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Brown: Atlas of Regional Anesthesia, 3rd ed., Copyright © 2006 Saunders, An Imprint of Elsevier
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Chapter 46 – Spinal Cord Stimulation

James P. Rathmell, M.D.

PERSPECTIVE

The idea that direct stimulation of the ascending sensory tracts in the spinal cord might interfere with the perception of chronic pain is founded on everyday observations. We are all familiar with the fact that rubbing an area that has just been injured seemingly reduces the amount of pain coming from that injured region. The advent of transcutaneous electrical nerve stimulation (TENS), wherein 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. 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 non-noxious stimulation can reduce pain perception. 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. If these same neurons receive input from other sensory fibers entering via the same set of neurons in the spinal cord, the non-noxious input can effectively close the gate, preventing simultaneous transmission of noxious input. 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. Based on this theory, investigators developed the concept of direct activation of the ascending fibers in the dorsal columns that transmit nonpainful cutaneous stimuli (e.g., light touch) as a means of treating chronic pain. We have learned much about the anatomy and physiology of pain perception since the gate control theory was first proposed. It is unlikely that the simplistic notion of a gate in the dorsal horn is responsible for our observations, but the theory served as a useful concept in the development of spinal cord stimulation. Both the peripheral nerve fibers and second-order neurons in the dorsal horn that transmit pain signals become sensitized following injury; and anatomic changes, cell death, and altered gene expression are all likely to have a role leading to chronic pain. Direct electrical stimulation of the dorsal columns, referred to as spinal cord stimulation (SCS) or dorsal column stimulation, has proven effective, particularly in the treatment of chronic radicular pain. The mechanism remains unclear, but direct electrical stimulation in the dorsal columns may produce retrograde changes in the ascending sensory fibers that modulate the intensity of incoming noxious stimuli.

Patient selection for SCS is empirical and remains a subject of some debate. In general, SCS is reserved for patients with severe pain that does not respond to conservative treatment. The pain responds best when relatively well localized because the success of SCS is dependent on the ability to cover the entire painful region with electrical stimulation. Attaining adequate coverage is more difficult when pain is bilateral, often requiring two leads, one to each side of the midline. When the pain is diffuse, it may be impossible to obtain effective coverage with stimulation using SCS. Among the best-established indications for SCS is chronic radicular pain with or without radiculopathy in either the upper or lower extremities. Use of SCS to treat chronic, axial low back pain has been less satisfactory, but recent results seem to be improving with the advent of dual lead systems and electrode arrays that allow for a broad area of stimulation. Randomized controlled trials comparing SCS with repeat surgery for patients with failed back surgery syndrome have demonstrated greater success in attaining satisfactory pain relief in those treated with SCS. Recent small randomized, controlled trials also suggest significantly improved pain relief and physical function in patients with complex regional pain syndrome (CRPS) who are treated with SCS in conjunction with physical therapy when compared with physical therapy alone. Prospective observational studies indicate an overall success rate of about 50% (defined as at least 50% pain reduction and ongoing use of SCS 5 years following implantation) in mixed groups of patients with ongoing low back or extremity pain (or both) following prior lumbar surgery. The usefulness of psychological screening prior to SCS remains controversial; some investigators have suggested that screening for patients with personality disorders, somatoform disorder, or hypochondriasis may improve the success rate of SCS.

Once a patient is selected for therapy with SCS, a trial is carried out. Most physicians now conduct trials by placing a temporary, percutaneous epidural lead and conducting the screening using an external device as an outpatient procedure to judge the effectiveness of this therapy before a permanent system is implanted. Some carry out the SCS trial using a surgically implanted lead that is tunneled using a lead extension that exits percutaneously. The strictly percutaneous trial lead is simpler to place and does not require a full operating room setup, but the lead must be removed and replaced surgically following a successful trial. The surgically implanted trial lead requires placement in the operating room, with surgical removal if the trial is unsuccessful. If the trial is successful, the implanted trial lead can remain and the second procedure to place the impulse generator is brief, not requiring placement of a new epidural lead. In either case, after successful trial stimulation, a permanent system is placed, and the lead is positioned to produce the same pattern of stimulation that afforded pain relief during the trial stimulation.

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