessment of the patient’s response to analgesic therapy and can guide future treatment.
Changing between types of pain scales for individual patients is not advised, as the variation will
prevent temporal comparison.
Visual analogue scale
A visual analogue scale (VAS) consists of a horizontal line, normally 100mm in length. The zero
end of the scale represents no pain while 100mm represents the worst pain imaginable (Figure 2).
The observer marks this line corresponding to their perception of the animal’s pain. This can be
quantified by measuring the distance in millimetres from zero.
Arbitrary cut-off values can be set for determining analgesia administration. Studies in human
medicine suggest values lower than 35mm are consistent with mild pain, while above this value
pain becomes progressively more severe (Boonstra et al, 2014). Regular, repeated assessment
and recording of values allows response to analgesia to be determined.
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Table 2. Possible behavioural effects of pain.
Although simplistic and rapid to use, some degree of training is required for use, as observers need
to be familiar with pain behaviours.
When used repeatedly by the same observer, VAS shows good reproducibility. However, if multiple
people are involved in pain assessment there is significant interobserver variability (Holton et al,
1998), reducing their usefulness and having implications for analgesia provision in clinical settings.
Numerical rating scale
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Numerical rating scales (NRS) are comparable to VAS, with similar caveats, although the scale is
drawn with discrete numbers, from zero to 10, on the illustrated line (Figure 1).
As with a VAS scale, zero represents no pain and 10 represents the worst pain imaginable. Such a
system may have a reduced sensitivity compared to VAS, as only whole integers are used.
Simple description scales
Simple description scales (SDS) have many parallels to NRS, the difference being that descriptors,
rather than numbers, are used. Such descriptors may be linked to a number to convert the result to
a pain score.
A widely used example in human medicine for ages three and older is the Wong-Baker FACES
pain rating scale (Figure 3). The diagrammatic and descriptive representation makes it easier to
understand and self-report.
Similar scales have been adapted for veterinary patients, including those produced by Colorado
State University, using multiple SDS to assess acute pain in cats, dogs and horses (Colorado State
University, 2006b; Colorado State University, 2007).
Dynamic interactive visual analogue scale
Dynamic interactive visual analogue scale (DIVAS) is an expanded version of the VAS. It uses the
same principle of marking the perceived pain level along a 100mm line. However, DIVAS also
relies on the handler interacting with the patient and gently palpating surgical wounds to provide a
complete score.
Multi-dimensional composite pain scales
VAS and NRS are considered unidimensional, as they only assess pain intensity. However, as
Tables 1 and 2 show, pain affects an animal in multiple ways; therefore, multidimensional
composite scales have been developed to incorporate such factors, assessing not only the physical
pain intensity, but also emotional effects. Such scales often weigh behaviours and variables
differently, which may help to minimise the interobserver variability seen with more basic scales.
Some multidimensional composite pain scales take account of both physiological and behavioural
aspects while some focus primarily on behavioural changes, usually with and without handler
interaction. As with other pain scales, these rely on user experience of normal behaviour for the
particular species.
The short form of the Glasgow composite measure pain scale for canine acute pain is probably the
best known multidimensional pain scale. This was preceded by the University of Melbourne’s pain
scale (Firth and Haldane, 1999).
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Figure 2a. Visual analogue scale.
Figure 2b. Numerical rating scale.
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Figure 3. The Wong-Baker FACES pain rating scale – a simple descriptive scale for assessment of
pain in humans. Used with permission from www.WongBaker FACES.org
Pain faces
There is a growing amount of research and evidence assessing how facial expressions change
when an animal is in pain. This has been a well-known method of pain assessment in rats and
mice in laboratory settings for many years, with the rat grimace scale and mouse grimace scale
including assessment of the tightening of the orbital area, flattening of the nose and cheek, and
changes in the ear and whisker position (Sotocinal et al, 2011; Matsumiya et al, 2012; Oliver et al,
2014).
Research is being performed to develop similar methods for species such as horses (Dalla Costa
et al, 2014; Gleerup et al, 2015), rabbits (Keating et al, 2012) and cats (Holden et al, 2014).
Validation
Table 3. Three aspects requiring assessment to ensure validity of a pain scale.
Pain scales can either be non-validated or validated. The process of validation involves extensive
testing of the pain scale to ensure it is reliable, giving expected results, including those across
different patients and users (Brondani et al, 2013).
