Visceral pain: mechanisms of peripheral and central sensitization.

This paper describes the responses of peripheral and central visceral nociceptive systems to acute injury and discusses these observations in relation to the concept of 'pre-emptive analgesia'. Visceral nociceptors are known to respond to injury but are also known to become sensitized to non-noxious stimuli during the inflammatory process that follows intense noxious stimulation. The afferent barrages triggered in visceral nociceptors by the acute injury and the enhanced responses evoked in sensitized nociceptors during the repair process can, in turn, increase the excitability of central nociceptive systems. The maintenance of central hypersensitivity is, however, dependant on the continuing presence of afferent volleys from sensitized nociceptors because the central changes cannot be sustained in the absence of a peripheral drive. Therefore it is proposed that the concept of 'pre-emptive analgesia', as such, has no neurophysiological basis. Any analgesic procedure aimed at reducing postoperative pain must not only prevent the arrival in the CNS of the initial afferent barrage evoked in nociceptive endings but also reduce or eliminate the persistent discharges of sensitized nociceptors during the inflammatory repair process that are critically important for the maintenance of the central pain state.