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Nervous System
Traditionally, the distinction between acute and
chronic pain has relied on a single continuum of time
with some interval (e.g., 6 months) since the onset of
pain used to designate the onset of acute pain or the tran-
sition when acute pain became chronic.
16
A more recent
conceptualization includes both time and pathophysio-
logic dimensions. Some conditions such as osteoarthritis
exhibit dimensions of both acute and chronic pain.
Acute Pain.
Acute pain is generally of short duration
and remits when the underlying pathologic process has
resolved.
17
Besides alerting the person to the existence
of actual or impending tissue damage, acute pain typi-
cally prompts a search for professional help. The pain’s
location, radiation, intensity, and duration, as well as
those factors that aggravate or relieve it, provide essen-
tial diagnostic clues.
Acute pain is elicited by surgery or trauma to body
tissues and activation of nociceptive stimuli at the site
of tissue damage.
16,18
This may induce an early wave of
hyperexcitability of neurons within the CNS. The devel-
opment of an inflammatory reaction to the tissue injury,
with sensitization of peripheral receptors, often results
in a second wave of longer-acting afferent input and a
new increase in central hyperexcitability.
18,19
The result-
ing hyperalgesia can lead to increased postoperative
and post-traumatic pain, usually about the second or
third day, and in some cases an increased likelihood of
developing chronic pain. A number of chronic pain syn-
dromes, including whiplash injury and phantom pain,
develop after trauma and surgery.
18
Because acute pain is self-limited, in that it resolves
as the injured tissues heal, long-term therapy usually is
not needed. However, pain from acute illness, trauma,
surgery, or medical procedures should be aggressively
managed and preemptive analgesia provided before the
pain becomes severe. The use of preemptive and multi-
modal therapy (two or more analgesics with differing anal-
gesic mechanisms) not only allows the person to be more
comfortable and active, but also helps to prevent periph-
eral sensitization of pain receptors and hyperexcitability
of central pain centers.
19
Usually, less medication is needed
when the drug is given before the pain becomes severe and
the pain pathways become sensitized. Interventions that
alleviate acute pain also relieve the anxiety and musculo-
skeletal spasms, as well increasing mobility and respira-
tory movements such as deep breathing and coughing.
Chronic Pain.
Chronic pain is pain that persists longer
than might be reasonably expected after an inciting event
and is sustained by factors that are both pathologically
and physically remote from the originating cause.
17,20,21
Chronic pain is highly variable. It may be unrelenting and
extremely severe, as in metastatic bone pain. It can be rel-
atively continuous with or without periods of escalation,
as with some forms of back pain. Some conditions with
recurring episodes of acute pain are particularly problem-
atic because they have characteristics of both acute and
chronic pain, as in sickle cell crisis or migraine headaches.
The biologic factors that contribute to chronic pain
include peripheral mechanisms, peripheral–central mech-
anisms, and central mechanisms.
20
Peripheral mecha-
nisms result from persistent stimulation of nociceptors,
termed
peripheral sensitization
. Inflammatory media-
tors released from injured tissues increase the sensitiv-
ity of the C fibers and lead to increased duration of
pain associated with chronic musculoskeletal, visceral,
and vascular disorders. Peripheral–central mechanisms
involve abnormal function of the peripheral and central
portions of the somatosensory system.
21
They typically
TABLE 35-1
Characteristics of Acute and Chronic Pain
Characteristic
Acute Pain
Chronic Pain
Onset
Recent
Continuous or intermittent
Duration
Short (<6 mo)
6 mo or more
Autonomic responses
Consistent with sympathetic
fight-or-flight response
Increased heart rate
Increased stroke volume
Increased blood pressure
Increased pupillary dilation
Increased muscle tension
Decreased gut motility
Decreased salivary flow
(dry mouth)
Absence of autonomic
responses
Psychological component
Associated anxiety
Increased irritability
Associated depression
Somatic preoccupation
Withdrawal from outside
interests
Decreased strength of
relationships
Other types of response
Decreased sleep
Decreased libido
Appetite changes
