Pain
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The effects of electrical stimulation of cervical vagal afferents (VAS) on the background activity and on the responses of 25 spinothalamic tract (STT) neurons to noxious stimuli were studied in anesthetized rats. Background (spontaneous) activity of 9 (36%) STT neurons was inhibited by all intensities of VAS. 6 (24%) units were facilitated at lesser and inhibited at greater intensities of VAS, 5 (20%) units were only facilitated by all intensities of VAS, and 5 (20%) units were not affected by VAS. ⋯ There were no significant differences in VAS-produced modulatory effects between STT neurons and 16 unidentified lumbar spinal dorsal horn neurons studied under the same conditions. These results reveal that descending facilitatory and inhibitory pathways engaged by activation of vagal afferents modulate rostrally projecting nociceptive transmission neurons in the spinal cord, constituting an important regulatory network for nociception.
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Repetitive stimulation of small diameter primary afferent fibres produces a progressive increase in action potential discharge (windup) and a prolonged increase in the excitability of neurones in the spinal cord following the stimulus. Previous studies have demonstrated that windup is the consequence of the temporal summation of slow synaptic potentials and that the slow potentials and windup are reduced by pretreatment with N-methyl-D-aspartic acid (NMDA) antagonists. We have now examined whether primary afferent induced hypersensitivity states in flexor motoneurones are also dependent on the activation of NMDA receptors and whether windup is a possible trigger for the production of the central hypersensitivity. ⋯ When the the MK-801 and the D-CPP were administered once a state of central facilitation had been induced by prior treatment with mustard oil, they returned the facilitated reflex to its pretreatment level. These results indicate that NMDA receptors are involved in the induction and maintenance of the central sensitization produced by high threshold primary afferent inputs. Because central sensitization is likely to contribute to the post-injury pain hypersensitivity states in man, these data have a bearing both on the potential role of NMDA antagonists for pre-emptive analgesia and for treating established pain states.
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Spinal cord stimulation has evolved over the past 20 years into an easily implemented technique, with low morbidity, for the treatment of intractable, chronic pain in properly selected patients. We report our experience with a series of 62 patients implanted between 1983 and 1987, with percutaneous and laminectomy electrodes, and with single- and "multi-channel" (programmable, multi-contact) devices. Fifty had chronic, intractable low back and leg pain ("failed back surgery syndrome," lumbar arachnoid fibrosis), five had spinal cord injuries, and seven "peripheral" pathology or stump pain. ⋯ Univariate and multivariate analysis of patient characteristics and technical factors as predictors of outcome demonstrated significant advantages for female patients, and for patients implanted with "multi-channel" devices. With these devices, electrode geometries with central cathode(s) flanked by rostral and caudal anode(s) were favored disproportionately. Technical improvements in implanted spinal cord stimulation devices, in particular the development of multi-contact percutaneous electrode arrays and supporting programmable electronics, have significantly improved clinical results.
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In a prospective open study we tested the Edmonton injector (EI), a device designed by our group for subcutaneous injection of narcotics. In 25 patients, the EI was used for patient-controlled analgesia (PCA); mean duration of treatment was 28 +/- 10 days. Mean equivalent daily dose of morphine was 160 +/- 85 mg. ⋯ Narcotics were administered every 4 h and extra doses were administered on a 'PRN' basis. Waiting time for a 'PRN' dose in 8 patients before starting the EI was 18 +/- 7 min; after the EI was started, it was 3 +/- 4 min (P less than 0.01). We conclude that the EI is safe, effective and inexpensive, and that it will be particularly useful in smaller hospitals and developing countries.