Recognizing pain is the cornerstone of effective pain management in veterinary practice. Recognizing pain in animals is not intuitive, particularly by individuals unfamiliar with normal behavior for a species or individual.

Numerous factors complicate the evaluation of pain in animals. Any pain scale should consider the following characteristics: species, breed, environment and rearing conditions, age, gender, cause of pain (eg, trauma, surgery, pathology), body region affected (eg, abdominal pain, musculoskeletal pain), character of pain state (eg, acute, chronic), and pain intensity. Differences in pain tolerance have been demonstrated experimentally in people and animals, and play an important role in the clinical management of pain.

Definition

• Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage^[1]. Pain is perceived first by sensory nerves and experienced as distress by the limbic system of the brain. Pain terms are very variable and people may talk of acute or chronic pain, or sharp or dull pain, for example. Pain is NOT solely physical NOR psychological, it is both. Pain is divided into two broad categories:

(1) sensory-discriminative, which indicates the site of origin and the stimulus giving rise to the pain

(2) motivational-affective in which the severity of the stimulus is perceived and the animal’s response is determined^[2].

• Anxiety – an emotional state involving increased arousal and alertness prompted by an unknown danger that may be present in the immediate environment^[3]. Unlike pain, anxiety is a diffuse sensation that has no specific location in the body. Unlike pain investigators, scientists who study anxiety have not developed a code of conduct to limit the extent of anxiety an animal may experience.
• Fear – an emotional state involving increased arousal and alertness prompted by an experienced or known danger present in the immediate environment.
• Distress – a state in which the organism is unable to escape from acute stressors or adapt to an altered external or internal environment. In acute distress, the organism will try to escape but in chronic distress, the organism will commonly engage in maladaptive (e.g. learned helplessness) behaviors^[4].
• Suffering – a highly unpleasant emotional response usually associated with pain and/or distress^[5].

The USDA guideline on pain in animals states that if one has reason to believe that a stimulus would be painful to humans, it should also be regarded as painful to animals.

Measurement

No “gold standard” exists to measure pain in animals or to compare one type of scale or measurement instrument with another. All of the pain scales used in animals rely on the recognition and/or interpretation of some behavior and are subject to some degree of variability among observers. Pain scales that are based on the determination of the presence or absence of species-specific behaviors, and that minimize the interpretation of those behaviours, are likely to be more accurate than generic scales that rely heavily on subjective assessment and interpretation. All current methods used to measure pain in animals are prone to errors of under- or overestimation. Even if the amount of pain is correctly estimated, determining how well the individual animal is coping with the pain may be difficult. This is particularly true if the animal is removed from its normal environment. Finally, all current methods assess the effects of physical pain; none has been designed to evaluate mental or psychological dimensions of pain that an animal may experience.

Physiologic parameters (eg, changes in heart rate, respiratory rate, arterial blood pressure, pupil dilation) may be used to assess responses to an acute noxious (painful) stimulus, particularly during anesthesia, and to assess pain in some clinical situations (eg, horses with acute colic pain). However, physiologic measurements often do not differentiate between animals that have undergone surgery and are experiencing pain and those that did not undergo surgery. Likewise, animals experiencing chronic pain may have normal physiologic parameters. Lack of change in physiologic responses should not be construed to mean there is no pain if other clinical signs suggest otherwise. Physiologic parameters are not specific enough to differentiate pain from other stressors such as anxiety, fear, or physiologic responses to metabolic conditions (eg, anaemia).

Unfamiliarity with normal behaviors typical of a particular species or breed makes recognition of pain-induced behaviors difficult or impossible. Behavioral changes indicative of pain may be too subtle or take too long to recognize under routine clinical situations in both large and small animals. Sporadic observation of animal behavior may not reveal signs of pain. Except in the most severe circumstances, signs of pain may be “masked” by behaviour that is stereotypical of the species being observed. For instance, dogs may wag their tails and greet observers in spite of being in pain. Flock animals, such as sheep, may be startled when an observer approaches and attempt to conceal any signs of pain by staying bunched up with the rest of the flock. Behavioral changes indicating pain may not be what we expect. A cat sitting quietly in the back of the cage after surgery may be painful; however, pain would not be recognized if the caregiver expects to see more active signs of pain such as pacing, agitation, or vocalization.

In general, responses to acute surgical and traumatic pain are likely to be more marked and readily recognizable than clinical signs associated with chronic pain. Clinical criteria used to assess chronic pain, eg, lack of activity, lack of grooming, decreased appetite, weight loss, are not specific signs of pain and point only to an underlying problem in need of further diagnosis. Evaluating the degree of lameness of affected limb(s) is often used to assess chronic orthopedic pain. For socialized animals, observations of owners are essential to detect more subtle signs of chronic pain such as changes in attitude or interaction with family members. Response to therapy, such as increased activity after administering an NSAID, may provide important information regarding the role that pain has played in behavioral changes.

Cancer pain may have components of acute pain (eg, expansion of a tumor or secondary responses to surgical, radiation, or chemotherapy treatment) and components of chronic pain. Thus, assessment of cancer pain requires the caregiver to use methods that can detect behavioral changes associated with both acute and chronic pain.

References

1. ↑ IASP definition
2. ↑ Kitchell RL, Erickson NH, Carstens E, et al (1983) Animal pain: perception and alleviation. Bethesda: American Physiological Society
3. ↑ Kitchen, H., et al (1987) Panel report on the Colloquium on recognition and alleviation of animal pain and distress. Journal of the American Veterinary Medical Association 191:1186-1191
4. ↑ ILAR. (1992) Recognition and Alleviation of Pain and Distress in Laboratory Animals. Washington, DC; National Academy of Sciences, p:137
5. ↑ Mellor, D. J. & Reid, C.S.W. (1994) Concepts of animal well-being and predicting the impact of procedures on experimental animals. In Improving the Well-being of Animals in the Research Environment, editors R.M. Baker, D. Jenkin and D.J. Mellor, pp. 3-18. Glen Osmond, South Australia: ANZCCART.