Spinal segmental and supraspinal mechanisms underlying the pain-relieving effects of spinal cord stimulation: An experimental study in a rat model of neuropathy

Integrative approaches in spinal cord stimulation: Neuropathic pain management and motor recovery in spinal cord injury. A narrative review

Introduction

Spinal cord stimulation is a widespread treatment of chronic neuropathic pain from different conditions. Several novel and improving technologies have been recently developed to increase the effect of neuromodulation in patients refractory to pharmacological therapy.

Research question

To explore spinal cord stimulation's mechanisms of action, indications, and management.

Material and methods

The paper initially explores the mechanism of action of this procedure based on the generation of an electric field between electrodes placed on the posterior dural surface of the spinal cord probably interfering with the transmission of pain stimuli to the brain. Subsequently, the most consolidated criteria for selecting patients for surgery, which constitute a major issue of debate, were defined. Thereafter, the fundamental patterns of stimulation were summarized by exploring the advantages and side effects. Lastly, the most common side effects and the related management were discussed.

Results

Proper selection of the patient is of paramount importance to achieve the best results from this specific neuromodulation treatment. Regarding the different types of stimulation patterns, no definite evidence-based guidelines exist on the most appropriate approach in relation to the specific type of neuropathic pain. Both burst stimulation and high-frequency stimulation are innovative techniques that reduce the risk of paresthesias compared with conventional stimulation.

Discussion and conclusion

Novel protocols of stimulation (burst stimulation and high frequency stimulation) may improve the trade-off between therapeutic benefits and potential side effects. Likewise, decreasing the rates of hardware-related complications will be also useful to increase the application of neuromodulation in clinical settings.

Evoked compound action potential (ECAP)-controlled closed-loop spinal cord stimulation in an experimental model of neuropathic pain in rats

BACKGROUND

Preclinical models of spinal cord stimulation (SCS) are lacking objective measurements to inform translationally applicable SCS parameters. The evoked compound action potential (ECAP) represents a measure of dorsal column fiber activation. This measure approximates the onset of SCS-induced sensations in humans and provides effective analgesia when used with ECAP-controlled closed-loop (CL)-SCS systems. Therefore, ECAPs may provide an objective surrogate for SCS dose in preclinical models that may support better understanding of SCS mechanisms and further translations to the clinics. This study assessed, for the first time, the feasibility of recording ECAPs and applying ECAP-controlled CL-SCS in freely behaving rats subjected to an experimental model of neuropathic pain.

METHODS

Adult male Sprague-Dawley rats (200-300 g) were subjected to spared nerve injury (SNI). A custom-made six-contact lead was implanted epidurally covering T11-L3, as confirmed by computed tomography or X-ray. A specially designed multi-channel system was used to record ECAPs and to apply ECAP-controlled CL-SCS for 30 min at 50 Hz 200 µs. The responses of dorsal column fibers to SCS were characterized and sensitivity towards mechanical and cold stimuli were assessed to determine analgesic effects from ECAP-controlled CL-SCS. Comparisons between SNI rats and their controls as well as between stimulation parameters were made using omnibus analysis of variance (ANOVA) tests and t-tests.

RESULTS

The recorded ECAPs showed the characteristic triphasic morphology and the ECAP amplitude (mV) increased as higher currents (mA) were applied in both SNI animals and controls (SNI SCS-ON and sham SCS-ON). Importantly, the use of ECAP-based SCS dose, implemented in ECAP-controlled CL-SCS, significantly reduced mechanical and cold hypersensitivity in SNI SCS-ON animals through the constant and controlled activation of dorsal column fibers. An analysis of conduction velocities of the evoked signals confirmed the involvement of large, myelinated fibers.

CONCLUSIONS

The use of ECAP-based SCS dose implemented in ECAP-controlled CL-SCS produced analgesia in animals subjected to an experimental model of neuropathic pain. This approach may offer a better method for translating SCS parameters between species that will improve understanding of the mechanisms of SCS action to further advance future clinical applications.

Optimizing Preoperative Chronic Pain Management in Elective Spine Surgery Patients: A Narrative Review of Outcomes with Opioid and Adjuvant Pain Therapies

» Chronic preoperative opioid use negatively affects outcomes after spine surgery, with increased complications and reoperations, longer hospital stays, decreased return-to-work rates, worse patient-reported outcomes, and a higher risk of continued opioid use postoperatively.» The definition of chronic opioid use is not consistent across studies, and a more specific and consistent definition will aid in stratifying patients and understanding their risk of inferior outcomes.» Preoperative weaning periods and maximum dose thresholds are being established, which may increase the likelihood of achieving a meaningful improvement after surgery, although higher level evidence studies are needed.» Spinal cord stimulators and intrathecal drug delivery devices are increasingly used to manage chronic back pain and are equivalent or perhaps even superior to opioid treatment, although few studies exist examining how patients with these devices do after subsequent spine surgery.» Further investigation is needed to determine whether a true mechanistic explanation exists for spine-related analgesia related to spinal cord stimulators and intrathecal drug delivery devices.

Pharmacological and Nonpharmacological Treatments for Painful Diabetic Peripheral Neuropathy

Diabetic peripheral neuropathy (DPN) is one of the most prevalent chronic complications of diabetes. The lifetime prevalence of DPN is thought to be >50%, and 15%-25% of patients with diabetes experience neuropathic pain, referred to as "painful DPN." Appropriate treatment of painful DPN is important because this pain contributes to a poor quality of life by causing sleep disturbance, anxiety, and depression. The basic principle for the management of painful DPN is to control hyperglycemia and other modifiable risk factors, but these may be insufficient for preventing or improving DPN. Because there is no promising diseasemodifying medication for DPN, the pain itself needs to be managed when treating painful DPN. Drugs for neuropathic pain, such as gabapentinoids, serotonin-norepinephrine reuptake inhibitors, tricyclic antidepressants, alpha-lipoic acid, sodium channel blockers, and topical capsaicin, are used for the management of painful DPN. The U.S. Food and Drug Administration (FDA) has approved pregabalin, duloxetine, tapentadol, and the 8% capsaicin patch as drugs for the treatment of painful DPN. Recently, spinal cord stimulation using electrical stimulation is approved by the FDA for the treatment for painful DPN. This review describes the currently available pharmacological and nonpharmacological treatments for painful DPN.

Network model of nociceptive processing in the superficial spinal dorsal horn reveals mechanisms of hyperalgesia, allodynia, and spinal cord stimulation

The spinal dorsal horn (DH) processes sensory information and plays a key role in transmitting nociception to supraspinal centers. Loss of DH inhibition during neuropathic pain unmasks a pathway from nonnociceptive Aβ-afferent inputs to superficial dorsal horn (SDH) nociceptive-specific (NS) projection neurons, and this change may contribute to hyperalgesia and allodynia. We developed and validated a computational model of SDH neuronal circuitry that links nonnociceptive Aβ-afferent inputs in lamina II/III to a NS projection neuron in lamina I via a network of excitatory interneurons. The excitatory pathway and the NS projection neuron were in turn gated by inhibitory interneurons with connections based on prior patch-clamp recordings. Changing synaptic weights in the computational model to replicate neuropathic pain states unmasked a low-threshold excitatory pathway to NS neurons similar to experimental recordings. Spinal cord stimulation (SCS) is an effective therapy for neuropathic pain, and accumulating experimental evidence indicates that NS neurons in the SDH also respond to SCS. Accounting for these responses may inform therapeutic improvements, and we quantified responses to SCS in the SDH network model and examined the role of different modes of inhibitory control in modulating NS neuron responses to SCS. We combined the SDH network model with a previously published model of the deep dorsal horn (DDH) and identified optimal stimulation frequencies across different neuropathic pain conditions. Finally, we found that SCS-generated inhibition did not completely suppress model NS activity during simulated pinch inputs, providing an explanation of why SCS does not eliminate acute pain.NEW & NOTEWORTHY Chronic pain is a severe public health problem that reduces the quality of life for those affected and exacts an enormous socio-economic burden worldwide. Spinal cord stimulation (SCS) is an effective treatment for chronic pain, but SCS efficacy has not significantly improved over time, in part because the mechanisms of action remain unclear. Most preclinical studies investigating pain and SCS mechanisms have focused on the responses of deep dorsal horn (DDH) neurons, but neural networks in the superficial dorsal horn (SDH) are also important for processing nociceptive information. This work synthesizes heterogeneous experimental recordings from the SDH into a computational model that replicates experimental responses and that can be used to quantify neuronal responses to SCS under neuropathic pain conditions.

