Assessing post-operative pain

Introduction

When examining animals, we often interpret certain clinical signs as suggesting the presence of pain based on how humans behave when in pain. Unfortunately, this anthropomorphic view of pain is flawed. Many animals do not respond to conditions and procedures that would cause pain in humans in a way that is immediately apparent as pain-related behaviour. 

The key to introducing effective pain control is to improve our methods of pain recognition and assessment. Pain assessment is important not simply because it encourages greater use of analgesics, but because it enables more appropriate use of these drugs. 


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A number of different approaches to pain assessment in animals have been suggested, but progress in developing and validating scoring systems has been slow. A proposal to develop more robust scoring schemes was published by Morton and Griffiths, but initial attempts to apply this were largely unsuccessful, primarily because the variables selected for inclusion were not fully identified and the scales used (0–3) not sufficiently well characterised. The scheme has proven much more successful when applied as a means of developing more humane endpoints for studies. These problems were identified by the original authors, but indiscriminate application of the system seems to have led to failure in identifying animals in pain, and pain scoring systems are almost certainly still under-used in most facilities. 

Measurements of body weight and food and water intake have been proposed as potential indicators of postoperative pain and the efficacy of analgesic therapy. These measures are objective, but they are retrospective measures and so could not be used to modify analgesic therapy for a particular animal. They are also relatively non-specific measures, that can be influenced by numerous factors in addition to postoperative pain. They can, however, be used as a simple measure of post-operative recovery, and as a means of adjusting future analgesic and post-operative care regimens for similar animals undergoing similar surgical procedures.

Behaviour-Based Pain Scoring Systems

More recently, behaviour-based schemes for assessing animal pain have been developed and this approach offers a step forward in developing a practically useful scoring system to evaluate pain and analgesic efficacy after at least some types of surgery in a range of species including rat, mouse, rabbit, guinea pig, dog, cat and some other larger species.

Pain Faces

The assessment of pain by evaluating facial expressions has been widely used in infants and children and recently, a means of analysing pain faces in mice, using a Mouse Grimace Scale, was described and similar scales have been developed in other species. The method has been applied successfully to assess post-surgical pain in a range of species. Most studies using the technique have obtained high-resolution images and scored them retrospectively, but initial work in a number of establishments has indicated that “cage-side” assessment is possible. 

It is important to appreciate that the use of this approach is still at an early stage of development. It is not yet certain what factors other than pain can influence facial expression in many species, and sedation persisting after anaesthesia appears to have significant effects in increasing grimace scores. It is also clear that different strains of mice have varying grimace scores when pain-free and varying degrees of response following surgery. Using grimace scoring as the sole means of evaluating animals after surgery may lead both to a failure to detect animals in need of analgesic treatment, and administration of analgesics to animals that do not require them. However, when used in combination with other measures, grimace scoring is likely to prove valuable. Grimace scores may also be a method of pain assessment that can be applied following a wide range of different surgical procedures. 

Other behavioural assessments

Unalleviated post-operative pain is likely to inhibit performance of a range of normal behaviours, and some highly motivated behaviours may be sensitive measures of the efficacy of analgesic use. Use of these measures is also at an early stage, but some studies suggest that nest-building in mice, or the time to commence nest building, may be useful assessments. Similarly, burrowing behaviour has been shown to be both a measure of general welfare in rats and mice and also, in rats, to be reduced in frequency by pain, with this effect reversed by analgesics. Burrowing may also be a useful measure of post-operative pain in mice. As with the other measures described above, a number of factors other than pain may influence both nest-building and burrowing, and in mice considerable strain variation occurs. However, it is relatively easy to provide these assessments in rodents, by including nesting material and burrowing opportunities within their cages. For burrowing, a water-bottle filled with food pellets provides an easy means of assessment in mice. Although more sophisticated means of using these assessments are likely to be developed, at present they can form a useful overall assessment of animals following surgery. For example, if a mouse was forming a qood quality nest pre-operatively and is failing to do so post-surgery, this should trigger careful evaluation. Similarly, a reduction in burrowing activity should give cause for concern. 

General assessments of behaviour

Although detailed well-controlled studies are lacking in most animals, a number of behavioural changes may occur following surgery, and some of these may be pain related. The general types of changes that may be assessed are described below:

Activity

As mentioned above, the overall level of activity of an animal suffering pain is often reduced, and most laboratory species will tend to remain motionless in a corner of their cage. Occasionally, an animal may show unusual restlessness and may seem unable to relax. When the animal moves, its posture or gait may be altered. This is most obviously seen when limb pain is present but is often noted following laparotomy, when the back may be arched to reduce tension on the abdominal muscles. This altered posture, coupled with a tendency to shorten the length of each stride, can be seen both in rodents and rabbits and also in dogs, cats and farm animals. Pain from an abdominal incision may also affect the frequency of urination and defaecation in species in which this process requires marked abdominal muscle contraction. Particular behaviours such as climbing, rearing up onto the hindlimbs, stretching and scratching may also be affected, but careful observation by an experienced assessor may be necessary before such changes are noticed. A further complication in assessing behaviour is that the animal may change its responses in the presence of an unfamiliar observer. In addition, some species are nocturnal, and observation of normal behaviour will require attendance during the dark phase of its photoperiod. Both of these problems can be solved to some extent by using video cameras to monitor the animal’s behaviour.

