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AnalgesicsPain 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 initi-
ate (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 Aδfibres (diameter 2–5μm) that conduct sharp pain rapidly
(40 m.s−^1 )unmyelinated C fibres (diameter < 2μm) 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). The substantia gelatinosa (lamina
II) integrates these inputs, from where second-order neurones form the ascending
spinothalamic and spinoreticular pathways on the contralateral side. Descending
pathways and the larger Aβfibres conducting ‘touch’ stimulate inhibitory interneu-
rones within the substantia gelatinosa and inhibit C fibre nociceptive inputs. This
forms the basis of the ‘gate theory’ of pain (Figure9.1).
Pain may be modified by altering the neural pathway from its origin at the noci-
ceptor to its interpretation within the central nervous system. The commonly used
agents are discussed below under the following headings:
Opioids and related drugs
Non-steroidal anti-inflammatory drugs (NSAIDs)