Comparison of clinical outcomes associated with spinal cord stimulation (SCS) or conventional medical management (CMM) for chronic pain: a systematic review and meta-analysis

OBJECTIVE

This study aims to evaluate the efficacy and safety of spinal cord stimulation (SCS) compared to conventional medical management (CMM) for patients diagnosed with chronic pain. Furthermore, the study seeks to compare the utilization of analgesics, as well as the long-term outcomes in terms of quality of life and functional capacity.

DATA SOURCES

We systematically searched Cochrane Library, Web of Science, PubMed, and EMBASE for randomized controlled trials from inception up to February 2022.

REVIEW METHODS

Inclusion and exclusion criteria were set according to the PICOS criteria. We searched for studies in which SCS was compared with CMM alone for chronic pain. Two reviewers independently identified eligible studies and extracted data. Risk of bias assessments were performed according to Cochrane review criteria and Interventional Pain Management Techniques-quality Appraisal of Reliability and Risk of Bias Assessment (IPM-QRB) criteria.

RESULTS

The present meta-analysis comprised eight studies and included a total of 893 patients. Our findings demonstrate that spinal cord stimulation (SCS) in combination with conventional medical management (CMM) is associated with a significant reduction in visual analogue scale (VAS) pain intensity (P = 0.0005) and decreased scores on the McGill Pain Questionnaire (MPQ) (P < 0.0001). Moreover, SCS plus CMM was found to improve patients' quality of life, as evidenced by improvements in SF-36 scores (P < 0.00001), EQ-5D utility index (P = 0.008), and Oswestry Disability Index (ODI) (P < 0.00001). Based on the results of four high-quality randomized controlled trials (RCTs), the level of evidence supporting the efficacy of SCS for the treatment of painful neuropathy is graded as level I to II. In contrast, there is currently only low-level evidence to support the use of high-frequency stimulation and other chronic pain conditions, which can be attributed to a lack of sufficient randomized controlled trials.

LIMITATIONS

The principal limitation of our study is the significant heterogeneity observed among the cohorts investigated. The primary source of this heterogeneity is the fact that spinal cord stimulation is indicated for the treatment of multiple chronic pain conditions. Moreover, variations in the stimulation parameters, differences among manufacturers, and the specific surgical implantation settings contribute to the increased heterogeneity observed in our analyses. To address this issue, we conducted a subgroup analysis based on specific situations and performed evidence synthesis to mitigate the potential impact of heterogeneity. These approaches allow for a more precise interpretation of the results and a more accurate evaluation of the quality of the included studies.

CONCLUSIONS

SCS is an effective treatment to relieve the pain level of chronic pain, decrease analgesic usage, and increase long-term quality of life and functional capacity.

Evaluation of External Trigeminal Nerve Stimulation to Prevent Cerebral Vasospasm after Subarachnoid Hemorrhage Due to Aneurysmal Rupture: A Randomized, Double-Blind Proof-of-Concept Pilot Trial (TRIVASOSTIM Study)

Cerebral vasospasm remains the most frequent and devastating complication after subarachnoid aneurysmal hemorrhage because of secondary cerebral ischemia and its sequelae. The underlying pathophysiology involves vasodilator peptide release (such as CGRP) and nitric oxide depletion at the level of the precapillary sphincters of the cerebral (internal carotid artery network) and dural (external carotid artery network) arteries, which are both innervated by craniofacial autonomic afferents and tightly connected to the trigeminal nerve and trigemino-cervical nucleus complex. We hypothesized that trigeminal nerve modulation could influence the cerebral flow of this vascular network through a sympatholytic effect and decrease the occurrence of vasospasm and its consequences. We conducted a prospective double-blind, randomized controlled pilot trial to compare the effect of 10 days of transcutaneous electrical trigeminal nerve stimulation vs. sham stimulation on cerebral infarction occurrence at 3 months. Sixty patients treated for aneurysmal SAH (World Federation of Neurosurgical Societies scale between 1 and 4) were included. We compared the radiological incidence of delayed cerebral ischemia (DCI) on magnetic resonance imaging (MRI) at 3 months in moderate and severe vasospasm patients receiving trigeminal nerve stimulation (TNS group) vs. sham stimulation (sham group). Our primary endpoint (the infarction rate at the 3-month follow-up) did not significantly differ between the two groups (p = 0.99). Vasospasm-related infarctions were present in seven patients (23%) in the TNS group and eight patients (27%) in the sham group. Ultimately, we were not able to show that TNS can decrease the rate of cerebral infarction secondary to vasospasm occurrence. As a result, it would be premature to promote trigeminal system neurostimulation in this context. This concept should be the subject of further research.

Spinal Cord Stimulation for Neuropathic Pain following a Spinal Cord Lesion with Past Spinal Surgical Histories Using a Paddle Lead Placed on the Rostral Side of the Lesion: Report of Three Cases

Spinal cord parenchymal lesions may induce intractable neuropathic pain. However, the efficacy of conventional spinal cord stimulation for the neuropathic pain following spinal cord lesions remains to be controversial. In this study, we present three cases of spinal cord stimulation using a paddle lead at the rostral side of the spinal lesion causing pain symptoms. Good pain reductions were achieved using conventional stimulation in one case and using differential target multiplexed stimulation in two cases. Case 1: A 55-year-old man presented with neuropathic pain affecting his bilateral upper extremities due to a traumatic cervical spinal cord injury. Conventional stimulation via a paddle-type electrode was able to reduce the pain from 8 to 4 via a visual analog scale. Case 2: A 67-year-old man had undergone three spinal surgeries. He presented with pain and numbness of bilateral lower extremities due to a spinal cord lesion by thoracic disc herniation. Differential target multiplexed stimulation via a paddle-type electrode achieved excellent pain reduction, that is, from 9 to 2 on the visual analog scale. Case 3: An 80-year-old man presented with pain in his bilateral upper extremities due to a cervical spinal cord lesion caused by compression and spinal canal stenosis. Posterior cervical decompression and paddle-type electrode placement were performed simultaneously. Differential target multiplexed stimulation was able to achieve excellent pain reduction, from 7 to 2 on the visual analog scale. Spinal cord stimulation using a paddle lead at the rostral side of the spinal lesion and differential target multiplexed stimulation may provide significant opportunities for patients with intractable neuropathic pain following spinal cord lesions.

Case report: The promising application of dynamic functional connectivity analysis on an individual with failed back surgery syndrome

Failed back surgery syndrome (FBSS), a chronic neuropathic pain condition, is a common indication for spinal cord stimulation (SCS). However, the mechanisms of SCS, especially its effects on supraspinal/brain functional connectivity, are still not fully understood. Resting state functional magnetic resonance imaging (rsfMRI) studies have shown characteristics in patients with chronic low back pain (cLBP). In this case study, we performed rsfMRI scanning (3.0 T) on an FBSS patient, who presented with chronic low back and leg pain following her previous lumbar microdiscectomy and had undergone permanent SCS. Appropriate MRI safety measures were undertaken to scan this subject. Seed-based functional connectivity (FC) was performed on the rsfMRI data acquired from the FBSS subject, and then compared to a group of 17 healthy controls. Seeds were identified by an atlas of resting state networks (RSNs), which is composed of 32 regions grouped into 8 networks. Sliding-window method and k-means clustering were used in dynamic FC analysis, which resulted in 4 brain states for each group. Our results demonstrated the safety and feasibility of 3T MRI scanning in a patient with implanted SCS system. Compared to the brain states of healthy controls, the FBSS subject presented very different FC patterns in less frequent brain states. The mean dwell time of brain states showed distinct distributions: the FBSS subject seemed to prefer a single state over the others. Although future studies with large sample sizes are needed to make statistical conclusions, our findings demonstrated the promising application of dynamic FC to provide more granularity with FC changes associated with different brain states in chronic pain.

