Review: Bioelectrical mechanisms in spinal cord stimulation.

OBJECTIVES: The aim of this review is to make specialists in a variety of disciplines familiar with basic aspects of spinal cord stimulation and the role of mathematical modeling in understanding its mechanisms of action and the solution of basic problems. METHODS: The paper is divided into five sections. The content of each section also covers aspects of various disciplines. Most aspects are presented in an unusual way, likely resulting in new viewpoints and further developments in the growing field of neuromodulation. RESULTS: A special, integrating role is the mathematical modeling of spinal cord stimulation and the simulation studies of various aspects, such as the stimulation in axial low-back pain. CONCLUSIONS: In particular the conclusions from several computer simulation studies are relevant and of interest to specialists in many disciplines.

Performance of transverse tripoles vs. longitudinal tripoles with anode intensification (AI) in spinal cord stimulation: computational modeling study.

OBJECTIVE: In spinal cord stimulation, anodes tend to have a strong effect over the area of dorsal column (DC) activation, when configured as both longitudinal guarded cathodes (LGCs) and transverse tripoles (TTs). Inclusion of a small spacing step (LGC+) in the center-center (C-C) spacing of the LGC can be an efficient method to study the local effects around the electrode. The primary aim of this computer modeling study is to investigate if enhanced DC recruitment is achieved when anodal currents in TT and LGC combinations (both LGC and LGC+) are increased up to 30% with respect to the cathodal current. Secondly, the merits of anodal intensification (AI) are evaluated by comparing the DC recruitment areas (S(RA)) and energy consumption (EDT ) of LGC+ with AI, against stimulation using an LGC without AI. MATERIALS AND METHODS: The commercially available LGC and LGC+, with 4.0 and 4.5 mm C-C, respectively, were modeled on a single percutaneous lead at the low-thoracic vertebral region (T10-T12). Transverse tripolar stimulation (TTS) was modeled on triple percutaneous leads. RESULTS: TTS with 10% AI recruited a smaller S(RA) as compared with TTS with no AI. AI of LGC and LGC+ resulted in increasing SRAs respectively to that of LGC and LGC+ without AI. Also, AI of LGC+ recruited a larger S(RA) and usage range (UR) at lower E(DT) compared with that of LGC without AI. CONCLUSIONS: AI of TTS is not advantageous. LGC and LGC+ with AI allow additional DC stimulation, which may increase the likelihood of activating fibers inaccessible with conventional programming. LGC+ with AI can be more efficient than LGCs without AI, as a larger SRA and UR is achieved at lower EDT .

Experimental spinal cord stimulation and neuropathic pain: mechanism of action, technical aspects, and effectiveness.

Spinal cord stimulation (SCS) is a valuable treatment for chronic intractable neuropathic pain. Although SCS has gone through a technological revolution over the last four decades, the neurophysiologic and biochemical mechanisms of action have only been partly elucidated. Animal experimental work has provided some evidence for spinal as well as supraspinal mechanisms of neuropathic pain relief of SCS. A SCS computer model of the electrical properties of the human spinal cord revealed many basic neurophysiologic principles that were clinically validated later on. The main question in clinical SCS is how to further improve the effectiveness of SCS as there is still a significant failure rate of 30%. In this context, experimental studies are needed to elucidate which target pain neuron(s) are involved, as well as with what exact electrical stimulation this target neuron can be influenced to produce an optimal supapression of neuropathic pain. This article reviews the basic clinical and experimental technical aspects in relation to the effectiveness of SCS in view of recent understanding of the dorsal horn pain circuit involved. These data may then result in experiments needed for an improved understanding of the mechanisms underlying SCS and consequently lead to improvement and increased effectiveness of SCS in neuropathic pain as a clinical therapy.

Spinal cord stimulation with hybrid lead relieves pain in low back and legs.

