Assessment of pain in dogs and cats

The treatment of pain in animals must nowadays be considered one of the most important duties of every veterinarian, first from an ethical point of view and then from a professional one. The Association of American Veterinary Medical Colleges, outlining the qualities that a veterinarian should have (Willis et al 2007), listed the following: compassion, expertise, humaneness, judgment, care andunderstanding. All of these qualities are important in attempting to reduce the exposure to pain of each animal entrusted to us. According to the International Association for the Study of Pain (IASP): “the inability to communicate in no way negates the possibility that an individual is experiencing pain and is in need of appropriate pain-relieving treatment”. This statement is very important in veterinary medicine because it emphasises the need to treat pain even in patients not able to communicate verbally.

Nevertheless, pain should be treated in pets not only for ethical reasons but also because it can affect the animal’s normal behaviour (reduced intake of water and food, aggressiveness, depression, changes in urination and defecation, changes in relationships with humans and with other animals). It also decreases respiratory movements, promoting pulmonary atelectasis, decreases mobility, facilitating ankylosis and muscular atrophy, and reduces gastrointestinal activity. Pain increases sympathetic nervous system activity, promoting the release of catecholamines, which predisposes to the onset of tachycardia and hypertension and alters the normal perfusion of organs, centralising the blood flow. This causes continuous stress to the cardiovascular system. Pain, especially if persistent, alters normal metabolism and neuroendocrine activity. The endocrine effects of pain and stress are well known. The secretion of cortisol and glucagon promotes a catabolic state, delaying or impairing the body’s healing processes; in addition, the mobilisation of fatty acids from the reserves causes a progressive decline in general conditions.

PATHOPHYSIOLOGY OF PAIN

Before continuing with the discussion of pain assessment, it is worth emphasising that the first step when dealing with pain is to characterize its pathophysiogy. It is, therefore, important to clarify that assessing all types of pain with the rating scales normally reported in veterinary medicine would be harmful for the patient and frustrating for the clinician. Three types of pain are currently recognized: acute pain, persistent pain and chronic pain. Acute pain is nociceptive pain, of brief duration and with an obvious cause/effect relationship. It ends when the application of the nociceptive stimulus stops or the damage responsible for the stimulus is repaired. Post-operative pain, renal and biliary colic and traumatic pain are examples. A basic characteristic of acute pain is that it responds to antinociceptive measures: this characteristic is shared by persistent pain but not by chronic pain. Persistent pain is caused by the permanence of the nociceptivestimulus or of the mechanisms that sustain neuropathic pain. As in acute pain, there is usually a positive response to analgesics and to other antinociceptive measures or to drugs for neuropathic pain, respectively. Chronic pain is ordinarilydefined based on a time criterion; some define pain that lasts longer than 6 months as “chronic”. In reality, modern algology rejects this definition. Indeed, the pain from hip arthritis, for example, can persist for months or years, maintaining its characteristics of acute pain, without becoming chronic, and in this case the definition of persistent pain is appropriate; pain caused by a bone fracture, on the other hand, which persists for more than a few weeks, in other words after the fracture has healed, can already be chronic before the 6 months of the previous definition. So it should be clarified that pain, to be considered chronic, must have the following three characteristics: first, the lack of a cause-effect relationship between nociception (or better “algogenic injury”) and the pain; second, there must be a relationship between the pain and the plastic modifications of the central nervous system induced by the nociception; third, there is usually a relationship between the pain and the changes in the patient’s character and lifestyle. Thus, pain must first of all be properly contextualised, then the appropriate methods for assessing its extent must be used and then it can be treated in a logical sequence.

