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<?xml version="1.0" ?> <tei> <teiHeader> <fileDesc xml:id="0"/> </teiHeader> <text xml:lang="en"> <p>M elzack and Wall <ref type="biblio">[37]</ref> stimulated new interest<lb/> in pain research and therapy using electrical<lb/> modalities. The first use of electrical stimulation for<lb/> the spinal cord was reported nearly 30 years ago by<lb/> Shealy et al <ref type="biblio">[54,55]</ref>. Today, spinal cord stimulation<lb/> (SCS) has become a common and effective method<lb/> of treating chronic pain. Over time, the results of<lb/> SCS have improved because of better patient selec-<lb/>tion <ref type="biblio">[26,33,34]</ref>, improvements in matching elec-<lb/>trode placement to sites of pain <ref type="biblio">[26,45,46,48]</ref>, and<lb/> the advent of multipolar stimulation devices to re-<lb/>place unipolar devices <ref type="biblio" >[26,41,42]</ref>. Incorporation of<lb/> the trial stimulation period has also improved se-<lb/>lection of long-term patients for SCS <ref type="biblio" >[26,27,41,42]</ref>.<lb/></p> <p>This study reviews our experience with SCS over<lb/> the past 15 years. Follow-up averaged 66 months<lb/> and was performed by a disinterested third party<lb/> not involved in patient care. We analyzed our series<lb/> for prognostic factors such as the age, sex, etiology,<lb/> type of electrode used, duration, work status and<lb/> involvement of Workman's Compensation Board<lb/> benefits, and laterality of pain as well as relation-<lb/>ships to previous operative procedures and their<lb/> response to SCS. Some of these factors will prove<lb/> useful to the clinician in selection of patients for<lb/> this procedure.<lb/></p> <head>Materials and Methods<lb/></head> <head>PATIENT SELECTION<lb/></head> <p>The 235 patients in this series were categorized by<lb/> etiology of pain as shown in <ref type="table">Table 1</ref>. Forty-one<lb/> other patients were not included because of inabil-<lb/>ity to contact them or because of patient death<lb/> (eight cases) unrelated to their SCS. One hundred<lb/> fifty men and eighty-five women were included, with<lb/> an age range of 23– 85 years and a mean age of 51.4<lb/> years. All patients had used narcotic medication for<lb/> pain relief before SCS. They were treated in a mul-<lb/>tidisciplinary pain clinic for a minimum of 6 months<lb/> and were found to be refractory to conservative<lb/> modalities of pain relief; after failure, they were<lb/> referred for SCS. All 235 patients met the following<lb/> criteria: 1) A defined non-malignant, organic cause<lb/> of pain; 2) failure of conservative pain control meth-<lb/>ods; 3) absence of a major psychiatric disorder; 4)<lb/> removal of inappropriate drug use before implanta-<lb/>tion; and 5) capacity to give informed consent for<lb/> the procedure. Patients who obtained 50% or<lb/> greater pain relief during trial stimulation subse-<lb/>quently received permanent implantation.<lb/></p> <head>PSYCHIATRIC CONSIDERATIONS<lb/></head> <p> During the treatment phase at the multidisciplinary<lb/> pain clinic, all patients received psychiatric screen-<lb/>ing. During their workup, patients who were found<lb/> to have a major refractory psychiatric disorder and<lb/> those found to have significant somatization were<lb/> excluded from this study.<lb/></p> <head>CAUSES OF PAIN<lb/></head> <p>The most common pain syndrome as an indication<lb/> for SCS was failed back syndrome, a total of 114<lb/> patients. Patients comprising this group had previ-<lb/>ous lumbar disc excisions or laminectomy with or<lb/> without foraminotomies for various surgical pathol-<lb/>ogies. Subsequently, all of these patients had un-<lb/>dergone magnetic resonance imaging (MRI) or com-<lb/>puted tomography (CT), which confirmed the<lb/> presence of epidural fibrosis, perineural fibrosis, or<lb/> arachnoiditis, which was assumed to be responsi-<lb/>ble for their continuing symptoms. These patients<lb/> were identified to have no further surgically cor-<lb/>rectable pathology.<lb/></p> <p>Another common cause of chronic pain was pe-<lb/>ripheral vascular disease (PVD) for 39 patients iden-<lb/>tified <ref type="biblio">[22]</ref>. All patients with PVD considered for SCS<lb/> had diffuse vascular disease confirmed by angiog-<lb/>raphy to involve both large and small vessels. Each<lb/> patient's vascular disease was determined to be<lb/> non-reconstructable.<lb/></p> <p>Peripheral neuropathy was another indication for<lb/> SCS in a total of 30 patients. This subgroup of pa-<lb/>tients has been discussed in detail in a previous<lb/> communication by the authors <ref type="biblio">[23]</ref>.