Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage. Since pain is so highly subjective, it may also be described as being what the patient says it is.

Pain may be classified according to its presumed aetiology. Nociceptive pain is the result of the stimulation of nociceptors by noxious stimuli, whilst neuropathic pain is the result of dysfunction of the nervous system. These may exist together as mixed pain. There is also visceral pain, the clearest example being that associated with gallstones.

An alternative classification is based on chronicity. The point at which acute pain becomes chronic has been suggested at about 12 weeks or when the pain is no longer thought to be due to the initial insult.

Physiology

Nociceptive impulses are triggered by the stimulation of nociceptors that respond to chemical, mechanical or thermal damage. The chemical mediators that initiate (H+, K+, acetylcholine, histamine, serotonin (5-HT), bradykinin), and sensitize (prostaglandins, leukotrienes, substance P, neurokinin A, calcitonin gene-related peptide) the nociceptors are legion. Two types of primary afferent fibres exist:

• small myelinated Aδ fibres (diameter 2–5 μm) that conduct sharp pain rapidly (40 m.s−1)

• unmyelinated C fibres (diameter < 2 μm) that conduct dull pain slowly (2 m.s−1)

These fibres enter the dorsal horn of the spinal cord and synapse at different sites (Aδ at Rexed laminae II and V; C at Rexed laminae II).