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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. There are four basic types of chronic pain: pain persisting beyond the normal healing time for a disease or injury, pain related to a chronic degenerative disease or persistent neurologic condition, cancer-related pain, and pain that emerges or persists without an identifiable cause. Chronic pain differs from acute pain in its function. Acute pain is an essential biologic signal to warn the individual to stop a potentially injurious activity or to prompt one to seek medical care. Chronic pain serves no obvious biologic function.

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Complete eradication of pain is not a reasonable end point in most cases. 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. Drug-seeking patients are also discussed.

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Chronic nonmalignant pain is a common problem affecting 11% to 24% of the general population.1–3 Back pain is the most common site for chronic pain, followed by the neck, extremities, and head.2 Prescription opioid use is four times more common in patients with chronic pain (12%) than in the general population (3%).4 Prevalence of neuropathic pain is 8% of the population.5 Seventy-nine percent of patients who began with pain of 3 months’ duration will still have pain 4 years later.6 Risk factors for chronic pain include increasing age, female gender, higher body mass, and chronic illness.1,3 Compared to those without pain, patients with chronic pain report higher usage of health care services, yet have lower satisfaction with the care provided.2 The cost of common pain conditions is estimated to be $61.2 billion (U.S.) per year in lost work productivity.7 Assessing noncritical ED patients, as much as 40.5% have chronic pain,8 and an exacerbation of chronic pain represents 11% to 14% of ED visits.9 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.4,8 Only 25% of these patients have been seen by pain specialists.8

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Nociceptive pain is mediated by receptors on A delta and C fibers, which respond to mechanical stretching, compression, thermal changes, and chemical substances, such as prostaglandins, bradykinins, and other mediators of inflammation. For many chronic pain syndromes, such as fibromyalgia, the pathophysiology is not understood. There is some evidence for increased sensitization of peripheral nociceptors in chronic myofascial syndromes,10 but increased central sensitivity has also been proposed.11 Increased central sensitization may play a role in other chronic pain disorders, such as transformed migraine.12

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There are several underlying mechanisms known for neuropathic pain: direct stimulation, deafferentation, automatic firing of damaged nerves, and sympathetically mediated ...

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