Innovations in spinal cord stimulation for pain

A Visual and Narrative Timeline Review of Spinal Cord Stimulation Technology and US Food and Drug Administration Milestones

OBJECTIVES

The aim of this study was to present key technologic and regulatory milestones in spinal cord stimulation (SCS) for managing chronic pain on a narrative timeline with visual representation, relying on original sources to the extent possible.

MATERIALS AND METHODS

We identified technical advances in SCS that facilitated and enhanced treatment on the basis of scientific publications and approvals from the United States (US) Food and Drug Administration (FDA). We presented milestones limited to first use in key indications and in the context of new technology validation. We focused primarily on pain management, but other indications (eg, motor disorder in multiple sclerosis) were included when they affected technology development.

RESULTS

We developed a comprehensive visual and narrative timeline of SCS technology and US FDA milestones. Since its conception in the 1960s, the science and technology of SCS neuromodulation have continuously evolved. Advances span lead design (from paddle-type to percutaneous, and increased electrode contacts) and stimulator technology (from wireless power to internally powered and rechargeable, with miniaturized components, and programmable multichannel devices), with expanding stimulation program flexibility (such as burst and kilohertz stimulation frequencies), as well as usage features (such as remote programming and magnetic resonance imaging conditional compatibility).

CONCLUSIONS

This timeline represents the evolution of SCS technology alongside expanding FDA-approved indications for use.

Comparative Analysis of the Efficacy of Spinal Cord Stimulation and Traditional Debridement Care in the Treatment of Ischemic Diabetic Foot Ulcers: A Retrospective Cohort Study

BACKGROUND AND OBJECTIVES

Spinal cord stimulation (SCS) is an effective treatment for diabetic peripheral neuropathy. The purpose of this study was to investigate the effectiveness of SCS in the treatment of ischemic diabetic foot ulcers.

METHODS

In this retrospective study, the SCS group comprised 102 patients with ischemic diabetic foot who were treated with SCS for foot ulcers and nonhealing wounds due to severe lower limb ischemia. The traditional debridement care (TDC) group comprised 104 patients with ischemic diabetic foot who received only TDC. Strict screening criteria were applied. The assignment of patients to either group depended solely on their willingness to be treated with SCS. Secondary end points were transcutaneous partial pressure of oxygen (PtcO 2 ), ankle-brachial index (ABI), and color Doppler of the lower limb arteries in the feet at 6 months and 12 months after treatment. The primary end point was the amputation.

RESULTS

The dorsal foot PtcO 2 and ABI of the patients in the SCS group were significantly improved at 6 months and 12 months postoperation ( P < .05). The therapeutic efficacy was significantly better than that of the TDC group over the same period of time ( P < .05). The degree of vasodilation of the lower limb arteries (ie, femoral, popliteal, posterior tibial, and dorsalis pedis arteries) on color Doppler was higher in the SCS group than in the TDC group ( P < .05). The odds ratios for total amputation at 6 and 12 months postoperatively in the SCS group were 0.45 (95% CI, 0.19-1.08) and 0.17 (95% CI, 0.08-0.37), respectively, compared with the TDC group.

CONCLUSION

SCS improved symptoms of lower limb ischemia in ischemic diabetic feet and reduced the rate of toe amputation by increasing PtcO 2 , ABI, and arterial vasodilation in the lower limbs.

Circuit-Specific Deep Brain Stimulation Provides Insights into Movement Control

Deep brain stimulation (DBS), a method in which electrical stimulation is delivered to specific areas of the brain, is an effective treatment for managing symptoms of a number of neurological and neuropsychiatric disorders. Clinical access to neural circuits during DBS provides an opportunity to study the functional link between neural circuits and behavior. This review discusses how the use of DBS in Parkinson's disease and dystonia has provided insights into the brain networks and physiological mechanisms that underlie motor control. In parallel, insights from basic science about how patterns of electrical stimulation impact plasticity and communication within neural circuits are transforming DBS from a therapy for treating symptoms to a therapy for treating circuits, with the goal of training the brain out of its diseased state.

