The human experience of chronic pain is a complex bio-psychosocial disorder and effective management requires concerted effort from multiple sources. Implanted pain therapy devices have gained significant popularity as useful adjuncts in managing severe chronic pain. Spinal cord stimulator and implanted intrathecal drug delivery devices are particularly successful when used in conjunction with other therapy services, procedures, counseling, surgical interventions, and medications.
The use of spinal cord stimulation and implanted intrathecal drug infusion for managing chronic pain and spasticity offers distinct advantages and challenges. Both therapies are considered reversible and minimally invasive. Sufficient surgical skill and clinical knowledge required for successful implantation and long-term management by the skilled physician can be learned in a proctored environment with diligent study and dedication. This chapter will discuss spinal cord stimulation and implanted intrathecal drug infusion devices and offer some guidance on patient selection, trial processes, permanent implantation, operative procedures, and postimplant patient management.
Spinal Cord Stimulation
Passing an appropriately configured low voltage alternating current through the dorsal spinal cord can induce a tingling or “pleasant paresthesia”. When this generated paresthesia covers or overlays areas of pain, the paresthesia it is said to be “concordant” with the pain. This concordant paresthesia may substantially reduce the perception of pain and is the goal of spinal cord stimulation (SCS) therapy. This reduction in pain perception is often maintained for prolonged periods lasting years if appropriately managed.
The use of electricity to mitigate the experience of pain is long and varied in human history. After Melzack and Wall’s 1965 publication in Science , , in which they proposed the gate control theory of pain reduction, there appears to have been renewed interest in electricity as an adjunct in pain management. Transcutaneous nerve and muscle stimulation has enjoyed a long history of use. The effectiveness of these devices for managing chronic pain is debated.
Spinal cord stimulation as a modern pain management practice was introduced by Shealy and colleagues in 1967. Shealy reported reduced pain in a patient by placing electrodes in the intrathecal space adjacent to the dorsal column of the spinal cord. Various mechanisms of spinal cord stimulation have been proposed and remain an area of ongoing research. Likely, multiple mechanisms play a role.
Current spinal cord stimulator systems consist of an epidural array of contacts and a power source or “pulse generator”. Typically patients are afforded a trial period during which the pulse generator is an external device connected to percutaneously inserted epidural electrodes. When the system is permanently implanted, the power source is surgically implanted into a subcutaneous pocket and is called an implanted pulse generator (IPG). Various stimulating parameters are adjustable on both external and internal power sources and represent the “programming” aspect of spinal cord stimulator management. For both trial and permanent implantation, electrode arrays are implanted into the epidural space as either “catheter-” or “paddle”-shaped devices. The catheter-shaped electrode or “percutaneous lead” is inserted through a specially configured needle-type introducer. The introducer is advanced into the epidural space utilizing fluoroscopic guidance and typically with a loss of resistance technique. The introducer may be inserted through the skin (percutaneous) or placed after an incision is made. One or more percutaneous leads are implanted to create the epidural array of metal electrode contacts. “Paddle” electrodes or laminotomy leads are much larger plastic substrate devices to which multiple metal electrode contacts are attached in various configurations. Figures 41-1 and 41-2 are images of current Medtronic and St. Jude Medical (Advanced Neuromodulation Systems, Inc. [ANS]) percutaneous and laminotomy leads. Because of their size, paddle electrodes require laminotomy or laminectomy for placement. Early electrode arrays were 2 or 4 contacts and available power sources contained only non-rechargeable batteries or required an inductive coil held over an implanted receiver coil for continuous power. At this writing, implanted power sources are capable of connecting up to 16 contacts and contain rechargeable or non-rechargeable batteries. Figures 41-3 and 41-4 are images of Medtronic and St. Jude Medical (ANS) implanted pulse generators.
Often, two and on occasion three percutaneous leads are implanted to create an array of contacts, whereas paddle leads have 4, 8, or 16 contacts arranged in various configurations of columns and rows.
Electrode arrays are most commonly placed in the epidural space between the second cervical and the eleventh thoracic vertebra levels over the middle portion of the cord. Retrograde electrode placement to stimulate sacral and lower lumbar nerve roots, lateral lead placement to stimulate nerve roots within the spinal canal, peripheral nerve stimulation, and subcutaneous “field” stimulation are all areas of ongoing investigation. This discussion is limited to traditional “dorsal column” stimulation, which represents the most common current usage of the SCS equipment.
Appropriate patients for SCS would include those whose pain is not acceptably managed with less invasive therapies and those for whom a definitive surgical procedure is not offered or is not desired. Often reserved for use later in a pain management continuum, spinal cord stimulation may be offered early in some cases, especially in the management of predominantly neuropathic and complex regional pain. Spinal cord stimulation has FDA indications in the management of pain of the trunk and or limbs. This somewhat nonspecific indication reflects the broad array of painful conditions that have been helped using spinal cord stimulation. Good to excellent long-term pain relief has been obtained in multiple chronic pain syndromes including failed back surgery syndrome (FBSS), complex regional pain syndrome (CRPS), postherpetic neuralgia, and radicular pain secondary to central and foraminal stenosis in the nonoperated patient.