Spinal Cord Stimulation Alleviates Neuropathic Pain by Attenuating Microglial Activation via Reducing Colony-Stimulating Factor 1 Levels in the Spinal Cord in a Rat Model of Chronic Constriction Injury

BACKGROUND

Spinal cord stimulation (SCS) is an emerging, minimally invasive procedure used to treat patients with intractable chronic pain conditions. Although several signaling pathways have been proposed to account for SCS-mediated pain relief, the precise mechanisms remain poorly understood. Recent evidence reveals that injured sensory neuron-derived colony-stimulating factor 1 (CSF1) induces microglial activation in the spinal cord, contributing to the development of neuropathic pain (NP). Here, we tested the hypothesis that SCS relieves pain in a rat model of chronic constriction injury (CCI) by attenuating microglial activation via blocking CSF1 to the spinal cord.

METHODS

Sprague-Dawley rats underwent sciatic nerve ligation to induce CCI and were implanted with an epidural SCS lead. SCS was delivered 6 hours per day for 5 days. Some rats received a once-daily intrathecal injection of CSF1 for 3 days during SCS.

RESULTS

Compared with naive rats, CCI rats had a marked decrease in the mechanical withdrawal threshold of the paw, along with increased microglial activation and augmented CSF1 levels in the spinal dorsal horn and dorsal root ganglion, as measured by immunofluorescence or Western blotting. SCS significantly increased the mechanical withdrawal threshold and attenuated microglial activation in the spinal dorsal horn in CCI rats, which were associated with reductions in CSF1 levels in the spinal dorsal horn and dorsal roots but not dorsal root ganglion. Moreover, intrathecal injection of CSF1 completely abolished SCS-induced changes in the mechanical withdrawal threshold and activation of microglia in the spinal dorsal horn in CCI rats.

CONCLUSIONS

SCS reduces microglial activation in the spinal cord and alleviates chronic NP, at least in part by inhibiting the release of CSF1 from the dorsal root ganglion ipsilateral to nerve injury.

Spinal Cord Stimulation to Treat Unresponsive Cancer Pain: A Possible Solution in Palliative Oncological Therapy

(1) Background: Treatment of cancer-related pain is still challenging, and it can be managed by both medical and interventional therapies. Spinal Cord Stimulation (SCS) is a minimally invasive technique, and its use is rapidly increasing in the treatment of chronic pain. (2) Materials and Methods: Our study aims to perform a review of the pertinent literature about current evidences in cancer pain treatment by Spinal Cord Stimulation. Moreover, we created a database based on case reports or case series (18 studies) in the literature. We analyzed a clinical group of oncological patients affected by intractable pain undergoing SCS implantation, focusing on outcome. (3) Results: The analysis of the 18 included studies in our series has shown a reduction in painful symptoms in 48 out of 56 treated patients (87.51%); also 53 out of 56 patients (96.64%) have shown an improvement in their Quality of Life (QoL). (4) Conclusions: Spinal Cord Stimulation can be considered an efficient method in the treatment of cancer-related pain. However, literature regarding SCS for the treatment of cancer-related pain is largely represented by case reports and small case series, with no effective population studies or Randomized Controlled Trials demonstrating the efficacy and the level of evidence. Further prospective studies are needed.

Comparison of Spinal Cord Stimulation vs. Dorsal Root Ganglion Stimulation vs. Association of Both in Patients with Refractory Chronic Back and/or Lower Limb Neuropathic Pain: An International, Prospective, Randomized, Double-Blinded, Crossover Trial (BOOST-DRG Study)

While spinal cord stimulation (SCS) is a well-established therapy to address refractory persistent spinal pain syndrome after spinal surgery (PSPS-T2), its lack of spatial selectivity and reported discomfort due to positional effects can be considered as significant limitations. As alternatives, new waveforms, such as burst stimulation and different spatial neural targets, such as dorsal root ganglion stimulation (DRGS), have shown promising results. Comparisons between DRGS and standard SCS, or their combination, have never been studied on the same patients. "BOOST DRG" is the first prospective, randomized, double-blinded, crossover study to compare SCS vs. DRGS vs. SCS+DRGS. Sixty-six PSPS-T2 patients will be recruited internationally in three centers. Before crossing over, patients will receive each stimulation modality for 1 month, using tonic conventional stimulation. After 3 months, stimulation will consist in switching to burst for 1 month, and patients will choose which modality/waveform they receive and will then be reassessed at 6 and 12 months. In addition to our primary outcome based on pain rating, this study is designed to assess quality of life, functional disability, psychological distress, pain surface coverage, global impression of change, medication quantification, adverse events, brain functional imaging and electroencephalography, with the objective being to provide a multidimensional insight based on composite pain assessment.

24-Month Effectiveness of Periduoscopic Adhesiolysis in Reducing the Use of Spinal Cord Stimulation in Patient With Chronic Lumbar Pain: A Possible Therapeutic Regimen?

Objectives

Epiduroscopy is a minimally invasive procedure that is used in pain therapy to treat lumbar and root pain that is resistant to medical and infiltrative therapies. The indications for periduroscopy are partly shared with those of spinal cord stimulation (SCS): failed back surgery syndrome (FBSS) and stenosis of the vertebral canal in particular. The costs and risks of periduroscopy are considerably lower than those of SCS. The purpose of this study is to evaluate the clinical and economic advantages of integrating periduroscopy as a step prior to SCS for patients with severe lumbar or radicular pain that is unresponsive to pharmacological and infiltrative treatments.

Materials and Methods

Patients were enrolled if they had FBSS and spinal stenosis with indications for SCS and accepted periduroscopy treatment before the possible SCS trial. Patients were followed up for 24 months with evaluations of clinical data on the day after the procedure and at one and 24 months. The pain trend, satisfaction with the periduroscopy procedure, and the incidence of SCS implants in the study period were analyzed.

Results

A total of 106 patients were enrolled. Immediately after the procedure and in the first month, the reduction of pain and the level of patient satisfaction were high, but they were drastically reduced at 24 months with a progressive reappearance of symptoms that substantially overlapped with the pre-surgery levels. At 24 months, 48% of the patients underwent a neurostimulation trial, and a significant percentage of them were able to avoid the implantation of an SCS.

Conclusions

Periduroscopy appears to be rational as a step prior to SCS in terms of the improvement of pain symptoms in the short term, the definitive results in a significant percentage of patients, and the significant economic savings for the health system.

Effects of Dorsal Column Spinal Cord Stimulation on Neuroinflammation: Revisiting Molecular Mechanisms and Clinical Outcomes on Chronic Lumbar/Leg Pain and Failed Back Surgery Syndrome

OBJECTIVE

In this narrative review, we reviewed and discussed current literature describing the molecular mechanisms leading to neuroinflammation and its role in the onset and progression of chronic neuropathic lumbar and leg pain in patients with persistent spinal pain syndrome. In addition, we reviewed the proposed mechanisms and impact of spinal cord stimulation (SCS) on neuroinflammation.

METHODS

A broad search of current literature in PubMed, Embase, Scopus, Cochrane library, Medline/Ovid, and Web of Science was performed using the following terms and their combinations: "biomarkers", "chronic back and leg pain", "cytokines", "neuroinflammation", "spinal cord stimulation (scs)," and "spinal cord modulation". We selected: 1) articles published in the English language between January 2000 and July 2020 2) preclinical and clinical data 3) case reports 4) meta-analysis and systematic reviews and 5) conference abstracts. Manuscripts not disclosing methodology or without full-text availability were excluded.

DISCUSSION

SCS techniques have gradually evolved since inception to include novel methods such as burst-SCS, high frequency SCS, and differential targeted multiplexed SCS. The incidence of chronic pain after spine surgery is highly variable, with at least one third of patients developing persistent spinal pain syndrome. Novel SCS techniques have been associated with improved clinical and functional outcomes thus increasing patient quality of life.

CONCLUSION

Currently, health care providers rely on different options and methods for SCS when treating patients with refractory chronic lumbar pain and persistent spinal pain syndrome. Nevertheless, compelling clinical trials remain necessary to elucidate the long-term benefits and mechanisms of neuromodulation of all different types of SCS.

