Spinal Cord Stimulation Technique
- Author: Anthony H Wheeler, MD; Chief Editor: Kim J Burchiel, MD, FACS more...
Preparation for placement of a permanent spinal cord stimulator usually necessitates a trial procedure, which allows both the SCS team and the subject to determine whether or not a permanent implant would provide substantial or adequate pain relief and improvements in QOL.
The SCS team is composed of a trained interventional pain physician, his/her medical team, a fluoroscopy technologist, and a manufacturer’s representative, who is a trained SCS technologist. The trial helps determine the optimal number of leads and electrodes and optimal placement for pain coverage. Usually, trial leads are cylindered or wire-like and have 4-8 electrode sites separated by ≥4mm. These 1-column leads with eight electrode sites are more versatile for trial use, because they allow more precise signal positioning across the dorsal columns and can span up to 2 vertebral body segments. Often, column leads with 8-electrode sites can provide concordant pain coverage with a single ipsilateral lead placement, and they can be reprogrammed to recapture paresthesia for pain coverage if lead migration occurs during the trial.
Because lead migration is commonly encountered, a second epidural lead, placed by an ipsilateral skin-entry point 1-segment above the first lead’s skin-entry or at a contralateral skin-entry site at the same segmental level, creates redundant coverage should such slippage occur. Furthermore, a second lead may provide improved or more comfortable pain coverage, increased ability to try various programs, and improved pain reduction during the trial.
The trial allows the SCS team to evaluate patient anatomy and potential anatomic barriers, eg, the capacity of the epidural space, CSF depth (greater distance allows more dispersion of signal), and patient-response to trial leads. The patient’s response to trial stimulation may be optimized by changing electrode geometry or tissue surface impedance; changing the frequency or intensity of the signal, a function of higher voltage or current; or changing the applied pulse width or duration.
The latter factors are important for determining the battery-type, such as a primary cell battery for lower requirements, or a rechargeable battery when higher needs are required. Rechargeable batteries require better patient compliance with supporting technologists, because recharging visits will need to occur on a more frequent and regular basis. In these cases, more rigorous compliance requirements may adversely affect SCS outcome due to ongoing or progressive physical, cognitive or financial limitations (eg, transportation due to no vehicle or gas money).
Other advantages of percutaneous trials are that they can be performed less expensively outside of an operating room in relatively aseptic conditions. Trials typically last between 3-10 days, long enough to eliminate any possible placebo effect and with instructions that help the patient focus on pain reports specifically related to the pretrial targeted neuropathic pain, not related to any soreness or pain that was caused by trial needle and lead placement.
During the SCS trial, epidural leads are attached to an external programmable pulse generator, which operates on alkaline batteries. Because the trial is performed in an outpatient setting, eg, the world of physical exertion, work, play, and social demands that are specific to this patient-participant’s lifestyle, a more realistic evaluation of the patient’s pain relief and QOL improvements can be achieved.
Most of the trial implants are placed through percutaneous tissues into the epidural space. The patient is usually premedicated with a sedative, often from the diazepam family (examples include alprazolam or lorazepam) and an adequate fast, short-acting pain medication, typically hydrocodone or oxycodone preparations. Most experts feel that these medications should be dosed for patient comfort but should do not obtund the patient’s capacity to report pain during needle placement that may indicate possible tissue injury, or to blunt the patient’s capacity to determine whether or not the trial successfully produces paresthesia coverage of the painful body regions.
The administration of oral or parenteral prophylactic antibiotics 30-120 minutes before the procedure is advocated by some. A consensus of studies and experts suggest cefazolin 1-2 g or cefuroxime 1.5 g IV 30 minutes prior to incision or surgical implant is commenced. If the patient is allergic to beta-lactams, substitute clindamycin 600 mg IV (30 minutes prior to incision). If the patient is colonized with MRSA then the use of vancomycin 1 g IV 30-120 minutes prior to incision is recommended. Antibiotic coverage beyond 24 hours after administration has not been shown to provide additional benefit.
