Introduction

Spinal cord stimulation (SCS) is a treatment for chronic intractable pain that was introduced by Shealy in 1967. With technological advances, SCS has become an increasingly effective treatment for those with many types of chronic pain conditions. In 2007 it is estimated that a total of 27,484 SCS implants occurred over all major payer types in the United States. The majority of patients have a positive outcome involving a reduction of pain with no untoward events. Unfortunately complications of SCS occur just as complications occur with all other interventional procedures. The goal of this chapter is provide information regarding the types and prevalence of complications and trends noted. Anecdotal information can also be useful as a way to increase awareness. Increased awareness of potential contributors of untoward events may help to decrease the likelihood of complications. When appropriate, recommendations will be proffered.

In an epidemiological report published in 1999, more than 1 million injuries and nearly 100,000 deaths occur annually as a result of errors in medical care. According to the systematic review by Turner and associates of 22 studies involving patients with diagnoses, including failed back surgery syndrome or complex regional pain syndrome ( Table 15-1 ), complication rates associated with SCS are noted to be 34%.

Cameron reported a mean complication rate of 36% in her review of the literature involving 68 studies and 3679 patients. She divided the complications into the categories of technical (27.2%), biological (4%), and others (5%). Different case series report that between one quarter and one third of patients undergo at least one SCS revision surgery to reverse a complication. The majority of complications associated with SCS are treatable conditions that may require simple treatments or minor surgical revisions. Timely recognition and treatment of complications reduce the likelihood that the condition progresses into an untreatable or permanent complication.

Prospective studies regarding complications of interventional procedures are rare. A summary of device-related complications from three prospective randomized SCS studies is presented in Table 15-2 .

Kumar and associates published a prospective study regarding SCS in 2007. They compared conventional medical management (CMM) to SCS for patients with failed back surgery syndrome resulting in residual predominant leg pain. One hundred patients were randomized to SCS or CMM, and cross over was permitted. Ultimately 84 patients received an electrode during the 12 months of the study. A total of 27 (32%) patients experienced a total of 40 device-related complications. Surgery was required to resolve the issues of 20 (24%) patients. Hardware-related issues, including lead migration, damage to the leads, and generator migration, accounted for 13 of the events. Six patients had loss of therapeutic effect, loss of paresthesia, or unpleasant paresthesia. Biological complications totaled 16 events and included infection or wound breakdown, pain at the incision site, and fluid collection in the pocket. Iatrogenic issues related to technique numbered five events: dural tear during implant, lead damage during implant, anterior migration of lead during implantation, suboptimal connection of the extension to the generator, and a cap not installed on the generator when one lead was implanted. This prospective study demonstrates the effectiveness of SCS while illustrating the point that complications can be a significant limiting aspect of the modality and must be factored into the decision-making process by both physician and patient when SCS is being considered as a treatment.

Complication rates are often extrapolated from observations in prospective studies, case reports, retrospective reviews, and closed claim studies. Avoidance of complications requires awareness and vigilance. Strict adherence to evidence-based surgical techniques is also mandatory to give the patient the best opportunity for a successful outcome. Most complications of SCS are not life threatening and can be resolved by explantation of the system.

Revisions of SCS systems are minimally invasive procedures that are rarely done on an emergent basis, and such procedures are associated with little permanent morbidity. However, repeated procedures expose patients to further risk and continuing disability and increase costs to the health care system. In the retrospective analysis by Rosenow and associates, the authors reviewed the charts of 289 patients who underwent 577 procedures. Hardware revision was required in 46% of patients, and nearly half (48.9%) of those patients underwent more than one revision.

In the 2-year follow up to the randomized controlled trial of effect of SCS on chronic reflex sympathetic dystrophy, Kemler reported that complications occurred in 38% of patients. However, he also states that, because SCS is a lifelong therapy, it is recognized that the incidence of complications is reduced after the first year. He concludes by asserting that SCS is safe and effective if there is careful patient selection and test stimulation.

Furthermore, Kumar reported that biological complications are more prevalent within the first 3 months after implantation, whereas hardware-related complications are likely to occur in the first 2 years after implantation. Despite an overall complication rate of 35%, incidence of life-threatening complications has been shown to be very rare in two large retrospective studies.