<lb/></p> <p>Multiple sclerosis was responsible for chronic<lb/> pain in 13 cases in this series. All of these patients<lb/> had spinal cord and brain lesions confirmed by<lb/></p> <ref type="table">1 Categorization of Pain Syndromes and Their Response to SCS<lb/> ETIOLOGY OF PAIN<lb/> NUMBER<lb/> TRIAL STIMULATION PAIN<lb/> RELIEF<lb/> LONG-TERM PAIN RELIEF<lb/> SUCCESS<lb/> FAILURE<lb/> SUCCESS<lb/> FAILURE<lb/> Failed back syndrome<lb/> 114<lb/> 101<lb/> 13<lb/> 52<lb/> 49<lb/> Peripheral vascular disease<lb/> 39<lb/> 29<lb/> 10<lb/> 20<lb/> 9<lb/> Peripheral neuropathy<lb/> 30<lb/> 19<lb/> 11<lb/> 14<lb/> 5<lb/> Multiple sclerosis<lb/> 13<lb/> 12<lb/> 1<lb/> 8<lb/> 4<lb/> Reflex sympathetic dystrophy<lb/> 13<lb/> 13<lb/> 0<lb/> 13<lb/> 0<lb/> Spinal cord lesion<lb/> 10<lb/> 4<lb/> 6<lb/> 2<lb/> 2<lb/> Perirectal pain<lb/> 5<lb/> 3<lb/> 2<lb/> 2<lb/> 1<lb/> Cauda equina lesion<lb/> 5<lb/> 5<lb/> 0<lb/> 0<lb/> 5<lb/> Bone and joint syndromes<lb/> 3<lb/> 3<lb/> 0<lb/> 0<lb/> 3<lb/> Stump pain/phantom limb pain<lb/> 3<lb/> 0<lb/> 3<lb/> —<lb/> —<lb/> Total<lb/> 235<lb/> 189<lb/> 46<lb/> 111<lb/> 78<lb/> magnetic resonance imaging. The spinal cord le-<lb/>sions were considered the cause of the patient's<lb/> limb pain.<lb/></ref> <p>Thirteen patients presented with reflex sympa-<lb/>thetic dystrophy (RSD) <ref type="biblio">[24]</ref>. All cases of RSD were<lb/> sympathetically mediated and met the following<lb/> clinical criteria: (1) history of limb trauma; (2) pain<lb/> in the limb was outside the distribution of a single<lb/> peripheral nerve; (3) objective physical findings<lb/> suggestive of RSD: edema, cyanosis, temperature<lb/> changes, or soft tissue atrophy; and (4) positive<lb/> response to sympathetic blockade. The criteria for<lb/> our diagnosis were derived from guidelines devel-<lb/>oped by Blumberg and Janig <ref type="biblio">[2]</ref>.<lb/></p> <p>The remainder of patients in this series had pain<lb/> secondary to lesions of the spinal cord, perirectal<lb/> pain, cauda equina lesions, bone and joint syn-<lb/>dromes, and stump or phantom limb pain.<lb/></p> <head>ELECTRODE IMPLANTATION<lb/></head> <p> All electrodes were Medtronic systems (Medtronic,<lb/> Inc., Minneapolis, MN); electrodes were implanted<lb/> percutaneously<lb/> (Pisces-Sigma<lb/> and<lb/> Pisces-<lb/>Quadripolar) or through a small laminotomy in the<lb/> case of the Resume systems. Techniques for im-<lb/>plantation have been described previously <ref type="biblio">[27]</ref>.<lb/> Electrodes were placed so that stimulation-induced<lb/> paresthesia covered the anatomic distribution of<lb/> the patient's pain. The location of the stimulating<lb/> tips was usually between the C5 and T1 vertebral<lb/> bodies for upper limb pain and between T9 and T11<lb/> for lower extremity pain. Pisces-Sigma and Pisces-<lb/>Quadripolar electrodes were inserted percutane-<lb/>ously through Touhy type needles and were placed<lb/> under fluoroscopic guidance. Sigma-Pisces systems<lb/> were only used in the earlier years of this study.<lb/> Resume electrodes were placed through small lami-<lb/>notomies and were guided visually. Resume elec-<lb/>trodes were selected in patients for whom percuta-<lb/>neous placement was technically difficult because<lb/> of post-surgical scarring or after fractures or re-<lb/>peated displacements of previous percutaneously<lb/> implanted electrodes. All surgeries were performed<lb/> under sterile conditions with patients mildly se-<lb/>dated and using lidocaine for local anaesthesia.<lb/></p> <p>After initial electrode implantation, all patients<lb/> were given a 3-to 7-day trial period of stimulation to<lb/> determine whether satisfactory pain relief could be<lb/> obtained. In patients where adequate pain relief<lb/> was achieved, the electrodes which were originally<lb/> implanted were internalized.<lb/></p> <p>Pulse generators used were Medtronic Itrel I, Itrel<lb/> II, or X-trel systems. Parameter settings were usu-<lb/>ally between 55– 60 Hz, with pulse widths of 210 to<lb/> 300 s, and amplitudes between 1.5– 6.0 V. Cycling<lb/> modes used were variable. In most cases, stimula-<lb/>tion units cycled on for 1 minute, and cycled off for<lb/> 10 minutes. Various combinations of bipolar or mul-<lb/>tipolar electrodes were used to determine the best<lb/> pain coverage. For upper limb implants, pulse gen-<lb/>erators were placed in subcutaneous pockets below<lb/> the clavicle, whereas thoracic and lower limb im-<lb/>plants had pulse generators located either in the<lb/> right iliac fossa below the belt line, or in the supe-<lb/>rior gluteal area.<lb/></p> <head>CLINICAL EVALUATION<lb/></head> <p> Pain relief was scored by personal interviews with a<lb/> disinterested third party physician who was not<lb/> involved in the direct care of patients in this study.<lb/> A modified visual analogue scale (VAS) <ref type="biblio">[5,15]</ref> with a<lb/> numerical scale of descriptors ranging from 0 to<lb/> 100% was used to allow patients to quantify pain<lb/> levels before and after SCS. The interviews to assess<lb/> pain relief were performed at every 6-month inter-<lb/>val of self-stimulation for each patient. Patients<lb/> were graded according to their pain control using<lb/> the following criteria: 1) less than 50% relief (poor);<lb/> 2) 50 to 75% relief (good); and 3) greater than 75%<lb/> relief (excellent). Both good and excellent results<lb/> were considered successful in this study.