All animals feel pain, but not all of them show it in the same way. Manifestations of pain not only vary between species, age and breed, for example, but there is also interindividual variability and the fact that different types of pain can cause different clinical manifestations should be kept in mind. Dogs and cats show signs of pain, but often in different ways. Dogs are more inclined to let their owners know when they are in pain, while cats more often tend to isolate themselves. However, the lack of obvious signs cannot automatically exclude that an animal is in pain or is undergoing negative consequences resulting from pain. Even the evolution of the pain following surgery varies from subject to subject; some animals already appear pain-free in the first few hours after surgery, while others experience pain for a long time. Thus, from the point of view of post-operative pain-relieving treatment, the same procedure in two different subjects might require completely different treatments, both in terms of dosage and in terms of active substance, method and frequency of administration. The individual response to a surgical insult and to analgesic drugs will profoundly condition the variability of the post-operative pain even in the veterinary patient. Part of this variability also depends on the possible presence of several types of pain at the same time, in the same subject. For example, an old dog with osteoarthritis operated on to remove a mass might have musculoskeletal pain caused by the position on the operating table, as well as pain caused by the surgery per se. Likewise, a dog undergoing a hemilaminectomy to remove a chronic herniated disc may have both nociceptive and non-nociceptive pain.

Behavioural evaluation is an integral part of the clinical assessment of each animal. Knowing an animal’s normal behaviour and discovering any peculiarities are essential for treating the pain and implementing the appropriate analgesic strategy. The information provided by the owner in this regard is essential for identifying unusual behaviours that could be explained as possible signs of pain. These can be lack of expected reactions or development of unusual behaviours; nevertheless, such situations can go unobserved or be excessively emphasised by both the owner and the veterinary team. A systematic and holistic approach, that does not consider just the animal but also the context in which it is integrated, is essential in order to recognize changes in behaviour and in physiological parameters. Respiratory frequency and heart rate, systemic arterial pressure and temperature are values that rise during painful states, as well as in many other situations. However two main factors make the evaluation of the above parameters an unreliable strategy for assessing pain: one is stress and the other is represented by the drugs administered to animals during the perioperative period. Obviously, animals perceive veterinary facilities as hostile environments, which cause anxiety and, thereby, increase sympathetic tone, altering cardiorespiratory values, with or without the contribution of pain. Not only, stress also induces behaviours that can overlap with those caused by pain. On the other hand, anaesthetic drugs profoundly modify the sympathetic system, once again causing alterations that have little to do with pain.

Another significant problem, once the suspicion that the patient might be in pain has arisen, is to be able to quantify it and consequently establish the most appropriate treatment. It is not always easy to “weigh” the manifestations of pain and make therapeutic decisions appropriate to the situation. The most effective response to this series of problems is provided by pain rating scales. Pain rating scales are instruments with the purpose of providing an objective measure of the extent of pain by evaluating various parameters and behaviours, reaching a decision on the need to administer a pain-relieving treatment. To get around the difficulties described above and thus treat pain effectively, these scales should be used in clinical practice. In light of the above, an assessment using measurement scales designed specifically for this purpose is essential to be able to offer the patient suitable pain control. The systematic detection of pain aids proper administration of analgesics, reduces the sensation of pain experienced by the small patient and contributes to its satisfaction with regards to the treatments received, improving the therapeutic outcomes and reducing the appearance of disturbances/side effects associated with the drugs. In contrast, untreated pain can worsen, giving rise to complications that make the patient’s clinical picture worse. Pain assessment in dogs and cats is still under discussion: there are several pain rating scales; nevertheless, very often these systems are not properly tested and validated and thus have limited applicability. Obviously, the biggest problem in assessing pain in animals is that the patients are not able to communicate verbally.

GENERAL RECOMMENDATIONS REGARDING PAIN ASSESSMENT

The principle that must guide all operators who take an interest in pain is that any being that is in pain deserves a prompt diagnosis and effective treatment. To ensure this, pain assessment must be a routine practice, repeated frequently enough even in a short lapse of time and must be documented. In subjects that cannot communicate verbally, pain must not be assessed with just one method (only observation of behaviour or stimulation of the intervention site). The following indications should be followed:

• Look for the potential causes of pain. Pathological conditions known to be able to cause pain, recent surgical procedures and a medical history of persistent or chronic pain should in any case promote an assessment of pain, even when there are no changes in behaviour.  On the other hand, changes in behaviour can help to exclude ongoing states of pain. When the above-mentioned conditions occur, we must assume that pain is present and look for elements to exclude it rather than the opposite. When legitimate doubt exists, it can be appropriate to administer a painkiller and monitor the effect. With regards to correct “preventive analgesia”, all subjects that undergo pain-causing procedures should receive analgesics before the procedure.
• Behaviour. The observation of behaviour provides an enormous amount of information on a patient's state of pain. However, the extent of changes in behaviour is not always proportional to the intensity of the pain; furthermore, the origin of these changes is often not the pain but rather physiological or mental stress. In this context it is important to underline that breed and interindividual differences play an important role. Some breeds manifest signs of anxiety in a very obvious way (Dalmatian, Weimaraner, Dobermann, etc.), while others tend to mask any reaction to states of discomfort (Setter, Breton, etc.). Obviously, there is wide interindividual variability within each breed.
• History. When the information obtained from the owners is considered reliable, it can be a precious resource both for diagnosis and as feedback during the therapeutic procedure.
• Analgesic trial. The administration of analgesics may be an integral part of the diagnostic procedure, when there are well-grounded reasons to believe that the subject being examined is suffering from ongoing pain. Two aspects should always be kept in mind when performing an analgesia trial. The first is that the trial must start with well-known drugs of known efficacy, such as non-steroidal anti-inflammatory drugs and opioids, before using drugs whose fields of use are notoriously narrower and whose side effects are potentially more severe. The second aspect to keep in mind is that many analgesic drugs, opioids above all, have a wide range of efficacy. This implies that in trials in which drugs such as opioids are administered, the drug must be given at increasing doses until the desired analgesic effect is achieved or side effects begin to appear.

Pain rating scales are among the most useful instruments for measuring pain in animals and for deciding whether or not the subject should be treated. The main aim of pain scales is that of providing a method for objective assessment of pain. Pain must be ascertained by observing the patient left undisturbed and then its interactions with people and surrounding objects. Scales based simply on the clinician’s opinion, and thus on subjective elements of opinion, should be viewed critically. On the other hand, it is common opinion that physiological indicators (heart rate, respiratory frequency and blood pressure) are not sensitive enough to distinguish the presence of pain from other states of distress such as anxiety. The absence of changes in vital signs does not exclude the presence of pain. Another aspect to be noted is that these scales can be an important source of documentation for monitoring pain (and thus analgesia); their use should be scheduled at regular intervals, which can be as short or long as necessary. It cannot, however, be denied that the use of pain rating scales in the daily clinical practice of a veterinary facility is not without problems. Several pain assessment systems have been proposed:

• Physiological parameters. It is well known that pain induces immediate activation of the sympathetic system and a neuroendocrine response of slower onset and longer duration mediated by activation of the hypothalamic-pituitary-adrenal axis. This response can be recognized clinically and through laboratory tests: there is an increase of heart rate, blood pressure, circulating catecholamines and blood cortisol levels. Other possible altered physiological parameters are pupil diameter and respiratory frequency. Changes in physiological parameters are not, however, specific to pain, since they simply indicate the presence of stress. Just the fact of being admitted to hospital or the presence of the veterinarian can activate a stress response.
• Subjective evaluation by the operator using a visual analogue scale (VAS). Transposed from human medicine, in which it plays an important role, the VAS consists of a 10 cm line whose end-points are “No pain” and “Unbearable pain”. In human medicine, the patient puts a vertical line at the point along a horizontal line that best reflects his/her perception of the pain. The system is interesting because it allows the patient to subjectively express the perception of pain. Unfortunately, in veterinary medicine the VAS cannot be used by the patient, so the observer chooses where to put the vertical line, attempting to estimate, based on his/her own experience, the pain perceived by the animal. The result of this "interpretation” is that the repeatability of the evaluation is low, both between different operators, but also by the same operator at different times. In spite of this, and in spite of the existence of more sensitive and specific systems, the VAS is still widely used in veterinary medicine.
• Simple descriptive scales: composed of four definitions, usually “No pain”, “Slight pain”, “Moderate pain” and “Intense pain”. Based on his/her own experience, the operator decides the intensity of the pain. The repeatability is moderate, as for the VAS, and the subjective evaluation by the operator strongly affects the result.
• Numerical scales composed of multiple items that assign scores based on the presence/absence/alteration of the item considered.The total score is then added up and analgesics are administered when the score exceeds a certain threshold (established a priori or based on a previous assessment of the same subject). The system devised and used at the University of Colorado is an example.
• Numerical scales that describe the characteristics of the patient in detail based on the intensity of the pain.A typical example is the scale drawn up by Mathews which can, nevertheless, be difficult to use because of very subtle nuances between one class and another.
• Considering that pain affects the patient’s behaviour to a considerable degree, some authors have devised assessment scales based on the identification of the presence of behavioural patterns that are specific and sensitive to pain (thus distinguishing, for the first time, between acute and chronic pain, which induce different behavioural alterations) or the absence of behaviours considered normal. A typical example is the evaluation system for acute pain in dogs proposed by the Universityof Glasgow, whose efficacy was scientifically validated and which makes it possible to identify acute pain in dogs specifically and sensitively and also quite quickly. The composite evaluation form can be downloaded free of charge from the website of the Faculty of Veterinary Medicine of the University of Glasgow.

