Abstract
Spinal cord stimulation provides a nondestructive and adjustable method of pain relief for several neuropathic pain syndromes, including failed back surgery syndrome and complex regional pain syndrome. Optimizing pain relief in these patients relies on proper patient selection, understanding of the implanted hardware, and proper preoperative planning and intraoperative technique. Compared with percutaneously implanted cylindrical leads, paddle electrodes that are surgically implanted offer improved paresthesia and pain coverage, power consumption, and fewer needs for revision due to lead migration at the cost of additional anesthetic needs and a more invasive procedure. Several considerations must be made in the perioperative setting to minimize complications associated with surgical implantation and maximize patient satisfaction. This chapter will discuss common indications, pitfalls to avoid, and basic surgical technique for implantation of surgical paddle electrodes for spinal cord stimulation.
Keywords
Burst stimulation, Complex regional pain syndrome, Complication avoidance, Failed back surgery syndrome, HF10 therapy, Paddle electrode, Postlaminectomy syndrome, Spinal cord stimulation, Surgical leads
Introduction
Spinal cord stimulation (SCS) provides a nondestructive and adjustable method of providing symptomatic relief of neuropathic pain across several indications. Based on Melzak and Wall’s Gate Control Theory of Pain ( ), electrical stimulation of dorsal column fibers within the spinal cord replaces painful dysesthesia with a more tolerable tingling paresthesia ( ). Recently, paresthesia-free SCS has been explored, and perhaps a new mechanism of pain control will be established ( ). Many methods and configurations of SCS currently exist on the market, each with their unique advantages and limitations that can be selected to optimize coverage of these dorsal column fibers ( ). Understanding the principles that influence the neuromodulatory effect of SCS during the preoperative selection of patient and device and the operative implantation of each device within a specific patient’s anatomy is critical to optimizing pain relief and functional improvement.
Common Indications
Failed Back Surgery Syndrome
Failed back surgery syndrome (FBSS), or postlaminectomy syndrome, is a spectrum of chronic axial and limb pain refractory to, or exacerbated by, previous surgical interventions ( ). Typically, these patients describe axial low lumbar back pain, buttock pain, and limb pain without clear anatomic or dermatomal distributions or mechanical causes. Although poorly defined as a singular entity, chronic inflammation in the setting of arachnoiditis, radiculitis, microinstability, and recurrent disk herniations have been implicated in the pathophysiology of FBSS. However, patients who develop FBSS represents a group of chronic back pain patients who may have a neuropathic component to their pain complex and may respond to SCS therapy ( ). In randomized controlled trials completed to date, SCS has demonstrated significant benefit for patients with FBSS compared with repeat spine surgery in pain relief and opiate consumption at 3-year postoperatively ( ) and health-related quality of life measures at 6 months ( ).
Complex Regional Pain Syndrome
Complex regional pain syndrome (CRPS), or reflex sympathetic dystrophy (RSD), is chronic and progressive dysregulation of the peripheral autonomic nervous system resulting in hyperalgesia and allodynia, swelling, and progressive tissue injury of the affected limb ( ). Neurogenic inflammation leads to an abnormal vasomotor response within the tissue. CRPS type I occurs after a noxious stimulus, often nontraumatic, which initiates an inappropriate response to painful stimulation. CRPS type II occurs after nerve injury, resulting in a similar pattern of maladaptive nociception and physiologic response. Multiple trials support the use of SCS therapy for durable analgesia, which has resulted in significant improvements in patient quality of life ( ).
Patient Selection
Successful SCS implantation relies on several factors, but the most important may be the careful selection of patients who have a pain complex that can be reliably targeted without significant comorbidities that may interfere with a patient’s perception of pain and subsequent pain relief ( ). Careful history and physical examination to differentiate spinal origin pain disorders from other musculoskeletal and peripheral nerve pain disorders such as bursitis, piriformis syndrome, and fibromyalgia are essential in isolating the appropriate patient for SCS and guiding inappropriate patients toward other modalities of pain control.