A percutaneous trial should be aseptic. Aseptic techniques, sometimes called "clean technique," refer to methods that prevent microbial contamination of the environment. For example, the purpose of scrubbing for surgery is to render your hands and forearms as aseptic as possible. Asepsis should protect both the patient and caregiver from infection. A sterile field describes a state that is technically free of all living microorganisms, including spores. Some items, such as surgical tools, can be sterilized and reused, but they must be stored in sterile conditions. A sterile operating field and sterile technique are used to keep the number of microorganisms to a minimum in an otherwise clean and aseptic environment, such as a fluoroscopic suite or operating room.
Hand washing with soap and water removes dirt, organic substances, and loose hand flora, but it is ineffective for reducing antimicrobial activity. Alcohol, especially isopropanol, ethanol, and n -propanol alcohol are highly effective against both gram-positive bacteria and gram-negative bacteria, including multi-drug resistant pathogens and fungi. Antiseptic agents containing 60-90% of alcohol are most effective.
However, their effectiveness against spores, protozoan oocytes, and some viruses is poor. Although these solutions have a rapid onset antiseptic effect, they also evaporate rapidly, thereby, providing only short-term efficacy. However, the eventual regrowth of bacteria is slow. Alcohol produces its antimicrobial effect due to its ability to denature proteins. Iodine is an effective antiseptic that quickly kills a broad range of microorganisms. Unlike antibiotics, iodine is not associated with the development of resistant strains of microbes. Betadine, a solution of povidone-iodine is the iodophor most commonly used as an antiseptic and scrubbing agent. Iodophors are effective against gram-positive and gram-negative bacteria, as well as some spore-forming bacteria, as well as mycobacteria, viruses, and fungi.
The patient is placed in a prone position with the arms above the fluoroscopic field and a pillow or a surgically designed flexion frame placed underneath the abdomen to reduce lumbar lordosis and even produce slight lumbar flexion for lumbar and thoracic needle entry and lead placement. Similarly, cervical positioning is best achieved with a prone neutral lower thoracic and lumbar spine with a bolster or pillow(s) placed underneath the chest to increase cervical flexion for cervical lead placement.
Prior to lead placement, the skin over the lumbar and lower thoracic paraspinal muscles within the needle entry region is prepared, wiped, and draped in an aseptic fashion. Complete sterile technique, including mask and gown are next implemented. Fluoroscopic assistance is used to visualize the pedicles of the vertebral bodies in the targeted field of entry. The optimal level for midline epidural entry is determined. This is typically in the dorsal midline at the level that the spinal cord becomes the conus medullaris, where lumbosacral nerve roots disperse laterally to form the cauda equina, ideally at the level above or below L1. Anterior-posterior fluoroscopic imaging of the working site is first optimized by aligning the image so that the spinous processes bisect the pedicles. Then the C-arm is adjusted in a cephalad or caudal direction to square off the vertebral end plates.
The skin entry point is paramedian, usually 2 levels below the desired midline epidural entrance, adjacent the medial border of the ipsilateral pedicle. A shallow angle of entry will facilitate lead advancement. For dual lead placement, pedicles are marked for 2 consecutive levels or on the contralateral side. Following application of 1% lidocaine without epinephrine or preservative, skin and subcutaneous analgesia is achieved. Thereafter, a 14-16 gauge epidural access needle is used to identify the epidural space using a loss of resistance or hanging drop technique.
The epidural space is the target for threading the percutaneous trial electrode. The outermost membrane of the spinal cord is the fibro-elastic dura mater that extends from the foramen magnum to the second sacral vertebrae. The epidural space encircles the dura mater. It is ventral to the ligamentum flavum with its adjacent periosteum and dorsal to the posterior longitudinal ligament. Its lateral borders for needle placement are the vertebral pedicles and intervertebral foramina (see the image below).
The cervical epidural space extends from the dura of the foramen magnum to the inferior border of C7. The thoracic epidural space begins at C7 and extends to the upper margin of the L1 vertebrae. The lumbosacral epidural space extends from the upper margin of the L1 vertebra.
The size of epidural space, as measured by the distance between the ligamentum flavum and the dura, varies with location in the spinal column. The largest distance is at L2, where it can measure up to 5-6 mm. However, in the thoracic spine, the distance is reduced to approximately 3- 4 mm, and at C7 the distance is only 1.5-2 mm. In 40% of patients the anatomic (vertebral) and physiologic (spinal cord) midlines may differ by as much as 2 mm at all spinal cord levels. For these reasons, the first lead should always be tested for paresthesia location before the second lead is placed.