Several other painful conditions have been successfully managed with SCS including: pain secondary to inoperable ischemic cardiac pain, diabetic peripheral neuropathy, peripheral vascular ischemic pain, and Raynaud syndrome. Thoracic, abdominal and pelvic pain coverage with SCS has enjoyed only limited success but clinical efforts continue to afford SCS benefits to these patients. Early in its history, but rarely today, SCS has been used for spasticity management.
The application of spinal cord stimulation in primarily somatic pain conditions is often less successful than when used for radicular or neuropathic pain. Effective long-term axial low back and neck paresthesia coverage is an area of research and significant clinical efforts. Axial pain may often be somatic, neuropathic, or a combination and, as such, is more difficult to capture long term with dorsal column stimulation. However, because the causes of pain can be greatly varied and unknown, patients with a lower probability of success using SCS may derive considerable benefit. The opportunity to place a trial stimulator consisting of percutaneously placed electrodes connected to an external pulse generator is a tremendous advantage and is what allows the less-than-optimal patient to be considered.
The following situations would generally contraindicate SCS:
Known systemic bacterial infection or infection in the proposed implant region
Patients with an untreated or undiagnosed psychiatric condition
Posterior surgical interventions that obliterate the epidural space where the lead array needs to be placed or along the required implant path of the lead
Patients unwilling or unable to comprehend using the device
Anticoagulated patients where the anticoagulated state cannot be stopped for the trial and implant process
Previous DREZ lesions at or above the level of lead placement
Deafferentation or CNS damage such that paresthesia generation is not possible
Significant canal stenosis along the proposed lead location contraindicates percutaneous lead placement and would make laminotomy lead placement in the area of stenosis a concern. Posterior spine surgeries most often obliterate the epidural space and lead placement through the surgical area typically is not possible. Magnetic resonance imaging (MRI), or if MRI is contraindicated, computed tomography (CT) scan of the proposed lead placement regions are often warranted to screen for significant canal stenosis or other intraspinal anomalies.
At time of this writing, three companies manufacture and support the majority of spinal cord stimulator equipment: Medtronic Neurological, St. Jude Medical (ANS), and Boston Scientific. Each company has product sales and support personnel. Generally, these companies provide a variety of lead or electrode packages containing the electrode, introducer needle, various stylets, and anchors. Percutaneous leads typically have four or eight electrodes secured to a solid plastic catheter through which wires pass to connect the electrodes to metal contacts on the other end to connect to the power source. A trialing cable to connect the leads to a trial pulse generator may be included in the lead kit, or supplied as a separate item. The various implanted pulse generator packages contain a tool for securing the lead to the IPG. Many items are individually packaged and available when needed such as: various anchors, additional stylets, wrenches for the set screws on the IPG or extension, lead extensions, and others. When connecting electrodes placed in the cervical region and many laminotomy leads to an IPG located in the upper buttock or abdominal region, lead extensions are typically required.
The basis for all SCS systems is essentially the same: Various forms of a low voltage alternating current are passed from the generator to an array of electrode contacts in the epidural space to generate an electric field within the spinal cord. This electric field affects a change in the central nervous system and, when effective, the experience of pain. Each company has claims to unique benefits, programmability, battery life, stimulation parameters, constant voltage versus constant current and various equipment features. Implanter experience with the various products, individual bias, possibly geographic location due to product support issues, and experience of the manufacturers support personnel are important when choosing which company or companies to choose.
Each manufacturer’s trial and implanted generator is designed to connect only to that particular company’s leads. A battery powered external pulse generator is programmed to match those settings found to be most beneficial during the trialing process and is sent with the patient for the trial period. Trial leads are not appropriate or intended for permanent implantation.
Implanted pulse generators are used as the power source for permanent SCS systems. At this writing, IPG power sources are either rechargeable or non-rechargeable (primary cell). An inductive coil IPG was marketed by Medtronic and ANS (St. Jude Medical), but these are generally not used or are out of production. The decision to implant a primary cell powered versus a rechargeable device is made based on expected power requirements as determined during trialing of the array at time of implant, patient cognitive abilities, and implanter preference and experience. Power requirements for a trial implant array do not necessarily always predict requirements for an array placed during a permanent implant.
Current Medtronic rechargeable IPGs will stop functioning after 9 years of usage and require replacement at that time. Boston Scientific and St. Jude Medical rechargeable IPGs do not have specific time limits. The actual life of the rechargeable devices is dependent on the type of battery technology, the number of battery recharge cycles, and the efficiency of the device. As the battery life depletes, the frequency of required recharges increases. A primary cell, non-rechargeable IPG battery under typical usage is expected to last approximately 6 years. Less maintenance from the patient (in the form of recharges) is required with a non-rechargeable IPG. When choosing between rechargeable or non-rechargeable power sources, battery recharging requirements must be weighed against the advantage of the battery’s expected increased life. When power requirements were high during trialing for the permanent electrode placement, then a rechargeable device may be more appropriate. Laminotomy leads tend to have considerably lower power requirements and therefore non-rechargeable power sources may be adequate. In general, however, the greater the power usage, the more likely it will be for a practitioner to choose a rechargeable option.