Psychological Screening
Although formal psychological screening is not routinely performed on potential candidates for SCS, it is imperative to understand the psychological and psychiatric factors that contribute to a patient’s pain complex ( ). Several tools such as the Minnesota Multiphasic Personality Inventory (MMPI) and McGill Pain Questionnaire (MPQ) have been used in the past to quantify the psychological component of pain. Patients with higher scores within the MMPI depression subscale are more likely to report inadequate pain relief ( ). Further, major depressive disorder, somatization disorders, and psychosis can alter self-perception and reporting of pain severity and should be considered relative contraindications to successful implantation ( ). Social issues such as Worker’s Compensation and patients with unreasonable expectations should also be carefully counseled before attempting SCS because their functional recovery and satisfaction may be limited.
Defined expectations influence patient therapy outcome. Establishing clear-cut patient goals in chronic pain management is a core part of patient selection. Complete termination of pain is usually not a valid expected outcome, and it must be iterated that SCS is an important part of treatment for the chronic pain syndrome.
Opiate Use
Quantification of preoperative opiate consumption can be helpful in determining the expected response to SCS. In a recent presentation, it was noted that patients demonstrated increasing dosages of opioids the year before SCS implantation. After SCS was performed, SCS levels stabilized or decreased in all patients who had successful use of device. Patients who failed, or were explanted, continued high opioid use ( ).
Additionally, reducing opiate use can often be an important metric in measuring successful outcomes to stimulation. Analysis of SCS patients with high (>100 mg) versus low consumption (<100 mg) of morphine equivalents preoperatively shows that patients have an increased postoperative length of stay and need for patient-controlled analgesia consistent with decreased pain tolerance and increased opiate dependency ( ). Also, these patients with high opiate needs are more likely to develop postoperative fevers and incur higher risk of nosocomial complications such as pneumonia, deep vein thrombosis, or ileus. Preoperative weaning of opiates in coordination with pain management specialists can help reduce these risks and should be a goal before proceeding with permanent implantation.
Screening Trials
Although SCS is a reversible, nonlesional pain control technique, temporary implantation of percutaneous trial leads or implantation of paddle electrodes connected to an external pulse generator for trial stimulation is a reasonable method of further screening patients who are appropriate for SCS before permanent implantation of the pulse generation. Various stimulator programs are used to determine optimal pain relief before the final positioning of epidural electrodes and the implantation of a permanent pulse generator. Patients are typically maintained on oral antibiotics such as cephalexin to minimize infection risk with these external electrodes.
Anatomic Mapping
Low back pain is often ill defined and vague, and poor metrics exist to quantify and describe the anatomy location of symptoms and subsequent paresthesia coverage. One common definition of the back is from the 12th rib cranially to the top of the gluteal folds caudally and subsequently subdivided into nine anatomic regions that are separated by palpable landmarks ( Fig. 37.1 ). This provides consistent visual maps of preoperative pain and postoperative paresthesia and allows for analysis of stimulation coverage and comparisons to be made between successful SCS and SCS failure.
Lead Geometry and Canal Morphometry
Compared with cylindrical percutaneous electrodes, paddle electrodes offer better coverage and lower power consumption due to their intrinsic insulation and closer proximity to dorsal columns ( ). Importantly, paddle electrodes also appear to provide a more durable pain control over a 2-year period ( ). Currently, paddle electrodes are available on the market along a wide spectrum, including two-, three-, four-, and five-column designs ( Fig. 37.2 ) that are meant to provide greater coverage of the dorsal column with additional points of contact that enable precise shaping of the electric field. Despite the increase in programming flexibility, early outcomes comparing two-, three-, four-, and five-column leads demonstrates similar outcomes in reported differences in Visual Analog Scale and Oswestry Disability Index. Subtle differences exist in the distribution of paresthesia coverage with two-column leads demonstrating greatest paresthesia coverage in the legs, three-column leads in the low back and buttocks, and five-column leads in the low back, buttocks, and legs; however, none of these differences reach statistical significance in a small retrospective review ( ).