These systems were designed to assess acute pain in dogs. In some cases they have been adapted for use in cats, but often this translation has not been supported scientifically. The Glasgow research group is developing specific assessment systems for cats, a species that is notoriously difficult to evaluate.

PAIN ASSESSMENT IN DOGS

The assessment of pain in this species, although complex, is less frustrating than in other species. Dogs are social animals and thus have developed complex social behaviours that make them inclined to communicate with other members of their group (including their owners) or with animals outside the group (veterinarian). Their nature is also aided by being at the top of the food chain; since they have few natural enemies, they have not developed behaviours aimed at concealing a state of suffering, but quite the opposite: the manifestation of discomfort may allow them to receive help from the group in some situations. Pain generally causes a reduction in the search for interactions between dogs and humans, a decrease in the dog’s desire to come out of its kennel or basket and reduces appetite; it can also trigger aggressive reactions towards other subjects living in the same circle. The good knowledge that humans have about dogs and the social nature of these animals make it possible to notice even minimal variations in behaviour and character which are potentially attributable to pain.

As previously mentioned, dogs, because of their nature, tend to provide information; the difficulty in assessing pain comes from correctly interpreting the signals that are manifested. This is probably the source of many of the problems revolving around the assessment of pain in the canine species. This problem is manifested mainly by the difficulty in distinguishing signs of environmental stress, experienced intensely by a dog hospitalised in a veterinary facility for a brief or long period of time, from manifestations of a painful state. Consequently, it is not always easy to understand when a subject is really in pain or when it is in a state of agitation that can push it to behave very similarly to a subject experiencing pain. In fact, given the same pain stimulus, every subject reacts differently; some dogs appear indifferent to minor pain stimuli, while others might vocalise and react aggressively. Generally, all tend to avoid handling, which they fear could cause pain. Certain subjects seem to forget the pain if distracted by food or in some other way. This suggests, once again, that factors extrinsic to the dog itself may change the dog’s behaviour towards pain even radically.

There are fewer grey areas associated with intense pain and misinterpretation is unlikely. Whimpers, barks, aggressive reactions and attempts to escape are obvious manifestations that are often associated with ongoing or past pathologies. Nevertheless, persistent intense pain can immobilise and depress a dog, discouraging it from eating and drinking and, thereby, cause a decline in the animal’s general physical conditions and dehydration. In fact, very painful conditions in dogs can be characterized by subtle signs, which often require a close observation of behaviour, as well as careful interpretation. It is, therefore, important for pet owners to collaborate with the clinician and to be educated to do so correctly. Obviously not all owners are able or willing to control the emotionalism that the fear of not noticing their dog's suffering normally generates. However, all owners can provide important information. The assessment of pain in dogs and thus also the pain rating scales are based mainly on the observation of the animals’ behaviour, with special attention to particular attitudes, level of activity, posture and locomotion, grooming, vocalisation, as well as the avoidance of certain types of handling or games proposed by the owner.