Preoperative evaluation and preparation of a patient are critical to optimizing outcomes. Most implanters and payers will require a pre-implant psychological evaluation. This evaluation is best accomplished by a licensed psychologist or psychiatrist who has a clear understanding of issues associated with chronic severe pain and spinal cord stimulation. This evaluation helps the implanting physician identify psychological factors that invariably are present in patients with chronic severe pain. Occasionally, psychological issues are identified that require treatment prior to further consideration of spinal cord stimulator therapy. Good collaboration between the psychologist or psychiatrist and implanting physician is important and that all involved understand and address psychological issues. Long-term psychological needs may also be identified during this evaluation. Mostly the evaluation is an attempt to identify psychological issues that would preclude a patient from being considered for implantation. It is important that this evaluation be obtained before placement of the trial and incorporated in the overall pre-implant decision-making process.
An accurate and timely history and physical examination appropriate for a surgical patient is reasonable. This would include past surgeries, surgical complications, bleeding problems, drug allergies, current medications—with specific attention to those affecting coagulation, and appropriate review of systems. A physical examination related to the proposed procedure would include a focused neurologic examination, auscultation of the heart and lungs, abdominal palpation, and inspection of the proposed surgical sites for evidence of infection. A discussion with the patient and significant others regarding external wires and the trial pulse generator for a trial placement, expected incision locations for permanent implant, expected increased pain, and activity restrictions is prudent. Preoperative laboratory testing may be appropriate to rule out infections, bleeding problems, and chemical abnormalities. Chest radiographs and ECG may also be appropriate in patients with significant cardiac or pulmonary history. Specific tests and studies obtained are determined by individual patient considerations, facility requirements, and anesthesia needs.
Identification of possible immune deficiencies and, when indicated, evaluation of the immune status of patients is important. Patients with a history of recurrent infections, especially those known to have antibiotic-resistant organisms, are at particular risk. Diabetics, patients with poor personal hygiene, unsanitary living conditions, chronic renal failure, long-term steroid exposure, the elderly and/or debilitated, and most obviously, those with known immunoglobulin deficiencies require careful considerations. When in doubt, consulting an internist and/or infectious disease specialist is appropriate. Maximize immune function in those patients identified to be at risk when possible. Appropriate perioperative antibiotics in the at risk patient and possibly all patients are important in reducing operative infections. Most preoperative antibiotics are best given within 30 minutes prior to incision.
Surgical implantation of permanent SCS systems is a surgical procedure requiring adherence to the usual surgical precautions and needs. Implantation of trial percutaneous spinal cord stimulator leads is generally also considered a surgical procedure requiring similar surgical precautions. C-Arm fluoroscopy is most typically used in addition to an x-ray translucent table, free of metal components which might interfere with appropriate x-ray imaging. Monitoring equipment appropriate for specific patient needs would typically include noninvasive blood pressure, pulse oximetry, and ECG. As with all surgical interventions, a surgery suite meeting local Life Safety Code, conditioned and filtered air, and back-up electric power suitable for a surgery suite would be expected. Appropriate monitoring and resuscitation equipment to care for the surgical patient is also required. This would include airway management equipment and supplies, resuscitation drugs, cardiac defibrillator, as well as personnel trained in airway management and patient resuscitation.
Typically, for both trial lead placement and permanent system implantation, patients are positioned supine with or without a pillow under the abdomen as needed to reduce lumbar lordosis for lumbar entry. Padding the upper chest will allow the neck to flex for upper thoracic entry when cervical placement is planned. Skin preparation is tailored to patient requirements and may be accomplished by washing the area, shaving when needed with an electric shaver, and final prepping with an applied iodine or chlorhexidine surgical prep. Plastic barrier drapes impregnated with iodine or chlorhexidine applied over the incision and introducer insertion areas will reduce local skin bacterial contamination. Appropriate draping of the patient and equipment including the C-arm is important.
Adherence to strict aseptic techniques by all personnel is critical to reducing infections. All room personnel must wear clothing appropriate for an operating room environment with surgical masks and hair caps. As in all implant surgeries, minimizing the handling of the sterile implanted devices lessens the chance of contamination. Liberal antibiotic irrigation may reduce the incidence of infection. Electrocautery is cautiously used by many implanters during the permanent implant process. Never cauterize near the introducer needle because severe shock and damage to the spinal cord may occur. Avoid cautery near any component leads or wires because the current may be transmitted down the wire and shock or damage neural structures. Heat from cautery will damage leads, extensions, and other components possibly causing failure. Cautery current has the potential to damage the electronic components of implanted pulse generators. Some implanters prefer using bipolar cautery to mitigate, but not eliminate, cautery risks. Heat from cautery damages surrounding tissues, which must go through a healing process. Increased seroma formation, wound healing complications, and infections are noted with excessive cautery usage.
Good surgical techniques, with proper wound closures in layers when needed, will reduce wound-healing complications. Additionally, local skin flaps when needed, creation of a generous IPG pocket, and closure of deeper fascial planes will help reduce tension across wound closures, thereby lessening wound healing complications. The use of absorbable suture in deeper layers is typical. A less reactive suture material such as PDS II (polydioxanone) may reduce the incidence of stitch abscess and superficial wound complications. Final skin closure with staples, nylon, or tissue glue such as DERMABOND adhesive may also provide an added level of skin approximation. Unlike nonimplant surgical procedures, superficial skin infections and wound healing complications may lead to involvement of deeper layers, exposing the implanted devices to infection. When deeper layers of the back incision or the pocket become infected, very often the entire system must be explanted to appropriately treat the infection.