<lb/></p> <head>Results<lb/></head> <head>PAIN RELIEF<lb/></head> <p>Two hundred and thirty-five patients were in-<lb/>cluded in this study; 189 patients (80%) experi-<lb/>enced satisfactory initial pain relief and had their<lb/> systems internalized. The minimum follow-up pe-<lb/>riod was 6 months and the maximum follow-up<lb/> period was 179 months. After an average of 5.6<lb/> years follow-up, 111 (59%) internalized patients<lb/> continue to receive satisfactory pain relief. Of the<lb/> 111 patients considered successful, 48 (43%) re-<lb/>ported excellent pain relief, while the other 63<lb/> (57%) patients reported good pain relief. Thus,<lb/> out of 235 patients in this study, 47% experienced<lb/> 50% or greater long-term pain relief with SCS. At<lb/> the time of this study, 145 patients continued to<lb/> use their systems, including 34 who reported<lb/> poor ( 50%) pain relief. <ref type="figure" >Figure 1</ref> demonstrates<lb/> the Kaplan-Meier survival curve for all 235 pa-<lb/>tients who entered the study (including 46 early<lb/> failures and 189 patients who subsequently re-<lb/>ceived internalization of SCS), suggesting a slow,<lb/> gradual decline in pain control over the duration<lb/> of follow-up.<lb/></p> <head>STATUS OF PATIENTS WHO FAILED<lb/>TRIAL STIMULATION<lb/></head> <p> A total of 46 patients failed trial stimulation and<lb/> were not internalized. Seven of these patients sub-<lb/>sequently proceeded to deep brain stimulation <ref type="biblio" >[25]<lb/></ref> and two patients received intrathecal morphine<lb/> pump implants. During the period of this study, five<lb/> patients died of unrelated causes, and 11 have been<lb/> lost to follow-up. Of the remaining 21 patients, all<lb/> continue to use narcotic analgesia and only three,<lb/> who were all previously employed, continue to en-<lb/>gage in part-time employment. None of these 21<lb/> patients have increased activity levels and none are<lb/> able to engage in activities of daily living without<lb/> the use of narcotics.<lb/></p> <head>ETIOLOGY OF PAIN<lb/></head> <p> Pain syndromes are grouped by etiology in <ref type="table">Table 1</ref>.<lb/> By far the most common indication for SCS was<lb/> failed back syndrome with 114 patients. Other syn-<lb/>dromes showing favorable response to SCS were<lb/> reflex sympathetic dystrophy, peripheral vascular<lb/> disease, and multiple sclerosis. Syndromes with<lb/> moderate success included peripheral neuropathy,<lb/> spinal cord lesions, and perirectal pain. Syndromes<lb/> with very poor response to SCS included cauda<lb/> equina syndrome and bone and joint pain<lb/> syndromes.<lb/></p> <head>CLINICAL PROGNOSTIC FACTORS<lb/></head> <p>The average age of patients included in this study<lb/> was 51.4 years. There were 117 patients under the<lb/> age of 51 years, and 96 of these patients were inter-<lb/>nalized with 56 patients (58%) reporting successful<lb/> results. In patients over the age of 51 years, 93 of<lb/> 118 patients were internalized and 55 (59%) of these<lb/> patients experienced successful control of pain. No<lb/> significant difference based on age was noted.<lb/></p> <p> One hundred and fifty male patients were<lb/> screened, with 122 (81%) internalized, and 73 cases<lb/> were successful (60%). Sixty-seven of eighty-five fe-<lb/>male patients (79%) were internalized with 38 suc-<lb/>cessful cases (57%). No significant difference based<lb/> on sex was noted.<lb/></p> <p>Unilateral pain was reported by 123 patients, of<lb/> whom 60 (49%) currently receive satisfactory pain<lb/> relief. The remaining 112 patients reported bilateral<lb/> pain, and 51 (46%) of these patients currently re-<lb/>port satisfactory pain relief using SCS. No signifi-<lb/>cant difference based on uni-versus bilaterality was<lb/> noted.<lb/></p> <p>Analysis of the number of pre-implant operations<lb/> was performed to examine possible prognostic fac-<lb/>tors. One-hundred and forty-four patients under-<lb/>went various forms of surgery that failed to relieve<lb/> or aggravated their pain syndromes. These patients<lb/> underwent an average of 3.3 2.3 surgeries with a<lb/> maximum of 19 surgeries in one case. <ref type="table">Table 2</ref> re-<lb/>ports the relationship between the number of sur-<lb/>geries and the success of pain relief with SCS. Out of<lb/> 120 patients with previous surgeries and successful<lb/> trial stimulation, 64 (53%) had successful long-term<lb/> pain relief. In contrast, 91 patients who had not<lb/> undergone prior surgical procedures had success-<lb/>ful long-term pain relief in 68% of those with suc-<lb/>cessful trial stimulation. The majority of patients<lb/> with previous operations belong to the failed back<lb/> syndrome group. Thus, analysis of the number of<lb/> operations as a prognostic factor has the confound-<lb/>ing element of diagnosis of failed back syndrome,<lb/> which itself may be a prognostic factor.