Complications to be discussed include those related to the technology itself, biological complications, and other complications. Technical complications include electrode migration, electrode breakage, hardware malfunction, and other complications related to the hardware. Biologic complications include infection, seroma, headache, neurological sequelae, gastroenterological effects, urologic effects, pain at the implantation site, and allergic reaction. Other complications include undesirable stimulation, changes in stimulation with position changes, system tolerance, and skin erosion.

Technical Complications

Technical complications are typically issues related to the hardware itself. The complications may occur because of operator error or negligence, but they also may occur as a result of limitations inherent in the SCS systems that are available at the time of implantation. Technological improvements are capable of reducing the incidence of complications. Van Buyten discussed the effect of technological improvements and their effect on the overall complication rate. Devulder and associates published a report of 69 patients with implanted SCS systems who were followed for up to 8 years. Of these 69 patients, there were 174 revision surgeries. Battery replacements accounted for 67 of the surgeries. Electrode migration, insufficient stimulation paresthesias in the painful area, technical failure, and electrode breakage made up the remaining 107 surgeries. Most cases of migration of the electrodes occurred in cases in which the electrodes were implanted in the cervical spine. During revision surgeries for lead migration, attempts at anchoring the leads to bony or ligamentous elements helped to ensure that the lead would not migrate again. Epidural fibrosis tended to respond to lead revision as well.

Andersen reviewed the complications associated with 60 patients with intractable angina who had SCS systems implanted. The first 22 patients had unipolar electrodes implanted, whereas the remaining 38 had quadripolar systems. The most frequent complication was electrode migration. The unipolar systems migrated in 10 patients; 11 of the quadripolar electrodes migrated. This difference is not statistically significant. All of the unipolar lead migrations required reoperation, but only 4 of the 11 patients with quadripolar electrode migrations underwent reoperation. By increasing the number of contacts, the area that is covered by stimulation increases; thus the placement of the electrode is more forgiving if there is a minor migration. The technological advance of progressing from unipolar to quadripolar electrodes reduces the number of reoperations, which in turn decreases the risk of further surgical complications as a larger area of the spinal cord is covered.

In Cameron’s literature review of 2972 patients treated with SCS, the most common complication was electrode migration. Kumar and associates noted that electrodes in the cervical spine were twice as likely to migrate as electrodes in the lumbar spine. This occurs because of the increased mobility of the cervical spine compared to the lumbar spine. Directional forces on the lead determine the direction of the displacement, whether laterally or longitudinally. Displacement occurs when the tensile load on the electrode exceeds the capacity of the anchor to stabilize it. Tensile load changes with range of motion of the spine, the position of the generator, and the elasticity of the electrode and the surrounding tissues. When the generator was implanted in the gluteal region (3 [21%] of 14), the electrodes were more likely to move than when the generator was implanted in the abdominal wall (15 [10%] of 146). The authors postulate that there is more traction on the electrode with lumbar range of motion if the generator is in the gluteal region.

In 2006 Pyles presented the notion of a single incision for the implantation of the SCS device in the lumbar spine. Kumar suggested that the traditional placement of the generator in the buttock pocket might lead to increased strain and fulcrum effect on the leads and anchors. Having only one incision may decrease the amount of postoperative pain that patients experience ( Fig. 15-1 ). There is also reduced operating time that may reduce the likelihood of time-sensitive complications such as infection. A potential complication that is unique to this technique is that revision surgery may be more difficult if the generator is enveloped in the same planes of scar tissue as the electrodes and the anchors.

A, One-incision technique above a laminectomy surgical incision. B, One-incision technique.

Another technique to avoid electrode migration was described by Kumar. He warns that migration is more likely if the nose of the anchor is not pushed through the deep fascia layer and the anchor is secured to that layer. Adding a strain relief loop may also decrease the likelihood of lead migration by reducing tension on the electrode when the patient is moving.

In his review of 10 years of experience at a single center in Belgium, Van Buyten found that most complications were of a technical nature. Fracture of the electrode occurred in 5% of the patients; fracture of the extension cable also occurred in 5%. Almost 8% of patients had a fracture in the temporary wires, and just over 8% had a dislocation of the electrode. Eleven percent experienced pain at the site of the electrode-extension connection. One of 24 patients in the Kemler and associates’ study of chronic reflex sympathetic dystrophy had a defective lead implanted that was corrected with a revision surgery. In his review of problems encountered with SCS devices published in 1974, Fox noted that lead breakage is more likely to occur during revision surgery since the lead is less likely to be identified among scar tissue.

