Spinal Cord Stimulation: Fundamentals

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Spinal Cord Stimulation: Fundamentals Interventional pain specialists offer an overview of spinal cord stimulation (dorsal column neuromodulation) fundamentals that referring physicians can use in clinical practice.

Elmer G. Pinzon, MD, MPH Minimally Invasive Interventional Spine and Musculoskeletal Specialist President, Medical Director, Owner University Spine & Sports Specialists, PLLC Clinical Assistant Professor Department of Surgery Division of Surgical Rehabilitation University of Tennessee Graduate School of Medicine Knoxville, Tennessee James A. Killeffer, MD Assistant Professor Department of Surgery Division of Neurosurgery University of Tennessee Graduate School of Medicine Knoxville, Tennessee

pinal cord stimulation (SCS) has been in visual analog pain scores, and improvement in activities used in pain management since C. of daily living. The hardware is more durable, more effec- Norman Shealy, MD, PhD, implanted tive, more maneuverable, and provides a greater range of the first neuroaugmentive device in a coverage for the affected area. The devices can be implanted cancer patient in 1967.1-3 Since then, sev- percutaneously under fluoroscopic guidance (especially for eral studies have examined the long-term the trial leads placement), which allows operator–patient S effects of SCS in pain management, with verbal interaction and more accurate positioning of spi- variable outcomes and success rates.4-8 As with many novel nal cord stimulator leads for trial and eventual permanent procedures, initial problems with SCS included poorly placement. In addition, more than three decades of expe- designed hardware, inadequate patient selection criteria, rience have provided improved patient selection criteria, and suboptimal surgical techniques.7,9-11 which is paramount in effecting a positive eventual out- Significant advances in SCS, however, have been made in come. The net result is an improved capability to control recent years. Postoperative outcomes of the procedures have various chronic pain conditions, especially those that are shifted to more positive outcomes in the field of neuroaug- peripherally referred as opposed to centrally referred.8 mentation, especially with respect to such practical factors This article will discuss the pathophysiology, mechanism as return to work, reduction in medication use, reduction of action, and clinical applications of SCS; as well as

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success of a spinal cord stimulator Table 1. Mechanistic Theories for SCS system.7,9-11 Technical advances leading to improved hardware, coupled • Gate control theory—segmental, antidromic activation of A-ß efferents with improved patient selection, have • SCS blocks transmission in the spinothalamic tract improved the rate of long-term effi- • SCS produces supraspinal pain inhibition cacy of SCS to approximately 70% today, up from approximately 40% • SCS produces activation of central inhibitory mechanisms influencing sympathetic 3,4,8 efferent neurons since the 1970s and 1980s. A spinal cord stimulator neuro- • SCS activates putative neurotransmitters or neuromodulators modulation system should be consid- SCS, spinal cord stimulation ered for patients who have failed all reasonable conservative care includ- ing appropriate diagnostic, therapeu- current clinical results, and potential changes in blood flow and skin tem- tic, and rehabilitative techniques, and future trends in SCS, also known as perature from SCS may affect noci- have been given a reasonable period dorsal column neuromodulation. ception at the peripheral level.16-20 of time to recover from the condi- This postulation is further supported tion.8 A reasonable time period is at SCS Mechanism of Action in part by data from Marchand et al, least 6 to 12 months of conservative, Although the exact mechanism for who investigated the effects of SCS pain-relieving, minimally invasive pain control from SCS is not entirely on chronic pain using noxious ther- treatments, and/or failure of surgical understood, it is believed to result mal stimuli.21 Since it was discovered treatments, with persistent extremity from direct or facilitated inhibition of that SCS causes vasodilation in ani- pain greater than axial spine pain. pain transmission.3,5-7,12,13 Table 1 lists mal studies, clinicians have used this An ideal patient should be moti- the five mechanistic theories for why modality for the treatment of chronic vated, compliant, and free of drug SCS works.13 pain due to peripheral vascular disease dependence.33 Psychological screen- The gate control theory motivated and is the leading indication for SCS ing is recommended but not manda- Shealy et al to apply SCS as a means in Europe today.13,15,22-26 The precise tory to exclude conditions that predis- to antidromically activate the tactile action of pain modulation by SCS is pose to failure of the procedure (see myelinated A-β fibers through dorsal still in debate. A better understanding article on psychological evaluation in column stimulation.1,2 Shealy rea- of the pain system may lead to more SCS patients on page 35). Diagnoses soned that sustained stimulation of effective stimulators and allow for that are typical indications for this the dorsal columns would keep the even greater success. procedure include chronic radiculop- gate closed and provide continuous Today, the most common indica- athy, perineural fibrosis, neuropathic pain relief. While the theoretical “gate tion for SCS is for the treatment of pain, and complex regional pain control” model put forth by Melzack chronic low back and lower extrem- syndrome.34-38 In the United States, and Wall has been shown not to be ity pain due to chronic radiculopa- peripheral vascular disease is not an precisely correct, pain gating or pain thy or postlaminectomy lumbar pain FDA-approved indication. control has been shown to exist.5-7,12 syndrome despite adequate surgical When considering pain topogra- Others believe that pain relief from intervention.27-32 This population phy, extremity pain responds better SCS results from direct inhibition of represents the primary indication for than axial pain, and the more distal pain pathways in the spinothalamic SCS in our practice and has provided the extremity pain the greater the tracts and not secondary to selective us with an effective treatment option. clinical response.27,39 Middle and large fiber stimulation.14 This theory Table 2 contains a list of commonly upper lumbar pain as well as tho- has been supported by Hoppenstein, accepted and potential indications, racic, cervical, and chest wall pain who showed that the posterolateral in addition to commonly accepted are difficult to adequately control stimulation of the spinal cord pro- contraindications. and maintain long term. Pain due to vided effective contralateral pain relief severe nerve damage superimposed on with substantially less current than Selection Criteria cutaneous numbness (ie, anesthesia posterior stimulation.15 As noted, proper patient selec- dolorosa) is also difficult to treat with Some investigators think that the tion is essential to the long-term SCS. Central pain syndromes do not