<lb/></p> <p>The time between the first operation performed<lb/> on the body part experiencing chronic pain and<lb/> implantation was also analyzed in this study. The<lb/> average length of time was 9.8 years 7.7 years,<lb/> with a minimum duration of 6 months and a maxi-<lb/>mum duration of 47 years in one case. <ref type="table">Table 3<lb/></ref> reports the results associated with this delay and<lb/> future success of SCS; <ref type="figure">Figure 2</ref> exemplifies this<lb/> trend. The success rate of SCS drops from a maxi-<lb/>mum of 93% for those with less than a 3-year delay,<lb/> to 9% for those with greater than a 12-year waiting<lb/> period. Student's t-test analysis proved significant<lb/></p> <figure>1<lb/> Kaplan-Meier Survival Curve for all 235 patients re-<lb/>ceiving spinal cord stimulation. The statistical end-<lb/>point for patient survival was (1) failure of trial stimula-<lb/>tion; (2) loss of satisfactory pain relief despite corrective<lb/> procedures; (3) refusal of the patient to undergo correc-<lb/>tive procedures; and (4) gradual loss of satisfactory pain<lb/> relief in the presence of a technically functioning elec-<lb/>trode. The curve illustrates an initial loss in survival<lb/> because of failed trial stimulation and a subsequent grad-<lb/>ual loss of pain control during the period of study.<lb/></figure> <p>at p 0.001, with the majority of these cases diag-<lb/>nosed as failed back syndrome, which may be a<lb/> possible confounding factor.<lb/></p> <head>WORK STATUS<lb/></head> <p> Fifteen (14%) out of our 111 internalized patients<lb/> had been working before implantation and all 15 of<lb/> these patients reported significant increases in<lb/> their level of work. After internalization, 25 (23%)<lb/> more patients reported gainful employment. Fur-<lb/>thermore, five successfully internalized patients, all<lb/> over the age of 60 years, reported a significant in-<lb/>crease in activities of daily living. Thus, a total of 45<lb/> (41%) patients with successful implantation re-<lb/>ported a significant increase in daily activities<lb/> which is analogous to gainful employment.<lb/></p> <head>ROLE OF WORKMAN'S<lb/> COMPENSATION BOARD CLAIMS<lb/></head> <p>Sixty-nine patients had claims involving the Work-<lb/>man's Compensation Board (WCB) at the time of<lb/> implantation. Of these patients, 51 patients (74%)<lb/> were internalized with 27 patients (53%) receiving<lb/> successful pain relief. Of 166 patients not involved<lb/> with WCB, 138 patients (83%) were internalized<lb/> with 84 patients (61%) achieving pain relief. There<lb/> was no significant statistical difference between the<lb/> two groups. Patients involved in active litigation<lb/> were deferred from admission to the study until<lb/> after the litigation was settled.<lb/></p> <head>ELECTRODES USED<lb/></head> <p>Electrode types in our series consisted of Resume,<lb/> Pisces-Quadripolar, and Sigma-Pisces systems.<lb/> There were 66 Sigma-Pisces devices, 74 Resume<lb/> devices, and 141 Quadripolar-Pisces devices used,<lb/> giving a total of 281 devices installed in a total of 235<lb/> patients in this series. <ref type="figure">Figure 3</ref> illustrates the<lb/> Kaplan-Meier Survival Curve for all three electrode<lb/> types. Both Resume and Quadripolar-Pisces elec-<lb/>trodes were significantly more reliable than Sigma-<lb/>Pisces electrodes (hazard ratio 0.47; p 0.001<lb/> and hazard ratio 0.48; p 0.001 respectively).<lb/> <ref type="figure">Figure 4</ref> demonstrates the Kaplan-Meier Survival<lb/> Curve for multipolar and unipolar systems. Multi-<lb/>polar systems, including Resume and Quadripolar-<lb/>Pisces electrodes, as a group were significantly<lb/> more reliable than unipolar systems (Sigma-Pisces<lb/> electrodes) (hazard ratio 0.49; p 0.001). There<lb/> was no significant difference in reliability between<lb/> Resume and Quadripolar-Pisces systems.<lb/></p> <figure type="table">2 Number of Operations Before SCS as a Prognostic Factor for SCS<lb/> NUMBER OF<lb/> OPERATIONS<lb/> TRIAL STIMULATION<lb/> PAIN RELIEF<lb/> LONG-TERM<lb/> PAIN RELIEF<lb/> AVERAGE<lb/> FOLLOW-UP<lb/> PERIOD<lb/> (YEARS)<lb/> SUCCESS<lb/> FAILURE<lb/> SUCCESS<lb/> FAILURE<lb/> 0<lb/> 69<lb/> 22<lb/> 47 (68%)<lb/> 22<lb/> 5.1<lb/> 1<lb/> 15<lb/> 7<lb/> 7 (47%)<lb/> 8<lb/> 6.3<lb/> 2<lb/> 27<lb/> 4<lb/> 14 (52%)<lb/> 13<lb/> 5.6<lb/> 3<lb/> 42<lb/> 3<lb/> 26 (62%)<lb/> 16<lb/> 5.7<lb/> 4<lb/> 17<lb/> 8<lb/> 7 (42%)<lb/> 10<lb/> 6.2<lb/> 4<lb/> 19<lb/> 2<lb/> 10 (53%)<lb/> 9<lb/> 5.8<lb/> Total:<lb/> 189<lb/> 46<lb/> 111 (59%)<lb/> 78<lb/> Success rates for long-term pain relief are calculated as percentages of the number of patients achieving success with trial stimulation.