Defining Short- and Long-Term Programming Requirements in Patients Treated With 10-kHz Spinal Cord Stimulation

OBJECTIVES

High-frequency spinal cord stimulation (10-kHz SCS) has been shown to be an effective treatment for refractory low back pain and neck pain with and without limb pain in clinical trial and real-world studies. However, limited information is available in the literature on the type and frequency of programming parameters required to optimize pain relief.

MATERIALS AND METHODS

Retrospective trial and postimplant clinical and system device data were analyzed from consecutive patients with neck pain and low back pain, with and without limb pain, from a single clinical site, including both thoracic and cervical lead placement. Best bipole, stimulation parameters, and outcomes, including pain relief, change in opioid medication use, sleep, and daily function, were analyzed.

RESULTS

Of the 92 patients in the trial, 70 received a permanent implant. Of these, the mean duration of follow-up was 1.8 ± 1.3 years. Pain relief of ≥50% at the last follow-up was achieved by 64% of patients implanted; in addition, 65% reduced their opioid medication use; 65% reported improved sleep, and 71% reported improved function. There was some consistency between the "best" bipole at trial and permanent implant, with 82% of patients within one bipole location, including 54% of permanent implants who were using the same best bipole as at trial. After permanent implant, device reprogramming was minimal, with ≤one reprogramming change per patient per quarter required to maintain pain outcomes.

CONCLUSIONS

In the study, 10-kHz SCS was an effective therapy for treating chronic pain, whereby a high responder rate (≥50% pain relief) was achieved with short time to pain relief in trial and maintained with limited device programming after permanent implant. The data presented here provide insight into the programming required during the trial and implant stages to obtain and maintain therapeutic efficacy.

Electrical neuromodulation for the treatment of chronic pain: derivation of the intrinsic barriers, outcomes and considerations of the sustainability of implantable spinal cord stimulation therapies

INTRODUCTION

For over 60 years, spinal cord stimulation has endured as a therapy through innovation and novel developments. Current practice of neuromodulation requires proper patient selection, risk mitigation and use of innovation. However, there are tangible and intangible challenges in physiology, clinical science and within society.

AREAS COVERED

We provide a narrative discussion regarding novel topics in the field especially over the last decade. We highlight the challenges in the patient care setting including selection, as well as economic and socioeconomic challenges. Physician training challenges in neuromodulation is explored as well as other factors related to the use of neuromodulation such as novel indications and economics. We also discuss the concepts of technology and healthcare data.

EXPERT OPINION

Patient safety and durable outcomes are the mainstay goal for neuromodulation. Substantial work is needed to assimilate data for larger and more relevant studies reflecting a population. Big data and global interconnectivity efforts provide substantial opportunity to reinvent our scientific approach, data analysis and its management to maximize outcomes and minimize risk. As improvements in data analysis become the standard of innovation and physician training meets demand, we expect to see an expansion of novel indications and its use in broader cohorts.

Surface-Grafted Biocompatible Polymer Conductors for Stable and Compliant Electrodes for Brain Interfaces

Durable and conductive interfaces that enable chronic and high-resolution recording of neural activity are essential for understanding and treating neurodegenerative disorders. These chronic implants require long-term stability and small contact areas. Consequently, they are often coated with a blend of conductive polymers and are crosslinked to enhance durability despite the potentially deleterious effect of crosslinking on the mechanical and electrical properties. Here the grafting of the poly(3,4 ethylenedioxythiophene) scaffold, poly(styrenesulfonate)-b-poly(poly(ethylene glycol) methyl ether methacrylate block copolymer brush to gold, in a controlled and tunable manner, by surface-initiated atom-transfer radical polymerization (SI-ATRP) is described. This "block-brush" provides high volumetric capacitance (120 F cm─3), strong adhesion to the metal (4 h ultrasonication), improved surface hydrophilicity, and stability against 10 000 charge-discharge voltage sweeps on a multiarray neural electrode. In addition, the block-brush film showed 33% improved stability against current pulsing. This approach can open numerous avenues for exploring specialized polymer brushes for bioelectronics research and application.