In general, dogs show three types of behavioural changes in response to pain, depending on whether the pain is acute, subacute (or persistent) or chronic. When experiencing acute pain, dogs usually exhibit reactions of refusal or escape, with the strength of these reactions increasing with the severity of the pain stimulus. Vocalisation may appear in the most severe cases, and sudden episodes of aggression are not infrequent, especially when the pain catches the dog by surprise. Vocalisation can be barking, growling, whimpering or yelping or any combination of them, while an aggressive reaction to palpation of the affected part of the body can be manifested as an attempt to bite or attack animals, humans or other. As the pain becomes more persistent, dogs usually begin to develop a more protective behaviour, in order to minimise any stimulus of the painful area. In this condition, many signs can be indicators of pain. There can be vocalisation or lamentation, which often stops when the dog is comforted. Protective behaviours include frequent changes in body position, to find the position that causes the least discomfort, as well as reluctance to use the sore part of the body, with consequent abnormal or unusual posture or gait. For example, a dog with a sore leg might limp in order to minimise the weight on the affected part, while dogs with abdominal or chest pain might be reluctant to lie down and may remain standing for hours until they become exhausted. Variations in activity level may appear as restlessness, agitation and disturbed sleep or, at the other end of the spectrum, as lethargy, weakness and refusal to move and play. “Self-awareness” behaviours, such as protecting, licking, chewing or looking at the sore area of the body are always reliable indicators of pain.

The question of how to recognize pain in dogs becomes more difficult when chronic pain is involved. This is ordinarily defined as pain that lasts longer than 3 months, but this time criterion is incorrect and misleading. More precisely, chronic pain is pain that is not associated with an obvious external injury, but rather arises from pathological conditions of the nociceptive system and has an impact on the subject’s quality of life. In this case, the behavioural signs of pain are usually subtle and non-specific, with an insidious onset and often go unnoticed by the owner or are mistaken for signs of normal aging. Depression, irritability, decreased appetite, reduced activity, no or decreased grooming, sleep disturbances, lack of reaction to environmental stimuli and poor interaction with owners can all be signs of chronic pain in dogs; in the more advanced phases they culminate in lethargy, weight loss, muscular atrophy and, therefore, in a decline in the animal’s general physical condition. Because of the debilitating nature of chronic pain, it is important for pet owners to learn to recognize pain in dogs in order to avoid the devastating consequences of this syndrome on their pets’ quality of life. It is also important for owners to keep track of any suspicious behavioural changes in their dogs, because these may be apparent at home and yet be masked by misleading behaviours during routine veterinary check-ups.

Other typical signs of chronic pain include a lowered pain threshold and the resulting increase in responses to painful stimuli (hyperalgesia), as well as the occurrence of reactions to stimuli that normally do not cause pain (allodynia). Hyperalgesia and/or allodynia are the result of profound alterations in the nociceptive system at the peripheral or central level. These pain mechanisms may be present in acute pain, as transient states or in non-nociceptive pain even for prolonged periods. Both these alterations of the nociceptive system, if present, cause particular clinical pictures, both because of the location of the body areas involved and because of the peculiar pain characteristics manifested by dogs.One of the salient characteristics of these conditions is that they are not resolved by using non-steroidal anti-inflammatory drugs or opioids. Patients with persistent pain that does not respond to common treatments should be referred to centres where there are people who, through their expertise and experience, are capable of establishing an appropriate treatment.

PAIN ASSESSMENT IN CATS

Cats have physiological particularities that predispose them to the development of complications, which can be serious, if pain is not treated. The fact that inadequate control of pain in cats leads to a considerable reduction in eating is especially dangerous. The limited capacity of cats to accumulate glycogen stores in the liver is responsible for the early and massive lipid mobilisation observed in this species after 2-3 days of fasting (for comparison, lipid mobilisation starts to become significant after 7-9 days in dogs). The result is the rapid onset of hepatic lipidosis (fatty liver), which can impair liver function. Cats also develop progressive depression, which makes the return to an adequate diet unlikely. In case of protracted pain and stress, for example in cats hospitalised for weeks or in intensive care, it is not unusual to witness the onset of progressive anaemia, probably of immune-mediated origin which, in some cases, makes it necessary to administer blood products, with related increases in morbidity and costs of hospitalisation. The impairment in immune system function following stress/chronic pain and anorexia also increases the risk of infectious diseases. It is obvious that the treatment of pain in this species, as in others, is justified not only by ethical reasons, but also by a possible impact on morbidity and peri-operative mortality and, last but not least, by economic factors.