Trial Process Considerations
The implantation of electrodes on a trial basis offers the patient, caregivers, and managing physician an opportunity to evaluate the effects of spinal cord stimulation prior to permanent implant consideration. Meaningful application and evaluation of the trial process is a combination of patient expectations, physician and staff experience, proper lead placement, programming of the electrode array, and careful evaluation of the stated results.
As with many therapies, there is a substantial placebo response which must be considered when evaluating patient response to a trial of spinal cord stimulation. Concordant paresthesia generated over the area of pain, which is reported by the patient to significantly reduce pain perception, is the goal of the trial. The length of time the trial leads remain implanted varies among practitioners and mitigating issues such as the need for anticoagulation medications and immune status. Most generally, the trial period should be long enough to allow the patient to use the stimulator while engaging in their usual activities of daily living. Typically, the trial duration is 5 to 14 days. Some practices occasionally use an “on the table” trial where the system is permanently implanted if the patient reports good relief with initial lead placement. This practice is most appropriate in situations where percutaneous lead placement is not possible and a laminotomy is required. An experienced implanter assesses the reported paresthesia and considers the risks and benefits of immediately implanting the permanent system. Permanent implantation of a laminotomy lead connected to an extension passed through the skin and attached to a trial generator has been used. An IPG and new extension can later be implanted if the trial is successful.
Long-term success of spinal cord stimulation cannot be completely predicted by the trial process. Positive placebo response with a trial implant can lead to poor permanent implant results. A positive placebo benefit will lessen over time and may account for many early nonimplant-related failures that appeared to have initially functioned well. If the trial protocol requires a very high reported level of benefit before a permanent implant is offered, improved long-term success would generally be expected. For example, patients who report only a 40% reduction in pain may be experiencing significant placebo response, and may not be the best candidates for permanent implantation. Factors to consider in the determination as to the effectiveness of the trial would include reported effects such as increase in activities of daily living; reduction in oral pain medication usage; family members reporting improvement in activities and mood; improved sleep pattern; and a reported decrease in level of pain during activities and at rest.
Unfortunately, even in the best of circumstances, placebo response cannot be completely controlled. Some permanent implantations will not be successful and will rapidly lose effectiveness secondary to waning placebo response. Keeping the expected level of benefit from the trial high and the duration long will help mitigate placebo response. However, consideration should be given that by using a strict protocol requiring a very high level of relief during the trial (i.e., greater than 80%), some patients will be denied spinal cord stimulation that could otherwise benefit.
The trial process has great impact on an individual patient and is the patient’s critical opportunity to gather information to make an informed decision whether to continue to permanent implant. Considerable care must be given to this very important process. Preprocedure education, expert psychological evaluation, and careful discussion with all concerned regarding the process and expectations will help reduce disappointment and failure. Poor lead placement and/or programming that fails to provide optimal paresthesia coverage is not acceptable and will doom the trial from the beginning. For the patient’s benefit, effort must be given to provide as good a trial as is reasonably practical.
Percutaneous trial leads are placed through needle type introducers inserted through the skin. Lead location is adjusted based on the patient’s reported paresthesia when the lead is connected to a trial generator. The trial leads may be secured using various techniques. Figures 41-5, 41-6, and 41-7 demonstrate one method whereby the leads are sutured to the skin with 0-silk, tincture of benzoin applied and Steri-Strips are placed to create a loop of lead to act as a strain relief. A 4 × 4 dressing is applied and secured with wide 3M Medipore H tape. The trialing cable is also secured with tape to reduce stress at the lead cable connection.
In the recovery area, final adjustments are made to the trial generator parameters when needed, and instructions are given to the patient and caregivers. Although use of the trial generator is straightforward, considerable time may be needed to assure and instruct the patient and care givers in its use. The patient is most frequently discharged to home and given instructions including a contact number to call when questions arise. Patients are cautioned against twisting and bending movements which might cause the leads to move from their implanted location. However, the patient is encouraged to engage in usual activities of daily living as practical to best assess the stimulator’s effectiveness. Although a reduction in pain medication requirements is one indication of effectiveness, if pain medication is abruptly discontinued during the trial implant, confusion may arise as to effectiveness. Patients are asked not to get the dressings wet and to contact the physician for any dressing changes that may be needed.
After completion of the trial, the patient and care givers are questioned regarding perceived benefits, and if a pain diary was kept, it is reviewed. The percutaneous trial leads are most generally removed by gentle traction on the lead with the patient in a slightly flexed sitting position. If unusually severe or radicular pain is experienced when attempting removal, the patient is repositioned prior to further attempts.
