Definition and Terminology

Pain is an unpleasant sensory and emotional experience that is associated with actual or potential tissue damage or described in terms of such damage (The International Association for the Study of Pain). Different categories of pain can be defined according to the duration, etiology, or perception of the painful experience.

1. Acute pain is pain that is secondary to physical injury to body tissues and usually resolves as the wound heals. Improved control of acute pain following surgery has become a focus for practice improvement measures leading to a more timely and diversified approach to pain therapy. Studies demonstrate that techniques that effectively reduce acute pain are associated with a lower incidence of subsequent chronic pain.
2. Chronic pain is pain that continues for months or longer and is often defined as pain persisting beyond 3 to 6 months. Common chronic pain conditions include low back pain, complex regional pain syndrome, postherpetic neuralgia, cancer pain, and myofascial pain.
3. Neuropathic pain results from pathologic function of the somatosensory system, either in peripheral elements (receptor or peripheral nerves) or in the central nervous system. The abnormal somatosensory function of neuropathic pain is the direct result of injury to the nervous system, and this type of pain persists even after tissue healing appears to be complete. It is most frequently described as burning, radiating, lancinating, or shooting in nature. Neuropathic pain can result in allodynia, which is the perception of pain from a normally innocuous stimulus, for example, light touch perceived as pain.
4. Nociceptive pain results from injury that activates peripheral nociceptors, which can be somatic or visceral in origin, and is the well-localized pain associated with most acute injuries. Visceral pain arises from distention or injury to the viscera and is usually less well localized than somatic pain, owing to the less dense innervation of the organs as compared to other tissues.
5. Inflammatory pain categorized as nociceptive pain in the presence of acute inflammation, but chronic inflammatory states can play mechanistic roles in neuropathic pain states (Loeser & Treede, Kyoto). Inflammation resulting from tissue damage can lead to hyperalgesia, which is the exaggerated painful perception of a known noxious stimulus.