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  • Pain is a subjective, unpleasant, sensory, and emotional experience associated with actual or potential tissue damage or abnormal functioning of nerves. It may be classified as acute, chronic, or cancer pain.


Adaptive (Physiologic) Pain

  • Nociceptive (eg, from touching something too hot, too cold, or sharp) and inflammatory pain (eg, trauma or surgery) are both adaptive and protective.

  • The steps in processing pain are:

    • ✔ Transduction—Stimulation of nociceptors.

      • Nociceptors, found in both somatic and visceral structures, are activated by mechanical, thermal, and chemical stimuli. Noxious stimuli may cause release of cytokines and chemokines that sensitize and/or activate nociceptors.

    • ✔ Conduction—Receptor activation leads to action potentials that continue along afferent fibers to the spinal cord. Stimulation of large-diameter, sparsely myelinated fibers evokes sharp, well-localized pain. Stimulation of small-diameter, unmyelinated fibers produces aching, poorly localized pain.

    • ✔ Transmission—Afferent nociceptive fibers synapse in the spinal cord’s dorsal horn, releasing excitatory neurotransmitters (eg, glutamate and substance P). The spinothalamic tract and other pathways bring the signal to the brain’s higher cortical structures.

    • ✔ Perception—The experience of pain occurs when signals reach higher cortical structures. Relaxation, meditation, and distraction can lessen pain, and anxiety and depression can worsen pain.

    • ✔ Modulation—Possible modulating factors include glutamate, substance P, endogenous opioids, γ-aminobutyric acid (GABA), norepinephrine, and serotonin.

  • The interface between neurons and immune cells in the central nervous system (CNS) may facilitate maintenance of chronic pain.

Maladaptive (Pathologic) Pain

  • Pathophysiologic pain (eg, postherpetic neuralgia, diabetic neuropathy, fibromyalgia, irritable bowel syndrome, chronic headaches) is often described as chronic pain. It results from damage or abnormal functioning of nerves in the CNS or peripheral nervous system (PNS). Pain circuits sometimes rewire themselves anatomically and biochemically, resulting in chronic pain, hyperalgesia, or allodynia.


Acute Pain


  • Acute pain can be sharp or dull, burning, shock-like, tingling, shooting, radiating, fluctuating in intensity, varying in location, and occurring in a temporal relationship with an obvious noxious stimulus. Infants and older individuals may present differently.


  • Hypertension, tachycardia, diaphoresis, mydriasis, and pallor. In some cases there are not obvious physical signs. There are no laboratory tests for pain.

Chronic Pain


  • Symptoms can be very similar to acute pain, and may change over time (ie, sharp to dull). Attention must also be given to emotional factors that alter the pain threshold.


  • In most cases there are no obvious signs. Depression, sleep disturbances, and anxiety are often present. No laboratory tests are diagnostic, but may identify etiology.


  • Pain is always subjective and best diagnosed based ...

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