<lb/></figure> <figure>3<lb/> Comparison of the Time Interval Between the First Operation to the Body Part Experiencing Chronic Pain and SCS<lb/> and the Subsequent Response to SCS<lb/> TIME BETWEEN FIRST<lb/> OPERATION AND<lb/> IMPLANT<lb/> NUMBER<lb/> TRIAL STIMULATION<lb/> PAIN RELIEF<lb/> LONG-TERM PAIN<lb/> RELIEF<lb/> AVERAGE<lb/> FOLLOW-UP<lb/> PERIOD<lb/> (YEARS)<lb/> SUCCESS<lb/> FAILURE<lb/> SUCCESS<lb/> FAILURE<lb/> 0–3 years<lb/> 15<lb/> 14<lb/> 1<lb/> 13 (93%)<lb/> 1<lb/> 5.2<lb/> 3–6 years<lb/> 34<lb/> 28<lb/> 6<lb/> 23 (82%)<lb/> 5<lb/> 5.8<lb/> 6–9 years<lb/> 34<lb/> 27<lb/> 7<lb/> 15 (56%)<lb/> 12<lb/> 6.3<lb/> 9–12 years<lb/> 29<lb/> 28<lb/> 1<lb/> 11 (39%)<lb/> 17<lb/> 5.9<lb/> 12 years<lb/> 32<lb/> 23<lb/> 9<lb/> 2 (9%)<lb/> 21<lb/> 5.7<lb/> Total:<lb/> 144<lb/> 120<lb/> 24<lb/> 64 (53%)<lb/> 56<lb/></figure> <head>COMPLICATIONS<lb/></head> <p>Complications within our series centered on diffi-<lb/>culties with hardware and are presented in <ref type="table">Table 4</ref>.<lb/> A total of 35 electrode repositionings, the majority<lb/> of which were unipolar Sigma electrodes, were un-<lb/>dertaken. In six cases, a Pisces system was replaced<lb/> by another Pisces system. In 24 situations, Pisces<lb/> systems were replaced by Resume systems. Rea-<lb/>sons for replacement included fracture of the elec-<lb/>trode, infection of hardware, fibrosis of the elec-<lb/>trode tip, or idiopathic failure. There were 11<lb/> infected systems (4% of total devices implanted),<lb/> three of which were successfully treated with anti-<lb/>biotics, whereas the other eight required removal.<lb/> There were eight cases of hardware malfunction, all<lb/> of which were repaired satisfactorily. Electrode<lb/> fracture occurred in eight cases, all of which were<lb/> repaired or replaced successfully. An electrical leak<lb/> was found in three cases. There were two cases of<lb/> cerebrospinal fluid leakage, which resolved sponta-<lb/>neously. One subcutaneous hematoma occurred at<lb/> the site of the pulse generator. There were no neu-<lb/>rological injuries or other anatomic complications<lb/> requiring surgical intervention.<lb/></p> <head>TOLERANCE<lb/></head> <p>Perhaps the most significant cause of loss of pain<lb/> control after long-term successful pain relief is tol-<lb/></p> <figure>2<lb/> Stacked-bar histogram demonstrating the relationship between the number of years since the first operation to<lb/> the body part experiencing chronic pain and the success of spinal cord stimulation.<lb/></figure> <figure>3<lb/> Kaplan-Meier Survival Curve for the electrode types<lb/> used in spinal cord stimulation (all 281 electrodes<lb/> included). The statistical endpoint for electrode survival<lb/> was (1) failure of trial stimulation; (2) any hardware com-<lb/>plication (fracturing, displacement, infection, malfunc-<lb/>tion) necessitating a corrective procedure or discontinu-<lb/>ation of stimulation; and (3) gradual loss of successful<lb/> pain relief in the presence of a technically functioning<lb/> electrode. The Resume electrode, represented by the up-<lb/>per curve, demonstrated the lowest failure rate, followed<lb/> by the Quadripolar-Pisces electrode. Both Resume and<lb/> Quadripolar-Pisces electrodes had significantly higher re-<lb/>liability compared with the Sigma-Pisces (hazard ratio<lb/> 0.47; p 0.001 and hazard ratio 0.48; p 0.001<lb/> respectively).<lb/></figure> <p>erance. This is also difficult to assess and treat.<lb/> Tolerance is defined in this series as the gradual<lb/> loss of pain relief for reasons not related to mechan-<lb/>ical problems. These patients continue to report<lb/> stimulation-induced paresthesia in their territory of<lb/> pain. Our series has now encompassed 15 years,<lb/> allowing a more complete study of tolerance. In our<lb/> study, tolerance is assumed to comprise most of<lb/> our 79 long-term failure cases. In our experience,<lb/> the use of amitriptyline or L-tryptophan has mini-<lb/>mal beneficial effects in these cases.<lb/></p> <head>Discussion<lb/></head> <p>Several reviews of spinal cord stimulation for the<lb/> control of chronic, intractable pain have been per-<lb/>formed over the past two decades. A comprehen-<lb/>sive review of long-term studies is provided in Ta-<lb/>ble 5. Most long-term studies (5 years) with a<lb/> significantly large patient database (100 patients)<lb/> tend to show success rates of 40 – 60%. These rates<lb/> are typically calculated by the number of patients<lb/> receiving implantation and not by the number of<lb/> patients screened for the procedure.<lb/></p> <p>During the past two decades, SCS has evolved as<lb/> better technology developed and as greater knowl-<lb/>edge of indications for SCS is acquired. The devel-<lb/>opment of multipolar stimulation devices to replace<lb/> unipolar devices has led to fewer occurrences of<lb/> electrode migration or malposition failure) <ref type="biblio" >[41]</ref>.