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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 ...