OBJECTIVE: The failed back surgery syndrome (FBSS) is the most common chronic pain syndrome. Whereas it is relatively easy to achieve pain relief in the lower limbs of FBSS patients with spinal cord stimulation (SCS), it is difficult to manage low back pain with SCS. The performance of a paddle-shaped SCS lead that can be inserted surgically as well as percutaneously (a hybrid lead) was evaluated in a prospective study on the relief of low back pain and leg pain in patients with FBSS. MATERIALS AND METHODS: Patients with FBSS being eligible for SCS were enrolled in the study, and a hybrid lead was placed surgically. Outcome measures included pain scores for low back and leg pain assessed by visual analog scale (VAS), pain medication, and patient satisfaction. These scores were assessed before and at regular intervals after implantation. RESULTS: It was shown that a single hybrid lead, generally positioned over the physiological midline of the spinal cord, is capable of alleviating both low back and leg pain in patients with FBSS. Forty-five subjects were eligible for SCS and received trial stimulation. Forty-two of them had a successful trial period and were converted to a permanent system. Their average VAS score at baseline was 8.0 for lower limb pain and 7.5 for low back pain. After six months of SCS, these average VAS scores were reduced to 3.2 and 3.5, respectively, and also pain medication was reduced significantly. CONCLUSION: SCS with a hybrid lead in subjects with FBSS is safe, and causes significant pain relief in both the low back and the lower limbs.

Triple leads programmed to perform as longitudinal guarded cathodes in spinal cord stimulation: a modeling study.

OBJECTIVE: In spinal cord stimulation, neurosurgeons increasingly tend to implant dual leads. Dual leads (longitudinal bipole/tripole) provide medio-lateral control over the recruited dorsal column (DC) area by steering the injected cathodal currents. However, the DC recruited area is suboptimal when dual aligned leads straddling the midline programmed as longitudinal guarded cathodes (+-+) are used instead of a single lead placed over the spinal cord midline with the same configuration. As a potential improvement, an additional third lead between the two aligned leads is modeled to maximize the medio-lateral extent of the DCs at the low-thoracic vertebral region (T10-T12). METHODS AND MATERIALS: The University of Twente Spinal Cord Stimulation software (UT-SCS) is used in this modeling study. Longitudinal guarded cathodes were modeled on the low-thoracic vertebral region (T10-T12) using percutaneous triple lead configurations. The central lead was modeled over the spinal cord midline and the two lateral leads were modeled at several transverse distances to the midline lead. Medio-lateral field steering was performed with the midline lead and the second lead on each side to achieve constant anodal current ratios and variable anodal current ratios. RESULTS: Reducing the transverse lead separation resulted in increasing the depths and widths of the recruited DC area. The triple lead configuration with the least transverse separation had the largest DC recruited area and usage range. The maximum DC recruited area (in terms of both depth and width) was always found to be larger under variable anodal current ratio than constant anodal current ratio conditions. CONCLUSIONS: Triple leads programmed to perform as longitudinal guarded cathodes provide more postoperative flexibility than single and dual leads in covering a larger width of the low-thoracic DCs. The transverse separation between the leads is a major determinant of the area and distribution of paresthesia.

Anodal vs cathodal stimulation of motor cortex: a modeling study.

OBJECTIVE: To explore the effects of electrical stimulation performed by an anode, a cathode or a bipole positioned over the motor cortex for chronic pain management. METHODS: A realistic 3D volume conductor model of the human precentral gyrus (motor cortex) was used to calculate the stimulus-induced electrical field. The subsequent response of neural elements in the precentral gyrus and in the anterior wall and lip of the central sulcus was simulated using compartmental neuron models including the axon, soma and dendritic trunk. RESULTS: While neural elements perpendicular to the electrode surface are preferentially excited by anodal stimulation, cathodal stimulation excites those with a direction component parallel to its surface. When stimulating bipolarly, the excitation of neural elements parallel to the bipole axis is additionally facilitated. The polarity of the contact over the precentral gyrus determines the predominant response. Inclusion of the soma-dendritic model generally reduces the excitation threshold as compared to simple axon model. CONCLUSIONS: Electrode polarity and electrode position over the precentral gyrus and central sulcus have a large and distinct influence on the response of cortical neural elements to stimuli. SIGNIFICANCE: Modeling studies like this can help to identify the effects of electrical stimulation on cortical neural tissue, elucidate mechanisms of action and ultimately to optimize the therapy.

The role of intra-operative motor evoked potentials in the optimization of chronic cortical stimulation for the treatment of neuropathic pain.