Exploration of the Supraspinal Hypotheses about Spinal Cord Stimulation and Dorsal Root Ganglion Stimulation: A Systematic Review

Despite the established efficacy and effectiveness of Spinal Cord Stimulation (SCS), there is still no consensus on the supraspinal mechanisms of action of this therapy. The purpose of this study was to systematically review previously raised hypotheses concerning supraspinal mechanisms of action of SCS based on human, animal and computational studies. Searches were conducted using four electronic databases (PubMed, EMBASE, SCOPUS and Web of Science), backward reference searching and consultation with experts. The study protocol was registered prior to initiation of the review process (PROSPERO CRD42020161531). A total of 54 publications were included, 21 of which were animal studies, and 33 were human studies. The supraspinal hypotheses (n = 69) identified from the included studies could be categorized into six groups concerning the proposed supraspinal hypothesis, namely descending pathways (n = 24); ascending medial pathway (n = 13); ascending lateral pathway (n = 10); affective/motivational influences (n = 8); spinal–cerebral (thalamic)-loop (n = 3) and miscellaneous (n = 11). Scientific support is provided for the hypotheses identified. Modulation of the descending nociceptive inhibitory pathways, medial and lateral pathways were the most frequently reported hypotheses about the supraspinal mechanisms of action of SCS. These hypotheses were mainly supported by studies with a high or moderate confidence in the body of evidence.

High-dose spinal cord stimulation for patients with failed back surgery syndrome: a multicenter effectiveness and prediction study

ABSTRACT

The use of high-dose spinal cord stimulation (HD-SCS) has increased drastically during the past few years, with positive results. However, there remains a deficit of real-world data of the effectiveness of HD-SCS. Therefore, the primary aim was to evaluate the effectiveness of HD-SCS in patients with failed back surgery syndrome (FBSS). The second aim was to develop a prediction model for a holistic responder. One hundred ninety-four patients were recruited to a multicenter real-world registry. Self-reporting outcome variables were evaluated at baseline (before SCS) and at 1, 3, and 12 months of HD-SCS implant. Outcome measures were the mean pain intensity over time, sleep quality, disability, health-related quality of life, and medication use. Besides the effectiveness, logistic regression and decision tree analysis were performed to define a holistic responder (pain intensity reduction, medication reduction, Oswestry disability index reduction, and EQ5D improvement) after 12 months of HD-SCS. Of 185 FBSS patients who underwent a baseline visit, 75.13% had a successful HD trial. At 12 months, 92 patients were still receiving HD-SCS. Both low back and leg pain significantly decreased at 12 months. All outcome measures revealed a significant time-dependent effect from baseline to 12 months. Holistic responders could be predicted with a sensitivity and specificity of 90%. Clinically significant and sustained pain relief over a period of 12 months was achieved with HD-SCS in patients with FBSS. In addition, HD-SCS also achieved an improvement in sleep quality, functionality, and a decrease in pain medication.

Do Thoracic Spinal Deformities Affect Outcomes of Spinal Cord Stimulators: A Retrospective Chart Review

STUDY DESIGN

Retrospective chart review.

OBJECTIVE

The aim of this study was to ascertain whether the presence of structural thoracic deformities affects outcomes of permanent SCS placement.

SUMMARY OF BACKGROUND DATA

Neural modulation via spinal cord stimulators (SCSs) has become an accepted treatment option for various chronic pain syndromes. In most cases, the surgeon desires accurate midline positioning of the paddle lead, allowing for flexibility of unilateral or bilateral coverage of pain patterns. Structural spinal deformities (scoliosis or kyphosis) often result from coronal, sagittal, and rotatory deformity that can make midline placement more difficult.

METHODS

Between 2013 and 2017, two-hundred forty-one charts of patients who underwent permanent SCS placement at our suburban hospital were reviewed. Demographic information, numerical rating system (NRS) pain scores, Oswestry Disability Index (ODI) scores, and opioid medication usage were recorded at baseline and after permanent stimulator placement. Thoracic scoliosis and kyphosis angles were measured using spinal radiographs. The effect of each structural deformity on NRS, ODI, and narcotic medication usage changes from baseline was analyzed.

RESULTS

Overall, 100 patients were included in our cohort. Fifty-six patients had measured thoracic spinal deformities (38% with scoliosis, 31% with kyphosis). There was no significant difference in NRS scores, ODI scores, or narcotic usage change between patients with scoliosis and those without (P = 0.66, P = 0.57, P = 0.75) or patients with kyphosis and those without (P = 0.51, P = 0.31, P = 0.63). Bivariate linear regression analysis showed that scoliotic and kyphotic angles were not significant predictors of NRS (P = 0.39, P = 0.13), ODI (P = 0.45, P = 0.07), and opioid usage (P = 0.70, P = 0.90) change, with multivariate regression analyses confirming these findings.

CONCLUSION

SCSs can be effective options for treating lumbar back pain and radiculopathy. Our study suggests that the presence of mild structural deformities does not adversely affect outcomes of permanent SCS placement and as such should not preclude this population from benefiting from such therapies.Level of Evidence: 4.

Overview of HF10 spinal cord stimulation for the treatment of chronic pain and an introduction to the Senza Omnia™ system

Chronic intractable pain affects a significant percentage of the worldwide population, and it is one of the most disabling and expensive health conditions across the globe. Spinal cord stimulation (SCS) has been used to treat chronic pain for a number of years, but high-frequency SCS was not the US FDA approved until 2015. In this review, we describe the history and development of high-frequency SCS and discuss the benefits of the Omnia™ implantable pulse generator. We also provide a thorough literature review of the published work, highlighting the efficacy and safety profiles of high-frequency SCS for the treatment of multiple chronic pain conditions. Lastly, we offer our outlook on future developments with the Omnia SCS system.

A Case Report on Spinal Cord Stimulation in an Atrophic Spinal Cord: What Exactly Are We Stimulating?

INTRODUCTION

We present a patient with paraplegia secondary to anterior spinal cord infarct below T6, with subsequent development of chronic low-back and bilateral lower-extremity neuropathic pain with an atrophic cord. In this patient with reduced spinal cord mass and modified neuroanatomy, spinal cord stimulation was surprisingly very effective.

OBJECTIVES

The primary objective of this report is to describe a case in which a patient benefited from spinal cord stimulator therapy in a way that is not explained by the traditionally accepted mechanism of action.

METHODS

A spinal cord stimulator was implanted with two 16-contact leads placed in series starting at the top of T6.

RESULTS

He reported complete resolution of low-back pain and about 50% resolution of bilateral lower-extremity pain.

CONCLUSION

Traditionally accepted mechanisms of action of dorsal column stimulation and suppression of wide-dynamic-range neurons are unlikely to explain the relief obtained in our patient with an atrophic spinal cord. No single consensus has been reached on the primary mechanism through which spinal cord stimulation renders its therapeutic effects. Spinal cord stimulator therapy is promising, and the future direction of its study is discussed.

1.2 kHz High-Frequency Stimulation as a Rescue Therapy in Patients With Chronic Pain Refractory to Conventional Spinal Cord Stimulation

OBJECTIVES

We aimed to investigate the efficacy of new subperception stimulation paradigms including 1.2 kHz-high-frequency stimulation (HFS) and advanced-HFS field-shaping algorithm (dorsal horn HFS [DHHFS]) in refractory cases which initially benefited from conventional spinal cord stimulation (SCS) and lost the effect throughout time.

MATERIALS AND METHODS

In the context of a rescue-therapy, patients underwent externalization of the implanted SCS-leads and were tested with multiple combinations of new SCS paradigms. Pain intensity was analyzed using the numeric rating scale (NRS), and data were collected preoperatively and at multiple postoperative follow-ups.

RESULTS

Thirty-seven patients underwent externalization of the leads. Mean preoperative NRS-score was 8.1/10 points (SD ± 0.9) for the ON-stimulation period. Patients received a combination of either tonic, burst and 1.2 kHz-HFS, or burst and 1.2 kHz-HFS, DHHFS, or 1.2 kHz-HFS and DHHFS, or 1.2 kHz-HFS alone. The mean postoperative NRS-score after the testing-phase was 3.8/10 points (SD ± 2.5), showing a 48.0% mean reduction (p < 0.001). In total, 29 patients reported a significant reduction above 50% in NRS-scores and therefore were reimplanted with new generators that could deliver the new paradigms. Eight patients underwent full SCS-system explantation. The patients who continued with the new paradigms (n = 29) reported mean NRS-scores of 3.5/10 points (SD ± 1.7) 12 months postoperatively, still showing a significant reduction of 43.3% when compared to preoperative scores (p < 0.001).

CONCLUSION

Rescue-therapy with combination of multiple waveforms, including tonic, burst, 1.2 kHz-HFS, and DHHFS, was associated with a significant pain relief in patients with failed conventional SCS. This approach is a safe and efficient and should be considered before explantation of the SCS-system.