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Table 2. SCS Indications and Contraindications

Commonly accepted indications • Chronic pelvic pain • Postherpetic neuralgia • Deafferentation pain • Intercostal neuralgia • Axial pain • Post-laminectomy (thoracic region) syndrome (ie, FBSS) • Thoracoabdominal aortic aneurysm • Post-laminectomy (lumbar region) syndrome (ie, FBSS) • Cerebral palsy • Cauda equina (chronic) injury syndrome • Multiple sclerosis • Chronic arachnoiditis • Spinal cord injury • CRPS of the upper limb Commonly accepted contraindications (Absolute) • CRPS of the lower limb • Sepsis • CRPS of other specified site • Coagulopathy • Phantom limb pain syndrome • Previous surgery or trauma that obliterates the spinal • Cardiovascular angina/ischemic pain canal • Atherosclerosis of the extremities with resting pain • Localized infection at the implantation site (ie, peripheral vascular disease) • Spina bifida • Brachial neuritis or chronic cervical radiculopathy Commonly accepted contraindications (Relative) • Thoracic or lumbosacral neuritis or chronic • Physical and/or cognitive/psychological disability that radiculopathy interferes with proper usage of and understanding of • Cervical nerve root injury the device • Thoracic nerve root injury • Significant somatization/somatoform disorders • Lumbar nerve root injury • Unmanaged substance abuse or cognitive disorders Other potential indications • Lack of social support • Chronic occipital neuralgia/cervicalgia

CRPS, complex regional pain syndrome; FBSS, failed back surgery syndrome; SCS, spinal cord stimulation respond to SCS and are best treated healthcare system. leads are sutured tightly with anchors by other modalities. The process of the trial percutane- to the skin. Subsequently, the com- ous spinal cord stimulator approach pleted circuits are taped securely to Percutaneous Trial should involve an alert and communi the skin and covered to prevent them The use of an outpatient percutane- cative patient who can provide the from accidently being pulled out but ous trial of between 3 to 7 days with practitioner with correct lead posi- still allow them to be attached to the an SCS system has been proven help- tioning. The patient should be made programmer. Then, to verify the final ful in determining which patients comfortable with local anesthesia electrode position, anteroposterior will respond well enough to war- infiltration at the insertion sites. The and lateral radiographs should be rant a permanent spinal cord stimu- interventional pain specialist can use obtained with the fluoroscopy films. lator implantation and determine the trial screening lead(s) during the the future permanent implantation screening trial. Once the screening Postprocedure Care levels.27,28,39,40 Absolute criteria that lead is positioned at the exact loca- The patient routinely recovers after must be present for a patient to have tions (determined from communi- 30 to 60 minutes in a postoperative a positive trial include tolerance cation between the patient and the recovery setting.7,9-11 Once the patient of paresthesia, >50% to 75% pain interventional spine specialist), then is awake and alert in the recovery relief, and overall patient satisfac- the temporary external power source area, the patient’s spinal cord stimulation. Relative requirements for a posi- (screener) is connected. When both tor settings should be optimized. The tive trial include improved functional the patient and physician are satisfied adjustable parameters of electrical level, reduced usage of pain medica- that the stimulation coverage is satis- stimulation in spinal cord stimulation, and reduced reliance on the factory, then the spinal cord stimulator tors are frequency (Hz), pulse width