The most simplistic approach to treating pain in cats is the anthropomorphic approach. Considering the similarity between the nervous systems of cats and humans and the notable behavioural complexity of cats, it is assumed that procedures that cause pain in humans also cause pain in cats and that the intensity of the pain following comparable procedures is similar in the two species. Consequently, considering the physiological similarity and the similar response to the majority of drugs, treating pain with similar protocols is justified. The advantage of this approach is that the presence of pain, following some procedures, is taken for granted and that treatment is initiated. The enormous disadvantage lies in the fact that many of the assertions on which this approach is based are actually approximate, in the best of cases. There is no doubt that cats feel pain following procedures which are painful in humans and there is probably an excellent degree of correlation between the intensity of the pain in the two species. The problem consists of the fact that, while this probably holds true with a good degree of approximation for acute pain, the same cannot be said for chronic pain. Other factors intervene in chronic pain, including psychological factors and interactions with the environment and other cats. Nevertheless, even when this approach is limited to the treatment of acute pain, major errors can still be committed when similar analgesic protocols are used in the two species. A typical example is the use of paracetamol, which is extensively used in human medicine to treat moderate pain, often in combination with opioids, but is certainly not indicated as the first choice in cats. Furthermore, the administration of drugs based on pharmacokinetics established in other species creates at least two other risks: inefficacy, with consequent peaks of pain followed by analgesia, and overdosage, which can cause side effects such as excessive sedation, respiratory depression or anorexia.

Attempts to ascertain the presence and measure the intensity of pain through the evaluation of physiological parameters, such as heart rate, respiratory frequency, blood pressure, temperature, production of urine and faeces, are unsuccessful. Not even the measurement of markers such as cortisol and catecholamines is specific, since in all cases the measured variables are associated with conditions of stress rather than of pain. In the case of cats, simple hospitalisation in an unfamiliar and sometimes hostile environment (presence of strange dogs and people) can cause substantial changes in these parameters.

A pain assessment system that has been studied fairly well in cats is variation in the response to pressure and thermal stimuli in the area affected by the trauma. Using systems designed for this purpose, progressive pressure is exerted (with an inflatable cuff) or the temperature is increased locally (using heater elements) above the traumatised part. The thermal or pressure threshold at which the cat responds, usually by moving away from the stimulus, is then recorded. Normal thermal and pressure values for cats are known and can, therefore, be used for comparisons. Alternatively, a distant region of the body can serve as control; however, it has been hypothesised that lowering of the thermal and pressure threshold might not be limited exclusively to the traumatised part, especially in cases of chronic pain. This system, although quite objective and exploited to compare the efficacy of different analgesic drugs, is not easy to use in clinical practice, not only because of the cost of the equipment, but mainly because cats have to be trained not to react to the presence of the equipment.

The assessment of behaviour appears to be a more promising method, even if cats have some peculiarities which, in some cases, make it difficult to assess pain. Behavioural assessment is based on the identification of behaviours or behavioural patterns whose expression is closely associated with the presence of pain or, conversely, on the absence of normal behavioural patterns. An essential condition for being able to evaluate changes in a cat’s behaviour is that the environment is suitable. Cats do not exhibit normal behaviours in a hostile environment and will attempt to mask any signs of pain. In addition, such an environment will cause a significant stress response, which is a physiological mechanism of adaptation to a hostile environment. Cats should be hospitalised separately from dogs, in a specific environment. The cage, of suitable dimensions, should be able to accommodate a litter box and a small basket in which the cat can rest, as well as food and water bowls. Obviously, the cage must be large enough so that the food and basket are not too close to the litter box. In cases of prolonged hospitalisation, and when there is enough space, a two-storey cage with some toys enables the cat to exhibit behaviours closer to normal. When assessing cats, it is very important to have detailed information on their behaviour in their habitual environment. The probability of being able to correctly identify behavioural patterns that indicate pain in a hospitalised cat may be higher when a cat’s habitual environment is a home rather than in the case of a semi-wild cat or a particularly aggressive one. Pheromone-based sprays are useful as they can help cats to become familiar with a new environment. In a suitable environment, and in the absence of stress and pain, cats should have a friendly attitude towards operators; for example, they will approach the front of a cage as soon as they notice the presence of a nearby person. When food is offered, they will eat promptly and, if stimulated with toys, they will be dynamic and interactive. Obviously, this does not allow any conclusions to be drawn on the possible behaviour of cats when, for example, an intravenous cannula is placed or a blood sample is taken! In such cases even animals that are not stressed and that are pain free might exhibit intense defensive behaviour although, in the author’s opinion, a lot depends on the operators’ familiarity with this species.

