It is now a settled matter that the adaptive capacities of animals, coupled with the innate biases of human observers, seriously
impairs our ability to "know" which of our patients are in pain, how much they are in pain, and sometimes, even where they
are in pain. Historically the absence of behaviors easily associated with pain (crying, whimpering, etc.) has been equated
with the absence of pain. In fact, animals can lay quietly in pain, or conversely act painful until the approach of a person
whereupon the painful behavior is replaced by a happy, tail-wagging greeting. Therefore it is first incumbent for veterinary
clinicians to operate under the assumption that some procedures and conditions are inherently painful, without the patient
having to "prove" they are in pain. Any surgical procedure, trauma, and many medical conditions such as gastroenteritis,
pancreatitis, cystitis, inflammatory bowel disease, and certain neoplasms are all examples of where it is incumbent upon the
clinician to include pain management in their treatment plan.
Pain is complex, experiential, individual, and multifactoral in its expression. Nevertheless, scoring pain is increasingly
recognized as the "4th vital sign" in animals after temperature, pulse, and respiration (in humans it is the 5th, after blood pressure). Indeed the AAHA certification guidelines now require this assessment on every patient (MA23, PM1).
Overt signs of pain vary by species, and even within species some breeds and individuals have enhanced local and descending
inhibitory mechanisms when compared others. Also, realize that pet owners may be better at "reading" their pet than a stranger.
Listed below, are just some of the "new onset pain behaviors" shown by animals in acute severe pain. (adapted from Karol Matthews excellent chapter in the Veterinary Clinics of North America Small Animal Management of
Pain, July 2000 Vol 30 issue entitled: Pain Assessment and General Approach to Management). However, the observer must be
very astute if they are to pick up signs of moderate or mild pain, and are exhibited not by the onset of new behaviors, but
rather absence of usual behaviors.
DOGS: abnormal sitting or lying posture, restlessness, thrashing, splinting of abdomen, "prayer position," whining, groaning
or otherwise vocalizing, limping, unwilling to get up, unwilling to lie down, lack of appetite, trembling, increased respiratory
rate, expiratory gruntbulging eyes, dilated pupils, aggression, resents being touched, dull behavior, won't wag tail, licking
or biting at affected area, lack of grooming
CATS: can be seen with many of the signs listed above for dogs, and cats are more likely to be in a crouched position or
grumpy when they are in pain. However, some cats actually purr when they are distressed or in pain! Cats in severe postoperative
pain may become aggressive, tearing at the bandage, frantic and vocalizing. Also, cats are more likely to exhibit withdrawn
behavior – crouching in the back of the cage, unwilling to use the litter box, etc., than dogs.
An investigational tool in human medicine is evaluation of facial expressions, on the premise that certain muscle movements
universally occur in our species when pain is present; this has been called "the primal face of pain." Computer software
programs are being developed to detect these expressions that are then translated into a automated score. Similar primal facial
expressions of pain are thought to exist in animals, e.g. furrowed brow, squinted eyes, ears turned back or away from the
forward position, and even without a computer program these subtle changes can provide additional information to the veterinary
Scoring pain in non-verbal patients is a special challenge, examples of which includes not only animals but neonates infant
children and incapacitated (physically or mentally) adult humans. Measuring objective physiologic parameters has proven to
be unreliable as indicators of pain largely because of the influence of other non-pain influences e.g. stress, distress, anxiety,
and normal biologic variation. Therefore we are left with subjective evaluations and such scoring systems should meet the
- Interobserver variability and observer bias is minimized
- Distinguish between varying levels of pain intensity
- The degree of "importance" of pain to the subject is detected.