Surgical Technique and Patient Selection for Spinal Cord Stimulation for Chronic Pain
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.302462
Source of Support: None, Conflict of Interest: None
Keywords: Chronic low back pain, complex regional pain syndrome, failed back surgery syndrome, SCS, spinal cord stimulation
Spinal cord stimulation (SCS) in the epidural space for the treatment of chronic pain was introduced clinically in the 1970s. Over the ensuing decades, it has become widely available and is widely used to treat chronic low back pain (CLBP), chronic neuropathic pain (CNP), chronic regional pain syndrome (CRPS), and failed back surgery syndrome (FBSS) among other indications. Initially, SCS was developed as an intradural, direct stimulation treatment; however, over time, epidural electrode placement was developed via laminotomy and eventually through percutaneous methods. Recent innovations to SCS have been independent current control of electrodes, revised contact spacing and design, novel waveforms, rechargeable batteries, and newer surgical and anesthetic techniques among many others.
Evidence of the effectiveness of SCS has grown over time as well. SCS is superior to medical management alone for FBSS as well as CNP and CLBP. Different studies have shown possible benefits of certain waveforms or programming technologies compared to others,, but these results have not been universally confirmed on repeat studies., While it is widely accepted that SCS is effective for the treatment of pain, the exact mechanism still remains unclear.
There are many proposed mechanisms of SCS. Traditional or low-frequency (2–100 Hz) SCS relies on producing paresthesias in the areas of pain by activating A-beta fibers in the dorsal columns. These A-beta fibers trigger neurotransmitter release that modulates the painful response. This activation can reduce long-term potentiation related to chronic pain as well as activating other pain relevant systems in the spinal cord. High-frequency stimulation (>1000 Hz) at subperceptible amplitudes may work by activating/modulating interneurons in Rexed lamina I, thereby improving pain without paresthesias., SCS burst waveforms (consisting of five monophasic wave bursts with 40 Hz interburst intervals and 500 Hz intraburst) potentially works by activating the sensorimotor cortex rather than working locally in the spinal cord. Significant research is ongoing and the exact mechanisms at play have yet to be fully elucidated. It is also likely that different mechanisms and programming paradigms are more effective for different pathologies and patients.
The increasing complexity of this therapy as well as the inherent complexity of the patient population make the successful application of SCS therapy difficult. The remainder of this report will focus on patient selection and technical details associated with SCS to maximize patient outcomes and patient and provider satisfaction.
SCS, like all therapies, depends on accurate patient selection for its ultimate success. The first step in patient assessment is identifying the patient's symptoms/syndrome to determine their similarity to patients involved in randomized controlled trials. Common exclusion criteria in pivotal trials were evidence of spinal instability, spine malignancy, recent surgery, a moderate Oswestry Disability Inventory (ODI), and a maximum of morphine daily equivalents.,, Despite each trial having specific criteria, actual inclusion and exclusion criteria vary widely between specific studies.
In general, a patient would be appropriate for consideration of SCS if their pain is present for >90 days. Besides, they should have attempted less invasive conservative therapy for relief. Opiate medications are not a prerequisite, and the ongoing opiate use disorder in the United States provides some evidence that early and widespread use of opiates for chronic pain can have significant negative effects on individual patients as well as society at large. Further, the patient should have screening imaging to ensure that there is no ongoing mechanical issue that should be addressed (spinal instability, severe spinal stenosis, large herniated discs, malignancy, fracture, etc.).
Similar to other surgeries, there are several absolute and contraindications to surgery. Bleeding risk should be evaluated and minimized before conducting any epidural surgery. Infection risk can be minimized by appropriate management of diabetes, smoking cessation, and appropriate preoperative antibiotics based on the local antibiogram. Anesthetic risk should also be assessed including cardiopulmonary risk factors, other conditions, etc.
If the patient's syndrome is appropriate and they have no excluding factors for SCS, the next step is neuropsychological evaluation. Reportedly, 50–80% of pain patients have comorbid psychiatric disease, making neuropsychological evaluation for all patients undergoing any surgery for pain neuromodulation mandatory. A neuropsychological evaluation can help predict patients who will or will not respond to SCS before surgery as well as identify some “red flags” and “yellow flags” inherent to the patient's psychological status that may prevent a good result. Common traits associated with poor outcomes are substance abuse, pain catastrophizing, depression/anxiety, and many others. Owing to the wide variety of factors and the expertise needed for accurate psychological diagnosis, the help of a dedicated expert will dramatically improve diagnostic accuracy.
After a thorough workup, if the patient is a potential candidate for SCS, a presurgical trial should be performed.