<lb/></p> <p>Percutaneous trial stimulation methods are used<lb/> in most reports of SCS, but at least two studies have<lb/> not used a trial period <ref type="biblio">[18,44]</ref>. We continue to use<lb/> the trial stimulation technique as the mainstay of<lb/> the screening process in deciding which patients<lb/> will receive permanent implants. We find it to be of<lb/> special benefit in peripheral vascular disease <ref type="biblio" >[22]<lb/></ref> and peripheral neuropathy <ref type="biblio">[23]</ref> where the yield of<lb/> permanent implantation is lower than in other pain<lb/> syndromes. We believe that the trial stimulation is<lb/> of benefit even in pain syndromes where permanent<lb/> implantation yields are high such as with failed<lb/> back syndrome, multiple sclerosis, and reflex sym-<lb/>pathetic dystrophy. The low morbidity of percuta-<lb/>neous electrode implantation encourages the use of<lb/> trial stimulation. The drawback of trial stimulation<lb/> is the addition of an extra procedure, making SCS<lb/> implantation a two-step procedure that might in-<lb/></p> <figure>4<lb/> Kaplan-Meier Survival Curve for the multipolar and<lb/> unipolar electrodes used in spinal cord stimulation<lb/> (all 281 electrodes included). The statistical endpoint for<lb/> electrode survival was (1) failure of trial stimulation; (2)<lb/> any hardware complication (fracturing, displacement, in-<lb/>fection, malfunction) necessitating a corrective proce-<lb/>dure or discontinuation of stimulation; and (3) gradual<lb/> loss of successful pain relief in the presence of a techni-<lb/>cally functioning electrode. Programmable multipolar<lb/> (Quadripolar-Pisces or Resume) systems had a signifi-<lb/>cantly greater clinical reliability than unipolar systems<lb/> (hazard ratio 0.49; p 0.001).<lb/></figure> <table>4 Complications of Spinal Cord Stimulation<lb/> COMPLICATIONS<lb/> NUMBER OF<lb/> PATIENTS<lb/> COMMENT<lb/> Displaced electrode<lb/> 65<lb/> 35 Sigma electrodes were repositioned, 6 were replaced<lb/> by a Pisces-Quadripolar electrode, and 24 were<lb/> replaced by Resume systems.<lb/> Infection<lb/> 11<lb/> Three resolved with antibiotics, eight required removal.<lb/> Fractured electrode<lb/> 8<lb/> All replaced or repaired satisfactorily. Site of fracture<lb/> was usually at fixation point to deep fascia.<lb/> Hardware malfunction<lb/> 8<lb/> All were repaired satisfactorily.<lb/> Electrical leak<lb/> 3<lb/> All receivers replaced satisfactorily. Usual site was<lb/> junction of connector cord to pulse receiver.<lb/> Subcutaneous hematoma<lb/> 1<lb/> Occurred at site of pulse generator.<lb/> Cerebrospinal fluid leak<lb/> 2<lb/> Both resolved satisfactorily.<lb/></table> <p>crease the rate of infection. However, we believe<lb/> that the enhanced predictive value for efficacy in<lb/> individual patients makes this step worthwhile. The<lb/> trial stimulation period allows for a period of pa-<lb/>tient self-education in the presence of the neurosur-<lb/>gical team. During the trial stimulation period, the<lb/> patient can be followed on an outpatient basis to<lb/> allow experimentation with their stimulation pro-<lb/>gram in familiar environments. The positioning of<lb/> the spinal electrode should be precise to allow<lb/> overlap of the pain area and paresthesias during<lb/> trial stimulation. This fact has been shown to be<lb/> important for long-term efficacy of SCS <ref type="biblio">[26,29,42]</ref>.<lb/> Therefore, we continue to use the trial stimulation<lb/> period to provide the best possible decision on<lb/> internalization.<lb/></p> <p>Most complications within our study were<lb/> caused by difficulties with hardware and are pre-<lb/>sented in <ref type="table">Table 5</ref>. The incidence of electrode mi-<lb/>gration, fracture and displacement was much<lb/> higher during the first half of this study when Sigma-<lb/>Pisces unipolar electrodes were the main type of<lb/> electrode available.<lb/></p> <p>Our study has now encompassed 15 years, allow-<lb/>ing for a more complete study of tolerance. Toler-<lb/>ance, as discussed earlier, is the best way to ex-<lb/>press loss of pain control without mechanical<lb/> failure. Tolerance has also been implicated in deep<lb/> brain stimulation studies for chronic pain as the<lb/> main reason for long-term failure <ref type="biblio">[25]</ref>. As stated in<lb/> previous studies <ref type="biblio">[26]</ref>, it is believed that tolerance is<lb/> a main factor in long-term failure and is attributable<lb/> to 1) fibrotic changes surrounding the electrode tip<lb/> causing insulation of electrical signals <ref type="biblio" >[20,26,48]</ref>; or<lb/> 2) plasticity of pain pathways, which has been dem-<lb/>onstrated in the spinal cord <ref type="biblio">[9]</ref>, thalamus <ref type="biblio">[17]</ref>, and<lb/> cortex <ref type="biblio">[11]</ref> in humans.