A successful trial is defined differently by various physicians. Most physicians consider at least a 50% reduction in pain, improvement in activities of daily living, and some medication reduction to be important. The determination to proceed with permanent implant may best be delayed for several days following removal of the trial. This period without the SCS effect allows time for comparison and gives the patient and physician additional information. It is important that the patient, care givers, and family members have reasonable long-term expectations. Clear understanding of the surgical implant process including short and long-term risks as well as postimplant requirements is important. The patient is made aware of the possible need for further surgeries for lead revision, equipment failure, and eventual IPG replacement.
Percutaneous Lead Placement
Lead placement whether for trial or permanent system implantation is preformed using minimal, if any, conscious sedation with appropriate amounts of local anesthetic. Moderate or heavy sedation is discouraged to lessen complications associated with introducer placement and lead advancement. Reported paresthesia location, quality, and benefit by overly sedated patients are suspect and difficult to interpret. Paresthesia qualities will often change as the level of sedation lessens. Clear verbal communication with the patient is critical during the lead placement process. Lingering sedation given for local infiltration, incision, introducer placement, and lead advancement can significantly affect the patient’s perception and reported paresthesia. Reassuring words along with slow infiltration of reasonable quantities of local anesthetic solutions into the appropriate region will greatly reduce sedation requirements. In permanent implantation, when the leads are anchored and trialed to ensure an appropriate stimulation pattern, patient communication becomes less important and increased sedation, if needed, can be given.
Percutaneous leads are inserted through a specially designed large-bore (approximately 14 gauge) introducer needle. These introducers allow leads to emerge from the tip, to be advanced and carefully manipulated during the placement process. Leads can be damaged and even sheared while withdrawing through the introducer. When there is resistance to withdrawal, slight advancement and rotating the lead may allow the lead to be successfully withdrawn. When there is concern that damage to the lead may occur, removal of the introducer and lead together with subsequent reinsertion of the introducer is indicated. For lumbar radicular pain, the expected region of the spinal cord best stimulated typically will reside between the sixth and eleventh thoracic vertebral level. Needle entry would be at T12-L1 or L1-2 when practical. Motion within lower lumbar segments may increase lead failures such as fracture and dislodgment. For cervical lead placement, introducer insertion at C7-T1 or below is best—again due to motion and typically a more generous epidural space. Cervical lead tips are positioned somewhere below the C2 level.
The spinal area being considered for placement of the introducer needle is imaged most generally with C-arm fluoroscopy. The C-arm is adjusted in oblique and tilted projections to provide an optimal view of the intended entry interspace. Inspection of the interspace may reveal boney changes that could make introducer placement or subsequent lead advancement difficult. Often, declining the fluoro beam to more closely match the needle entry angle can demonstrate obstruction or anatomic variations. Paramedian insertion of the introducer needle is preferred starting approximately one spinal level below. The entry angle of the introducer relative to the spine should be approximately 45 degrees when practical to allow for the lead to optimally emerge from the tip. This angle also improves the ability to “steer” or to control the lead tip as it is advanced. The introducer tip target is slightly ipsilateral and below the spinous process. Figure 41-8 shows a left paramedian introducer placement at T12-L1. Air or saline loss of resistance technique is most often used along with anterior-posterior and lateral fluoroscopy imaging as the needle is advanced to assist in identifying the epidural space. Nonionic contrast may also be employed if needed to help confirm epidural space placement.
Most introducer placements are at spinal levels where the spinal cord is present. Great care is exercised to have exacting needle control as the introducer is advanced so as not to cause damage to the spinal cord. Figure 41-9 demonstrates one technique of holding the syringe and needle. Notice the operator’s left thumb and index fingers grasping the needle at the skin level while the right hand gently “bounces” the syringe plunger providing pressure for the loss of resistance. The introducer is advanced only by the fingers pinching the needle while the right hand assists in directing the introducer. Using this technique, the introducer is less likely to be accidentally advanced into the dural space possibly causing spinal cord or nerve root injury.
Once the introducer is in place, an electrode or lead is inserted through it and advanced staying midline or slightly off midline to a level above the expected final implant level. When practical, it is best that the electrode emerge from the introducer directly cephalad and not angled to either side. Fig. 41-10 demonstrates the lead emerging from the introducer directed cephalad. Lumbar and thoracic leads can easily stray laterally when advanced and pass laterally and then into the anterior epidural space where paresthesia is not pleasant. Depending on implanter experience and the desired paresthesia, the lead may be placed directly midline or slightly off midline.
Once the lead is initially placed, it is connected to a trial generator using a cable passed from the sterile field. Some of the lead contacts are selected, for example, three in the middle portion on an eight contact electrode, in a +,−,+ configuration. The power is increased on the trial generator until the patient reports tingling or paresthesia, which they are asked to describe. The lead is slowly withdrawn while the patient reports changes in the quality and location of the paresthesia. If in this process a very good paresthesia is obtained, which the patient assuredly reports to be beneficial, the lead may be left at that location. When the lead is withdrawn to a spinal level at which useful paresthesia is no longer reported, the lead is advanced back into a position where the most optimal paresthesia was reported. These position adjustments are made slowly and in cooperation with the person controlling the trial generator to minimize unpleasant or very strong stimulation. This “trolling” technique can reduce the number of adjustments required for optimal final lead placement. Trial generator parameters may be adjusted as needed to improve paresthesia coverage, but fine-tuning of the parameters for optimal coverage is generally undertaken at a later time. Generator parameters include pulse width, frequency, power (voltage or current), and lead contact configuration (each contact can be set to +, −, or off).