To cover the desired dermatomal area with paresthesia, it is necessary to place electrodes over the dorsal columns several segments cephalad to that level.
With lead placement slightly off the midline between C2 and C4, stimulation can be obtained to alleviate shoulder pain. When coverage that encompasses the entire upper extremity is desirable, this is usually accomplished when leads are placed just off the physiologic midline between C3 and C4 and then moving inferiorly, as necessary, for stimulation of the more medial forearm and hand. Placing 2 leads, each slightly off the physiologic midline to the right and left between C4 and C6, is successful when bilateral upper extremity stimulation is desirable.
Lateral fluoroscopy allows the operator to document lead placement in the dorsal epidural space. Stimulation of the back and both lower extremities is often intended following failed lumbar surgery for treatment of lower torso and extremity neuropathic pain. Most patients can achieve stimulation coverage when the leads are placed in the midline between T8 and T9. The current medical literature supports the placement of a single midline lead for coverage of low back pain; however, many experienced practitioners place an additional lead, so paresthesia coverage can be achieved with reprogramming in cases when lead migration occurs.
Placing two leads slightly off the physiologic midline to the right and left between T8 and T10 will often allow for both lower back and lower extremity stimulation. Coverage of midline low back pain with current steering SCS systems can be achieved with leads that are less than 4 mm apart so that current can be directed to the midline to obtain low back stimulation.
Often unilateral lower extremity stimulation can be produced by lead placement slightly off the midline between T9 and T11. Distal lower extremity stimulation (eg, the foot) can be achieved with leads placed as low as T12 and L1, usually 1- 4 mm off the midline. Retrograde lead placement is sometimes necessary to achieve foot or pelvic stimulation. Furthermore, numerous more complex techniques that require retrograde electrode placement for lumbosacral nerve root stimulation are sometimes necessary when attempting pain coverage in some cases of rectal, perineal, or coccygeal pain.
For more information about the relevant anatomy, see Topographic and Functional Anatomy of the Spinal Cord, Cervical Spine Anatomy, and Lumbar Spine Anatomy.
A trial of spinal cord stimulation consists of 3 parts: The lead that will be implanted in the epidural space, the external pulse generator, and extension wires that connect the lead to the pulse generator. Programming units are necessary so that stimulation variables can be adjusted. Implantation kits include leads, equipment, and other tools necessary to implant the SCS system and are available from the manufacturers.
Epidural leads have stimulating electrodes at the distal end and a connector at the proximal end. Percutaneous leads are cylindrical catheters (1-column leads) with sequentially spaced electrodes at the distal end of the catheter. The number of electrodes and spacing between electrodes vary depending on the model and manufacturer.
A percutaneous electrode design allows current to flow symmetrically, thereby, creating 360° stimulation. Cylindrical spacers separate each electrode on the catheter. Percutaneous leads are placed through a Tuohy needle with a large flat bevel that is suitable for percutaneous trials, tunneled trials, or permanent implantation.
Placing more than one lead in order to provide adequate stimulation and provide redundancy in the event of minor lead migration is common.
Paddle leads deliver focused energy ventrally and contain up to 3 columns. When using 3-column leads, the outer columns can provide anodal blocking, whereby, stimulation is confined to the midline. This is especially useful in cases of refractory midline back pain. Some suggest that paddle leads are more efficient because the electrical energy is focused in 1 direction, perhaps, prolonging battery life. Paddle leads are placed using a small laminotomy so that the lead can be secured to the supraspinous ligament at the level determined most effective following a percutaneous trial. A head-to-head trial revealed comparable clinical outcomes at 3 years following the procedure.
Programmable pulse generators and neurostimulators provide the electrical power for stimulation. Extension wires connect the lead(s) to the pulse generators. One extension wire is necessary per implanted electrode lead. Manufacturer representatives are usually available to assist in selecting a combination of anodes and cathodes, amplitude, pulse width, and stimulation frequency that produce a comfortable and concordant paresthesia and provides the best coverage of the targeted pain area with the highest degree of pain relief. At the present time, most test screeners are integrated into a laptop computer or PDA device.