<lb/></p> <p>The cause of failure of trial stimulation in cases<lb/> otherwise considered to be suitable is not well un-<lb/>derstood. These patients may be examples of a<lb/> population whose primary pain mechanisms are<lb/> unsuitable for this treatment, or whose epidural<lb/> anatomy in some way obviates correct electrode<lb/> positioning. More sensitive screening methods<lb/> might be used, perhaps along the lines of somato-<lb/>sensory evoked potential monitoring <ref type="biblio" >[12,13,38]</ref>.<lb/></p> <p>The majority of patients in this series suffered<lb/> from failed back syndrome secondary to multiple<lb/> surgeries or radiculopathy. These patients had a<lb/> very high early success rate (89%) and a successful<lb/> internalization rate of 52%. This seems to be in<lb/> accordance with other long-term studies <ref type="biblio">[6,41]</ref>. A<lb/> list of recent studies presenting results with SCS for<lb/> failed back syndrome is provided in <ref type="table">Table 6</ref>.<lb/></p> <p>Peripheral vascular disease was another frag-<lb/>ment indication for SCS in our study. An interesting<lb/> ancillary effect of SCS is improvement of lower limb<lb/> perfusion in peripheral vascular disease <ref type="biblio">[3,10,16,<lb/> 22]</ref>. In our series, we had an internalization rate of<lb/> 74% and a success rate of 69% among the 29 inter-<lb/>nalized patients.<lb/></p> <p>Peripheral neuropathy as an indication for SCS in<lb/> our series involved 30 patients. Cases showing the<lb/> best results were causalgic pain (100%) and dia-<lb/>betic neuropathic (75%) pain syndromes. Less re-<lb/>sponsive cases included post-herpetic neuralgia<lb/></p> <figure type="table">5 Summary of Results of Spinal Cord Stimulation Reported in the Literature<lb/> SOURCE<lb/> NUMBER<lb/> SCREENED<lb/> NUMBER<lb/> IMPLANTED<lb/> >50% LONG-TERM<lb/> PAIN RELIEF<lb/> AVERAGE FOLLOW-UP<lb/> PERIOD<lb/> RANGE<lb/> MEAN<lb/> deVera et al [7]<lb/> 124<lb/> 110<lb/> 75%<lb/> Devulder et al [8] 69<lb/> 55%<lb/> 8 yr<lb/> Gonzalez-Darder et al [14]<lb/> 46<lb/> 41<lb/> 50–100%<lb/> 5–27 mo<lb/> Krainick and Thoden [21]<lb/> 126<lb/> 91<lb/> 18%<lb/> 5 yr<lb/> Kumar et al [present study]<lb/> 235<lb/> 189<lb/> 59%<lb/> 6 mo–15 yr<lb/> 66 mo<lb/> Kupers et al [28]<lb/>700<lb/> 52%<lb/> Meyerson et al[39]<lb/> 84<lb/> 67%<lb/> 16 yr<lb/> Nielson et al [40]<lb/> 221<lb/> 130<lb/> 49%<lb/> 1–35 mo<lb/> North et al [41]<lb/> 205<lb/> 171<lb/> 52%<lb/> 2–20 yr<lb/> 7.1 yr<lb/> Sandez-Ledesma et al [53]<lb/> 49<lb/> 36<lb/> 57% (75% pain relief)<lb/> 5.5 yr<lb/> Shimoji et al [56]<lb/> 454<lb/> 83% (30% pain relief)<lb/> Siegfried and Lazorthes [57]<lb/> 191<lb/> 89<lb/> 37%<lb/> 1–8 yr<lb/> 4.0 yr<lb/> Simpson [58]<lb/> 60<lb/> 47%<lb/> 2 wk–9 yr<lb/> 29 mo<lb/> Spiegelman and Friedman [59]<lb/> 43<lb/> 30<lb/> 63%<lb/> 3–33 mo<lb/> 13 mo<lb/> Sweet and Wepsic [60]<lb/> 100<lb/> 98<lb/> 21–42%<lb/> van de Kelft and de la Porte [61]<lb/> 116<lb/> 84<lb/> 54%<lb/> 47 mo<lb/></figure> <p>(25%) and intercostal neuralgia (25%) pain syn-<lb/>dromes <ref type="biblio" >[23]</ref>.<lb/></p> <p>Multiple sclerosis also showed high early pain<lb/> relief success rates, producing a 92% internalization<lb/> rate. However, possibly because of the effects of<lb/> tolerance, satisfactory long-term pain relief fell to<lb/> 67% for these patients.<lb/></p> <p>Reflex sympathetic dystrophy showed a remark-<lb/>able response to SCS, as reported by us <ref type="biblio">[24]</ref> and<lb/> others in detail elsewhere <ref type="biblio" >[1,52]</ref>. All 13 of the pa-<lb/>tients in our series had satisfactory relief of their<lb/> pain of reflex sympathetic dystrophy. All had inter-<lb/>nalization of their SCS systems. At an average of 40<lb/> months follow-up, all patients were continuing to<lb/> use their stimulators regularly and only two were<lb/> using occasional narcotic medications.<lb/></p> <p>Spinal cord lesions ranged from traumatic injury<lb/> to low-grade malignancy of the spinal cord. Lesions<lb/> of the spinal cord which showed favorable early<lb/> responses included two compression fractures, an<lb/> ependymoma of the cord, and a case of myelopathy.<lb/> All cases demonstrating satisfactory pain relief had<lb/> incomplete paraplegia with the majority of pain felt<lb/> below the level of the lesion. Cases of complete<lb/> paraplegia showed no benefit with SCS with either<lb/> pain at the level of injury or diffuse pain below the<lb/> level of injury.<lb/></p> <p>Cases of perirectal pain as an indication for SCS<lb/> included two successful cases of post-prostatectomy<lb/> pain. Cauda equina lesions and bone and joint pain<lb/> syndrome cases responded in an interesting<lb/> fashion—all cases had successful early pain relief,<lb/> but all subsequently lost pain relief. All cauda<lb/> equina lesions were of traumatic origin. Cases of<lb/> stump pain or phantom limb pain responded poorly<lb/> in our limited experience (three patients) with no<lb/> such cases internalized. Success using SCS for am-<lb/>putation syndromes has been reported in earlier<lb/> papers <ref type="biblio">[19]</ref>, but was not reproduced in this study.