Another similar held belief is that larger and wider electrodes reduce the effective distance to dura and minimize stimulation thresholds to achieve activation of dorsal column fibers. One measure of contact distance is the surgical paddle electrode effect ratio (SPEER), quantified as the radiographic distance from the implanted electrode to the anterior aspect of the spinal canal as a ratio of the preoperative spinal canal diameter. Larger paddle electrodes had smaller SPEERs, suggesting closer contact with the spinal canal; however, subjective outcomes were similar across all types of lead geometries (60% vs. 50% vs. 57% positive response in two-, three-, and five-column leads, respectively), suggesting other factors may play a role in determining successful implantation ( ).
Surgical Technique
Paddle electrodes are implanted under direct visualization of the epidural space via single- or multiple-level laminotomies at, or just inferior to, the desired spinal level of pain coverage. The preoperative workup should include thoracic (or cervical) spine magnetic resonance imaging to ensure that the spinal canal and epidural space can accommodate the desired electrode and to avoid anatomic barriers such as ligamentous hypertrophy, osteophytes, or severe scoliotic deformities. It is also useful to obtain plain radiographs of the lumbar and thoracic spine as there can be segmentation anomalies and normal variation in the total number of lumbosacral vertebra and ribs, which can significantly impact intraoperative localization. During permanent implantation, it is important to replicate the same spinal level as the trial stimulation to maximize efficacy of stimulation. For example, in one patient with 11 ribs and a 12th thoracic vertebra without an associated rib, it is important to recognize this anatomic variation before implantation of permanent electrodes ( Fig. 37.3A and B ).
Practitioners have the choice of implantation with monitored sedation (local anesthesia and intravenous sedation) or general anesthesia ( ). Monitored sedation allows for immediate feedback from the patient regarding stimulation effects and can help optimize paresthesia coverage to the painful region. This requires careful titration of intravenous sedation during the surgical exposure to minimize discomfort while at the same time allowing for a cooperative and coherent patient when desired. However, patients have unpredictable responses to sedation and often wake up during the procedure confused and disoriented, unable to cooperate fully with the required questions. Patients will often react to undesired stimulation and move unexpectedly, resulting in a stressful experience for the patient and challenging the surgeon to properly position the electrodes. This leads to an increased risk of inappropriate positioning, lead migration, and, ultimately, treatment failure ( ).
The trend for many practitioners is to place electrodes with the patient under general anesthesia, providing reliable analgesia and better control over final positioning ( ). Rather than relying on patient-reported coverage, electrodes are placed in a midline position at preplanned spinal levels corresponding to the patient’s pain relief during screening trials and tested physiologically with intraoperative neuromonitoring via electromyography (EMG) and somatosensory evoked potentials (SSEPs) ( ). With the patient under general anesthesia, conditions are kept optimum for visualizing the placement of the electrode, thus minimizing the risk of lead migration. Once in position, EMG stimulation with symmetric activation into bilateral extremities confirms a physiologic midline position along the dorsal columns. Paddle electrodes with 16, 20, or 32 electrode contact points allow various implantable pulse generators (IPGs) increasingly more freedom to tailor paresthesia coverage in a postoperative, postanesthetic setting rather than requiring the patient to report pain relief intraoperatively.
Regardless of technique, either monitored sedation with awake testing or electrophysiologic positioning with trial reproduction strategy is one of the most common techniques used to confirm paddle level implantation.
The surgical approach for paddle insertion, regardless of type of anesthesia, remains relatively consistent ( ). Fluoroscopy or plain radiography is obtained with the patient in a prone or lateral position localizing to the lower thoracic levels. Typical levels of implantation are between T8 and T10 ( Fig. 37.4 ) depending on preoperatively generated pain maps and successful trial stimulation positions. Recent results with HF10 therapy demonstrate that paddle placement centered at T9-T10 provides the best efficacy for non–paresthesia-based SCS ( ). To land the electrode at the appropriate position, laminotomies should be performed at T11 and T12. Laminotomies at T10 and T11 can also be used, but the superior aspect of T11 lamina will often need to be removed to allow the paddle to appropriately span the T9-T10 level.
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