Biophysics of Frequency-Dependent Variation in Paresthesia and Pain Relief during Spinal Cord Stimulation

The neurophysiological effects of spinal cord stimulation (SCS) for chronic pain are poorly understood, resulting in inefficient failure-prone programming protocols and inadequate pain relief. Nonetheless, novel stimulation patterns are regularly introduced and adopted clinically. Traditionally, paresthetic sensation is considered necessary for pain relief, although novel paradigms provide analgesia without paresthesia. However, like pain relief, the neurophysiological underpinnings of SCS-induced paresthesia are unknown. Here, we paired biophysical modeling with clinical paresthesia thresholds (of both sexes) to investigate how stimulation frequency affects the neural response to SCS relevant to paresthesia and analgesia. Specifically, we modeled the dorsal column (DC) axonal response, dorsal column nucleus (DCN) synaptic transmission, conduction failure within DC fiber collaterals, and dorsal horn network output. Importantly, we found that high-frequency stimulation reduces DC fiber activation thresholds, which in turn accurately predicts clinical paresthesia perception thresholds. Furthermore, we show that high-frequency SCS produces asynchronous DC fiber spiking and ultimately asynchronous DCN output, offering a plausible biophysical basis for why high-frequency SCS is less comfortable and produces qualitatively different sensation than low-frequency stimulation. Finally, we demonstrate that the model dorsal horn network output is sensitive to SCS-inherent variations in spike timing, which could contribute to heterogeneous pain relief across patients. Importantly, we show that model DC fiber collaterals cannot reliably follow high-frequency stimulation, strongly affecting the network output and typically producing antinociceptive effects at high frequencies. Altogether, these findings clarify how SCS affects the nervous system and provide insight into the biophysics of paresthesia generation and pain relief.

Peripheral direct current reduces naturally evoked nociceptive activity at the spinal cord in rodent models of pain

Objective.Electrical neuromodulation is an established non-pharmacological treatment for chronic pain. However, existing devices using pulsatile stimulation typically inhibit pain pathways indirectly and are not suitable for all types of chronic pain. Direct current (DC) stimulation is a recently developed technology which affects small-diameter fibres more strongly than pulsatile stimulation. Since nociceptors are predominantly small-diameter Aδand C fibres, we investigated if this property could be applied to preferentially reduce nociceptive signalling.Approach.We applied a DC waveform to the sciatic nerve in rats of both sexes and recorded multi-unit spinal activity evoked at the hindpaw using various natural stimuli corresponding to different sensory modalities rather than broad-spectrum electrical stimulus. To determine if DC neuromodulation is effective across different types of chronic pain, tests were performed in models of neuropathic and inflammatory pain.Main results.We found that in both pain models tested, DC application reduced responses evoked by noxious stimuli, as well as tactile-evoked responses which we suggest may be involved in allodynia. Different spinal activity of different modalities were reduced in naïve animals compared to the pain models, indicating that physiological changes such as those mediated by disease states could play a larger role than previously thought in determining neuromodulation outcomes.Significance.Our findings support the continued development of DC neuromodulation as a method for reduction of nociceptive signalling, and suggests that it may be effective at treating a broader range of aberrant pain conditions than existing devices.