OBJECTIVE: To explore the significance of intra-operative motor evoked potentials (MEPs) obtained by monopolar and bipolar stimulation in determining the location of the electrode(s) giving most pain relief in chronic motor cortex stimulation (MCS). METHODS: Eight patients with chronic refractory neuropathic pain were implanted epidurally with two parallel leads of four electrodes each and placed normal to the central sulcus (CS). We measured the peak-peak amplitude (V(p-p)) of the MEPs recorded intra-operatively at the contralateral hand with the same stimulus delivered by each single electrode used as an anode or a cathode. Those electrodes giving the largest MEPs in monopolar stimulation were also tested in bipolar stimulation with an adjacent electrode located on the same or the other lead. It was analyzed whether a relation was present between the electrode providing the largest V(p-p) in the monopolar condition and the bipolar combination selected for chronic stimulation. RESULTS: In monopolar stimulation the median amplitude of MEPs evoked with an anode was 59% larger than with a cathode. The mean amplitude of the bipolarly evoked MEPs was only 21% and 37%, respectively, of the corresponding monopoles when the anode and cathode were separated by 6mm and by more than 8mm. A significant pain relief was obtained in 5 out of 8 patients post-operatively. In all these patients, one of the cathodes used in chronic stimulation was one of the anodes producing the largest MEP intra-operatively. Conversely, in the 3 patients who did not benefit from MCS, one of the cathodes used in chronic stimulation was one of the cathodes producing the largest MEPs intra-operatively. CONCLUSIONS: Monopolar stimulation should be applied in intra-operative neurophysiological testing because, contrary to bipolar stimulation, the corresponding MEPs are unambiguously related to a single stimulating electrode and their amplitude is not affected by the anode-cathode distance. The anode providing the largest MEPs intra-operatively should be selected as the cathode in chronic stimulation. However, implantable pulse generators allowing monopolar (cathodal and anodal) stimulation for MCS should become available to compare the respective analgesic efficacy of monopolar and bipolar chronic cortical stimulation. SIGNIFICANCE: Intra-operative MEP recordings can predict which electrode should be used as the cathode to obtain the best analgesic effect with chronic MCS.

Application of correlation techniques in the analysis of corpus cavernosum electromyographic signals.

AIM: To establish an objective, easy-to-use and comprehensive method to analyze corpus cavernosum electromyographic signals (CC-potentials). METHODS: CC-potentials were recorded during flaccidity in 23 young healthy volunteers, with surface electrodes placed on the penile shaft bilaterally. Based on the correlation function of Matlab software, an application program for the analysis of CC-potentials was developed. Individual CC-potentials and their autocorrelation function were evaluated, yielding parameters amplitude (A), duration (D), and dominant frequency (DF). The cross-correlation function of both longitudinal and bilateral pairs of adjacent electrodes was calculated to assess the similarity and mutual delay of CC-potentials recorded simultaneously from different parts of the CC. The parameters derived were squared maximum cross-correlation coefficient (Rmax) and delay (tau). Based on the absolute value of tau and the corresponding inter-electrode distance, propagation velocity (PV) was calculated. RESULTS: The values of the parameters were determined automatically. No significant difference related to the locations of the electrodes for parameters A, D, and DF was detected. The cross-correlation showed that both longitudinal and bilateral CC-potential pairs had highly similar waveforms (the absolute values of Rmax were 0.80 +/- 0.05 and 0.87 +/- 0.06, respectively). PV of longitudinal pairs was estimated as 6.15 +/- 3.98 cm/s. CONCLUSION: The application program for correlation analysis of CC-potentials is a comprehensive and versatile method to analyze corpus cavernosum electromyographic recordings. Its objectiveness makes multi-center application possible.

Motor Cortex Stimulation in Patients Suffering from Chronic Neuropathic Pain: Summary of Expert Meeting and Premeeting Questionnaire, Combined with Literature Review.

