Pain Management in Veterinary Practice

Christine M. Egger, Lydia Love, Tom Doherty, Christine M. Egger, Lydia Love, Tom Doherty

• Publisher: Wiley-Blackwell
• Year: 2013
• Print ISBN: 9780813812243
• eBook ISBN: 9781118761601
• Edition: 1
• Topic: Medicine
• Subtopic: Veterinary Medicine

Pain Management in Veterinary Practice provides veterinary practitioners with the information needed to recognize and manage pain in a wide range of large, small, and exotic animal species. Encompassing acute, adaptive, and chronic, maladaptive pain, the book provides an up-to-date review of the physiology and pathophysiology of pain. Pain Management in Veterinary Practice offers specific strategies for addressing pain in animals, including local and regional analgesia, continuous rate infusions, and novel methods of analgesic drug delivery.

With comprehensive information on the pharmacokinetic and pharmacodynamic characteristics of analgesic drugs, the book goes beyond pharmaceutical options to incorporate scientific information on techniques for complementary treatment, including physical therapy, acupuncture, chiropractic techniques, and nutritional strategies. Pain Management in Veterinary Practice is a valuable resource for developing pain management protocols in the veterinary clinic.

Section 1

Introduction and Anatomy, Physiology, and Pathophysiology of Pain

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Chapter 1. Introduction: Pain: An Issue of Animal Welfare
Alice Crook

Chapter 2. Anatomy, Physiology, and Pathophysiology of Pain
Yael Shilo and Peter J. Pascoe

Chapter 3. Mechanisms of Cancer Pain
Cholawat Pacharinsak and Alvin J. Beitz

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1

Introduction: Pain: An Issue of Animal Welfare

Alice Crook

There has been considerable progress since the early 1990s in pain research in animals and in our understanding of related physiology and pharmacology, enabling great strides to be made in pain management. But pain is still a huge welfare issue for animals: farm animals are routinely subjected to painful husbandry procedures with no anesthesia or analgesia; perioperative pain management in small and exotic animals is inconsistent; and management of cancer-related and chronic pain remains a challenge. Pain can diminish animal well-being substantially due to its aversive nature, the distress arising from the inability to avoid such sensations, and the secondary effects that may adversely affect the animal's quality of life (QOL). Pain may affect an animal's appetite, sleep habits (e.g., fatigue), grooming (e.g., self-mutilation), ability to experience normal pleasures (e.g., reduced play and social interaction), personality and temperament, and intestinal function (e.g., constipation), and may prolong the time needed for recovery from the underlying condition (ACVA, 1998; McMillan, 2003). Untreated pain may also result in systemic problems; for example, hepatic lipidosis in cats as a result of inappetance and inadequate caloric intake (Mathews, 2000).

Much is known about the recognition and assessment of pain in animals; however, more work is needed to develop valid and reliable pain scoring systems for all species that are practical in real-life situations. Perception of animal pain directly affects analgesic usage, and there is a wide range in attitudes among veterinarians, farmers, and pet owners. This can best be addressed through education. There are also economic, regulatory, and other constraints to effective pain management, particularly in large animals.

RECOGNITION AND ASSESSMENT OF PAIN IN ANIMALS

Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage (IASP, 1994). The experience of pain is always subjective. Self-reporting is the gold standard in people, yet how can we know the experience of animals?

Three approaches are used in the recognition and measurement of pain in animals. The first approach includes measures of general body function or productivity (e.g., food and water intake, weight gain) that are relatively easy to quantify; such measures reflect what was happening to the animal over the period between observations. The second approach includes physiological measures (such as changes in heart rate or cortisol concentrations) that are widely used in studies assessing pain in animals (Stafford & Mellor, 2005; Vickers et al., 2005; Whay et al., 2005) and are, in principle, particularly useful in prey species that are considered stoic and therefore unlikely to show pronounced behavioral responses until injuries are advanced (Phillips, 2002; Rutherford, 2002). However, the physical restraint required to obtain such measurements may itself be stressful and confound the results (Weary et al., 2006). Also, while cortisol measurements are useful for comparing treatments and controls, they are not useful in assessing the degree of pain an individual animal is experiencing (Rutherford, 2002).