Trialing involves the placement of percutaneous dorsal epidural stimulator leads with the distal connections externalized. This allows delivery of therapy in a minimally invasive way to ensure that the patient has a good response before inserting the permanent system. Before trialing, imaging of the target area (thoracic or cervical spine) with magnetic resonance imaging (MRI) or computed tomography (CT) myelogram is advised to ensure there are no vascular or mechanical contraindications to lead placement such as arteriovenous malformations or areas of severe stenosis or cord compression.
Traditionally, trials are inserted in sedated, but awake patients using a standard epidural loss of resistance technique as an outpatient procedure. To accomplish this, a Touhy needle is inserted under local anesthesia toward the interlaminar space. The Tuohy needle is advanced with stylet under fluoroscopy until the tip engages with the ligamentum flavum/interspinous ligament. At this stage, a glass syringe filled with sterile, preservative-free saline, or air is attached. Gentle pulsing pressure with the thumb is used on the syringe while it is carefully advanced. When the tip of the needle is engaged in the ligament, the syringe will give backpressure and the saline or air will not flush through the needle. Once the tip of the needle enters the epidural space, loss of resistance will allow the saline or air to flush easily confirming entry. If saline is used, suction can be applied to the needle hub to ensure that any clear fluid dripping from the needle represents saline and not cerebrospinal fluid (CSF). If CSF is observed, judged by continuous dripping of fluid after suction, the needle should be withdrawn with the stylet in place and new access attempted, preferably at a different level.
Once the needle opening is in the epidural space, a lead with stylet can be advanced through the needle and steered under fluoroscopic guidance to the appropriate level. Anterior-posterior and lateral views must be used to ensure the dorsal placement of the lead [Figure 1]. Once the lead(s) are in position, the patient can be awoken from sedation and testing carried out to ensure paresthesia coverage of the painful areas. Alternatively, placement can be guided by radiology only (especially if trialing high-frequency SCS) or based on neurophysiologic testing. For CLBP/FBSS or leg pain, the leads are typically placed in the midline, spanning across the T8–T10 vertebral bodies. For thoracic pain or refractory angina, the T4 vertebral body is a good target. For neck pain/arm neuropathic pain, C2–C4 is a good starting location.
After the leads are in place, a sterile dressing is applied and the patient is assessed in recovery for initial programming. The patient will then have the device in place for 5–10 days depending on surgeon preference. Upon conclusion of the trial, the patient should be reassessed to determine the level of pain relief. Minimum relief should consist of 50% pain relief 50% of the time or more. Often, functional goals can be assessed as well, including quality/quantity of sleep, ability to ambulate, mood, need for short-acting opiates, etc. Trialing leads can be pulled in the office and the patient observed for 30 min to 1 h afterward before being discharged. If the trial is successful, plans can be made for permanent placement.
Several decisions must be made after the initial decision to proceed with implantation. The final location of the electrode should be determined from the programming done during the trial. This allows permanent placement to be guided by radiographs under general anesthesia, awake testing, or with neurophysiologic monitoring. In general, the location under the active contact(s) during the trial should be centered, as much as possible, in the permanent electrode array.
Another consideration is lead type and number. When using percutaneous electrode arrays, one lead is as effective as two leads in providing pain relief. Despite this, the reliance on paresthesia mapping for guiding low-frequency SCS therapy has made two lead systems preferred. Epidural paddle leads are another option. Epidural paddles have the theoretical advantages of being wider, having more predictable coverage, and less risk of migration and hardware complications. They also have a higher risk of perioperative complications, are more invasive, and have a more painful recovery for the patient. Despite the different advantages and disadvantages of the various choices, peri-procedural and long-term outcomes are largely similar despite the lead type chosen.,
The technique for open placement is similar to the technique for a routine thoracic laminotomy. First, the laminotomy is localized by centering the incision on the vertebral body one level below the target area of stimulation. Care should be taken to obtain accurate Anterior-Posterior (AP) images before beginning the procedure. This includes crisp superior and inferior endplates at the level of interest and the spinous process being midway between the two pedicles. Then a standard, midline incision is made and a bilateral, subperiosteal dissection is carried out. A midline laminotomy is then carried out at the interspace below the targeted stimulation area. Usually, the spinous process below the laminotomy is removed as well, to reduce the acuteness of the insertion angle of the paddle lead which reduces the risk of spinal cord injury. After the laminotomy is completed, careful cephalad dissection of the epidural space with small up-going curettes or angled dissectors is carried out. When the epidural space is free of any adhesions, the paddle is gently slid into position under the lamina superiorly until the totality of the paddle is in the epidural space. Intraoperative X-rays are taken to ensure midline placement. Under most circumstances, lateral views are not necessary. After the lead is in position, testing, whether awake or with neurophysiology, is carried out to confirm placement. Then lead anchors are applied and sutured to the supraspinous ligament or paraspinal musculature. After being anchored in position, a final X-ray is taken [Figure 1]. The thoracic fascia can then be closed in an interrupted fashion with the leads exiting inferiorly. The skin can be undermined just above the fascia and a strain relief loop can be created and anchored to the fascia.