If on initial trialing, the patient reports a sharp biting pain at a very low power setting, the lead may be intrathecal. The lead is withdrawn and an attempt may be made to reinsert at this or a different level. On occasion, CSF in the epidural space from a dural puncture by the introducer may make trialing the lead difficult because the CSF interferes with the conductance. In this situation, the procedure is best abandoned and again tried at a later date. If the lead is being placed in the thoracic region, and the patient reports sharp pain within the chest wall or abdomen, the lead may be in the anterior epidural space or lateral in the posterior epidural space stimulating the nerve roots. A lateral fluoroscopic image is often useful in diagnosing these placements. Stimulation of the ligamentum flavum may be reported as a sharp or biting posterior sensation as the power is increased. Repositioning of the lead may reduce this undesired stimulation. Implantation of laminotomy leads in lieu of percutaneous leads is thought to reduce this ligamentum stimulation.
Many implanters repeat this lead implantation process with additional leads to provide optimal paresthesia coverage and afford more programming flexibility. With changes in pain patterns, electrode movement, and scar formation, reprogramming with multiple contacts is generally more successful. Two percutaneous leads are generally placed in this fashion to provide an electric field across the spinal cord. Occasionally, three electrodes are implanted and are thought to possibly offer some advantage from a “tri-pole” electrical field. Fig. 41-11 is an AP radiograph showing the electrode positions following trialing but before the introducers were removed and Figure 41-12 shows a lateral projection of the same leads after one introducer was removed.
The epidural space contains nerve roots, fat, connective tissue, lymphatics, venous vessels, and small arteries. These small arteries supply posterior spinal structures and do not supply the spinal cord. Tissue adhesions within the epidural space may make passage of the electrode more difficult. When a patient is in the prone position, contact between the ligamentum flavum and the dura may be less consistent. Tissues contained within the epidural space may reduce effective contact between the dura and the electrode. Often stimulation is stronger when the patient lies supine owing to improved electrode contact and the spinal cord’s posterior movement secondary to gravity.
Surgical Implantation of Percutaneously Placed Leads
When permanent percutaneous leads are placed, a midline incision at the expected lead implant spinal level may be made prior to placing the needle introducer(s). The advantage of this technique is that good exposure of the spine and hemostasis using cautery can more easily be accomplished prior to introducer placement. The disadvantage is that the incision may need to be extended when placement at the expected level cannot be accomplished or when a more lateral introducer insertion is required. Incisions may be made following placement of the leads through the skin. If the leads are placed in a bilateral paramedian approach, the incision is made between them. If they are placed on the same side, the incision is made alongside both introducers. Some implanters make a separate incision at each introducer location. When making two or more incisions, consideration must be given to wound healing complications owing to impaired blood supply to the skin areas between incisions. The dissection is made to the fascia overlying the spinous process and developed as needed for exposure. Dissection to the entry point of the introducer into the spinal fascia or ligament is required when the leads are placed percutaneously prior to incision. Following careful removal of the needle, the lead is drawn backwards through the skin puncture into the incision and anchored. When the incision is made prior to introducer placement, cautery may be used before introducer placement.
Lead movement from the initial implant location is the most common cause of lead failure. This typically arises from failure at the anchor. Leads can move laterally or medially without failure at the anchor site. Device companies have made good progress in developing new anchors that securely hold the lead with minimal circumferential pressure. Proper use of anchors, referencing manufacturer technique recommendations, and suturing to appropriate structures are critical to long- and short-term success. The anchor is generally sutured along the posterior lateral aspect of the spine, such that the leads are not sharply bent. This reduces stress points that can cause failure of the internal wires. Anchors are applied as close as practical to the point where the lead enters the interspinal ligament or fascia. Figure 41-13 shows an anchor sutured at each end and a tie being passed around the anchor to apply circumferential pressure securing the lead to the anchor. Leads may pull back with spinal flexion-extension movement and emerge between the fascia entry point and the anchor, especially if this distance is great. A nonabsorbable, purse-string suture placed around the introducer needle prior to removal and tightened after final verification of lead position may reduce this complication.
Stress points along the leads are reduced by providing room for them to coil and to lie flat within tissue planes. Generally, a flap of skin or deeper tissue is developed in the inferior aspect of the incision to allow the lead to curl in this area and provide strain relief.
Permanent Laminotomy Lead Implantation
Laminotomy or surgical leads are also referred to as paddle leads because of their shape. A laminotomy or laminectomy is generally required to provide enough room for insertion into the epidural space. Electrode contacts reside on the surface toward the spine and are, therefore, insulated on the posterior surface. Generated electric fields with these leads are unidirectional toward the dura and spinal cord. The posterior surface against the ligamentum flavum is insulated, so stimulation of the ligament is unlikely. These leads tend to be more efficient and require less power to produce paresthesia. Current laminotomy leads have 4 to 16 contacts arranged in various configurations of contact size, spacing, and orientation. It is likely that their larger size makes them less prone to movement when implanted.