Spinal Cord Stimulation Enhances Microglial Activation in the Spinal Cord of Nerve-Injured Rats

Microglia can modulate spinal nociceptive transmission. Yet, their role in spinal cord stimulation (SCS)-induced pain inhibition is unclear. Here, we examined how SCS affects microglial activation in the lumbar cord of rats with chronic constriction injury (CCI) of the sciatic nerve. Male rats received conventional SCS (50 Hz, 80% motor threshold, 180 min, 2 sessions/day) or sham stimulation on days 18-20 post-CCI. SCS transiently attenuated the mechanical hypersensitivity in the ipsilateral hind paw and increased OX-42 immunoreactivity in the bilateral dorsal horns. SCS also upregulated the mRNAs of M1-like markers, but not M2-like markers. Inducible NOS protein expression was increased, but brain-derived neurotrophic factor was decreased after SCS. Intrathecal minocycline (1 μg-100 μg), which inhibits microglial activation, dose-dependently attenuated the mechanical hypersensitivity. Pretreatment with low-dose minocycline (1 μg, 30 min) prolonged the SCS-induced pain inhibition. These findings suggest that conventional SCS may paradoxically increase spinal M1-like microglial activity and thereby compromise its own ability to inhibit pain.

A Review of Clinical Data on Salvage Therapy in Spinal Cord Stimulation

BACKGROUND

Since its introduction in 1967, neuromodulation through spinal cord stimulation (SCS) or dorsal root ganglion stimulation (DRGs) has advanced significantly in both the technology and indications for use. There are now over 14,000 SCS implants performed worldwide every year. This review focuses on mechanisms behind the loss of efficacy in neuromodulation and current data on salvage therapy, defined as the conversion of a neuromodulation device to an alternative SCS or DRG stimulation, in the event of loss of efficacy or failure of a trial.

STUDY DESIGN

A narrative review of clinical studies regarding habituation, explant data, and salvage therapy with SCS.

METHODS

Available literature was reviewed on spinal cord stimulation technology and salvage therapy. Data sources included relevant literature identified through searches of PubMed, MEDLINE/OVID, SCOPUS, and manual searches of the bibliographies of known primary and review articles.

OUTCOME MEASURES

The primary outcome measures were to understand the mechanisms of loss of efficacy, provide a review of explants due to failure in treatment, and summarize the data on current salvage therapy in SCS.

RESULTS

A total of eight studies and four abstracts/poster presentations were identified and reviewed. Of the eight studies, only one was a randomized controlled trial.

CONCLUSIONS

There is limited evidence for the appropriate treatment alternatives, but from data currently available the conversion from conventional tonic stimulation to burst, high frequency (10 kHz), multiple wave forms, and/or DRGs may be appropriate in select patients and will require further research to determine the most appropriate first line salvage in the context of the underlying pain pathology.

A Novel Approach in Spinal Cord Stimulation for Enhancing Gastric Motility: A Preliminary Study on Canines

Background/Aims

Gastroparesis is commonly seen in patients with diabetes and functional dyspepsia with no satisfactory therapies. Dysautonomia is one of the main reasons for the imbalanced motility. We hypothesized that spinal cord stimulation (SCS) is a viable therapy for gastroparesis via the autonomic modulation to improve gastric motility. The aim is to find an optimal method of SCS for treating gastroparesis.

Methods

Eight healthy-female dogs were implanted with a gastric cannula, a duodenal cannula, 2 multi-electrode spinal leads, and an implantable pulse generator. Gastric motility index (MI) was used to determine the best stimulation location/parameters of SCS. Optimized SCS was used to improve glucagon-induced gastroparesis.

Results

With fixed parameters, SCS at Thoracic 10 (T10) was found most effective for increasing gastric MI (37.8%, P = 0.013). SCS was optimized with different parameters (pulse width: 0.05–0.6 msec, frequency: 5–500 Hz, motor threshold: 30–90%) on T10. Our findings revealed that 0.5 msec, 20 Hz with 90% motor threshold at T10 were the best parameters in increasing MI. Glucagon significantly delayed gastric emptying, and this inhibitory effect was partially blocked by SCS. Gastric emptying at 120 minutes was 25.6% in the control session and 15.7% in glucagon session (P = 0.007 vs control), while it was 22.9% with SCS session (P = 0.041 vs glucagon). SCS with the optimal parameters was found to maximally enhance vagal activity and inhibit sympathetic activity assessed by the spectral analysis of heart rate variability.

Conclusions

SCS with optimized stimulation location and parameters improves gastric motility in healthy-dogs and accelerates gastric emptying impaired by glucagon via enhancing vagal activity.

Relief of Neuropathic Pain After Spinal Cord Stimulator Implantation in a Patient With Idiopathic Thoracic Transverse Myelitis: A Case Report

Transverse myelitis (TM) is a rare neurologic disorder of acute inflammation resulting in spinal cord injury. Chronic pain in TM is a significant detriment to quality of life. Spinal cord stimulation (SCS) is an emerging treatment that has shown significant efficacy in neuropathic pain. We present a 37-year-old man with a history of idiopathic thoracic TM and refractory chronic neuropathic pain who underwent an SCS trial. He reported 70% improvement during the trial and was subsequently implanted with an SCS. He continues to experience significant pain relief and functional improvement (>80%) with conventional paresthesia programming at the 9-month follow-up.

Spinal cord stimulation prevents paclitaxel-induced mechanical and cold hypersensitivity and modulates spinal gene expression in rats

Supplemental Digital Content is Available in the Text.

Introduction:

Paclitaxel-induced peripheral neuropathy (PIPN) is a common dose-limiting side effect of this cancer treatment drug. Spinal cord stimulation (SCS) has demonstrated efficacy for attenuating some neuropathic pain conditions.

Objective:

We aim to examine the inhibitory effect of SCS on the development of PIPN pain and changes of gene expression in the spinal cord in male rats after SCS.

Methods:

We examined whether traditional SCS (50 Hz, 6–8 h/session daily for 14 consecutive days) administered during paclitaxel treatment (1.5 mg/kg, i.p.) attenuates PIPN-related pain behavior. After SCS treatment, we performed RNA-seq of the lumbar spinal cord to examine which genes are differentially expressed after PIPN with and without SCS.

Results:

Compared to rats treated with paclitaxel alone (n = 7) or sham SCS (n = 6), SCS treatment (n = 11) significantly inhibited the development of paclitaxel-induced mechanical and cold hypersensitivity, without altering open-field exploratory behavior. RNA-seq showed that SCS induced upregulation of 836 genes and downregulation of 230 genes in the spinal cord of paclitaxel-treated rats (n = 3) as compared to sham SCS (n = 5). Spinal cord stimulation upregulated immune responses in paclitaxel-treated rats, including transcription of astrocyte- and microglial-related genes, but repressed transcription of multiple gene networks associated with synapse transmission, neuron projection development, γ-aminobutyric acid reuptake, and neuronal plasticity.

Conclusion:

Our findings suggest that traditional SCS may attenuate the development of pain-related behaviors in PIPN rats, possibly by causing aggregate inhibition of synaptic plasticity through upregulation and downregulation of gene networks in the spinal cord.

Burst and Tonic Spinal Cord Stimulation Both Activate Spinal GABAergic Mechanisms to Attenuate Pain in a Rat Model of Chronic Neuropathic Pain

BACKGROUND

Experimental and clinical studies have shown that tonic spinal cord stimulation (SCS) releases gamma-aminobutyric acid (GABA) in the spinal dorsal horn. Recently, it was suggested that burst SCS does not act via spinal GABAergic mechanisms. Therefore, we studied spinal GABA release during burst and tonic SCS, both anatomically and pharmacologically, in a well-established chronic neuropathic pain model.

METHODS

Animals underwent partial sciatic nerve ligation (PSNL). Quantitative immunohistochemical (IHC) analysis of intracellular GABA levels in the lumbar L4 to L6 dorsal spinal cord was performed after 60 minutes of burst, tonic, or sham SCS in rats that had undergone PSNL (n = 16). In a second pharmacological experiment, the effects of intrathecal administration of the GABA_A antagonist bicuculline (5 μg) and the GABA_B antagonist phaclofen (5 μg) were assessed. Paw withdrawal thresholds to von Frey filaments of rats that had undergone PSNL (n = 20) were tested during 60 minutes of burst and tonic SCS 30 minutes after intrathecal administration of the drugs.

