Chronic pain is a painful condition that lasts >3 months, pain that persists beyond the reasonable time for an injury to heal, or pain that persists 1 month beyond the usual course of an acute disease. Chronic pain lacks the essential function of acute pain. Whereas acute pain is a vital biologic signal to stop the individual from a potentially injurious activity or to pursue medical care, chronic pain serves no obvious biologic function. Complete pain relief is unrealistic in cases of chronic pain. Rather, the goal of therapy is pain reduction and return to functional status. Chronic pain syndromes discussed in this chapter are divided into neuropathic and nonneuropathic conditions. Aberrant drug-related behavior, also called drug-seeking behavior, is discussed.
Chronic pain is a common problem affecting 30.7% of the U.S. population and is more prevalent in women (34.3%) than in men (26.7%).1 Back pain is the most common site for chronic pain, followed by the knee and neck.1 The prevalence of neuropathic pain is 6.9% to 10% of the population.2 Risk factors for chronic pain include increasing age, female gender, higher body mass, and chronic illness.3 An exacerbation of chronic pain is part of the presentation of 11% to 15% of ED visits.4 Compared to patients with acute pain, chronic pain patients are more likely to report their pain as severe and more likely to be frequent visitors to the ED.5
The pathophysiology of chronic pain is incompletely understood. Many chronic pain syndromes follow nerve or tissue injury, producing nerve dysfunction secondary to the mechanical injury or in response to chemical mediators released from adjacent cell injury. Peripheral nerves, the CNS, or both become abnormally sensitive and develop pathologic spontaneous activity through upregulation of sodium channels and receptors.6 Neuroplastic changes in the central descending pain modulatory systems, inhibitory or facilitatory, may lead to further hyperexcitability. These changes lead to hyperalgesia (exaggerated response to a normally painful stimulus) and allodynia (pain from a normally nonpainful stimulus). In several disorders, a history of injury may be lacking, such as for fibromyalgia, where central sensitization is thought to play a key role.7 Psychological factors frequently precede or follow the onset of chronic pain and frequently predispose individuals to physiologic changes, through the fear-avoidance model. The fear of pain may lead to disuse disability, which leads to nerve hyperexcitability and dysfunction, and ultimately, may result in a chronic pain syndrome.8
The nonneuropathic syndromes share certain characteristics, the most common feature being muscle-related pain (Table 38-1). A feature common to most neuropathic syndromes is allodynia (Table 38-2).
TABLE 38-1Symptoms and Signs of Nonneuropathic Pain Syndromes