Computational modeling of dorsal root ganglion stimulation using an Injectrode

Objective.Minimally invasive neuromodulation therapies like the Injectrode, which is composed of a tightly wound polymer-coated Platinum/Iridium microcoil, offer a low-risk approach for administering electrical stimulation to the dorsal root ganglion (DRG). This flexible electrode is aimed to conform to the DRG. The stimulation occurs through a transcutaneous electrical stimulation (TES) patch, which subsequently transmits the stimulation to the Injectrode via a subcutaneous metal collector. However, it is important to note that the effectiveness of stimulation through TES relies on the specific geometrical configurations of the Injectrode-collector-patch system. Hence, there is a need to investigate which design parameters influence the activation of targeted neural structures.Approach.We employed a hybrid computational modeling approach to analyze the impact of Injectrode system design parameters on charge delivery and neural response to stimulation. We constructed multiple finite element method models of DRG stimulation, followed by the implementation of multi-compartment models of DRG neurons. By calculating potential distribution during monopolar stimulation, we simulated neural responses using various parameters based on prior acute experiments. Additionally, we developed a canonical monopolar stimulation and full-scale model of bipolar bilateral L5 DRG stimulation, allowing us to investigate how design parameters like Injectrode size and orientation influenced neural activation thresholds.Main results.Our findings were in accordance with acute experimental measurements and indicate that the minimally invasive Injectrode system predominantly engages large-diameter afferents (Aβ-fibers). These activation thresholds were contingent upon the surface area of the Injectrode. As the charge density decreased due to increasing surface area, there was a corresponding expansion in the stimulation amplitude range before triggering any pain-related mechanoreceptor (Aδ-fibers) activity.Significance.The Injectrode demonstrates potential as a viable technology for minimally invasive stimulation of the DRG. Our findings indicate that utilizing a larger surface area Injectrode enhances the therapeutic margin, effectively distinguishing the desired Aβactivation from the undesired Aδ-fiber activation.

The role of stimulus periodicity on spinal cord stimulation-induced artificial sensations in rodents

Objective.Sensory feedback is critical for effectively controlling brain-machine interfaces and neuroprosthetic devices. Spinal cord stimulation (SCS) is proposed as a technique to induce artificial sensory perceptions in rodents, monkeys, and humans. However, to realize the full potential of SCS as a sensory neuroprosthetic technology, a better understanding of the effect of SCS pulse train parameter changes on sensory detection and discrimination thresholds is necessary.Approach.Here we investigated whether stimulation periodicity impacts rats' ability to detect and discriminate SCS-induced perceptions at different frequencies.Main results.By varying the coefficient of variation (CV) of interstimulus pulse interval, we showed that at lower frequencies, rats could detect highly aperiodic SCS pulse trains at lower amplitudes (i.e. decreased detection thresholds). Furthermore, rats learned to discriminate stimuli with subtle differences in periodicity, and the just-noticeable differences from a highly aperiodic stimulus were smaller than those from a periodic stimulus.Significance.These results demonstrate that the temporal structure of an SCS pulse train is an integral parameter for modulating sensory feedback in neuroprosthetic applications.

REPORT-SCS: minimum reporting standards for spinal cord stimulation studies in spinal cord injury

Objective.Electrical spinal cord stimulation (SCS) has emerged as a promising therapy for recovery of motor and autonomic dysfunctions following spinal cord injury (SCI). Despite the rise in studies using SCS for SCI complications, there are no standard guidelines for reporting SCS parameters in research publications, making it challenging to compare, interpret or reproduce reported effects across experimental studies.Approach.To develop guidelines for minimum reporting standards for SCS parameters in pre-clinical and clinical SCI research, we gathered an international panel of expert clinicians and scientists. Using a Delphi approach, we developed guideline items and surveyed the panel on their level of agreement for each item.Main results.There was strong agreement on 26 of the 29 items identified for establishing minimum reporting standards for SCS studies. The guidelines encompass three major SCS categories: hardware, configuration and current parameters, and the intervention.Significance.Standardized reporting of stimulation parameters will ensure that SCS studies can be easily analyzed, replicated, and interpreted by the scientific community, thereby expanding the SCS knowledge base and fostering transparency in reporting.

