Current opinion in neurology and neurosurgery
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Curr Opin Neurol Neurosurg · Apr 1993
ReviewThe spinal pharmacology of facilitation of afferent processing evoked by high-threshold afferent input of the postinjury pain state.
Repetitive C afferent input evokes a facilitated state of processing that results in increased receptive fields and exaggerated responses to afferent input ("wind-up"). These phenomena underlie the behavioral phenomena of secondary hyperalgesia and this in turn is an important component of postoperative pain. The initiation of this facilitated component is not well blocked by even higher concentrations of volatile anesthetics, but it can be prevented by pretreatment with agents known to block afferent input (local anesthetics) or C-fiber transmitter release (opiates) or to act at one of several links to block a complex spinal cascade involving the N-methyl-D-aspartate receptor, nitric oxide synthase, and cyclooxygenase. These fundamental mechanisms promise to have an impact on the management of postoperative pain.
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Neurogenic pain (encompassing all types of neuropathic and central pain) is discussed. Experimental work is presented in a model in which the rat sciatic nerve is loosely ligatured. In painful human neuropathies, tricyclic antidepressants have been found to be effective in proportion to the degree they facilitate monoaminergic activity. ⋯ In nociceptive pain, recent findings in humans emphasize the importance of both the retroinsular (SII) and the anterior cingulate cortices in the conscious appreciation of pain. Opioid studies have revealed individual differences in the metabolism of morphine to its 3- and 6-glucuronosides; patients with nociceptive pain who respond poorly to morphine or diamorphine probably have a high 3:6 ratio. It has been pointed out that methadone may be useful in such cases, as it is not broken down to glucuronosides.