Behavioral measures—the third approach—represent a way in which animals can “self-report.” Weary (2006) provides a comprehensive review of the ways such measures are used to recognize and quantify animal pain, and discusses the evidence necessary to ensure that the measures are valid (i.e., that the measure provides useful information about the pain the animal is experiencing) and reliable (i.e., repeatable). The three main classes of behavior used in pain assessment are pain-specific behaviors (e.g., gait impairment in lame dairy cows (Flower et al., 2008) or head shaking and rubbing in dehorned dairy calves (Vickers et al., 2005)); a decline in frequency or magnitude of certain behaviors (e.g., locomotory behaviors in rats postoperatively) (Roughan & Flecknell, 2003); and choice or preference testing (e.g., hens' responses to different concentrations of carbon dioxide used in stunning) (Webster & Fletcher, 2004). Rutherford (2002) discusses the usefulness of behaviors associated with acute, subacute, and longer-lasting pain in assessing the experience of pain in animals, including specific parameters that may be useful for veterinarians in clinical assessment of pain and by scientists studying pain in animals. These include simple and more complex behavioral responses, both qualitative and quantitative, which may or may not be adaptive, such as behaviors associated with escape or avoidance, guarding or protection (e.g., postural changes), and depression or “learned helplessness.”

Pain Recognition Tools

Pain researchers and clinicians alike agree that there is a need for sensitive and specific measures that are practical for real-time assessments in a variety of animal settings including farms, veterinary clinics, and laboratories (Viñuela-Fernández et al., 2007). Multidimensional pain scales that integrate objective and subjective behavioral observations with various other measures can be used to characterize an individual animal's experience of pain (Rutherford, 2002). Another approach is to develop questionnaires for use by animal owners that can be used in the assessment of pain and its impact on QOL (McMillan, 2003; Wiseman-Orr et al., 2004; Yazbek & Fantoni, 2005). Wiseman-Orr (2006) provides a thorough discussion of the approaches and potential pitfalls of designing and validating questionnaires where self-reporting is not possible and the questionnaires are designed for use by a proxy, as in the case of animals. Work continues in the development of scientifically validated pain recognition tools for veterinarians for clinical assessment of pain and for scientists studying pain in large, small, exotic, and laboratory animals (Roughan & Flecknell, 2003; Wiseman-Orr et al., 2004; Yazbek & Fantoni, 2005; Morton, 2005; Wojciechowska et al., 2005; Föllmi et al., 2007; Flecknell et al, 2007; Weary & Fraser, 2008).

PAIN AND CONSCIOUSNESS

Pain is always subjective and psychological variables such as past experience, attention, and other cognitive activities affect the individual's experience of pain (Melzack, 1993). Self-reporting is the gold standard in people and, because of the subtlety of communication possible with language, the understanding of pain has been greatly advanced through human subjects' descriptions of pain and the effects of different modalities of analgesia (Johnson, 2008). However, “The inability to communicate verbally does not negate the possibility that an individual is experiencing pain and is in need of appropriate pain-relieving treatment” (IASP, 1994).

If we cannot know the subjective emotional experiences of other human beings, how can we possibly know the emotional experience of animals? For most people, the evidence that animals have nociceptive receptors and pathways, physiological responses, and behavioral reactions to pain similar to that of people, is sufficient to accept that animals experience pain and suffer as a result. However, some scientists, surprisingly, suggest that animals are not capable of experiencing pain. Psychologist Bermond (2001), for example argues that animals other than anthropoid apes have an “irreflexive consciousness” (a consciousness without past or future) due to the lack of a well-developed prefrontal cortex, and that reflection is a requirement to experience suffering and pain as unpleasant. Therefore, he distinguishes between “the registration of pain as a stimulus, which does not induce feelings of suffering and the experience of pain as an emotion, which does induce suffering” (Bermond, 2001).

