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. 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 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. 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. 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 the spinal cord, the nonnoxious 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. 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. 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 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). 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. 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. 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.

The epidural SCS lead is placed directly within the dorsal epidural space just to one side of 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 via the L1/L2 interspace, and those for upper extremity stimulation are placed via 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 2 to 3 mm to the left or right of 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 the T10 vertebral levels, whereas upper extremity stimulation usually requires lead placement between the occiput and the C3 vertebral levels. If the lead ventures too far from midline, stimulation of the spinal nerves will result. If the lead is placed too low, overlying the conus medullaris
(at or approximately below L1/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 spinal nerve entering the spinal cord toward their eventual paramedian location several levels cephalad.

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 relatively well localized because 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 to each side of midline. When the pain is diffuse, it may be impossible to get 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 the lower extremities. The use of SCS to treat chronic, axial low back pain has been less satisfactory, but more recently 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 (RCTs) 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. More recent, small RCTs also suggest significant improvement in 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 and/or extremity pain 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 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 trial of SCS 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 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 and 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 the 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 period of trial stimulation.

There are several hundred case reports and observational trials that report on the use of SCS for a variety of painful disorders that involve the trunk and extremities. There is a limited amount of high-quality evidence available about the technique. One RCT reports effective pain relief for patients with CRPS at follow-up assessment periods to 2 years when SCS in combination with physical therapy is compared with physical therapy alone. One RCT reports effective pain relief during an assessment period of 6 months when patients with persistent pain after prior lumbar spinal surgery are treated with SCS when compared with reoperation. Studies with observational findings report that SCS also provides pain relief for other conditions (e.g., peripheral neuropathic pain, peripheral vascular disease, or postherpetic neuralgia). Reported adverse effects and complications include insertion-site pain, infection, lead dislodgement, and device/lead failure.

Expert recommendations regarding the use of SCS are largely supportive of the use of this modality in selected
patients. The American Pain Society Low Back Pain Guideline Panel published a report in 2009, concluding, “In patients with persistent and disabling radicular pain following surgery for herniated disc and no evidence of a persistently compressed nerve root, it is recommended that clinicians discuss risks and benefits of spinal cord stimulation as an option (weak recommendation, moderate-quality evidence). It is recommended that shared decision-making regarding spinal cord stimulation include a discussion about the high rate of complications following spinal cord stimulator placement.” The American Society of Anesthesiologists Task Force on Chronic Pain Management published a 2010 Practice Guideline, offering the following recommendations: “Spinal cord stimulation may be used in the multimodal treatment of persistent radicular pain in patients who have not responded to other therapies. It may also be considered for other selected patients (e.g., those with CRPS, peripheral neuropathic pain, peripheral vascular disease, or postherpetic neuralgia). Shared decision making regarding spinal cord stimulation should include a specific discussion of potential complications associated with spinal cord stimulator placement. A spinal cord stimulation trial should be performed before considering permanent implantation of a stimulation device.”

SCS is an invasive and expensive treatment modality that provides moderate long-term pain reduction in selected patients, primarily those with persistent radicular pain after prior spinal surgery. The available evidence for long-term efficacy is modest and the associated risks are low. Expert opinion from different consensus groups is consistent but highlights the empiric nature of patient selection.

Placement of a percutaneous trial spinal cord stimulator lead can be carried out in any location that is suitable for epidural catheter placement. This may be done in the operating room, but it can easily and safely be carried out in any location that allows for adequate sterile preparation of the skin and draping of the operative field and that has fluoroscopy available to guide anatomic placement. Using a strictly percutaneous trial, the trial lead is placed in the same fashion 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, discuss with the patient the location of the pocket for the impulse generator. The regions most suitable for placement are the lower quadrant of the abdomen or the lateral aspect of the buttock. Once the site is determined, mark the proposed skin incision with a permanent marker while the patient is in the sitting position. The position of the pocket is deceptively difficult to determine once the patient is lying on his or her side. If the location is not marked, the pocket is often placed too far lateral within the abdominal wall. Placement of the impulse generator within the buttock allows for 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 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 the use of full surgical attire (Fig. 16-1). The procedure must be conducted using local anesthesia and light enough sedation that the patient can report where he or she feels the electrical stimulation during lead placement.

The patient is positioned on a radiolucent table in the prone position (see Fig. 16-1). Initial lead placement can be carried out with the patient in a lateral decubitus position; however, even small degrees of rotation along the spinal axis can make positioning of the lead difficult. The arms are extended upward so they are in a position of comfort well away from the surgical field. The skin is prepared, and sterile drapes are applied. For stimulation in the low back and lower extremities, the radiographic C-arm is positioned directly over the thoracolumbar junction to provide an anteroposterior view of the spine. Care must be taken to ensure the x-ray view is not rotated by observing that the spinous processes are in the midline, halfway between the vertebral pedicles (Fig. 16-2).

The L1/L2 interspace is identified using fluoroscopy. The epidural needle supplied by the device manufacturer must be used to ensure the lead will advance through the needle without damage. The needle is advanced using a paramedian approach, starting 1 to 1.5 cm lateral to the spinous processes and somewhat caudad to the interspace to be entered. The needle is directed to enter the spinal space in the midline, with an angle of entry no >45 degrees from the plane of the epidural space (Figs. 16-2 and 16-3). If the angle of attack of the needle on initial entry into the epidural space is too great, the epidural lead will be difficult to thread as it negotiates the steep angle between the needle and the plane of the epidural space. The shallower the angle of attack, that is, the closer that the plane of the needle shaft is to the plane of the posterior epidural space, the easier it will be to thread the lead directly along the midline of the posterior epidural space. The epidural space is identified using a loss-of-resistance (LOR) technique. The electrode is then advanced through the needle and is directed to remain to one side of midline in the dorsal epidural space as it is threaded cephalad under fluoroscopic guidance (Figs. 16-4 and 16-5). The electrode contains a wire stylette with a slight angulation at the tip; gentle rotation of the electrode as it is advanced allows the operator to direct the electrode’s path within the epidural space (Fig. 16-6). For stimulation in the low back and lower extremities, the electrode is initially positioned 2 to 3 mm from the midline on the same side as the patient’s pain between the T8 and the T10 vertebral levels (Fig. 16-7

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