March 2013 | Practical Pain Management 29 30 Image Copyright St. Jude Medical Neuromodulation, Inc., all rights reserved. Spinal Cord Stimulation:Fundamentals permanent implantation. and trial stimulation cord spinal for column dorsal the along leads 1. Figure experience. high, the patient may have anoxious When the amplitude is adjusted too through” thestimulationpattern. ing it harder for the pain to “break dense stimulationpattern,thus,mak- that thepatientexperiencesamore ulus. Clinically, thisusuallymeans sents theelectricalforce ofthestim- the lower back. The amplitude repre- esthesia patternmaythenincorporate width canbeincreased andthepar- but isonlycovering thehip. Thepulse lation patternneedstocover theback eficial when,forexample,thestimu- ment ofparesthesia. Thismaybeben- this usuallymeansabroader disburse- tion into the spinal cord. Clinically, which provides fordeeperpenetra- increases thedensityofstimulus, stimulus. Increasing thepulsewidth may beusedasastronger counter- 80 Hz,althoughahigherfrequency (volts). Atypicalfrequency is50to (stimulus duration), and amplitude Practical Pain Management | March 2013 Schematic representation of percutaneous Octrode dual Octrode of percutaneous representation Schematic or anyotherunusual occurrences. ulation pattern,signsofinfection, cian in case of any alteration in stim- thephysi- toalert also isinstructed their routine activitylevel. Thepatient cation consumptionandtomaintain tion, theyare toldnottoaltermedi- spinal cord stimulatorlead.In addi- or twistingasthismaydislodgethe toavoid excessiveinstructed bending otics are provided andthepatientis trial period.Prophylactic oralantibi- but totakespongebathsduringthe cifically toldnottobatheorshower andspe- stimulator area clean and dry patient istoldtokeepthespinalcord and anyquestionsare answered. The educated onhow to usethedevice, patient and/orpatient’s familyis tor programming isfine-tuned,the period, thespinalcord stimula- Duringinstructions. the recovery charged home with postoperative uneventful, thepatientcanbedis- As longastherecovery periodis countdifferential with nalysis, completeblood of theprocedure. Auri- hospital themorning surgicalcenteror latory brought intotheambu- tor implantationis spinal cord stimula- going apermanent The patientunder- Implantation Permanent Surgical spinal cord stimulator. ment ofapermanent proceed withplace- determine whetherto physician shouldthen is assessed,andthe the efficacyofSCS the leadisremoved, tion, atwhichpoint following implanta- within 3to7days Follow upisusually done eitherunder generalanesthesia previous trialstimulation. Thiscanbe cise level beingadaptedtoresults of T8-T10 laminectomy, withthepre - back pathologyisviaapproximately of sublaminar paddle leads for low better overall clinical outcomes. broader stimulationcoverage, with subject tomigrationandcanprovide of dural breach. Paddle leads are less ization, potentiallymitigatingtherisk dle leadsisdoneunderdirect visual- ment ofwire leads.Placement ofpad- advantages over percutaneous place- paddle leadplacementoffersseveral anesthesia (Figure 2). to undergoorunsuitableforgeneral erable inpatientswhoare unwilling taneous leadplacementmaybepref- minimally invasive nature ofpercu - the generatorincisionisclosed.The cutaneously placedgeneratorand are tunneledwithatrochar toasub- lead placementiscomplete,wires for permanent lead placement.After used intrialleadplacementis thesia. Thesamegeneraltechnique typically donewithoutgeneralanes- leads isminimallyinvasive andis ative site. installed generatorataseparateoper- nected by wires toasubcutaneously the sublaminarepiduralleadsare con- paddle leads(Figure 1).In eithercase, via openplacementofflatplateor round wireneously inserted leads,or systems canbeplacedwithpercuta- difficulty.monary signs orsymptomsofcardiac orpul- disease,orshow ongoing pulmonary ofcardiacyears old,have or ahistory in allpatientswhoare more than45 electrocardiogram shouldbeobtained the implantation.Achestx-rayand obtained within72hourspriorto and sedimentation rate, shouldbe A versatile techniqueforplacement Although more invasive, open- Percutaneous placementofwire Permanent spinalcord stimulator 41,42