What kind of observations can provide evidence for or against the experience of pain and other affective states in animals? The neurophysiologist Gentle (2001) carried out an elegant series of studies to provide information on cognitive perception of pain in chickens by looking at the effect of selective attention on pain-related behavior. Noting that the human experience of pain can be modulated by shifts in attention through such modalities as relaxation training, hypnosis, and other therapies, he reasoned that if a chicken's response to a painful event was simply an unconscious automatic reaction the response would not be influenced by shifting the bird's attention. On the other hand, if the bird actually felt the pain as an unpleasant experience, redirecting its attention might reduce the signs of pain, as in people (e.g., installation of overhead television screens in dental offices). In his work, Gentle induced gout in one leg of chickens by injecting sodium urate crystals. Chickens kept in barren cages avoided placing weight on the affected leg and, if encouraged to walk, did so with a limp. These pain-related behavioral signs were greatly reduced or eliminated in chickens given a variety of motivational changes including nesting, feeding, exploration, and social interaction. The shifts in attention not only reduced pain but also reduced peripheral inflammation.

This work has far-reaching consequences. The evidence that motivational changes, by altering the birds' attention, significantly altered pain-related behaviors, and hence probably the pain experience for the animal, indicates a cognitive component of pain in the chicken and provides evidence of consciousness. On a practical level, these results also reinforce the importance of environmental enrichment, which will promote shifts in attention and, thereby, potentially improve the welfare of birds suffering pain under commercial conditions. Strategies, such as distraction and refocusing attention through positive interaction, are very familiar to veterinarians and animal health technicians as adjuncts to pain management in small animals in clinical settings.

ATTITUDES TOWARD ANIMAL PAIN

“Freedom from pain, injury, or disease (by prevention or rapid diagnosis and treatment)” is one of the Five Freedoms widely accepted as the major components of good animal welfare (Farm Animal Welfare Council, 2009). The recognition and effective treatment of pain is central to animal welfare (Rutherford, 2002). There is a strong emphasis on pain among animal welfare researchers, with the number of pain-related articles in scientific journals considerably outweighing articles on the other Freedoms (freedom to behave normally, freedom from fear and distress, freedom from hunger and thirst, and freedom from discomfort) (Phillips, 2008).

National animal welfare advisory bodies in Australia, New Zealand, and the European Union have recommended steps to avoid or minimize animal pain and associated suffering, and the World Organization for Animal Health (OIE) produced a special edition in its Technical Series on “Scientific assessment and management of animal pain” (Mellor et al., 2008). Veterinary associations commonly have positions or policies advocating the effective management of pain in animals (CVMA, 2007; AVMA, 2011).

In theory, then, we agree that animals should not be in pain, yet studies show that attitudes toward pain vary greatly among societal groups responsible for animal care, including veterinarians. Veterinary attitudes toward pain and pain management in companion and production animals have been studied in Canada (Dohoo & Dohoo, 1996; Hewson et al., 2006b, 2007a, 2007b), the United States (Hellyer et al., 1999), the United Kingdom (Lascelles et al., 1999; Capner et al., 1999; Huxley, 2006), Finland (Raekallio et al., 2003), Scandinavia (Thomsen et al., 2010), Europe (Hugonnard et al., 2004; Guatteo et al., 2008), and New Zealand (La...

Table of contents

• Cover
• Title Page
• Copyright
• Contributors
• Preface
• Section 1: Introduction and Anatomy, Physiology, and Pathophysiology of Pain
• Section 2: Pharmacology of Analgesic Drugs
• Section 3: Nonpharmacological Pain Therapy
• Section 4: Management of Pain in Veterinary Species
• Section 5: Incorporating Pain Management into Your Practice and Hospice and Palliative Care
• Index