BACKGROUND: Motor cortex stimulation (MCS) was introduced in the early 1990s by Tsubokawa and his group for patients diagnosed with drug-resistant, central neuropathic pain. Inconsistencies concerning the details of this therapy and its outcomes and poor methodology of most clinical essays divide the neuromodulation society worldwide into "believers" and "nonbelievers." A European expert meeting was organized in Brussels, Belgium by the Benelux Neuromodulation Society in order to develop uniform MCS protocols in the preoperative, intraoperative, and postoperative courses. METHODS: An expert meeting was organized, and a questionnaire was sent out to all the invited participants before this expert meeting. An extensive literature research was conducted in order to enrich the results. RESULTS: Topics that were addressed during the expert meeting were 1) inclusion and exclusion criteria, 2) targeting and methods of stimulation, 3) effects of MCS, and 4) results from the questionnaire. CONCLUSIONS: Substantial commonalities but also important methodologic divergencies emerged from the discussion of MCS experts from 7 European Centers. From this meeting and questionnaire, all participants concluded that there is a need for more homogenous standardized protocols for MCS regarding patient selection, implantation procedure, stimulation parameters, and follow-up-course.

Perception threshold and electrode position for spinal cord stimulation.

The perception threshold for epidural spinal cord stimulation in chronic pain management was analyzed on 3,923 testing data obtained from 136 implanted patients. The initial areas of paresthesiae due to stimulation were recorded and reported as the stimulation map according to the location of electrodes. Measurement of dorsal thickness of the cerebrospinal fluid (CSF) layer was obtained from 26 subjects using magnetic resonance imaging (MRI). The results indicate that the perception threshold is a function of the spinal level of the implanted electrodes, of the mediolateral position in the spinal canal and the contact separation of electrode. Differences in perception threshold at various vertebral levels are mainly due to varying depths of the dorsal CSF layer. The medially placed electrodes caudal to the mid-cervical levels have a higher perception threshold than more laterally placed ones. The electrodes at high and mid-cervical levels, however, have a smaller perception threshold if placed medially. The information obtained from this investigation has important implications for the design of a new-generation stimulation system and clinical application to maximize the longevity of the power source.

New trends in neuromodulation for the management of neuropathic pain.

Since its first application in 1967, the methodology and technology of spinal cord stimulation for the management of chronic, intractable pain have evolved continuously. Despite these developments and improved knowledge of the effects of spinal anatomy and epidural contact configuration on paresthesia coverage, the clinical results of spinal cord stimulation-particularly the long-term effects-are still unsatisfactory in many patients. This dissatisfaction has come primarily from the failure of single-electrode configurations to provide consistent paresthesia coverage of the entire painful area. Therefore, new approaches were developed during the late 1990s that attempted to selectively cover one or more dermatomes with paresthesia as well as to provide sequential stimulation of different anatomic sites. These approaches have been applied both intraspinally and extraspinally by stimulating either the spinal nerves or the dorsal columns. To target parts of the latter, different methods have been developed and tested using either two-dimensional contact configurations or electronic field steering. These developments hold promise for improving long-term outcomes as well as increasing the number of pain conditions that can be treated with neuromodulation therapy. In this review, the history, theoretical basis, and evolution of these methodologies, as well as the ways in which they represent new trends in neuromodulation, are discussed.

Staggered transverse tripoles with quadripolar lateral anodes using percutaneous and surgical leads in spinal cord stimulation.

BACKGROUND: In spinal cord stimulation for low-back pain, the use of electrode arrays with both low-power requirements and selective activation of target dorsal column (DC) fibers is desired. The aligned transverse tripolar lead configuration offers the best DC selectivity. Electrode alignment of the same configuration using 3 parallel percutaneous leads is possible, but compromised by longitudinal migration, resulting in loss of DC selectivity. This loss might be repaired by using the adjacent anodal contacts on the lateral leads. OBJECTIVE: To investigate if stimulation using adjacent anodal contacts on the lateral percutaneous leads of a staggered transverse tripole can restore DC selectivity. METHODS: Staggered transverse tripoles with quadripolar lateral anodes were modeled on the low-thoracic vertebral region (T10-T12) of the spinal cord using (a) percutaneous lead with staggered quadripolar lateral anodal configuration (PERC QD) and (b) laminotomy lead with staggered quadripolar lateral anodal configuration (LAM QD), of the same contact dimensions. The commercially available LAM 565 surgical lead with 16 widely spaced contacts was also modeled. For comparison with PERC QD, staggered transverse tripoles with dual lateral anodes were modeled by using percutaneous lead with staggered dual lateral anodal configuration (PERC ST). RESULTS: The PERC QD improved the depth of DC penetration and enabled selective recruitment of DCs in comparison with PERC ST. Mediolateral selectivity of DCs could not be achieved with the LAM 565. CONCLUSION: Stimulation using PERC QD improves anodal shielding of dorsal roots and restores DC selectivity. Based on our modeling study, we hypothesize that, in clinical practice, LAM QD can provide an improved performance compared with the PERC QD. Our model also predicts that the same configuration realized on the commercial LAM 565 surgical lead with widely spaced contacts cannot selectively stimulate DCs essential in treating low-back pain.