Long-term efficacy of spinal cord stimulation for chronic primary neuropathic pain in the contemporary era: a systematic review and meta-analysis

INTRODUCTION

Spinal cord stimulation (SCS) is a modern neuromodulation technique extensively proven to be an effective modality for treatment of chronic neuropathic pain. It has been mainly studied for complex regional pain syndrome (CRPS) and failed back surgery syndrome (FBSS) and recent data almost uniformly establishes its statistically significant positive therapeutic results. It has also been compared with other available treatment modalities across various studies. However, long term data on maintenance of its efficacious potential remains less explored. Few studies have reported data on long follow-up times (>= 12 months) and have compared its efficacy with other treatment options for chronic pain, respectively. Our study pools and analyzes the available data and compares SCS with other treatment options. It also analyzes the efficacy of SCS in long term management of patients with chronic pain.

EVIDENCE ACQUISITION

We reviewed all the data available on MEDLINE, Embase and Cochrane CENTRAL using a search strategy designed to fit our pre-set inclusion and exclusion criteria. Both single-arm and double-arm studies were included. The primary outcome was defined as decrease of visual analogue scale (VAS) by >50% at 6, 12 and/or 24 months after SCS.

EVIDENCE SYNTHESIS

According to the pooled data of double-arm studies, SCS has unanimously proven its superiority over other treatment options at 6 months follow-up; however it fails to prove statistically significant difference in results at longer treatment intervals. Dorsal root ganglion stimulation, a relatively recent technique with the same underlying physiologic mechanisms as SCS, showed far more promising results than SCS. Single-arm studies show around 70% patients experiencing greater than 50% reduction in their VAS scores at 6 and 12 months.

CONCLUSIONS

SCS is a viable option for management of chronic neuropathic pain secondary to FBSS and CRPS. However, data available for its long term efficacy remains scarce and show no further statistically significant results.

Examining the Type, Quality, and Content of Web-Based Information for People With Chronic Pain Interested in Spinal Cord Stimulation: Social Listening Study

BACKGROUND

The increased availability of web-based medical information has encouraged patients with chronic pain to seek health care information from multiple sources, such as consultation with health care providers combined with web-based information. The type and quality of information that is available on the web is very heterogeneous, in terms of content, reliability, and trustworthiness. To date, no studies have evaluated what information is available about neuromodulation on the web for patients with chronic pain.

OBJECTIVE

This study aims to explore the type, quality, and content of web-based information regarding spinal cord stimulation (SCS) for chronic pain that is freely available and targeted at health care consumers.

METHODS

The social listening tool Awario was used to search Facebook (Meta Platforms, Inc), Twitter (Twitter, Inc), YouTube (Google LLC), Instagram (Meta Platforms, Inc), blogs, and the web for suitable hits with "pain" and "neuromodulation" as keywords. Quality appraisal of the extracted information was performed using the DISCERN instrument. A thematic analysis through inductive coding was conducted.

RESULTS

The initial search identified 2174 entries, of which 630 (28.98%) entries were eventually withheld, which could be categorized as web pages, including news and blogs (114/630, 18.1%); Reddit (Reddit, Inc) posts (32/630, 5.1%); Vimeo (Vimeo, Inc) hits (38/630, 6%); or YouTube (Google LLC) hits (446/630, 70.8%). Most posts originated in the United States (519/630, 82.4%). Regarding the content of information, 66.2% (383/579) of the entries discussed (fully discussed or partially discussed) how SCS works. In total, 55.6% (322/579) of the entries did not elaborate on the fact that there may be >1 potential treatment choice and 47.7% (276/579) did not discuss the influence of SCS on the overall quality of life. The inductive coding revealed 4 main themes. The first theme of pain and the burden of pain (1274/8886, 14.34% coding references) explained about pain, pain management, individual impact of pain, and patient experiences. The second theme included neuromodulation as a treatment approach (3258/8886, 36.66% coding references), incorporating the background on neuromodulation, patient-centered care, SCS therapy, and risks. Third, several device-related aspects (1722/8886, 19.38% coding references) were presented. As a final theme, patient benefits and testimonials of treatment with SCS (2632/8886, 29.62% coding references) were revealed with subthemes regarding patient benefits, eligibility, and testimonials and expectations.

