Pain is ubiquitous in life. Inextricably bound to consciousness, it is an experience that all living creatures with advanced nervous systems share. For our ancestors, whose lives were fraught with danger, pain conferred an evolutionary advantage, signaling the need to separate oneself from its immediate source. However, evolution has failed to keep pace with biomedical and technological advances, allowing chronic pain (pain that persists beyond an acute injury or condition) to become a disease in itself.
It is difficult to overestimate chronic pain’s societal impact. According to the US Institute of Medicine, one in three people suffer from chronic pain — more than from heart disease, cancer and diabetes combined. Pain is the leading cause of disability, especially back pain among people under 45 years of age and joint pain in older individuals. In the US alone, chronic pain is estimated to cost more than US$600 billion annually.
Pain can be classified according to a variety of factors, such as duration or location, but the most useful categorization is based on mechanism. Nociceptive pain, which arises from damage to non-nervous tissue, occurs when, say, a person twists an ankle. An example of chronic nociceptive pain is arthritis. Neuropathic pain, by contrast, arises after a lesion or disease affects the nervous system. Nerve damage resulting from diabetes (diabetic neuropathy) and persistent pain after shingles (postherpetic neuralgia) are among the most common causes.
Chronic pain is difficult to treat; even the most effective medications provide only modest relief to a minority of patients. This can be explained partly by the subjective nature of pain, and partly by the fact that its source can be difficult to pinpoint.
Although neuroscientists are adept at studying pain, animal models fail to account for its “affective-motivational” component — that is, pain’s emotional, cognitive and contextual features. Indeed, physiological indicators have less of an impact on a patient’s prognosis after a painful injury than psychological and social factors, such as depression or poor coping skills. The problem is that subjective measures are much more difficult to study — not least because they are associated with high placebo response rates.
Unrealistic expectations exacerbate these psychological impediments to progress. In an era of instant access, people often expect immediate relief from symptoms, which is difficult to achieve when it comes to chronic pain.
For example, the best way to relieve back and neck pain is often to exercise, while treating underlying contributing factors like obesity. However, few people are willing to devote the time and effort that such a therapeutic plan demands; they would prefer an injection, operation or medication. When there is no instant fix available, they can become discouraged, hampering their recovery further.
Making matters worse, people are being inundated with information — and often misinformation — through TV, the Internet and other direct-marketing channels. This fuels misconceptions and, in many cases, gives people false hope about the kind of relief they can expect from a particular drug or treatment.
Not even physicians are immune to these influences; indeed, in some cases, they actually create the bias. For example, studies of epidural steroid injections for back pain have been shown to be almost three times more likely to yield positive results when conducted by doctors who routinely administer them.