Heidecke published a retrospective analysis of a group of 42 failed back surgery syndrome patients using a single percutaneous lead. The patients were followed for 6 to 74 months, and 12 of the 42 patients had systems that experienced hardware failures. Eight of the leads broke. The other four hardware issues included two broken extension cables and two cases of receiver insulation failure at the plug connector site. Electrode breakage or a disruption in stimulation should be suspected when a sudden disappearance of stimulation occurs. Six of the cases of electrode breakage were caused by disrupted insulation, and the other two cases of electrode dysfunction were without known cause. One of the eight cases was caused by trauma, whereas the other seven were spontaneous.

There were two cases of receiver failure because of short circuiting caused by leaks in the insulation at the plug connection. Special attention must be paid to all connections. Heidecke recommends securing connections with screws while still under pressure. In the cases of the disconnected extension cable at the junction of the lead and cable, the loosening was likely secondary to insufficient tightening of the connector screws that were in use at the time. Another possible problem with that scenario could have been increased traction on the extension cable that could be averted by forming a loop in the course of the electrode. Inadequate connections may be avoided by careful and diligent handling during implantation.

In his retrospective analysis of 160 SCS patients, Kumar and associates discovered nine patients whose electrodes were fractured. Of these patients, one had a paddle electrode; the remaining eight patients had percutaneous electrodes. In this series there were a total of 28 paddle electrodes and 132 percutaneous electrodes. The likelihood of an electrode fracture is higher in the percutaneous implants. The fracture developed in the percutaneous electrodes cephalad to where the lead is affixed to the deep fascia at the point at which the electrode enters the spinal canal.

However, in their article reviewing the charts and operative reports of 289 patients who had undergone SCS implantation between 1998 and 2002, Rosenow and associates reported that the rate of breakage of electrodes was twofold higher in laminotomy electrodes as compared to the percutaneous variety. They also reported that electrode migration was marginally higher in the laminotomy electrodes.

Many of the technical complications are avoidable by careful handling of the hardware during implantation. Many of the reports regarding complications resulting from technical issues related to hardware were older studies involving patients whose SCS systems were older and less technologically advanced.

Eisenberg and Waisbrod reported a case in which a patient experienced an electrical injury to the central nervous system as a result of the cervical SCS becoming activated uncontrollably by an antitheft device. Six months after implantation the patient entered a store that was protected by an antitheft device. The patient requested the store representative to deactivate the deterrent system. The patient entered the store at the direction of the representative. The patient recalls a shocklike sensation at the back of his skull before losing consciousness. The patient regained consciousness in the emergency room where he demonstrated moderate confusion, dysarthric speech, gait ataxia, bilateral upper-extremity intention tremor, and weakness of the left upper extremity. Brain computed tomography (CT) and electroencephalography were normal. After 6 months the patient’s condition improved; however, dysarthria, impaired memory, tremor of the right hand, and gait ataxia were still present. CT of the brain revealed an old infarction in the left basal ganglia.

Given the low voltage that the battery is able to generate, it is assumed that the antitheft device induced a sudden burst of electrical current. The low voltage was strong enough to cause neurological injury because of its proximity to the delicate structures and the low electrical resistance of the meninges. External electrocution is less likely to cause such damage at this voltage level because of the relatively high resistance of skin. Radiofrequency transmission is the likely mechanism that activated the SCS in an uncontrollable manner.

Biological Complications

Infection

Implanting hardware in patients bears a potential risk of infection. Infection in the spinal canal is rare, but superficial infections at the site of the generator or the connector between the generator and the electrode occur more frequently. The typical rate of infection is approximately 5%. According to Barolat, patients may present at any time with an infected device, whether it is within a few days of implantation or a few years. Risk factors for infections of implanted hardware include tobacco and alcohol use and immunosuppressive therapy. Co-morbidities such as diabetes mellitus and rheumatoid arthritis may also increase the risk.