RESULTS

Quantitative IHC analysis of GABA immunoreactivity in spinal dorsal horn sections of animals that had received burst SCS (n = 5) showed significantly lower intracellular GABA levels when compared to sham SCS sections (n = 4; P = 0.0201) and tonic SCS sections (n = 7; P = 0.0077). Intrathecal application of the GABA_A antagonist bicuculline (5 μg; n = 10) or the GABA_B antagonist phaclofen (5 μg; n = 10) resulted in ablation of the analgesic effect for both burst SCS and tonic SCS.

CONCLUSIONS

In conclusion, our anatomical and pharmacological data demonstrate that, in this well-established chronic neuropathic animal model, the analgesic effects of both burst SCS and tonic SCS are mediated via spinal GABAergic mechanisms.

Assessment of axonal recruitment using model-guided preclinical spinal cord stimulation in the ex vivo adult mouse spinal cord

Spinal cord stimulation (SCS) is used clinically to limit chronic pain, but fundamental questions remain on the identity of axonal populations recruited. We developed an ex vivo adult mouse spinal cord preparation to assess recruitment following delivery of clinically analogous stimuli determined by downscaling a finite element model of clinical SCS. Analogous electric field distributions were generated with 300-µm × 300-µm electrodes positioned 200 µm above the dorsal column (DC) with stimulation between 50 and 200 µA. We compared axonal recruitment using electrodes of comparable size and stimulus amplitudes when contacting the caudal thoracic DC and at 200 or 600 μm above. Antidromic responses recorded distally from the DC, the adjacent Lissauer tract (LT), and in dorsal roots (DRs) were found to be amplitude and site dependent. Responses in the DC included a unique component not seen in DRs, having the lowest SCS recruitment amplitude and fastest conduction velocity. At 200 μm above, mean cathodic SCS recruitment threshold for axons in DRs and LT were 2.6 and 4.4 times higher, respectively, than DC threshold. SCS recruited primary afferents in all (up to 8) caudal segments sampled. Whereas A and C fibers could be recruited at nearby segments, only A fiber recruitment and synaptically mediated dorsal root reflexes were observed in more distant (lumbar) segments. In sum, clinically analogous SCS led to multisegmental recruitment of several somatosensory-encoding axonal populations. Most striking is the possibility that the lowest threshold recruitment of a nonprimary afferent population in the DC are postsynaptic dorsal column tract cells (PSDCs) projecting to gracile nuclei.NEW & NOTEWORTHY Spinal cord stimulation (SCS) is used clinically to control pain. To identify axonal populations recruited, finite element modeling identified scaling parameters to deliver clinically analogous SCS in an ex vivo adult mouse spinal cord preparation. Results showed that SCS first recruited an axonal population in the dorsal column at a threshold severalfold lower than primary afferents. These putative postsynaptic dorsal column tract cells may represent a previously unconsidered population responsible for SCS-induced paresthesias necessary for analgesia.

Spinal cord stimulation in chronic pain: evidence and theory for mechanisms of action

Well-established in the field of bioelectronic medicine, Spinal Cord Stimulation (SCS) offers an implantable, non-pharmacologic treatment for patients with intractable chronic pain conditions. Chronic pain is a widely heterogenous syndrome with regard to both pathophysiology and the resultant phenotype. Despite advances in our understanding of SCS-mediated antinociception, there still exists limited evidence clarifying the pathways recruited when patterned electric pulses are applied to the epidural space. The rapid clinical implementation of novel SCS methods including burst, high frequency and dorsal root ganglion SCS has provided the clinician with multiple options to treat refractory chronic pain. While compelling evidence for safety and efficacy exists in support of these novel paradigms, our understanding of their mechanisms of action (MOA) dramatically lags behind clinical data. In this review, we reconstruct the available basic science and clinical literature that offers support for mechanisms of both paresthesia spinal cord stimulation (P-SCS) and paresthesia-free spinal cord stimulation (PF-SCS). While P-SCS has been heavily examined since its inception, PF-SCS paradigms have recently been clinically approved with the support of limited preclinical research. Thus, wide knowledge gaps exist between their clinical efficacy and MOA. To close this gap, many rich investigative avenues for both P-SCS and PF-SCS are underway, which will further open the door for paradigm optimization, adjunctive therapies and new indications for SCS. As our understanding of these mechanisms evolves, clinicians will be empowered with the possibility of improving patient care using SCS to selectively target specific pathophysiological processes in chronic pain.

Cortical Mapping in Conventional and High Dose Spinal Cord Stimulation: An Exploratory Power Spectrum and Functional Connectivity Analysis With Electroencephalography

OBJECTIVES

Spinal cord stimulation (SCS) is considered an effective pain-relieving treatment for patients with Failed Back Surgery Syndrome (FBSS). Despite the clinical effectiveness, it is unknown whether the altered functional connectivity in such patients, as compared to healthy persons, can be influenced by SCS. Therefore, the goal of this study is to evaluate whether brain connectivity assessed by EEG differs between baseline and SCS in patients with FBSS.

MATERIALS AND METHODS

Eight patients with FBSS underwent a resting-state EEG protocol before SCS, 1.5 months and 2.5 months after receiving SCS. At each frequency band, power spectrums were compared for no SCS, conventional (CON) SCS and High Dose (HD) SCS. Functional connectivity, with the aid of eConnectome was also calculated.

RESULTS

Significant differences in the average power density spectrum over the whole scalp were observed between no SCS, CON SCS and HD SCS in delta, theta and beta frequency bands (p < 0.01). The average power spectrum for CON SCS was significantly lower than the average power spectrum for HD SCS. Marked increases in strength of the information flow between electrode pair FC3-TP9 in the beta frequency band (p = 0.006) were found in favor of HD SCS.

CONCLUSIONS

The differences in power spectrum and connectivity between the three conditions lead to the hypothesis that HD SCS differs from CON SCS on average power spectrum, suggesting that HD SCS may have a higher contribution on the excitatory bottom-up pathway.

Modulation of Spinal Nociceptive Transmission by Sub-Sensory Threshold Spinal Cord Stimulation in Rats After Nerve Injury

OBJECTIVES

High-frequency spinal cord stimulation (SCS) administered below the sensory threshold (subparesthetic) can inhibit pain, but the mechanisms remain obscure. We examined how different SCS paradigms applied at intensities below the threshold of Aβ-fiber activation (sub-sensory threshold) affect spinal nociceptive transmission in rats after an L5 spinal nerve ligation (SNL).

MATERIALS AND METHODS

Electrophysiology was used to record local field potential (LFP) at L4 spinal cord before, during, and 0-60 min after SCS in SNL rats. LFP was evoked by high-intensity paired-pulse test stimulation (5 mA, 0.2 msec, 400 msec interval) at the sciatic nerve. Epidural SCS was delivered through a miniature electrode placed at T13-L1 and L2-L3 spinal levels. Four patterns of SCS (200 Hz, 1 msec; 500 Hz, 0.5 msec; 1200 Hz; 0.2 msec; 10,000 Hz, 0.024 msec, 30 min, bipolar) were tested at 90% Aβ-threshold as a subthreshold intensity. As a positive control, traditional SCS (50 Hz, 0.2 msec) was tested at 100% Aβ-plateau as a suprathreshold intensity.

RESULTS

Traditional suprathreshold SCS at T13-L1 level significantly reduced LFP to C-fiber inputs (C-LFP). Subthreshold SCS of 200 and 500 Hz, but not 1200 or 10,000 Hz, also reduced C-LFP, albeit to a lesser extent than did traditional SCS (n = 7-10/group). When SCS was applied at the L2-L3 level, only traditional SCS and subthreshold SCS of 200 Hz inhibited C-LFP (n = 8-10/group).

CONCLUSIONS

Traditional suprathreshold SCS acutely inhibits spinal nociceptive transmission. Low-frequency subthreshold SCS with a long pulse width (200 Hz, 1 msec), but not higher-frequency SCS, also attenuates C-LFP.