Percutaneous Trial of SCS Placement
As noted previously, percutaneous trial electrodes are placed through a 14-gauge needle. When oral analgesic and antianxiety agents are adequate, local anesthesia should be administered with preservative-free lidocaine with or without epinephrine. Lidocaine is preferred over bupivacaine because of reduced cardiotoxicity should an inadvertent intravascular injection occur. Minimal sedation is used during the procedure since the patient needs to be alert and communicative during the intraoperative stimulation procedure to ensure proper positioning of the leads. Airway equipment and resuscitation drugs, oxygen, and resuscitation equipment must be maintained and readily available.[13, 32, 33]
Following intradermal and subcutaneous injection of local aesthetic, some use a small skin incision with a #11 blade; however, with adequate anesthesia and multiple needle penetrations, using up to an 18-gauge needle, the area can be adequately anesthetized for the necessary 14-guage needle puncture radius that is required. The skin is entered at the medial aspect of the pedicle with the Tuohy needle (see image below) usually angled 30-45° toward the midline for optimal entry into the epidural space at the interlaminar edge 1 or 2 levels of cephalad.
The 14-guage needle is then passed with the bevel facing up at a 30-45° oblique angle to reach the depth of the midline laminar target.
After contact between with the interlaminar bone, the needle stylet can be removed and a loss of resistance (LOR) syringe attached. The Tuohy can next be angled more steeply, walking down the bone to enter the epidural space (see the image below). Once LOR is achieved, the syringe is gently removed, and a trial electrode lead is slowly advanced through the needle into the epidural space. AP and lateral fluoroscopic guidance is used to verify correct course of the lead cephalad in the dorsal epidural midline. With dual leads, the second lead is usually placed opposite to the side of the same spinous process or on the same side at a higher or lower level.
Although significant variability exists from patient-to-patient with regard to ideal lead positioning, most commonly, patients with lower back and leg back find adequate coverage with a lead placed midline at T8. Placing 2 leads, each slightly off the physiologic midline to the right and left, between T8 and T10 will allow for both back and lower extremity stimulation. Leads should be no more than 2-4 mm off the midline. During the procedure, and at the end, AP and lateral fluoroscopic records should be obtained to document the level and correct placement of the leads within the epidural space.
When electrodes are satisfactorily placed and the manufacturer’s representative has calibrated the SCS to achieve the most comfortable and concordant paresthesia with the best possible coverage of the targeted pain area, with the highest degree of pain relief at an acceptable voltage, then the lead stylets are carefully removed and a fluoroscopic record of final electrode placement is obtained. The Tuohy needle is then carefully removed and the operative site is cleaned with chlorhexidine and alcohol.
Next, the leads must be anchored externally. Lead migration is a common cause for an inadequate or unsuccessful trial. Most manufacturers provide cylindrical sleeves that are designed to improve skin fixation and to reduce lead migration. Recent anchor designs are under analysis to determine whether newer designs can reduce lead migration and breakage. The lead is passed through the anchor sleeve, sometimes with silicone adhesive to assure a firm bond within the anchor.
Before the Tuohy needle is removed, a surgeon’s knot can be used to suture the lead to the skin and to assure fixation before passing the lead through the anchor (see the image below).
Throughout this process of securing the lead externally, fluoroscopic imaging is used to confirm that lead manipulation does not result in any positional change of the electrode tip. Next, the anchor can be attached to the skin using a figure-of-8 stitch with 2-0 non-absorbable sutures. The leads and anchors can be further secured by carefully applying Steri-Strips to the lead or to its anchor. Some use Steri-Strips alone to fixate the leads.
A tension loop is usually placed, and then a sterile occlusive dressing is applied. Each lead is attached to an extension wire, which connects the trial lead(s) to the pulse generators. One extension wire per implanted electrode lead is necessary. The leads are attached to a laptop or PDA, which can be programmed to adjust the aforementioned variables.
