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Myths and misconceptions in veterinary pain medicine (Proceedings)

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Aug 01, 2009

The major barrier to good pain management is the mythology that has persisted around animal pain behavior, physiology of pain, owner psychology and analgesic drugs and techniques. Veterinarians and technicians must work constantly to dispel the myths for themselves, colleagues, other staff members and pet owners.

Myth #1. Animals do not feel pain as people do.

From a physiologic standpoint, mammals and humans process pain in the same way; by nociception and cognition. Nociception, derived from the Latin word nocere (to injure), uses specific pathways β€” transduction, transmission, and modulation β€”for its physiologic processes. The pain pathway begins at the point of tissue trauma, such as a site of inflammation, injury, or surgical incision, where nociceptors (pain receptors) are stimulated. These specialized nerve endings convert mechanical, chemical, and/or thermal energy into electrical impulses (transduction). If the noxious stimulus is large enough to exceed the nociceptor's threshold, a nerve impulse is generated and transmitted along peripheral nerves to the spinal cord (transmission). Once at the spinal cord, a nerve impulse is either projected upward to the thalamus and then to other parts of the brain, including the cerebral cortex or it may be transmitted to an a nerve cell located entirely within the central nervous system that modifies nerve signals and links sensory and motor neurons that in turn activates sympathetic reflexes damping the pain sensation (modulation).

This understanding of pain physiology gives rise to the concept of multimodal analgesia. That is, attacking pain from many angles is more effective than from only one. Since the pain pathway has distinct phases, pain can be interrupted at various points. For example, we may want to do a local ore regional nerve block (transduction or transmission) in addition to pre-emptive NSAIDs (transduction and modulation) as well as postoperative opioid administration (modulation and perception). Using drugs from three different classes provides better pain control and has the added benefit of allowing us to use lower doses of individual agents thereby reducing side effects. Effective analgesia can also reduce the amount of gas anesthesia required for a procedure.

Myth #2. Animals tolerate pain better than people do or Tail wagging = comfort

In many cases animals do "appear" to tolerate pain better than humans. There may be several explanations for this. In contrast to pain-detection threshold, pain tolerance β€” the greatest intensity of pain that is voluntarily tolerated β€” varies widely between species and individuals within a species. Like humans, animals likely tolerate pain to a particular level before showing changes in behavior. Knowing that patients may exhibit a wide range of pain tolerances as well as a broad spectrum of behaviors can improve pain recognition and treatment.

In the past much attention has been given to recognizing the signs of pain in animal patients. This approach focuses on the individual and requires the patient "prove" he/she is in pain in order to receive treatment. By consensus we have concluded what we believe may be clinical signs of pain in our patients. Increased HR, increased RR, restlessness, increased temp, increased BP. abnormal posturing, inappetence, aggression, frequent movement, facial expression, trembling, depression and insomnia have all been listed among the top signs of pain by veterinary professionals. Anxiety, nausea, pupillary enlargement, licking/chewing/staring at site, poor MM color, salivation, decreased CO2 and head pressing are also sited by some observers. We see that many behaviors and clinical signs may be evidence of pain. Ultimately, we must conclude that any abnormal sign(s), in a veterinary patient, which cannot be attributed to another cause, are suspect of indicating pain. In actual practice, the clinical manifestations may be quite different between species and even among different members of the same species.

When evaluating animal pain we must always bear in mind that, much to their detriment (in a setting without predators) animals, in fact, attempt to hide pain from us. Science has shown us dogs observed by closed circuit camera post spay surgery. These dogs displayed many pain behaviors that abruptly abated when a human researcher entered the room. This type of research has led us to become less reliant on pain signs. Recently, the focus of much research in pain management has shifted toward identifying and even predicting known painful events. For example: Severe pain is expected with cervical disc herniation, extensive inflammation, medical or surgical, fracture repair, limb amputation, declaw, TECA etc... Moderate to mild pain is expected with cruciate repair, laparotomy, mass removal, castration, dental procedures, etc... This approach encourages us to treat patients who undergo painful procedures or diseases processes without requiring proof. It does not however, consider the vast variation in pain tolerance in the individual. It seems reasonable to incorporate both concepts to develop a truly effective analgesic plan. That is, to have direction given by what are known to be painful events and be prepared to provide adequate analgesia for the expected level of pain but also to look a the individual and tailor analgesic protocols accordingly.