Recognition and Assessment of Pain in Animals

Pain is a complex, multidimensional experience with sensory and behavior elements associated with actual or potential tissue damage. Recognizing pain in animals is not intuitive, particularly for those unfamiliar with normal species-specific or individual behaviors. In recent years, there has been an increased focus on determining and measuring such specific pain behaviors. These efforts should improve recognition and treatment of pain in animals as validated pain assessment tools are developed.

Numerous factors complicate the evaluation of pain in animals. Veterinary pain scales should consider the following characteristics:

• species

• breed

• environmental rearing conditions

• age

• gender

• origin of pain (eg, trauma, surgery, pathology)

• body region affected (eg, abdominal pain, musculoskeletal pain)

• type of pain (eg, acute, chronic)

• pain intensity

Any pain scale or methodology used for pain assessment must be able to distinguish individual sensitivities. Even when animals score low on pain scales, if pain is considered likely, analgesia should be administered. The most accurate way to determine whether a patient is in pain is to administer a dose of analgesic and monitor the patient's response.

There is no "gold standard" to assess pain in animals. Many scoring methods that include physiologic and behavioral variables have been published, but few have been validated. Most veterinary pain scales rely on the recognition and/or interpretation of some behavior and are subject to some degree of variability among observers. Pain scales based on the presence or absence of species-specific behaviors, which minimize the interpretation of those behaviors, 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 pain may be difficult. This is particularly true if the animal has been removed from its normal environment. Assessment systems must also consider the different types and sources of pain, such as acute versus chronic, visceral pain versus somatic pain, and neuropathic pain.

Physiologic parameters (eg, changes in heart rate, respiratory rate, arterial blood pressure, or 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 signify absence of 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, anemia).

Behavioral changes indicative of pain may be too subtle to detect under routine clinical situations in both large and small animals. With the exception of severe circumstances, clinical signs of pain may be masked by behavior that is stereotypical of the species being observed. For instance, dogs may wag their tails and greet observers despite being in pain. Flock animals, such as sheep, may be startled when an observer approaches and attempt to conceal pain by staying bunched up within 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 in pain; however, pain might not be recognized if the caregiver expects to see more active clinical signs of pain such as pacing, agitation, or vocalization. Because of these difficulties, recent research has focused more on sudden changes in facial expressions.

Grimace scales may be used to assess facial expressions specific to pain in humans and animals based on changing relative distances between facial features and changes in facial shape. They have been developed for mice, rats, rabbits, horses, sheep, lambs, piglets, ferrets, and cats. Along with the evaluation of behavioral changes, facial expressions can reflect the emotional experiences of animals and communicate their experience of pain.

In general, responses to acute surgical and traumatic pain are more likely to be readily recognizable than clinical signs associated with chronic pain. Often, clinical criteria used to assess chronic pain (eg, lack of activity, lack of grooming, decreased appetite, weight loss) are not specific clinical signs of pain and point only to an underlying problem in need of further diagnosis. Observations by owners are essential to detect more subtle clinical signs of chronic pain in animals, such as changes in attitude or interaction with family members or members of the herd or flock.

The Helsinki Chronic Pain Index(1,2,3) and the Canine Brief Pain Inventory(4,5) are owner-completed questionnaires designed to quantify the severity and impact of chronic pain in companion dogs. They have been validated for canine osteoarthritis.

The Canine Brief Pain Inventory, which is based on the human Brief Pain Inventory, also has been validated for canine bone cancer. A thorough history and physical examination are integral to the evaluation. Evaluating the extent of lameness and sensitivity to manipulation are critically important when assessing chronic orthopedic pain and pain of spinal origin. A comprehensive neurologic examination must be included for complete assessment and accurate diagnosis of any chronic pain syndrome. Lastly, response to treatment, such as increased activity after administering an NSAID, may provide important diagnostic information regarding the role of pain 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), chronic pain, and neuropathic pain (eg, nerve entrapment). Thus, assessment of cancer pain requires methods capable of detecting behavioral changes associated with both acute and chronic pain.