Electrical stimulation of the spinal cord: Neurophysiological mechanisms involved in the relief of ischemic pain

Abstract

Spinal cord stimulation is a spin-off of the well-known ‘Gate Control Theory of Pain’ presented by Melzack and Wall in 1965. SCS has been used predominantly in chronic intractable pain conditions. Careful evaluation has revealed that a significant number of patients reported pain relief accompanied by a feeling of warmth in the lower limbs during SCS. This observation has led to the use of SCS in the treatment of patients with severe ischemic rest pain. Several studies have evaluated the effects of SCS on relief of ischemic pain, microcirculatory blood flow and limb salvage. However, up to now very little is known about the physiological mechanisms behind the beneficial effects on ischemic pain. The ‘Gate Control Theory of Pain’ comprehends a model in which the nociceptive unmyelated afferents are inhibited by stimulation of non-nociceptive myelinated afferents. Although the concept of this theory is not longer tenable in all its aspects, the idea remain unchanged. At present, several hypotheses for the relief of ischemic pain under spinal cord stimulation are discussed. These mechanisms seem most dependent on activation of the lumbal segments of the spinal cord. However, supraspinal effects may also be involved. Inhibition of afferent activity in the spinothalamic tracts, suppression of sympathetic activity, antidromic effects on peripheral reflex circuits may take part in the relief of ischemic pain. Stimulation may also induce antidromically mediated release of vasoactive substances with vasodilatory effects. Besides pain relieving effects SCS has also a beneficial effect on the ischemic condition per se. Further experimental and clinical research is necessary to increase understanding of the neurophysiological and neurochemical changes activated by spinal cord stimulation. summary