Spinal Cord Stimulation: Fundamentals

or conscious sedation. In either case, this is easily accomplished in an outpatient setting. The patient is placed in the prone position on a surgical saddle frame. Using fluoroscopic guidance, a midline incision is made over the spinous process and is taken through the dorsal fascia. Paraspinous muscles are elevated off of the lamina, and a complete facet-sparing laminectomy is performed. The epidural space is developed using a dural dissector. If lower extremity peripheral nerve stimulation is desired, a lead may be placed caudally to cover the posterior spinal cord, conus medullaris, and anterior cauda equina. Otherwise, the lead is placed anteriorly over the appropriate thoracic level. Appropriate lead position is confirmed with anteroposterior and lateral fluoroscopy. If the procedure is done in an awake patient, test stimulation can be performed. Neurophysiological mapping may be used if desired for confirmation of localization with general anesthesia.43 Once a satisfac- tory position is confirmed, the wire leading into the lead is anchored to the remnant of the interspinous ligament or directly to the adjacent spinous process utilizing a sleeve and a non-absorbable suture. Wires are then passed through a trochar to the generator, which is implanted in a subcu- taneous pouch commonly just below the iliac crest. Incisions are closed in a layered fashion and the patient is transported to the post-anesthesia recovery unit, where appropriate stimulation coverage is confirmed once the patient is fully awake. An example of a patient in which the paddle lead is advantageous is one who has undergone previous instrumented fusion for spondylolisthesis. Although there is Figure 2. Schematic representation of permanent implantation of radiographic evidence of adequate fusion Eon IPG (implantable pulse generator) system (St. Jude Medical, and no evidence of neural compression, Inc.) along the dorsal column for spinal cord stimulation. the patient has persistent radicular lower extremity pain and disabling axial low back Image Copyright St. Jude Medical Neuromodulation, Inc., all rights reserved. pain. After a successful percutaneous lead trial, spinal cord stimulator placement using a single paddle lead centered at T9-T10 is performed. The lead consists of two or

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three rows of several electrodes whose to work, then aggressive rehabilitation an unknown source; and the third output can be adjusted individually should be performed. infection occurred 18 months follow- via a transcutaneous programming ing implantation. The latter infection device. This allows the patient to try, Potential Complications of SCS was apparently due to hematogenous noninvasively, a wide range of pro- There are rarely any serious complica- seeding when the patient broke an gramming options postoperatively tions from the temporary percutane- abscessed tooth after he bit down on until optimal stimulation coverage ous trial or permanent procedure for an apple the week before. In the first of low back and lower extremities spinal cord stimulator implantation.44 two cases of infection, the spinal cord is achieved without uncomfortable In one study, one nonfatal pulmonary stimulators were removed and the paresthesias. embolism and one case of paraplegia patients were placed on intravenous Preoperative and postoperative lasting 3 months were reported.45 The antibiotics without further sequelae. intravenous antibiotics are admin- latter resulted from a laminectomy In the third case, the spinal cord istered and following recovery, the that was used to place the stimulating stimulator was not removed and the patient is discharged with 7 to 10 lead. Other rare reported complica- patient was adequately treated with days of oral antibiotics, or kept for a tions include sphincter disturbance oral antibiotics and dental care. We 23-hour hospital observation (physi- and gait abnormality.46 have had no complications with any cian/surgical preference). Most complications from the tem- of the trial lead placements. porary or permanent devices include Postprocedure Care and Follow-up formation of scar tissue, poor local- Clinical Results Protocol ization of paresthesias, lead migra- The largest study of SCS includes Upon discharge, the patient is given tion, lead fracture, pain at the pocket 320 consecutive patients who under- verbal and written instructions to site or connection site, infection, went either temporary or permanent avoid excessive lifting, twisting, or nerve injury, and epidural hema- implantation at the Johns Hopkins bending, and to sponge bathe only for toma.24,25,29,44,47-52 In a comprehensive Hospital between 1971 and 1990.13 2 weeks. The first postoperative visit summary of different publications, This series includes follow up on 205 is 1 week following the permanent lead migration or displacement varied patients, the majority of whom had insertion. The surgical site is checked from 3.7% to 69%, although most the diagnosis of failed back surgery and any skin staples or sutures are studies reported migration between syndrome (FBSS). Permanent spinal removed. At that time, there may be 16% and 25%.44 Rates of lead frac- cord stimulator implants were placed slight swelling noted in the pocket. tures were reported in various series in 171 of these patients. At follow up This is probably a normal finding from <1% to >20% and superficial (mean interval 7.1 years, SD 4.5), and represents a seroma (a pocket of infections occurred in 2% to 12% 52% of patients had >50% contin- clear fluid secreted from the serous of cases. Serious surgical infections ued pain relief, and 58% had reduced glands that can develop post-surgery) were rare, as were clinically apparent or eliminated the use of medication. although the clinician should have epidural hematomas. In one study, About 54% of patients younger than appropriate suspicion for infection. A cerebrospinal fluid leakage was found 65 were working at the time of fol- seroma may last for 3 to 4 weeks and in 2% of patients. Avoiding compli- low up; 41% had been working may interfere with transmission to cations in spinal cord stimulation preoperatively. the radiofrequency-controlled devices should follow an analytical step-wise The percentage of patients having (eg, Eon, Genesis, and Renew devices approach. long-term pain relief is similar in the from St. Jude Medical; Interstim, In our clinical experience that has majority of large published studies Prime Advanced, and Restore devices involved >600 lead implants, the of spinal cord stimulator implants from Medtronic; and Precision device clinical practice has experienced only for FBSS. The success rate in most from Boston Scientific). Also during 3 in situ infections with permanent of these studies, which is gener- this visit, the spinal cord stimulator is devices.10 One infection resulted from ally reported as ≥50% pain relief, is reprogrammed as needed. The patient an occult bone stimulator infection approximately 50% to 60%.37,53-57 should be seen 2 weeks later and then due to a previous fusion and pre- Some studies report success rates again in 1 month. After that, the sented >6 months following implan- as high as 88% and others as low as patient should be seen as indicated. tation; the second infection occurred 37%.58,59 Although these latter studies If the patient has a goal of returning 2 ½ months after implantation from differ in implantation technique and