Appearance

Even when at rest, the animal’s overall appearance may be altered. The animal may adopt a hunched-up posture and position itself in a corner of its cage or pen. Pain may result in a reduction in grooming activity, which leads to the development of an unkempt appearance of the coat and soiling of the anus. Lack of grooming may also lead to the build-up of an encrusted discharge around the eyes, nose and mouth. Rats may develop dark encrustations around the eyes or nose. This material is porphyrin excreted from the Harderian glands, and if wiped with moist cotton wool, it has a red colour. The presence of porphyrin staining is a non-specific stress response but should alert the observer to the possibility that the stress involved may be pain.

Temperament

Changes in temperament often occur in animals experiencing pain. Previously tractable animals may become uncharacteristically aggressive and may bite or scratch. Alternatively, a previously active animal which showed obvious interest in its handler may appear completely apathetic. The animal may cower away from the handler and attempt to avoid being restrained. The interpretation of any of these types of behaviour will require not only a knowledge of the normal predicted behaviour of an animal of that particular age, sex and species but also prior knowledge of the normal behaviour of that particular individual. Clearly, close liaison with animal care staff is essential in attempting to assess the behaviour of an animal in the post-operative period.

Vocalisations

Acute pain can make an animal cry out, and handling an animal which is in pain may provoke such a response. The pitch of the cry may be abnormal and may be accompanied by attempts to bite the handler or to escape. Animals in pain rarely cry continuously, although on occasions dogs may howl or whimper for long periods and sheep and cattle may also make prolonged vocalization. When assessing pain in rodents, it is important to appreciate that many of their cries are at high sound frequencies which are inaudible to humans.

Feeding behaviour

Food and water intake are often markedly reduced if an animal is in pain. Severe pain is often associated with a complete cessation of eating and drinking. These changes in feeding may go unnoticed if the animal is fed ad libitum from a hopper, or if other animals that are feeding normally are present in the cage or pen. A reduction in body weight as a consequence of this inappetence can usually be readily detected, but normal day-to-day variations in body weight must also be appreciated. To improve the detection of changes in food and water intake, weighed quantities of food and water should be dispensed and daily intake measured. Weighing the food hopper and the water bottle provides a satisfactory means of monitoring intake in larger rodents. Care must be taken that spillage of food by the animal does not result in intake being erroneously assessed as normal. In addition to recording food consumption, the animal should be weighed each day to determine any changes in body weight.

A reduction in food and water intake will also be reflected in a reduction in faecal and urine output, but the latter may be difficult to detect. The onset of dehydration will be reflected in the clinical appearance of the animal. Loss of skin tone will cause it to tent and tend to remain elevated when a fold is twisted between the fingers.

Alterations in physiological variables

Pain generally causes changes in the respiration pattern and rate. This can be dramatic following thoracic surgery when the reduction in the depth of respiration can cause considerable concern. In other instances, the change may be less obvious and masked by the normal tendency of animals such as rodents or rabbits to respond to restraint or close observation with an increase in their respiratory rate. Pain may also affect the cardiovascular system. Frequently, the heart rate is increased, but the natural responses to handling may mask these changes. The other factors influencing these cardiorespiratory variables may render them of little use for routine assessment of post-operative pain.

Severe pain may cause the development of circulatory failure (shock), with blanching and chilling of the extremities and a decrease in the strength of the peripheral pulse.

A practical approach to pain assessment

Although well-validated, quantitative methods of post-operative pain assessment have yet to be developed for all species, virtually all studies of post-operative pain in animals have demonstrated a beneficial effect of analgesic therapy. It is not unreasonable, then, to suggest that most animals require some analgesics post-operatively.

As a general approach, it is recommended that attempts are made to assess pain using a modification of the Morton and Griffith scheme, using a range of variables to develop a scoring scheme adapted to the specific type of surgery and animal species concerned. The selection of variables should be made after observing a small number of animals following the particular surgery, to determining which variables are most affected by the procedure. The assessment may include the general types of changes outlined above and should incorporate specific pain-related behaviours when appropriate. These measures can be expanded to include the use of facial expression and behaviours such as nest-building and burrowing. 

Many of the changes that will be observed are primarily indicators of ‘abnormality’ and are not necessarily indicative of pain – they could be caused by the general response to surgery or anaesthesia, or could be the result of dehydration, hypothermia or other factors. However, noting a positive response to analgesic therapy helps indicate which measures can be useful, but remember that some analgesics (e.g. opioids) can alter behaviour. 

Developing a scoring scheme takes time and effort, and it is important to involve all of those who may carry out the assessments so that a standardised approach can be developed. If several members of staff assess the same animal together, then grading of the degree of abnormality will become more consistent. This approach is particularly valuable when different staff are involved in assessments on different occasions. Applying a scoring scheme, once it has been developed, also takes time, and the staffing resources needed should be included in the general infrastructure of the facility or the specific budget for each research project. 

Irrespective of whether a formal scoring system is used or not, pain assessment, whether based on formal scoring systems or more subjective evaluations will be facilitated by:

  • Good knowledge of the species-specific behaviours of the animal being assessed.
  • A knowledge and comparison of the individual animal’s behaviour and appearance before and after the onset of pain (e.g. pre- and post-operatively).
  • The use of palpation or manipulation of the affected area and assessment of the responses obtained.
  • Examination of the level of function of the affected area, for example, leg use following injury or limb surgery, together with a knowledge of any mechanical interference with function.
  • The use of analgesic regimens or dose rates that have been shown to be effective in controlled clinical studies, and evaluation of the changes in behaviour this brings about.
  • Knowledge of the non-specific effects of any analgesic, anaesthetic or other drugs that have been administered.

Next Article : Analgesics used for pain relief

Updated on 17th May 2020

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