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Pain management in large animals: Should we be doing more? (Proceedings)

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Aug 01, 2008

In 2001, a letter to the editor of Veterinary Record from a highly experienced equine practitioner suggested that horses experience little or no pain after castration and questioned the routine administration of postoperative analgesic medications for all equine castrations. This conclusion was based on the lack of classic behavioral signs of abdominal pain (sweating, rolling, stretching, etc) after castration. Rebuttal letters from other, equally experienced equine practitioners argued eloquently that horses do, indeed, experience significant pain after castration but that pain is manifest in more subtle behavioral alterations. What is the truth? Despite the universality of pain, it is a uniquely personal experience that is difficult or impossible to accurately recognize and quantify. None of us can know the extent to which another individual or animal truly feels pain in any given situation. The neural pathways and higher centers of perception in horses bear enough similarity to those of humans to convince most knowledgeable individuals that the physiology and perception of pain is very similar in our equine patients. However, domestic animal species probably have significantly less ability to appreciate the psychological component of fear of death or "impending doom" that contributes greatly to the emotional response to pain in injured human patients. It is impossible to know to what extent this difference in emotional response impacts the physiological responses to painful stimuli in domestic animals.

It seems intuitive that the pain response must be present in horses as an important evolutionary response to facilitate recognition of potential tissue damage. However, it seems equally intuitive that there has been evolutionary pressure in prey animals to minimize the outward expression (behavioral indicators) of pain and injury. Most equine veterinarians readily recognize signs of acute severe abdominal pain (pawing, rolling, flank gestures, etc.) or injury to the appendicular skeleton (varying degrees of lameness) but our ability to recognize more subtle pain or pain secondary to surgery or trauma is less certain. There is a paucity of objective scientific data regarding recognition and interpretation of pain behaviors in horses. This lack of basic information is compounded by the recognition that the intensity of pain perception and the subsequent behavioral responses after trauma or surgery can vary greatly depending upon genetic and environmental factors. The net result is that the degree of pain perceived by a patient and the extent of behavioral changes exhibited as a result of that perceived pain can vary markedly between individual patients after apparently identical noxious stimuli such as routine castration of a healthy colt.

What is Pain?

The International Association for the Study of Pain has proposed the following definition of pain:

"Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage" and "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 definition implies that perception of pain is a sensory and emotional event but that the inability to verbalize a response to pain does not negate the experience or the need for therapy. While written in relation to human patients these concepts have equal applicability in veterinary medicine. In 1997, Moloney proposed the following definition for animal pain (Moloney V et al., J An Sci. 1997;75:266-272):

"Animal pain is an aversive, sensory experience representing awareness by the animal of damage or threat to the integrity of its tissues (note that there might not be any damage). It changes the animal's physiology and behaviour to reduce or avoid the damage, to reduce the likelihood of its recurrence and to promote recovery. Non-functional (non-useful) pain occurs when the intensity or duration of the experience is not appropriate for damage sustained (especially if none exists) and when physiological and behavioural responses are unsuccessful in alleviating it."

Pain differs depending on its origin, severity, and the degree and type of tissue damage. It may be classified on the basis of inciting event or purpose of the response (physiologic vs pathologic pain), anatomic location of the painful event (superficial somatic, deep somatic, visceral or neuropathic pain) and duration (acute or chronic).

  • Physiologic pain: Occurs when a stimulus that induces minimal or no tissue damage activates high-threshold sensory nerve fibers, warning the organism of potentially tissue damaging events (also known as nociceptive pain).
  • Pathologic pain: Occurs when excessively intense or prolonged stimuli induce tissue damage that results in extended discomfort and abnormal sensitivity (also known as clinical pain).
  • Somatic pain: Arises from skin, bone, joint, connective tissue, and muscle. This type of pain, especially if superficial, tends to be well-localized and constant. Superficial somatic pain (cutaneous tissues) tends to be sharp, well-localized and constant. It may have a burning or pricking quality. Deep somatic pain (bones, joints, muscles) may more closely resemble visceral pain in being dull or aching in quality.
  • Visceral pain: Arises from infiltration, compression, extension or stretching of the thoracic, abdominal or pelvic visceral organs. This type of pain tends to be diffuse, dull, and poorly localized. It may be referred to other deep or cutaneous sites.
  • Neuropathic pain: Pain initiated or caused by a primary lesion or dysfunction in the nervous system. It is usually described as burning or tingling pain.
  • Acute pain: Pain of short duration typically occurring secondary to surgery or trauma; usually lasts hours to days.
  • Chronic pain: Pain persisting longer than the expected time frame for healing or pain associated with progressive, nonmalignant disease (e.g. chronic osteoarthritis); usually pain of greater than one month's duration.