Supraspinal Mechanisms of Spinal Cord Stimulation for Modulation of Pain: Five Decades of Research and Prospects for the Future

The field of spinal cord stimulation is expanding rapidly, with new waveform paradigms asserting supraspinal sites of action. The scope of treatment applications is also broadening from chronic pain to include cerebral ischemia, dystonia, tremor, multiple sclerosis, Parkinson disease, neuropsychiatric disorders, memory, addiction, cognitive function, and other neurologic diseases. The role of neurostimulation as an alternative strategy to opioids for chronic pain treatment is under robust discussion in both scientific and public forums. An understanding of the supraspinal mechanisms underlying the beneficial effects of spinal cord stimulation will aid in the appropriate application and development of optimal stimulation strategies for modulating pain signaling pathways. In this review, the authors focus on clinical and preclinical studies that indicate the role of supraspinal mechanisms in spinal cord stimulation-induced pain inhibition, and explore directions for future investigations.

Neurostimulation for Intractable Chronic Pain

The field of neuromodulation has seen unprecedented growth over the course of the last decade with novel waveforms, hardware advancements, and novel chronic pain indications. We present here an updated review on spinal cord stimulation, dorsal root ganglion stimulation, and peripheral nerve stimulation. We focus on mechanisms of action, clinical indications, and future areas of research. We also present current drawbacks with current stimulation technology and suggest areas of future advancements. Given the current shortage of viable treatment options using a pharmacological based approach and conservative interventional therapies, neuromodulation presents an interesting area of growth and development for the interventional pain field and provides current and future practitioners a fresh outlook with regards to its place in the chronic pain treatment paradigm.

Advanced methods of spinal stimulation in the treatment of chronic pain: pulse trains, waveforms, frequencies, targets, and feedback loops

INTRODUCTION

Spinal cord stimulation has emerged as a state-of-the-art evidence-based treatment for chronic neuropathic pain and mixed nociceptive-neuropathic pain. In recent years, several newer devices and treatment algorithms have provided unique and effective ways of treating chronic pain by spinal cord stimulation. In a previous review, the authors commented on the 5-year forecast for high frequency and Burst waveforms, as the only two paresthesia independent SCS strategies. Over the last 5 years, there has been considerable addition to the outcome data related to these modalities. Additionally, new treatment algorithms and modalities for spinal cord stimulation have emerged. In this review, the authors provide an up to date summary of these modalities of treatment, indications, and evidence on all different modalities and programming paradigms that are available today.

AREAS COVERED

A literature review was performed using key bibliographic databases to find outcomes related studies pertaining to spinal cord stimulation, limited to the English language and human data, between 2010 and 2018. The literature search yielded the following based on our inclusion criteria; six articles on burst stimulation, three articled on high density/high dose stimulation, six articles on Dorsal Root Ganglion stimulation, nine articles on high-frequency stimulation, and one article on closed-loop stimulation. We have also included in the discussion some smaller and anecdotal studies.

EXPERT COMMENTARY

The evidence to support outcomes of spinal cord stimulation has evolved considerably since our last review in 2014. New targets, frequencies and pulse trains, and feedback appear to have advanced the efficacy of spinal cord stimulation. Future developments aim to continue to refine patient selection and maintenance of patients in therapy.

Spinal Cord Stimulation 50 Years Later: Clinical Outcomes of Spinal Cord Stimulation Based on Randomized Clinical Trials-A Systematic Review

To assess the efficacy of spinal cord stimulation (SCS) for each indication, one must critically assess each specific clinical outcome to identify outcomes that benefit from SCS therapy. To date, a comprehensive review of clinically relevant outcome-specific evidence regarding SCS has not been published. We aimed to assess all randomized controlled trials from the world literature for the purpose of evaluating the clinical outcome-specific efficacy of SCS for the following outcomes: perceived pain relief or change pain score, quality of life, functional status, psychological impact, analgesic medication utilization, patient satisfaction, and health care cost and utilization. Interventions were SCS, without limitation to the type of controls or the type of SCS in the active arms. For each study analyzed, a quality assessment was performed using a validated scale that assesses reporting, external validity, bias, confounding, and power. Each outcome was assessed specific to its indication, and the primary measure of each abovementioned outcome was a summary of the level of evidence. Twenty-one randomized controlled trials were analyzed (7 for trunk and limb pain, inclusive of failed back surgery syndrome; 8 for refractory angina pectoris; 1 for cardiac X syndrome; 3 for critical limb ischemia; 2 for complex regional pain syndrome; and 2 for painful diabetic neuropathy). Evidence assessments for each outcome for each indication were depicted in tabular format. Outcome-specific evidence scores were established for each of the abovementioned indications, providing both physicians and patients with a summary of evidence to assist in choosing the optimal evidence-based intervention. The evidence presented herein has broad applicability as it encompasses a breadth of patient populations, variations of SCS therapy, and comparable controls that, together, reflect comprehensive clinical decision making.

Spinal Cord Stimulation for Pain Treatment After Spinal Cord Injury

In addition to restoration of bladder, bowel, and motor functions, alleviating the accompanying debilitating pain is equally important for improving the quality of life of patients with spinal cord injury (SCI). Currently, however, the treatment of chronic pain after SCI remains a largely unmet need. Electrical spinal cord stimulation (SCS) has been used to manage a variety of chronic pain conditions that are refractory to pharmacotherapy. Yet, its efficacy, benefit profiles, and mechanisms of action in SCI pain remain elusive, due to limited research, methodological weaknesses in previous clinical studies, and a lack of mechanistic exploration of SCS for SCI pain control. We aim to review recent studies and outline the therapeutic potential of different SCS paradigms for traumatic SCI pain. We begin with an overview of its manifestations, classification, potential underlying etiology, and current challenges for its treatment. The clinical evidence for using SCS in SCI pain is then reviewed. Finally, future perspectives of pre-clinical research and clinical study of SCS for SCI pain treatment are discussed.

Mechanisms of spinal cord stimulation for the treatment of pain: Still in the dark after 50 years

Background and Objective

Despite the value of spinal cord stimulation (SCS) in treating some patients with focal neuropathic pain, technological advances in stimulator design and treatment protocols have not correlated with significant improvements in clinical outcomes. This may be because incomplete understanding of the mechanisms underlying SCS precludes improvement in clinical efficacy. In this brief review, we (a) review phenomenological effects of SCS, (b) review the literature on proposed spinal sites of action of SCS and (c) propose a novel hypothesis of mechanism of action.

Results

Dorsal columns, dorsal roots and dorsal horns have each been proposed as spinal sites of action of SCS. We suggest that evidence in favour of the dorsal columns or dorsal roots as the primary mediators of SCS is weak and propose that the dorsal horn is the crucial site of action. Furthermore, we hypothesize that, based on their location, and neurochemical and morphological properties, dorsal horn islet cells may mediate the effects of SCS.

Conclusions

The precise spinal mechanisms of action of SCS are still unknown. Dorsal horn islet cells have properties that position them to play a key role in analgesic effects of electrical stimulation. Understanding the mechanisms responsible for positive SCS effects are needed for successful translation into clinical dividends.

Significance

We review possible spinal mechanisms of action of spinal cord stimulation for neuropathic pain, proposing that direct modulation of dorsal horn neurons is crucial. We suggest that mechanistic insights are needed for translation into more favourable clinical outcomes.

Duloxetine Improves Spinal Cord Stimulation Outcomes for Chronic Pain

OBJECTIVE

Spinal cord stimulation (SCS) has been shown to be effective in treating chronic pain in patients with varying etiologies. However, the impact of pharmacological treatment on augmenting response to SCS has not been previously studied.

METHODS

We enrolled 108 patients who had undergone SCS surgery and documented their pain preoperatively and at 12 months postoperatively using the Numeric Rating Scale (NRS), McGill Pain Questionnaire (MPQ), Beck Depression Inventory (BDI), Oswestry Disability Index (ODI), Pain Catastrophizing Scale (PCS), and Global Impression of Change (GIC). Pain outcomes were compared between patients receiving SCS alone and in addition to duloxetine.

RESULTS

At 1-year follow-up, patients receiving duloxetine and SCS (n = 41) had better pain relief in the affective component of MPQ (p < 0.05) than those receiving SCS alone (n = 71). Patients on duloxetine with SCS also were significantly more willing to receive SCS again (p < 0.01). This willingness appeared to be duloxetine dose dependent (p < 0.05). Patients receiving pregabalin or gabapentin with SCS did not have significantly more pain relief than patients receiving SCS alone.

CONCLUSION

This study shows the combination therapy to be an effective strategy to provide more holistic pain relief and further improve the quality of life of SCS patients.