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screening protocols, the success rate periods of disability, and the atten- devices and technology should assist for pain reduction generally remains dant sociobehavioral changes that in providing further options to be the same. take place. Despite this disparity, available for this select, but grow- More recent published reviews there is a general increase in function ing, population of chronic pain specifically have looked at the reduc- and activities of daily living. patients.7,9-11 tion in pain, reduction in opioid medication consumption, improve- The Future Authors’ Bios: Elmer G. Pinzon, ments in activities of daily living The future of SCS/neuromodu- MD, MPH, FABPMR, is the presi- function, and return to work sta- lation looks promising with the dent, medical director, and owner of tus.24,60-63 According to these stud- planned technological advances University Spine and Sports Specialists, ies, long-term pain reduction (at in these devices.37,39,64-66 Both St. PLLC. He also is a clinical assistant least 2 years after implantation) can Jude Medical and Medtronic have professor in the Department of Surgery, be expected to range from 50% to implanted pulse generators and lead Division of Surgical Rehabilitation at 70% in approximately 60% of SCS devices that allow an adequate power the University of Tennessee Graduate patients. In 50% to 90% of indi- supply for dual lead systems, which School of Medicine in Knoxville, viduals, there will be an elimination extends the life of the pulse genera- Tennessee. Dr. Pinzon specializes in or reduction in the use of opioids. tor. In addition, St. Jude Medical minimally invasive interventional spine The return to full employment rate has developed a pulse generator and pain medicine, and musculoskele- after SCS reported by two studies is that employs a capacitor instead of tal medicine. 25% to 59%, which is very signifi- a battery that is rechargeable by an James A. Killeffer, MD, a neurosur- cant when comparing it to the usual external radiofrequency-controlled geon, is a clinical assistant professor in return-to-work rate in this popula- device. With a coordinated program the Department of Surgery, Division tion of 1% to 5%.24,61 of multivariate treatment protocols, of Neurosurgery, at the University of Reasons for the disparity between as outlined in this spine-centered Tennessee Graduate School of Medicine. pain reduction and return-to-work orthopedic clinic setting model, fur- He is also owner of University rates appear to reflect the high per- ther coordinated improvements may Neurosurgery, PC, in Knoxville. centage of unskilled laborers among facilitate successful long-term out- Dr. Pinzon and Dr. Killeffer have no this population, the prolonged comes. Further neuromodulation financial information to disclose.

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