The effect of pulse width and contact configuration on paresthesia coverage in spinal cord stimulation.

BACKGROUND: In spinal cord stimulation for the management of chronic, intractable pain, a satisfactory analgesic effect can be obtained only when the stimulation-induced paresthesias cover all painful body areas completely or partially. OBJECTIVE: To investigate the effect of stimulus pulse width (PW) and contact configuration (CC) on the area of paresthesia (PA), perception threshold (VPT), discomfort threshold (VDT), and usage range (UR) in spinal cord stimulation. METHODS: Chronic pain patients were tested during a follow-up visit. They were stimulated monopolarly and with the CC giving each patient the best analgesia. VPT, VDT, and UR were determined for PWs of 90, 210, and 450 microseconds. The paresthesia contours at VDT were drawn on a body map and digitized; PA was calculated; and its anatomic composition was described. The effects of PW and CC on PA, VPT, VDT, and UR were tested statistically. RESULTS: Twenty-four of 31 tests with low thoracic stimulation and 8 of 9 tests with cervical stimulation gave a significant extension of PA at increasing PW. In 14 of 18 tests (low thoracic), a caudal extension was obtained (primarily in L5-S2). In cervical stimulation the extension was predominantly caudal as well. In contrast to VPT and VDT, UR is not significantly different when stimulating with any CC. CONCLUSION: PA extends caudally with increasing PW. The mechanism includes that the larger and smaller dorsal column fibers have a different mediolateral distribution and that smaller dorsal column fibers have a smaller UR and can be activated only when PW is sufficiently large. A similar effect of CC on PA is unlikely as long as electrodes with a large intercontact distance are applied.

Descending volleys generated by efficacious epidural motor cortex stimulation in patients with chronic neuropathic pain.

Epidural motor cortex stimulation (EMCS) is a therapeutic option for chronic, drug-resistant neuropathic pain, but its mechanisms of action remain poorly understood. In two patients with refractory hand pain successfully treated by EMCS, the presence of implanted epidural cervical electrodes for spinal cord stimulation permitted to study the descending volleys generated by EMCS in order to better appraise the neural circuits involved in EMCS effects. Direct and indirect volleys (D- and I-waves) were produced depending on electrode polarity and montage and stimulus intensity. At low-intensity, anodal monopolar EMCS generated D-waves, suggesting direct activation of corticospinal fibers, whereas cathodal EMCS generated I2-waves, suggesting transsynaptic activation of corticospinal tract. The bipolar electrode configuration used in chronic EMCS to produce maximal pain relief generated mostly I3-waves. This result suggests that EMCS induces analgesia by activating top-down controls originating from intracortical horizontal fibers or interneurons but not by stimulating directly the pyramidal tract. The descending volleys elicited by bipolar EMCS are close to those elicited by transcranial magnetic stimulation using a coil with posteroanterior orientation. Different pathways are activated by EMCS according to stimulus intensity and electrode montage and polarity. Special attention should be paid to these parameters when programming EMCS for pain treatment.

Electrode contact configuration and energy consumption in spinal cord stimulation.

