1) What is pain?

The Task Force on Taxonomy Committee of the International Association for the Study of Pain (IASP) defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.”

This definition of pain allows us to understand that pain causes suffering and adversely affects quality of life.

2) Physiology of Nociceptive Pain

When nociceptors are stimulated by touch, heat, or cold, the sensation is transmitted along sensory nerves, including unmyelinated slow-conducting C fibers and thinly myelinated larger-diameter Ad fibers to second-order neurons located in the dorsal horn of the spinal cord.

These second-order neurons, in turn, propagate the impulse along ascending tracts to the brain.

The impulse travels first to the thalamus, which provides a crude perception of pain, and then to the cerebral cortex, which identifies the precise location of the pain.

Pain perception in nociceptive pain is also modulated by descending (inhibitory) and segmental controls.

3) Other Pain Definitions

Pain term Definition
Allodynia Pain due to a stimulus that does not normally provoke pain
Analgesia Absence of pain in response to stimulation that would normally be painful
Hyperalgesia An increased response to a stimulus that is normally painful
Hyperaesthesia Increased sensitivity to stimulation, excluding the special senses
Hyperpathia A painful syndrome characterized by an abnormally painful reaction to a stimulus, especially a repetitive stimulus, as well as an increased threshold
Hypoalgesia Diminished pain in response to a normally painful stimulus
Hypoaesthesia Decreased sensitivity to stimulation, excluding the special senses

4) Classifications of Pain

Acute Pain:

  • Predictable
  • Associated with tissue damage
  • Circuitry well defined
  • Self limiting
  • Unimodal treatment
  • Responds predictably
  • Useful warning function

Chronic Pain:

  • Unpredictable
  • Relationship with tissue damage unclear
  • Circuitry unknown
  • Continues
  • Multimodal treatment
  • Difficult to treat
  • No useful warning function

Acute to Chronic Pain Continuum

The persistence of pain for more than 3 to 6 months is one criterion for classifying pain as chronic versus acute.

Traditionally, acute pain is defined as pain lasting less than 3 to 6 months. Most pain specialists view chronic pain as a pain response that persists for at least 3 to 6 months after the initial insult has resolved. In addition, the pain experienced between 1 month post-injury and less than 6 months post-injury is considered to be subacute pain.

Thus, the pain associated with the blisters that occur in patients with an acute herpes zoster infection is an example of acute pain. However, in patients who develop postherpetic neuralgia, pain persists after the blistering has resolved.

However, chronic pain can involve components of persistent nociceptive pain in response to ongoing nociceptive stimuli. An example of this is a chronic inflammatory disease such as osteoarthritis (OA).

Chronic neuropathic pain is the result of plastic nervous system changes involving either the peripheral or central nervous system.