<lb/></p> <p>Our results with different pain syndromes dem-<lb/>onstrate the variability of response to SCS. Cases of<lb/> failed back syndrome, multiple sclerosis, reflex sym-<lb/>pathetic dystrophy, peripheral neuropathy, or pain<lb/> caused by peripheral vascular diseases showed<lb/> good responses after successful implantations.<lb/></p> <p>Prognostic factors other than the type of pain<lb/> syndrome have been examined in several other pa-<lb/>pers <ref type="biblio">[18,26,35,41,42,44,50]</ref>.<lb/></p> <p>Significant differences in responses between<lb/> males and females have been reported in two stud-<lb/>ies, with females demonstrating superior results<lb/> [<ref type="biblio" >[41,50]</ref>, whereas other papers reported no signifi-<lb/>cant difference <ref type="biblio">[14,28]</ref>. In our study, there was no<lb/> significant difference between sexes.<lb/></p> <p>Age did not prove to be a significant determining<lb/> factor in our series. Laterality of pain has been<lb/> speculated on as a prognostic factor in several stud-<lb/>ies, with unilateral pain reported to respond to<lb/> treatment better than bilateral pain <ref type="biblio">[45,46,47,50]</ref>.<lb/> Our results did not show a significant increase in<lb/> response to SCS of unilateral pain as compared with<lb/> bilateral pain.<lb/></p> <p>Outcome of patients following prior operations<lb/> has been analyzed in several studies, with no rela-<lb/>tionship of outcome to number of operations re-<lb/>ported; <ref type="biblio">[36]</ref> one report has linked favorable out-<lb/>come to a small number of previous surgeries <ref type="biblio">[41]</ref>.<lb/> Our study did not show any relationship of number<lb/> of surgeries to outcome, although those patients<lb/> not receiving surgery showed a superior response<lb/> to SCS (68%) as compared with those who had had<lb/> previous surgeries (53%).<lb/></p> <p>The duration of pain prior to SCS surgery has<lb/> been examined in several studies. We found this<lb/></p> <figure type="table">6 Summary of Results of Spinal Cord Stimulation for Failed Back Syndrome as Reported in the Literature<lb/> SOURCE<lb/> NUMBER<lb/> SCREENED<lb/> NUMBER<lb/> IMPLANTED<lb/> >50% LONG-TERM<lb/> PAIN RELIEF<lb/> AVERAGE FOLLOW-UP<lb/> PERIOD<lb/> RANGE<lb/> MEAN<lb/> Burton [4]<lb/> 198<lb/> 186<lb/> 43%<lb/> 1 yr<lb/> de la Porte and van de Kelft [6]<lb/> 78<lb/> 64<lb/> 55%<lb/> 1–7 yr<lb/> 4 yr<lb/> Kumar et al [present study]<lb/> 114<lb/> 101<lb/> 52%<lb/> 6 mo–15 yr<lb/> 66 mo<lb/> LeDoux and Langford [30]<lb/> 33<lb/> 26<lb/> 75%<lb/> 1–2 yr<lb/> LeRoy [32]<lb/> 49<lb/> 49<lb/> 60%<lb/> 1–63 mo<lb/> 31 mo<lb/> Long and Erickson [34]<lb/> 69<lb/> 54<lb/> 18%<lb/> 12–35 mo<lb/> North et al [41]<lb/> 153<lb/> 138<lb/> 1.5–20.4 yr<lb/> 7.1 yr<lb/> Pineda [43]<lb/> 76<lb/> 56<lb/> 43%<lb/> Probst [44]<lb/> 112<lb/> 67%<lb/> 4.5 yr<lb/> Ray et al [49]<lb/> 78<lb/> 50<lb/> 49%<lb/> 3–64 mo<lb/> 19 mo<lb/> Richardson and Shatin [51]<lb/> 136<lb/> 136<lb/> 67%<lb/> 45 mo<lb/></figure> <p>difficult to examine due to its inherent subjectivity.<lb/> Instead, we examined the duration of time between<lb/> the first operation on the body part experiencing<lb/> chronic pain and time of implantation and this re-<lb/>lationship to subsequent success of pain relief with<lb/> SCS. <ref type="table">Table 3</ref> displays this relationship and <ref type="figure">Figure 2<lb/></ref> presents a stacked bar-graph representation. A<lb/> striking relationship emerges suggesting that the<lb/> longer the duration of time between the first sur-<lb/>gery and SCS, the poorer the response to SCS. This<lb/> suggests that pain becomes firmly established over<lb/> time, leading to difficulty in modification via SCS. In<lb/> our practice, this information has become an im-<lb/>portant prognostic factor in screening patients for<lb/> SCS.<lb/></p> <head>Conclusions<lb/></head> <p>Efficacy of spinal cord stimulation for chronic,<lb/> intractable pain has increased with the introduc-<lb/>tion of multipolar devices <ref type="biblio">[41,42]</ref>. The use of mul-<lb/>tipolar devices significantly improves results of<lb/> stimulation-induced pain relief. A better under-<lb/>standing of prognostic factors should lead to better<lb/> patient selection and therefore, improved results.<lb/> The low morbidity of SCS when compared with<lb/> destructive surgery or other pain relief procedures<lb/> makes SCS an attractive alternative. Further studies<lb/> that improve our understanding of pain mecha-<lb/>nisms and behavior will refine the process of pa-<lb/>tient selection, leading to greater efficacy of SCS as<lb/> a long-term therapy to relieve pain.</p> </text> </tei>