The epidural SCS lead is placed directly within the dorsal epidural space just to one side of midline using a paramedian, interlaminar approach. The mechanism remains unclear, but direct electrical stimulation within the dorsal columns may produce retrograde changes within the ascending sensory fibers that modulate the intensity of incoming noxious stimuli. Direct electrical stimulation of the dorsal columns (referred to as SCS or dorsal column stimulation) has proven effective, particularly in the treatment of chronic radicular pain. Both the peripheral nerve fibers and the second-order neurons within the dorsal horn responsible for pain transmission become sensitized following injury, and anatomic changes, cell death, and altered gene expression are all likely to have a role in chronic pain. It is unlikely that the simplistic notion of a gate within the dorsal horn is responsible for our observations, but the theory served as a useful concept in the development of spinal cord stimulation (SCS). We have learned much about the anatomy and physiology of pain perception since the gate-control theory was first proposed. From this theory, investigators developed the concept of direct activation of the ascending fibers within the dorsal columns that transmit nonpainful cutaneous stimuli (e.g., light touch) as a means of treating chronic pain. Thus, the light touch of rubbing an injured region or the pleasant electrical stimulation of TENS closes the gate to the noxious input of chronic pain. If these same neurons receive input from other sensory fibers entering the spinal cord, the nonnoxious input can effectively close the gate, preventing simultaneous transmission of noxious input. In their theory, they proposed that second-order neurons at the level of the spinal cord dorsal horn act as a “gate” through which noxious stimuli must pass to reach higher centers in the brain and be perceived as pain. In 1965, Patrick Wall, a neurophysiologist exploring the basic physiologic mechanisms of pain transmission, and Ronald Melzack, a psychologist working with patients who had chronic pain, together proposed the gate-control theory to explain how nonnoxious stimulation can reduce pain perception. The advent of transcutaneous electrical nerve stimulation (TENS), whereby a light, pleasant electrical current is passed through surface electrodes in the region of ongoing pain, reinforced the observation that stimulation of sensory pathways reduces pain perception in chronic pain states. We are all familiar with the fact that rubbing an area that has just been injured seemingly reduces the amount of pain coming from the injured region. The idea that direct stimulation of the ascending sensory tracts within the spinal cord might interfere with the perception of chronic pain is founded in everyday observations.
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