The placement of paddle electrodes may be accomplished using minimally invasive spinal retractors and access systems or via a traditional open laminotomy. Figure 41-14 shows a laminotomy lead being implanted using a minimally invasive technique. To confirm useful concordant paresthesia, the patient is questioned during trial stimulation whenever a lead array is being implanted. Intraoperative questioning requires the laminotomy be performed using spinal or epidural block and local anesthesia infiltration with sedation when required. Spinal and epidural block anesthesia along with local anesthesia infiltration can be used when correct paresthesia reported. The location of optimal laminotomy lead placement can often be well approximated by a previously performed percutaneous trial. When the laminotomy lead is placed under general anesthesia at the level as determined by the trial, a risk is taken that, upon patient awakening, less than optimal coverage will be afforded. Figure 41-15 shows a tripole laminotomy lead implanted slightly right of midline.
Considerable radiation exposure may be had by the implanting physician during introducer placement and lead manipulation. This is especially true during the early phases of learning. Techniques used to reduce radiation exposure include using a modern C-arm fluoroscopy machine in the pulse and low-dose modes when appropriate; using collimation to view only those areas needed to be imaged; using AP imaging with the x-ray source under the patient; keeping hands out of the x-ray beam; positioning the image so that the area of interest is at the bottom of the image screen; lowering the image intensifier as practical; and proper lead shielding of the surgeon. Proper lead shielding includes leaded goggles to protect the eyes; thyroid shield; full lead apron; and lead batons hanging from the side of the table. A piece of lead sheeting may be fashioned to provide an area of minimal radiation to protect the hands. This shield is sterilized and laid over the drapes. All these maneuvers play a role in reducing lifetime radiation exposure. With practice and experience, typically radiation exposure becomes much less. Figure 41-16 shows fluoroscopy positioning, lead apron, thyroid shield, lead eye protection, and the use of a sheet of lead to protect the hands. Some of these efforts will also reduce radiation exposure to the patient.
Pocket Creation and IPG Implantation
After placement of the epidural lead array, a subcutaneous area or pocket is created to accommodate an IPG. Following infiltration of the proposed pocket area with local anesthetic containing epinephrine to reduce bleeding, a horizontal incision is made to the subcutaneous fat layer. Blunt finger dissection and electrocautery is used inferior to the incision to develop a pocket. Pockets are most often developed in the upper buttocks just lateral to the upper sacrum. This area typically is below the belt line and pressure against a chair is minimal. Smaller power sources may allow for a suitable pocket to be developed adjacent to the lead implant spinal incision. Recharging may be more difficult with the IPG in these areas and should be considered when planning pocket location. Other IPG pocket sites include the abdomen or infraaxillary region. Placement under muscle or a fascial layer may be needed in extremely thin patients.
The pocket is usually developed inferior or below the incision such that the incision does not overlie the IPG. Rechargeable IPG pockets are best created under less subcutaneous tissue to reduce the distance between the IPG device and the recharging coil placed over it. Device company recommendations are followed in regard to pocket configuration and depth. If a non-rechargeable device is implanted, pocket depth can generally be greater—but not so great as to make communicating with it difficult.
A tunneling device is inserted through the subcutaneous tissues and passed between the pocket and the spine incision. The leads or lead extensions when required are passed or drawn into the pocket. Figure 41-17 shows tunneling between the spinal incision site and the IPG pocket. Connections are carefully dried and secured to the IPG using a special tool.
Most IPGs have two suture holes along the upper edge where nonabsorbable sutures are used to secure the IPG to a fascial plane. These sutures keep the IPG from turning in the pocket. Rotation and movement of the IPG in the pocket over time will stress the lead wires causing fracture and lead dislodgement. These sutures are best placed through a deep fascia layer and tied with 2 to 3 cm of slack. Figure 41-18 shows the IPG with silk sutures being placed. Figure 41-19 shows how placing an instrument into the loop of tie will allow the knots to be tied tightly while leaving slack in the loops to reduce failure. If these sutures are tied tightly, they tend to “saw” through the tissues over time and fail. The spinal incision and the pocket are closed with absorbable sutures in layers when appropriate.
Postoperative instructions include antibiotic coverage, dressing changes, wound care, pain medications, symptoms and signs for the patient to be vigilant of and for which the physician should be notified. For trial placements, the dressing may be left in place until the trial leads are removed. If a trial lead dressing becomes soiled or wet, dressing change by the physician or staff would reduce incidence of damage or lead dislodgement.
After surgical interventions, including permanent implants, system removal or revisions, dry gauze type dressings taped in place with nonplastic tape works well. Over time, occlusive dressings may hold moisture and become sites for bacterial entry. Patients implanted with permanent devices are instructed not to get the dressing wet; no soaking in water for a period of at least four weeks is recommended. Wrapping plastic kitchen wrap circumferentially around the low abdominal and midback regions will allow patients to shower without compromising the dressing. Otherwise, sponge bathing may be best. If the dressing becomes wet or soiled, it is to be changed. Wounds are cleansed only with mild soap and water or dilute hydrogen peroxide. Antibiotic ointments are avoided as this may introduce ointment into deep layers, impairing skin approximation and healing.