After the procedure, the manufacturer’s representative performs final programming, and the patient is instructed to minimize general activities on the day of the procedure. Thereafter, the patient is asked to minimize activities that might encourage lead migration, such as bending, squatting, or stooping. Some physicians insist that patients wear a cervical collar or lumbar brace to prohibit precarious trunk movements. As mentioned, the trial length is determined by the physician’s experience and practice, usually, with durations that vary between 3-8 days. During the trial, the responsible company’s representative/ technologist should maintain 24 hours/ day availability for continued optimal programming and for any emergency physician actions that might be necessary, including lead removal. Patients are usually given written instructions that include precautions such as getting the SCS entry site wet or symptoms that represent complications (eg, infections at the needleentrysite,chills, fever, headache, or increasing neck or back pain).
The goal of the trial is to evaluate the level of pain relief and any perceived positive changes in quality of life that can be potentially recaptured by electrical spinal neuromodulation. Diaries often help patients chronicle their pain scores, using various validated pain scales; reduction of medication usage, especially fast-acting opioids used for break-through pain; and functional improvements, such as daily activities of living. The goal of the trial is to evaluate the level of pain relief and any perceived positive changes in QOL that can be potentially re-captured by electrical spinal neuromodulation.
Current criteria used to proceed with a permanent implantation of an SCS device demand at least 50% reduction in pain intensity and reported improvement in the patient’s QOL during the SCS trial.[32, 33, 42, 43, 44] In addition to global impressions regarding outcome, a battery of physical and psychological self-assessment tests or psychometrics may allow better or more objective parameters by which to assess the failure or success of a trial.
Before the trial electrodes are removed, the position of the lead tips are reassessed by fluoroscopy and a visual record is established for permanent placement if the patient feels the trial was successful and chooses to proceed.
Permanent SCS Placement
Percutaneous circumferential electrode leads are placed with the same preparation, patient positioning, and technique described for the trial with some exceptions. Parenteral access is placed with a 22-guage heplock. IV prophylactic antibodies are administered at least 30 minutes prior to skin entry. An anesthesia care provider should be present to provide light sedation and cardiopulmonary monitoring during the procedure. Sedation can be lightened appropriately for intraoperative programming.
Following confirmation of lead placement and stimulation coverage, the lead stylets and Tuohy needles are removed. Next, skin along the superior portion of the pocket is infiltrated with local anesthetic, and a 4-cm scalpel incision forms the superior border of the IPG pocket. A subcutaneous pocket approximately 2 cm deep is formed using bipolar electrocautery and blunt dissection. Monopolar electrocautery should not be used and care should be taken not to damage SCS system components. Local anesthetic is infiltrated prior to additional incisions.
After infiltration of local anesthetic, the leads are secured by manufacturer-supplied anchors in the midline to the interspinous ligament (see the image below).
Next, a strain relief loop is placed, and a diagonal subcutaneous tunnel is created to pass the leads to the planned IPG pocket-site. Local anesthetic should be placed along the tunnel route, and IV sedation should be increased. A preassembled, manufacturer-supplied tunneling tool consists of a sharply tipped malleable metal shaft that is enclosed in a plastic cannula sleeve.
The tunneling tool is passed subcutaneously between IPG pocket-site and midline lead-anchor site above superficial muscular tissues. The handle of the tunneling tool is removed from its shaft, leaving the plastic sleeve within the tunnel.
The ends of the leads are introduced into and passed through the plastic sleeve. The plastic sleeve is withdrawn from the subcutaneous tunnel, and the ends of both leads are adjacent the IPG site. They should be irrigated with sterile water rather than saline to prevent corrosion and then securely connected to the pulse generator.
Following successful SCS testing, the set screws are tightened to secure the lead fixation to the IPG. When the pocket is clear of "bleeders" and irrigated, the battery is placed with the noninsulated lettered side of the unit facing out toward the skin. Both incision sites are irrigated with Bacitracin, and the wound is closed in layers with a 3-0 absorbable multifilament suture. Subcutaneous suturing is accomplished with a 4-0 absorbable monofilament suture and cyanoacrylate skin adhesive, followed by application of Steri-Strips. Meticulous hemostasis and suturing techniques prevent dead-space that predispose to seroma or hematoma formation and secondary infection. All surgical wounds are secured by sterile dressings, and the patient is taken to the recovery room.
When the patient is awake and alert, the unit is again activated, tested and programmed. The patient is instructed regarding limiting activities and braced per physician preference similar to post-trial instructions.
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