Spinal Cord Stimulation for Neuropathic Pain: Current Trends and Future Applications

The origin and the neural pathways involved in chronic neuropathic pain are still not extensively understood. For this reason, despite the wide variety of pain medications available on the market, neuropathic pain is challenging to treat. The present therapeutic alternative considered as the gold standard for many kinds of chronic neuropathic pain is epidural spinal cord stimulation (SCS). Despite its proved efficacy, the favourable cost-effectiveness when compared to the long-term use of poorly effective drugs and the expanding array of indications and technical improvements, SCS is still worldwide largely neglected by general practitioners, neurologists, neurosurgeons and pain therapists, often bringing to a large delay in considering as a therapeutic option for patients affected by neuropathic chronic pain. The present state of the art of SCS in the treatment of chronic neuropathic pain is here overviewed and speculations on whether to use a trial period or direct implant, to choose between percutaneous leads or paddle electrodes and on the pros and cons of the different patterns of stimulation presently available on the market (tonic stim, high-frequency stim and burst stim) are described.

Spinal Cord Stimulation: Clinical Efficacy and Potential Mechanisms

Spinal cord stimulation (SCS) is a minimally invasive therapy used for the treatment of chronic neuropathic pain. SCS is a safe and effective alternative to medications such as opioids, and multiple randomized controlled studies have demonstrated efficacy for difficult-to-treat neuropathic conditions such as failed back surgery syndrome. Conventional SCS is believed mediate pain relief via activation of dorsal column Aβ fibers, resulting in variable effects on sensory and pain thresholds, and measurable alterations in higher order cortical processing. Although potentiation of inhibition, as suggested by Wall and Melzack's gate control theory, continues to be the leading explanatory model, other segmental and supraspinal mechanisms have been described. Novel, non-standard, stimulation waveforms such as high-frequency and burst have been shown in some studies to be clinically superior to conventional SCS, however their mechanisms of action remain to be determined. Additional studies are needed, both mechanistic and clinical, to better understand optimal stimulation strategies for different neuropathic conditions, improve patient selection and optimize efficacy.

Burst and high frequency stimulation: underlying mechanism of action

INTRODUCTION

Paresthesia-free spinal cord stimulation (SCS) techniques, such as burst and high-frequency (HF) SCS, have been developed and demonstrated to be successful for treating chronic pain, albeit via different mechanisms of action. The goal of this review is to discuss the mechanisms of action for pain suppression at both the cellular and systems levels for burst and HF SCS. In addition, we also discuss the neuromodulation devices that mimic these paradigms.

AREAS COVERED

The authors performed a literature review to unravel the mechanisms of action for burst and HF SCS coupled with booklets and user manuals from neuromodulation companies to understand the programmable parameters and operating ranges. Burst SCS modulates the medial pathway to suppress pain. On cellular level, burst SCS is independent on activation of γ-aminobutyric acid (GABA) receptors to inhibit neuronal firing. HF SCS blocks large-diameter fibers from producing action potentials with little influence on smaller fibers, increasing pain suppression as frequency increases.

EXPERT COMMENTARY

The neuromodulation industry is in a phase of intense innovation characterized by adaptive stimulation to improve patients' experience and experiment with alternative frequencies and novel stimulation targets.

Activation of cannabinoid CB1 receptor contributes to suppression of spinal nociceptive transmission and inhibition of mechanical hypersensitivity by Aβ-fiber stimulation

Activation of Aβ-fibers is an intrinsic feature of spinal cord stimulation (SCS) pain therapy. Cannabinoid receptor type 1 (CB1) is important to neuronal plasticity and pain modulation, but its role in SCS-induced pain inhibition remains unclear. In this study, we showed that CB1 receptors are expressed in both excitatory and inhibitory interneurons in substantia gelatinosa (SG). Patch-clamp recording of the evoked excitatory postsynaptic currents (eEPSCs) in mice after spinal nerve ligation (SNL) showed that electrical stimulation of Aβ-fibers (Aβ-ES) using clinical SCS-like parameters (50 Hz, 0.2 millisecond, 10 μA) induced prolonged depression of eEPSCs to C-fiber inputs in SG neurons. Pretreatment with CB1 receptor antagonist AM251 (2 μM) reduced the inhibition of C-eEPSCs by Aβ-ES in both excitatory and inhibitory SG neurons. We further determined the net effect of Aβ-ES on spinal nociceptive transmission in vivo by recording spinal local field potential in SNL rats. Epidural SCS (50 Hz, Aβ-plateau, 5 minutes) attenuated C-fiber-evoked local field potential. This effect of SCS was partially reduced by spinal topical application of AM251 (25 μg, 50 μL), but not CB2 receptor antagonist AM630 (100 μg). Finally, intrathecal pretreatment with AM251 (50 μg, 15 μL) in SNL rats blocked the inhibition of behavioral mechanical hypersensitivity by SCS (50 Hz, 0.2 millisecond; 80% of motor threshold, 60 minutes). Our findings suggest that activation of spinal CB1 receptors may contribute to synaptic depression to high-threshold afferent inputs in SG neurons after Aβ-ES and may be involved in SCS-induced inhibition of spinal nociceptive transmission after nerve injury.

Spinal Cord Stimulation for Treating Chronic Pain: Reviewing Preclinical and Clinical Data on Paresthesia-Free High-Frequency Therapy

BACKGROUND

Traditional spinal cord stimulation (SCS) requires that paresthesia overlaps chronic painful areas. However, the new paradigm high-frequency SCS (HF-SCS) does not rely on paresthesia.

STUDY DESIGN

A review of preclinical and clinical studies regarding the use of paresthesia-free HF-SCS for various chronic pain states.

METHODS

We reviewed available literatures on HF-SCS, including Nevro's paresthesia-free ultra high-frequency 10 kHz therapy (HF10-SCS). Data sources included relevant literature identified through searches of PubMed, MEDLINE/OVID, and SCOPUS, and manual searches of the bibliographies of known primary and review articles.

OUTCOME MEASURES

The primary goal is to describe the present developing conceptions of preclinical mechanisms of HF-SCS and to review clinical efficacy on paresthesia-free HF10-SCS for various chronic pain states.

RESULTS

HF10-SCS offers a novel pain reduction tool without paresthesia for failed back surgery syndrome and chronic axial back pain. Preclinical findings indicate that potential mechanisms of action for paresthesia-free HF-SCS differ from those of traditional SCS.

CONCLUSIONS

To fully understand and utilize paresthesia-free HF-SCS, mechanistic study and translational research will be very important, with increasing collaboration between basic science and clinical communities to design better trials and optimize the therapy based on mechanistic findings from effective preclinical models and approaches. Future research in these vital areas may include preclinical and clinical components conducted in parallel to optimize the potential of this technology.

Spinal Cord Stimulation in Chronic Pain: Mode of Action

STUDY DESIGN

Literature review.

OBJECTIVE

A review of the literature that presents a perspective on mechanisms of actions behind spinal cord stimulation (SCS) therapy for chronic pain.

SUMMARY OF BACKGROUND DATA

SCS is an effective therapeutic alternative for the treatment of intractable chronic pain. Its application has been mostly based on the gate control theory of pain. Computational models have been fundamental on the understanding of clinical observations and the design of therapies that provide optimal neuromodulation. Research has provided insight into the involvement of specific neurotransmitters that support segmental and supraspinal mechanisms of action.

METHODS

A literature review was performed with emphasis on mechanisms of action for SCS including the effects of electrical fields on spinal cord structures based on computational models and preclinical and clinical explorations.

RESULTS

This review provides background on the development of SCS, which has been driven around a paresthesia-based paradigm as a result of the gate control theory. A review of computational models emphasizes their importance on our current understanding of the mechanism of action and clinical optimization of therapy. Electrophysiology and molecular biology have provided a closer, yet narrow, view of the effect of SCS on neurotransmitters and their receptors, which have led to the formulation of segmental and supraspinal mechanisms. Literature supporting the involvement of glial cells in chronic pain and their characteristic response to electrical fields should motivate further investigation of mechanisms involving neuroglia. Finally, a review of recent results paresthesia-free strategies should encourage research on mechanisms of action.

CONCLUSION

The mechanisms of SCS have been extensively studied and several consistent phenomena have emerged. The activation of A-beta fibers to induce paresthesia also involve neurotransmitter release via segmental and supraspinal pathways. Despite advancements, much remains to be understood, particularly as new stimulation strategies are developed.

LEVEL OF EVIDENCE

N /A.