Another risk factor for infection that was illustrated by Barolat is multiple surgeries. He reported that 3.4% of patients and 3.7% of implanted electrodes developed infection. In his review of his experience with 509 implanted plate electrodes, 12 patients developed infection involving a total of 19 plate electrodes. In four patients the infection presented after multiple revisions of the electrodes. He adds that two of the four patients had a history of infections in the course of previous surgical interventions.

According to Follett and associates, the most commonly reported organism cultured in infected SCS systems is Staphylococcus at 48%, followed by Pseudomonas at 3%. Of the remaining organisms, 24% were unknown or not reported, 18% demonstrated no growth, and 6% of cultures grew multiple organisms. They also found that the location of the infection is most likely to be the generator pocket at 54%, followed by the SCS electrodes at 17% and the lumbar incision site at 8%. Multiple sites were infected in 14%, and 8% were not reported.

In a study of 84 patients receiving a total of 92 dorsal column stimulators, Pineda reported that only one patient experienced an infection. The patient presented years after implantation with an infection at the site of the generator. S. aureus was isolated from the wound. According to the author, it was likely that the bacteria were sequestered in the hardware. The infection was treated with antibiotics, and the hardware was explanted. Pineda also reported the case of a patient whose electrode extruded through the skin at the site of the generator years after the implantation surgery. The extruded electrode ultimately became infected with Staphylococcus , and the system was removed.

Kemler and associates reported one infected SCS system out of 24 (4%) patients receiving SCS, and the system was explanted. The infection was not confirmed by culture. After the infection had resolved, the patient underwent a successful reimplantation. Devulder and colleagues reported that 2 of 69 (3%) patients had infected SCS devices. One of the patients had a successful reimplantation procedure. SCS was abandoned for the other patient since the infection was recurrent.

In Kay and associates’ review of 70 patients who were treated with SCS, six patients were diagnosed with an infection. Three of those patients had their SCS system explanted; the others were treated successfully with antibiotics. Prophylactic antibiotics were not administered to these patients, which the authors assert may have contributed to the infection rate of 9%. Three of the infected patients had previously undergone revision surgery. The additional surgical procedures exposed these patients to added risk and increased the likelihood of complications for these patients.

Meglio, Cioni, and Rossi reported a 9-year experience in their institution with SCS. Of the 109 patients treated with SCS, seven patients developed an infection related to the implanted hardware. One of the patients became paraplegic within a few days of the explantation of the SCS device. A myelographic block was noted at the level where the electrode had been implanted. A bacterial epidural and intradural abscess was discovered and drained. The recovery of this patient was not complete.

Prophylactic antibiotic administration during implantation procedures is an important step in reducing the likelihood of an infection. Optimal prophylaxis takes into consideration the appropriate choice of antibiotic, proper timing of administration, and limiting the duration of antibiosis. In the case of spinal cord stimulator implantation, a single dose of a cephalosporin is generally adequate unless there is history of an allergy to that class of antibiotic agent. To ensure maximal effect, adequate tissue levels must be attained before incision. The usual recommendation is that the drug should be administered between 30 minutes and 2 hours before incision. There is no advantage to prescribing postimplantation antibiotics, although this has not been prospectively tested for SCS implantation procedures.

Recommendations to reduce the likelihood of infection also include vigilance when preparing and draping the surgical area and gentle treatment of tissue. Electrocautery near the superficial tissues should be limited, and there should be a two- or three-layer closure to carefully approximate the edges of the incision. The implanting physician must maintain a low threshold for suspicion with new complaints of increased pain, new neurological deficits, and constitutional signs and symptoms. A significant increase in pain over the baseline level and new neurological deficits in patients with an implanted SCS device are harbingers of infection. Any suspicion of infection should initiate a complete work-up. Useful laboratory studies include a complete blood count with differential, erythrocyte sedimentation rate, C-reactive protein, a gram stain and cultures with sensitivities. A CT scan of the areas in which the hardware is implanted is also indicated.

If the infection is superficial, successful treatment is possible with oral or intravenous antibiotics. Explantation should not be delayed if there is lack of improvement or if the infection is neuraxial. A consultation from an infectious disease expert is recommended. Typically the device may be reimplanted after a period of 12 weeks. An approach that has been suggested in reducing the risk of infection is to soak the electrode in gentamicin solution before implantation ( Figs. 15-2 and 15-3 ).

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