Antibiotics are provided during the trial period and after permanent system implantation or revision according to the implanting physician’s experience, patient needs, and type of surgery. For the average-sized adult, an antibiotic such as Keflex (cephalexin), 500 mg PO, q 6 to 8 hours unless patient is allergic or allergic to a typically cross-allergic antibiotic is appropriate. For patients with impaired immune response or known chronic infections, consultation with an infectious disease specialist may be appropriate.
Patients may require additional pain medications. For trials, this would be minimal and care is given not to increase the amount to such a level as to impact the trial results. Some added discomfort may be expected for 1 to 2 days following a difficult percutaneous lead placement due to introducer placement. After this period, an attempt is made to return the patient to their baseline pain medications or even a reduced dose. In permanent system implantation, surgical pain is expected and may be considerable during the first 2 to 3 postoperative days. Increased pain medication requirement during the first week would be expected. Adding transdermal fentanyl for this postoperative pain is not appropriate and is avoided unless the patient had been using transdermal fentanyl as part of their general pain management regimen.
Activity is limited following permanent lead placements. Aggressive spine twisting and bending motions are discouraged for 6 to 8 weeks after which the leads are thought to be held in position by scar forming around the lead in the epidural space. Trial leads are left in place only days and during this brief period, significant scaring of the leads will not develop. Part of the trial process, however, is to encourage patients to engage in their usual activities. These activities may, of course, cause leads to move from their implanted locations and patients are counseled regarding this possibility. If trial leads move after sufficient time has elapsed, such that the patient can make a reasonable determination of effectiveness, then the trial need not be repeated. However early lead movements to a point where reprogramming is unable to regain paresthesia, may necessitate repeating the trial. Patients often report significant increased stimulation during acceleration in automobiles owing to improved contact and the cord moving closer to the electrodes. Therefore patients are advised not to drive with the stimulator powered on.
Postoperative instructions include some counseling regarding possible adverse events. Epidural hematoma occurs most often early with epidural abscess occurring somewhat later after implant and developing more slowly.
The trial generator and implantable pulse generator (IPG) have multiple programmable parameters. The IPG is programmed through the skin using a wireless device where the trial generator is external and can be directly programmed. Often programming and reprogramming of the IPG by the physician is assisted by a representative of the device company or other knowledgeable staff. After initial IPG parameter settings are programmed, future adjustments are expected in permanent systems due to minor lead movements, scar formation, or pain profile changes. During the trial period, reconfiguration of the trial generator settings may be required if stimulation pattern substantially changes.
Each device company has their individual programming system to communicate with the IPG for this programming or reprogramming. A trial generator connected to the lead array by means of a cable passed from the sterile field, is used intraoperatively for the trialing of leads. This is the case whether the lead is being placed for a trial or for permanent implantation. Each contact on the lead may be individually programmed to positive (+), negative (−), or (off) to not be used. The voltage or current range (amplitude), pulse frequency in hertz (Hz), and pulse width (microseconds) can be modified to generate the most effective paresthesia. At time of this writing, pulse generators can control as many as 16 contacts, which when adjusted with these various parameters, can create a nearly endless number of possible combinations. Experienced and knowledgeable physician and personnel are needed to optimally program these devices. Various electrode contact configurations along with other parameters are selected and the power is slowly increased while the patient reports specific paresthesia for that particular combination. Based on the location and quality of the reported paresthesia, changes are made to the programming and the power is again slowly increased. This process is repeated until the optional coverage is reported. This process may be time consuming to optimize all reasonable combinations, but with experience it is often manageable.
As with any surgical procedure, even in the most competent of hands, complications will and do occur. The frequency and severity of complications will vary depending on multiple factors, some of which can be mitigated. Severe complications are rare, but can include death, spinal cord injury, or nerve root injury that may occur during the placement of the introducer needle and passing the lead. Short-term complications tend to be related to, and associated with, the implant surgical procedure.
Wound healing complications include subcutaneous infection, wound dehiscence, stitch abscess formation, and deep infections. Most infections related to the surgical implant will develop in the first 4 weeks following surgery. If at any time the wound breaks down (dehisces) such that implanted components are visible, aerobic and anaerobic wound cultures must be obtained to help guide antibiotic coverage and to determine if the wound is infected or if mechanical factors led to the dehiscence. Typically, some skin contamination in a dehisced wound is cultured. If the wound is infected, most generally the system is explanted, the infection is cleared, and the patient is later evaluated for possible reimplantation. If wound dehiscence is caused by failure of the closure and the wound is not infected as determined by cultures, surgical excision of the affected skin margins, vigorous irrigation with antibiotic or dilute Betadine (povidone—iodine) solution, and secondary wound closure with PDS-type suture may be attempted. Local skin flap development will reduce the stress across an incision and lessen the incidence of dehiscence. Figure 41-20 shows a dehisced wound being marked for excision of the margins for secondary closure. The cultures and Gram stain along with clinical impressions suggested that this pocket was not infected. Abscess formation around suture material may remain limited to the subcutaneous tissue or can extend and enter deeper layers. When the device capsule is involved or if the deeper layers around a lead implant site are involved, resolution in the presence of the implanted devices is difficult.