Spinal Cord Stimulation

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51 Spinal Cord Stimulation

James P. Rathmell

Perspective

The idea that direct stimulation of the ascending sensory tracts of 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. With 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 through 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 after injury, and anatomic changes, cell death, and altered gene expression are all likely to have a role in the development of 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 empiric 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 it is relatively well localized because the success of SCS depends 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 on 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 more promising 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 who are treated with SCS in conjunction with physical therapy 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 after implantation) in mixed groups of patients with ongoing low-back or extremity pain (or both) after lumbar surgery. The usefulness of psychological screening before 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 after 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.

Placement

Anatomy

The epidural SCS lead is placed directly into the dorsal epidural space just to one side of the midline using a paramedian, interlaminar approach. Entry into the epidural space is performed several levels below the final intended level of lead placement. Typically, leads for stimulation of the low back and lower extremities are placed through the LI-L2 interspace, and those for upper extremity stimulation are placed through the C7-T1 interspace. Investigators have mapped the patterns of electrical stimulation of the dorsal columns and the corresponding patterns of coverage reported by patients with leads in various locations. In general, the epidural lead must be positioned just 2 to 3 mm to the left or right of the midline on the same side as the painful region to be covered. For lower extremity stimulation, successful coverage is usually achieved by placing the lead between the T8 and T10 vertebral levels, whereas upper extremity stimulation usually requires lead placement between the occiput and C3 vertebral levels. If the lead ventures too far from the midline, uncomfortable stimulation of the spinal nerves may result. If the lead is placed too low, overlying the conus medullaris (at or below Ll-L2), unpredictable patterns of stimulation may result. In the region of the conus, the fibers of the dorsal columns do not lie parallel to the midline; rather, they arc from the corresponding nerve entering the spinal cord toward their eventual paramedian location several levels cephalad.

Position

A percutaneous trial spinal cord stimulator lead can be placed in any location that is suitable for epidural catheter placement. This procedure may be done in the operating room but can easily and safely be carried out in any location that allows adequate sterile preparation of the skin and draping of the operative field and that has fluoroscopy available to guide anatomic placement. In a strictly percutaneous trial, the trial lead is placed in the same fashion as that used for permanent lead placement, but the lead is secured to the skin without any incision for the trial period.

Before permanent spinal cord stimulator implantation, one must discuss with the patient the location of the pocket for the impulse generator. The most suitable regions are the lower quadrant of the abdomen and the lateral aspect of the buttock. Once the site is determined, the proposed skin incision is marked with a permanent marker while the patient is in the sitting position. The position of the pocket is deceptively difficult to determine once a patient is lying on his or her side. If the location is not marked, the pocket is often placed too far laterally in the abdominal wall or buttock. Placing the impulse generator in the buttock allows the entire procedure to be carried out with the patient in the prone position and simplifies the operation by obviating the need to turn from the prone to the lateral position halfway through implantation.

Implantation of a spinal cord stimulator lead and impulse generator is a minor surgical procedure that is carried out in the operating room using aseptic precautions, including skin preparation, sterile draping, and full surgical attire (Fig. 51-1A). The procedure must be conducted using local anesthesia and sedation light enough that the patient can report feeling the electrical stimulation during lead placement. The patient is positioned on a radiolucent table in the prone position (see Fig. 51-1A

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