The technique for percutaneous placement is identical to the loss of resistance technique described above with minor adjustments. The authors prefer bilateral access for two lead systems and after the leads are in position, a horizontal incision is made connecting the two percutaneous insertion points. A number 15 scalpel is used to undermine the subcutaneous tissue along the path of both Tuohy needles until an opening of sufficient size and depth to accommodate a Weitlander retractor is created. After this, the Tuohy needles and stylet are removed from the leads, taking exquisite care to not disturb the position of the electrodes. Then the anchors are applied to the leads and they are sutured to the underlying fascia in the pocket just created. Another option for percutaneous placement involves making a linear incision down to the thoracolumbar fascia. The skin is circumferentially undermined at this level. Epidural access is then obtained by puncturing through the fascia. Once again, anchors are applied and secured down to the thoracolumbar fascia.
Once the electrode of either type is in the final position, attention is paid to subcutaneous tunneling of the leads to the subcutaneous pocket where the battery will be placed. In general, this should be high enough on the posterior flank to not interfere with sitting and should be away from the patient's beltline. If using a rechargeable battery, it should be shallow enough and lateral enough to allow the patient to recharge it independently.
Permanent placement is usually done as an outpatient procedure for both paddle leads as well as percutaneous leads. Initial programming can be carried out in the postoperative period once the patient has recovered from anesthesia enough to participate in the programming process.
In 2016, the Neurostimulation Appropriateness Consensus Committee (NACC), released a statement on complications of SCS and consensus advice on complication avoidance. The complications listed as most frequent for percutaneous and open SCS lead placement were infection (3–6% for both), epidural hematoma (0.75% for percutaneous, 0.19–0.63% for paddle), spinal fluid leak (0.3% percutaneous, 0.001–0.05% for paddle), spinal cord injury (0.03–2.35% percutaneous, 0.022-0.067% paddle), and serious neurologic complications (0–2.35% percutaneous, 0.54–1.71% paddle).
For spinal fluid leak avoidance, the NACC recommended a preoperative review of imaging to avoid areas likely to give complications. It is also recommended that spinal headaches be managed conservatively as most will resolve spontaneously within 7 days. If a spinal fluid leak is persistent, an epidural blood patch can be considered.
For the avoidance of spinal cord injury, the NACC recommended light sedation to allow patient feedback during access. This is not always necessary or advisable, especially in open paddle electrode placement. In those situations, neurophysiology may be used to monitor both spinal cord injury as well as to aid in optimal lead placement. The most common causes of spinal cord injury in both percutaneous and paddle lead placements are direct trauma and epidural hematoma. New neurologic deficits after surgery may represent a surgical emergency and should be worked up and treated as such. A noncontrast CT scan should be obtained immediately to rule out acute hematoma. If this is unrevealing, a STAT MRI may be obtained, if the system is MRI compatible. Surgical exploration will likely be required for significant neurologic deficits. Other interventions that can minimize the risk of spinal cord injury would be meticulous attention to technique. Besides, long or stiff instruments in the epidural space should be avoided as they can injure the spinal cord where it is not directly visible to the surgeon.
An epidural hematoma can best be avoided by carefully avoiding epidural access or surgery in patients with disturbances of coagulation from either medications, supplements, genetics, or other sources. Other than optimizing preoperative coagulation status, careful observation during open surgery or percutaneous access can give the surgeon initial clues that coagulation status is perturbed. Although not a recommendation of the NACC, the authors routinely use topical surgical thrombin mixtures in the epidural space to minimize the risk of epidural hematomas.
Infection is the most common complication of SCS. Infection risks can be minimized by optimal diabetes management and smoking cessation. Preoperative antibiotics should be selected based on the local antibiogram to maximize effectiveness. Intrawound vancomycin can also be considered for infection prevention, while the evidence for this practice is marginal. Although there is no strong evidence for this practice, the authors routinely prescribe antibiotics for 72–120 h after implantation depending on the risk status of the patient (diabetes, smoking status, urinary issues, etc.).
SCS is an effective neuromodulatory treatment for chronic pain. The evidence for SCS is strong and reproducible. Newer technology continues to offer potential improvements in the therapy, but evidence for increased efficacy of these developments lag behind their commercial availability. SCS can be provided to patients safely and effectively by adhering to best practices and meticulous surgical technique.
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