Some behaviours that can be easily identified and that indicate quite specifically that pain is present are the avoidance of using parts of the body (lameness, incorrect postures that alter the balance of weight) and protective behaviours (praying position, crouching posture with elbows wide apart, immobility). Even continuous licking of an area, perhaps at the expense of the rest of the body, should raise the suspicion that pain is present. Unlike in dogs, vocalisation is not a common means of communicating pain in cats. In contrast, facial expressions can be indicative. A cat in pain may have half-closed eyes and a dull gaze. In the case of stress, the eyes tend to be open, with dilated pupils and an alert stare. Stretching the limbs when the abdomen is touched may indicate slight abdominal pain, but can also be a normal behaviour. If the sore part is bothered, cats may growl, hiss or vocalise. In some cases, but not often, a cat may be aggressive when attempts are made to approach or touch it. In the case of moderate pain, cats tend to hide at the back of the cage or under a blanket, avoiding the front, exposed part of the cage. However, this behaviour, as well as taking refuge in the litter tray, can also be induced by stress alone (barking dogs, noisy environment). In general, a cat with moderate to severe pain has a passive attitude and a neglected appearance. Ruffled fur indicates that the fur is not being taken care of normally and is, therefore, an indicator of an abnormal attitude, which could be due to apathy or the impossibility of performing the movements required to carry out the action. Food and water intake is reduced and the cat shows no interest in food, even when tempted with highly appetising foods, and will stay at the back of the cage instead of approaching the food bowl. Cats will also avoid interacting with people and with their owner, if possible. They will refuse to play and, if picked up, may have a completely passive attitude with slack muscle tone, or become aggressive. An attitude assumed in the case of pain to the face or head, but also a sign of deep prostration, is a crouching posture with the head bent 90° on the neck, looking at the floor of the cage. In some cases the cat might even tremble. In the case of extreme pain, cats may become completely apathetic and not react even when the injured part is handled and refuse to eat and drink.

It can be very difficult to evaluate the adequacy of analgesia in an animal awakening from anaesthesia following surgery, distinguishing between the presence of pain and the “normal” manifestations of a very bad awakening. This judgement depends mainly on the experience of the anaesthetist, who will have to weigh the extent of the trauma in relation to the analgesics administered and the cat’s temperament.

In all questionable cases, the best way to exclude the presence of pain is to administer an analgesic. If the cat’s behaviour returns to normal, it is plausible to assume that pain was present.

Although the ability of cats to perceive pain is well known and many of the correlated manifestations have been codified, veterinary medicine has dedicated relatively little attention to the codification of an objective system of pain assessment. Subjective evaluation by the operator is still the standard in this species, even though some research groups are working to adapt objective systems for assessing pain in dogs so that they can be used in the feline species.

Considering the obstacle represented by the subjectivity of pain assessment, certain conditions must be met for correct management of analgesia:

• knowing the cat’s normal behaviour (obtaining the information, if necessary, from the owner when taking the medical history);
• avoiding creating stressful conditions during hospitalisation;
• evaluating the patient’s well-being at regular intervals (for example, every 4-6 hours), even if analgesics with assumed longer duration of action have been administered;
• if possible, getting the opinion of at least two independent operators and arranging that they are always the ones to assess the patient’s condition;
• accepting that the assessment of behaviour requires time – it cannot be done by simply observing the patient from outside the cage for a few seconds;
• observing the patient’s behaviour from afar, then moving close to the cage and assessing the response to this stimulus. Next opening the cage and interacting with the patient before giving an opinion;
• considering the intake of food and liquids, the condition of the fur and the use of the litter in questionable cases; considering that it is unlikely that pain will be manifested by a single behavioural alteration; it is more likely that there will be numerous changes in behaviour;
• if doubt persists, administering a test dose of analgesic and repeating the assessment, giving the drug time to work properly (for example, around 30 minutes after intramuscular administration).

As regards the assessment of acute pain in cats, the system proposed by the 4-a-Vet association is interesting because it assesses the presence of abnormal behaviours, which could be linked to the presence of pain.

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