The process of validation assesses three aspects (Brondani et al, 2011; Table 3). Validating
veterinary pain scales can be difficult, particularly for content validity (by relying on what has
deemed to be important by experts) and criterion validity (a lack of “gold standard” methods of
assessing pain).
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In most cases, particular pain scales have been validated only for specific species and, in some
instances, only for certain conditions or scenarios. Using a validated pain scale provides more
consistent and accurate results, allowing easier comparison between subjects and clinical studies.
Acute versus chronic pain scales
The pathophysiology of acute and chronic pain differs greatly. Acute pain is generally caused by
injury and often serves a protective or biological purpose. It is usually self-limiting and associated
with the timescale of normal healing. Chronic pain, however, is often considered a disease state in
its own right, doesn’t serve any biological purpose and has no recognisable end point (Grichnik
and Ferrante, 1991).
Both the nature of the two pain states and their behavioural indicators are different. Therefore, pain
scales designed for acute or chronic pain are not interchangeable. Acute pain scales are often
designed for use by veterinary personnel in a hospital environment, as this is where acute pain is
most likely to be experienced. Although it is always beneficial to know the pain-free behaviour of
the animal, it is possible for a naïve observer/handler who has knowledge of the general species’
behaviour to use an acute pain scale to judge the level of pain.
In contrast, behavioural changes associated with chronic pain can often be subtle and gradual in
onset. As a result, such alterations can only be recognised by someone who is very familiar with
the animal, usually the owner (Mathews et al, 2014). However, sometimes owners are unaware
subtle changes may be associated with pain and often it is only highlighted when there is an
improvement in patient demeanour during an analgesia trial.
Chronic pain scales revolve around quality of life questionnaires and functional assessment of the
animal. Factors considered often include:
mobility
daily activity levels
interest in food and water
grooming behaviour
changes in temperament and demeanour (Brown et al, 2007; Hielm-Björkman et al, 2009;
Mathews et al, 2014)
Ocular pain
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Figure 4. A dog with left ocular pain. Severe blepharospasm was present with a concurrent ocular
discharge. The dog resented gentle palpation in the area surrounding the left eye.
Patients with pain caused by ocular conditions are particularly challenging to assess. Clinical signs,
such as serous ocular discharge and photosensitivity, may be associated with the underlying
condition, but not necessarily pain. Conversely, other clinical signs, such as blepharospasm, are
often indicative of acute ocular pain (Figure 4).
Assessment can become more difficult with ocular conditions likely to cause chronic pain, such as
glaucoma. None of the veterinary acute pain scales are specific to ocular pain. However, a number
of publications have used non-validated pain scales to assess ocular pain (Myrna et al, 2010; Clark
et al, 2011).
Validated pain scales
A limited number of validated pain scales are available for companion animal use and may be for
use in specific scenarios.
Canine acute pain
Short form of the Glasgow composite measure pain scale (Morton et al, 2005). Validated for
assessment of acute pain associated with multiple conditions in dogs through a questionnaire
completed by veterinary personnel.
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Canine chronic pain
Liverpool osteoarthritis in dogs scale (Hercock et al, 2009; Walton et al, 2013)
Canine brief pain inventory (Brown et al, 2007; Brown et al, 2008; PennVet, 2006)
Helsinki chronic pain index (HCPI; Hielm-Björkman et al, 2009; University of Helsinki
(2012)
All three have been validated for chronic pain associated with osteoarthritis through the use of
owner questionnaires. The HCPI also uses a questionnaire completed by the veterinary surgeon.
Feline acute pain
São Paulo State University (UNESP) Botucatu multidimensional composite pain scale
(Brondani et al, 2013; Animal Pain). Validated for pain assessment in cats postovariohysterectomy via a questionnaire completed by veterinary personnel.
Glasgow composite measure pain scale – feline (Calvo et al, 2014; Holden et al, 2014;
NewMetrica). Validated for acute pain assessment in cats via a questionnaire completed by
veterinary personnel. Work is ongoing to define the analgesia intervention level.
Conclusion
Assessing pain in veterinary species can be challenging. Pre-existing knowledge of normal speciesspecific behaviour is essential. Pain scales enable a more objective assessment of pain and allow
serial assessments
to be compared.
Regular, repeated assessments with the same scale are required to determine if there has been a
positive response to analgesia provision and to guide further therapy. Use of appropriate pain
scales can help improve animal welfare in an acute, postoperative setting and animal quality of life
during chronic pain states.
Postoperative and acute pain management: part 1 (VT46.07)
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