Does it happen to you, that a client says: »No, my pet is not in pain. It never cries.« I patiently explain to them, that animals don’t complain as we do. That their expression of pain is often subtle and difficult to perceive. Accurately recognizing the pain of animals is challenging, but of the utmost importance for effective pain relief. Successful recognition and management of pain are gaining more attention and importance in veterinary medicine and results in improved patient welfare, lower incidence of chronic refractory pain and greater owner satisfaction.
Acute pain is generally associated with tissue damage as a result of a traumatic, surgical, medical or infectious event. It rapidly changes the animal’s behaviour to avoid or minimize further damage and optimize the healing process. It is usually self-limiting and stops when the healing is completed.
Objective measurements as heart and respiratory rates, arterial blood pressure, plasma cortisol and catecholamine levels can be used in the assessment of acute pain, but these changes can be affected by stress, fear and anaesthetic drugs. Therefore, the most accurate method for pain recognition in animals is not by physiological parameters, but by observation of behaviour. Pain can be expressed through the presence of a new or abnormal behaviour or disappearance of a normal one.
Many behavioural changes may be associated with pain: changes in body position, shaking, licking the painful area, lameness, reluctance to move, hiding, altered facial expression, slow breathing or tachypnoea, decreased appetite, depression, aggression, vocalization, touch avoidance, altered interaction with people and more. Reflexes (skin twitch or limb withdrawal) are not enough to imply the sensation of pain if they are not accompanied by voluntary behaviour (turning the head towards the testing site, vocalization …). It is helpful to know the patient’s normal behaviour and temperament to more easily recognize any deviations from it, although the presence of strangers, other animals, sedatives or stress may inhibit the expression of normal behaviour.
It is important to differentiate pain from fear and anxiety or other sources of discomfort. Be careful when the animal is already on analgesics that also produce sedation. Sometimes it can be difficult to determine if the animal is pain-free and sleeping or is deeply sedated but still in pain. Do not awake them to assess the pain – rest and sleep are signs of comfort, but be sure that the animal is resting in a normal, relaxed position. Other clinical conditions that may interfere with pain assessment, and should be ruled out, can be discomfort from bladder distension, placement of Elizabethan collars, placement of bandages, urge to defecate, hyperthermia, etc. Dysphoria should be considered when panting, nausea, vomiting or vocalization occur immediately after opioid administration or with restlessness and continuous activity in early the postoperative period (20-30 min).
Pain assessment should start with observing the patient in the cage/kennel/bed (posture, movement, facial expression) without interaction, then while interacting with a caregiver. In the end, palpate the patient’s painful (surgical) area if possible, and observe its response. It is recommended to adopt a specific protocol and use it in every animal in a consistent manner.
The patient should be regularly re-evaluated to ensure effective treatment. The interval depends on the procedure, expected duration of chosen analgesic and previous pain score. Ideally, the assessment should be performed every 15 minutes during the first hour after surgery, then hourly for the first 4 to 6 hours or more frequent, if the patient is not comfortable. After 6 hours, the frequency can be reduced in stable patients. It is wise to have always the same team member to assess the patient through the evaluation period.
Pain measurement tools – pain scales
There are many pain assessment tools, designed for the use in humans, but only a few have been adapted for the use in veterinary medicine. Most veterinary scales have been designed to assess acute somatic postoperative pain in a specific animal population and may not be as reliable if applied to other types of pain (visceral or chronic) or patient population (dogs vs. cats). There is no “gold standard” for assessing acute pain in animals – these methods are subjective, prone to error from underestimating or overestimating pain and show some variability when different observers use the same scale. None of the existing scoring systems is perfect and a full clinical examination and wound palpation are of paramount importance in the assessment of pain in small animal patients.
Unidimensional scales are simple tools, where the observer records a subjective score for pain intensity, based on observation and interaction with the animal. There are 3 types of unidimensional scales that have been described for the use in dogs.
- The Simple Descriptive Scale (SDS) is the most basic form of the pain scale. It generally includes 4 or 5 categories to choose from, e.g. no pain, mild, moderate and severe pain. It does not describe its categories and is not sensitive enough to detect small changes in pain behaviour.
- The Numerical Rating Scale (NRS) is used to numerically evaluate the pain intensity, e.g. 0 = no pain, 10 = maximum pain possible. It has better accuracy with more levels (10 instead of 4).
- Visual Analogue Scale (VAS) is a horizontal line, on which the observer places a mark to indicate the amount of pain. The left side of the line means “no pain” and right “maximum pain possible”. The measured distance between the left end of the line and the mark is the pain score.
Multidimensional composite pain scales include assessment of multidimensional pain experience and represent a significant improvement upon unidimensional ones. They are easy to use and include interactive components and behavioural categories. Their important objective is to determine an intervention level, at which a patient requires (additional) analgesia.
- The University of Melbourne Pain scale (UMPS) was developed as a multidimensional NRS for assessing dogs postoperatively. It combines physiologic data and behaviour responses, divided into 6 categories: physiological variables, response to palpation, activity, mental status, posture and vocalization.
- There are 2 versions of the Colorado State University Pain Scale – one to assess acute pain in dogs and other in cats. The observer selects the most appropriate description for behavioural and psychological signs and response to palpation. Additionally, body tension is assessed on an SDS. The scale gives the evaluation of pain from 0 to 4. The patient’s analgesia plan should be reviewed if the score is ≥ 2. These scales have not been officially validated.
- The original Glasgow Composite Measure Pain Score (GCMPS) was designed based on the human pain scoring instrument – McGill Pain Questionnaire. Its shorter form (CMPS-SF) was designed for quick and reliable assessment of acute pain in dogs in a clinical setting and is most widely used. It includes 30 numerically ranked descriptors inside 6 behavioural categories. The overall pain score is the sum of chosen descriptors and additional analgesia is required if the score is 6/24 or 5/20 (when mobility is impossible to assess).
- UNESP-Botucatu Multidimensional Composite Pain Scale is the first validated pain scale for postoperative pain in cats and has 10 items, distributed in 3 subscales. Each item can be rated from 0 to 3. The first subscale assesses psychomotor changes, the second relates to pain expressions and the third includes physiological variables. Analgesic intervention is recommended when the score is > 7/30.
Duke-Novakovski T., de Vries M., Seymour C.: BSAVA Manual of Canine and Feline Anaesthesia and Analgesia, 3rd Ed. BSAVA, 2016
WSAVA Guidelines for Recognition, Assessment and Treatment of Pain, 2014