OBJECTIVE: To test the hypothesis that in spinal cord stimulation, an increase in the number of cathodes increases the energy per pulse, contrary to an increase in the number of anodes, which decreases energy consumption per pulse. METHODS: Patients with an Itrel III (7425; Medtronic, Inc., Minneapolis, MN) implantable pulse generator and a Pisces-Quad (3487A; Medtronic, Inc.) implantable quadripolar lead were selected for this study. A set of 7 standard contact configurations was used for each patient. Resistor network models mimicking these configurations were constructed. The University of Twente's Spinal Cord Stimulation software was used to simulate the effect of these contact configurations on large spinal nerve fibers. To allow a comparison of the measured and modeled energy per pulse, all values were normalized. RESULTS: Both the empirical and the modeling results showed an increase in energy consumption with an increasing number of cathodes. Although the patient data with 1 and 2 cathodes did not differ significantly, energy consumption was significantly higher when 3 cathodes were used instead of 1 or 2 cathodes. The average energy consumption was significantly higher when bipolar stimulation was used instead of monopolar cathodal stimulation. An increasing number of anodes caused a decrease in energy consumption. CONCLUSION: When the paresthesia area can be covered with several configurations, it will be beneficial for the patient to program a configuration with 1 cathode and either no or multiple anodes.

A reproducibility study of corpus cavernosum electromyography in young healthy volunteers under controlled conditions.

INTRODUCTION: Although the corpus cavernosum electromyography (CC-EMG) has been studied already for 16 years, doubts regarding its reproducibility have remained. AIM: To assess the reproducibility of CC-EMG under controlled conditions and the influence of confounding factors. METHODS: Three CC-EMG recording sessions were performed in 13 healthy young men under the same conditions. Furthermore, the effects of potentially confounding factors, such as intake of caffeine, alcohol and smoking, and sexual activity, were investigated in the same population. Using auto- and cross-correlation techniques, CC-potentials were characterized with parameters amplitude (A), duration (D), dominant frequency (DF), maximum cross-correlation coefficient of longitudinal and bilateral CC-potential pairs (Rmax-lon. and Rmax-bi.), and propagation velocity (PV ). MAIN OUTCOME MEASURES: Comparison of CC-EMG parameters A, D, DF, Rmax-lon., and Rmax-bi. between three recording sessions and assessment of the impact of confounding factors on these parameters. RESULTS: DF, D, A, and Rmax-lon. showed significant correlations among three sessions; PV showed significant correlations between two sessions performed within the same day but not between those performed on different days; Rmax-bi. did not show significant correlations between any two sessions. Intake of caffeine, alcohol, and smoking did not affect CC-potentials, while the recordings shortly after ejaculation showed more irregular oscillations and less CC-potentials with smaller A. CONCLUSIONS: CC-potential parameters DF, D, A, and Rmax-lon. have been demonstrated to be reproducible. The results provide a basis for the clinical and scientific application of CC-EMG. CC-potentials are not sensitive to confounding factors such as intake of caffeine, alcohol, and smoking, while measurements shortly after ejaculation should be avoided.

Effects of electrode positioning on perception threshold and paresthesia coverage in spinal cord stimulation.

Objectives. This pilot study aims to validate the hypothesis that a smaller distance between spinal cord stimulation (SCS) lead and spinal cord results in more extensive paresthesia and less energy consumption. Materials and Methods. After insertion of a percutaneous SCS lead in patients with chronic pain (condition A), a first catheter was temporarily placed alongside the lead (condition B), and a second catheter was placed on the other side of the lead (condition C). In all three conditions paresthesia coverage, perception threshold (PT) of paresthesia, and maximum comfortable (MC) stimulus amplitude were determined and the catheters were subsequently removed. Results. Paresthesia coverage in all six patients was increased markedly in condition C when compared to condition A, whereas the mean values of PT, MC, and therapeutic range (MC/PT) dropped by 22%, 14%, and 13%, respectively. Conclusions. The results suggest that paresthesia coverage is increased when the space between the SCS lead and spinal cord gets smaller, whereas PT and energy consumption are reduced.

Theoretical investigation into longitudinal cathodal field steering in spinal cord stimulation.

