Pain is one of the most challenging problems in medicine and biology. It is a challenge to the sufferer who must often learn to live with pain for which no therapy has been found. It is a challenge to the physician or other health professional who seeks every possible means to help the suffering patient. It is a challenge to the scientist who tries to understand the biological mechanisms that can cause such terrible suffering. It is also a challenge to society, which must find the medical, scientific and financial resources to relieve or prevent pain and suffering as much as possible. (Melzack & Wall The Challenge of Pain, 1982)

Introduction

Last year, the International Association for the Study of Pain (IASP) introduced a revised definition of pain stating that pain is “an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage [1]. Pain is divided into two broad categories: acute pain, which is associated with ongoing tissue damage, and chronic pain, which is generally taken to be pain that has persisted for longer periods of time. Many injuries and diseases are capable of instigating acute pain with sources including mechanical tissue damage, inflammation and tissue ischemia. Similarly, chronic pain can be associated with other chronic diseases, terminal illness, or may persist after illness or injury with uncertain biological mechanisms. The point at which chronic pain can be diagnosed may vary with the injury or condition that initiated it; however, for most conditions, pain persisting beyond 3 months is reasonably described as a chronic pain condition. In some cases, one can identify a persistent pain condition much earlier, for example, in the case of post‐herpetic neuralgia subsequent to an attack of shingles, if pain persists beyond rash healing it indicates a persistent or chronic pain condition is present.

Exponential growth in pain research in the past five decades has increased our understanding regarding underlying mechanisms of the causes of chronic pain, now understood to involve a neural response to tissue injury. In other words, peripheral and central events related to disease or injury can trigger long‐lasting changes in peripheral nerves, spinal cord and brain such that the system becomes sensitized and capable of spontaneous activity or of responding to non‐noxious stimuli as if painful. By such means, pain can persist beyond the point where normal healing takes place and is often associated with abnormal sensory findings. In consequence, the scientific advances are providing a biological basis for understanding the experience and disabling impact of persistent pain. Table 1.1 presents definitions of pain terms relevant to chronic pain.

Traditionally, clinicians have conceptualized chronic pain as a symptom of disease or injury. Treatment was focused on addressing the underlying cause with the expectation that the pain would then resolve. It was thought that the pain itself could not kill. We now know that the opposite is true. Pain persists beyond injury and there is mounting evidence that “pain can kill.” In addition to contributing to ongoing suffering, disability and diminished life quality, it has been demonstrated that uncontrolled pain compromises immune function, promotes tumor growth and can compromise healing with an increase in morbidity and mortality following surgery [2, 3], as well as a decrease in the quality of recovery [4]

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