Chronic Pain in Small Animal Medicine PDF

PREFACE

There is no pain pathway! Pain is the result of a complex signaling network. The cognition of pain, like cognition in general, requires sophisticated neurological hardware.

Pain has many definitions because it's an intensely subjective experience that is filtered through our emotions as well as our anatomy. It's any sensation amplified to an uncomfortable level, and it's a plethora of negative emotions called ‘suffering'. No one patient feels pain the same - there is no single accepted pain experience. Like the perception of beauty, it's very real, but only in the eye of the beholder. Yet, pain is so fundamental to our wellbeing that it is added to heart rate, respiratory rate, temperature, and blood pressure as the ‘fifth vital sign'. Without a ‘pain thermometer' people in pain must rely on their language skills to describe what they are feeling. In human medicine pain is what the patient says it is, in veterinary medicine pain is what the assessor says it is! Trained as scientists, veterinarians are schooled to assess responses based on the mean ± standard deviation, yet effective pain management suggests we target the least-respondent patient within the population, so as to ensure no patient is declined the relief of pain it needs and deserves.

In its simplest sense, pain protects us from bodily harm, hence the proposal that pain is a teacher, the headmaster of nature's survival school. Dangerous things are noxious things, and pain punishes us if we take excessive risks or push ourselves beyond our physical limits. Further, pain often forces us to observe ‘recovery time'. Another way of understanding pain is that any stimulus - noxious or otherwise - can become painful if the patient's ability to cope with it has been diminished.

A working definition of chronic pain is that, unlike acute pain, it lasts beyond the time necessary for healing and resists normal treatment. The primary indicator of chronic pain is not how long it persists, but whether it remains long after it should have disappeared. As the father of pain medicine, John Bonica, explains, ‘Acute pain is a symptom of disease; chronic pain itself is a disease.'

The noxious stimuli that constitute pain can reconfigure the architecture of the nervous system they invade. Lasting noxious input can produce a neurobiological cycle of chemical and electrical action and reactions that becomes an automatic feedback loop: a chronic, self-perpetuating torment that persists long after the original trauma has healed.

From the human healthcare experience, pain, and in particular chronic pain, is a major problem for which current treatments are often inadequate. The tangible costs economically are in the many tens of billions of dollars, and the costs in terms of suffering are known all too well to practitioners who seek to help these patients. In veterinary medicine we are experiencing a surge of increasing focus on measuring and resolving pain and suffering, and indeed, this aspect is central to the veterinarian's oath. This focus is being supported by an increased understanding of pain neurophysiology, discovery of novel treatment targets, a greater offering of innovative pharmacologics, and consumer demand. The pharmaceutical industry has made important strides forward in bringing new therapies to address the problem of chronic pain, but to the suffering patient, this progress is glacially slow. Specific areas of exploration include peripheral nervous system targets, central nervous system targets, diseasespecific targets, and development of measurement tools and applications of new technologies.

In the 1880s Friedrich Bayer and Company commercialized Bayer Aspirin®. When aspirin (‘a' for acetyl, part of its chemical composition; ‘spir' from a plant that contained salicin; and ‘in' a popular medical suffix at the time) went over the counter in 1915, the mass production of pain alleviators for the general public was launched. Pain is the most common reason patients see a physician, while pain and pain relief are among the most robust areas of medical research.

Realistically, new discoveries and innovative drug formulations for veterinary patients will continue to lag considerably behind those for humans, despite the fact that animals are often used for the development of human therapies. This is a reality of present-day economics, appreciated as return on investment by the pharmaceutical industry. Accordingly, there are presently, and will likely in the future be, a limited number of agents and techniques actually labeled for 4 veterinary use. It is, therefore, incumbent on the veterinarian and veterinary healthcare professional to understand both the neurobiology of chronic pain, and the mode of action of various therapies so as to determine if the therapeutic agent or technique is likely to be safe and efficacious when utilized ‘offlabel'. Such insights may not be readily available for the proposed target patient, but would be ‘inferred' from data obtained from a different species. Herein comes the weighing of ‘species specificity' vs. ‘one science' in the clinical decision-making process.

This text was created for the veterinary healthcare professional seeking a greater depth of knowledge in mechanisms of pain accompanying chronic disease states, and potential targets for treatment. It aspires to go beyond the ‘cookbook protocols' found in many offerings, by providing contemporary understandings of ‘why and how to treat'.