Objective. When using spinal cord stimulation (SCS) for chronic pain management, precise longitudinal positioning of the cathode is crucial to generate an electrical field capable of targeting the neural elements involved in pain relief. Presently used methods have a poor spatial resolution and lack postoperative flexibility needed for fine tuning and reprogramming the stimulation field after lead displacement or changes in pain pattern. We describe in this article a new method, "electrical field steering," to control paresthesia in SCS. The method takes advantage of newer stimulator design and a programming technique allowing for "continuous" adjustment of contact combination while controlling stimulation current for each contact separately. Method. Using computer modeling we examined how stimulation of dorsal column (DC) and dorsal root (DR) fibers was influenced by changing the current ratio of the cathodes of a dual (--) and a guarded dual cathode (+--+) configuration programmed on a percutaneous lead with 9 and 4 mm center-to-center contact spacing. Results. A cathodal current ratio could be found for which DC or DR fiber recruitment and thus, most likely, paresthesia coverage was maximized. The DR threshold profiles shifted longitudinally, thus following the shift in the electrical field during steering. The profiles had a constant shape when the contact spacing was small and a varying shape for wider contact separation. Generally, the wider contact separation provided less DC and more DR fiber recruitment. Conclusions. By means of cathodal steering on a longitudinal contact array, the group of excited DC and DR fibers, and thus paresthesia coverage, can be controlled when using SCS. With widely spaced contacts, superposition of the electrical field from each steering contact is limited. To precisely control segmental paresthesia (DR stimulation), a small contact spacing is necessary.

Factors affecting impedance of percutaneous leads in spinal cord stimulation.

Objectives. Although the load impedance of a pulse generator has a significant effect on battery life, the electrical impedance of contact arrays in spinal cord stimulation (SCS) has not been extensively studied. We sought to characterize the typical impedance values measured from common quadripolar percutaneous SCS contact arrays. Methods. In 36 patients undergoing percutaneous trial stimulation for various chronic pain conditions, bipolar impedance between adjacent contacts of 64 leads with 9 mm center-to-center spacing was measured in two different vertebral level regions, cervical (C3-C7) and lower-thoracic (T7-T12). Multiple linear regression was applied to analyze the contribution of six variables to the biological tissue portion of the impedance (excluding the resistance of the lead wires). Results. The median impedance in the cervical region (351 ± 90 Ω) was significantly lower (36%, p < 0.001) than in the lower-thoracic region (547 ± 151 Ω). In addition, time since implant had a weaker but still significant effect on tissue impedance. Conclusions. Results from finite-difference mathematical modeling of SCS suggest that the difference in tissue impedance related to vertebral level may be due to the dorsoventral position of the lead in the epidural space. The presence of a larger space between the triangularly shaped dorsal part of the vertebral arch and the round shape of the dural sac in the lower-thoracic region increases the likelihood that the stimulating lead will not make dural contact, and thus "see" an increased impedance from the surrounding epidural fat. This implies that the energy requirements for stimulation in the thoracic region will be higher than in the cervical region, at least during the acute phase of implant.

Transverse tripolar spinal cord stimulation: results of an international multicenter study.

Experienced neurosurgeons at eight spinal cord stimulation centers in the United States, Canada, and Europe participated in a study from 1997 to 2000 investigating the safety, performance, and efficacy of a Transverse Tripolar Stimulation (TTS) system invented at the University of Twente, the Netherlands. This device was proposed to improve the ability of spinal cord stimulation to adequately overlap paresthesia to perceived areas of pain. Fifty-six patients with chronic, intractable neuropathic pain of the trunk and/or limbs more than three months' duration (average 105 months) were enrolled with follow-up periods at 4, 12, 26, and 52 weeks. All patients had a new paddle-type lead implanted with four electrodes, three of them aligned in a row perpendicular to the cord. Fifteen of these patients did not undergo permanent implantation. Of the 41 patients internalized, 20 patients chose conventional programming using an implanted pulse generator to drive four electrodes, while 21 patients chose a tripole stimulation system, which used radiofrequency power and signal transmission and an implanted dual-channel receiver to drive three electrodes using simultaneous pulses of independently variable amplitude. On average, the visual analog scale scores dropped more for patients with TTS systems (32%) than for conventional polarity systems (16%). Conventional polarity systems were using higher frequencies on average, while usage range was similar. Most impressive was the well-controlled "steering" of the paresthesias according to the dermatomal topography of the dorsal columns when using the TTS-balanced pulse driver. The most common complication was lead migration. While the transverse stimulation system produced acceptable outcomes for overall pain relief, an analysis of individual pain patterns suggests that it behaves like spinal cord stimulation in general with the best control of extremity neuropathic pain. This transverse tripole lead and driving system introduced the concept of electrical field steering by selective recruitment of axonal nerve fiber tracts in the dorsal columns.