CONCLUSIONS

Health care consumers have access to web-based information about SCS, where details about the surgical procedures, the type of material, working mechanisms, risks, patient expectations, testimonials, and the potential benefits of this therapy are discussed. The reliability, trustworthiness, and correctness of web-based sources should be carefully considered before automatically relying on the content.

A Definition of Neuromodulation and Classification of Implantable Electrical Modulation for Chronic Pain

OBJECTIVES

Neuromodulation therapies use a variety of treatment modalities (eg, electrical stimulation) to treat chronic pain. These therapies have experienced rapid growth that has coincided with escalating confusion regarding the nomenclature surrounding these neuromodulation technologies. Furthermore, studies are often published without a complete description of the effective stimulation dose, making it impossible to replicate the findings. To improve clinical care and facilitate dissemination among the public, payors, research groups, and regulatory bodies, there is a clear need for a standardization of terms.

APPROACH

We formed an international group of authors comprising basic scientists, anesthesiologists, neurosurgeons, and engineers with expertise in neuromodulation. Because the field of neuromodulation is extensive, we chose to focus on creating a taxonomy and standardized definitions for implantable electrical modulation of chronic pain.

RESULTS

We first present a consensus definition of neuromodulation. We then describe a classification scheme based on the 1) intended use (the site of modulation and its indications) and 2) physical properties (waveforms and dose) of a neuromodulation therapy.

CONCLUSIONS

This framework will help guide future high-quality studies of implantable neuromodulatory treatments and improve reporting of their findings. Standardization with this classification scheme and clear definitions will help physicians, researchers, payors, and patients better understand the applications of implantable electrical modulation for pain and guide informed treatment decisions.

Factors Predicting Clinically Relevant Pain Relief After Spinal Cord Stimulation for Patients With Chronic Low Back and/or Leg Pain: A Systematic Review With Meta-Analysis and Meta-Regression

RATIONALE

To optimize results with spinal cord stimulation (SCS) for chronic low back pain (CLBP) and/or leg pain, including persistent spinal pain syndrome (PSPS), careful patient selection based on proved predictive factors is essential. Unfortunately, the necessary selection process required to optimize outcomes of SCS remains challenging.

OBJECTIVE

This review aimed to evaluate predictive factors of clinically relevant pain relief after SCS for patients with CLBP and/or radicular leg pain, including PSPS.

MATERIALS AND METHODS

In August 2023, PubMed, Cinahl, Cochrane, and EMBASE were searched to identify studies published between January 2010 and August 2023. Studies reporting the percentage of patients with ≥50% pain relief after SCS in patients with CLBP and leg pain, including PSPS at 12 or 24 months, were included. Meta-analysis was conducted to pool results for back, leg, and general pain relief. Predictive factors for pain relief after 12 months were examined using univariable and multivariable meta-regression.

RESULTS

A total of 27 studies (2220 patients) were included for further analysis. The mean percentages of patients with substantial pain relief were 68% for leg pain, 63% for back pain, and 73% for general pain at 12 months follow-up, and 63% for leg pain, 59% for back pain, and 71% for general pain at 24 months follow-up assessment. The implantation method and baseline Oswestry Disability Index made the multivariable meta-regression model for ≥50% back pain relief. Sex and pain duration made the final model for ≥50% leg pain relief. Variable stimulation and implantation method made the final model for general pain relief.

CONCLUSIONS

This review supports SCS as an effective pain-relieving treatment for CLBP and/or leg pain, and models were developed to predict substantial back and leg pain relief. To provide high-grade evidence for predictive factors, SCS studies of high quality are needed in which standardized factors predictive of SCS success, based on in-patient improvements, are monitored and reported.

Model-based analysis of subthreshold mechanisms of spinal cord stimulation for pain

Objective.Spinal cord stimulation (SCS) is a common treatment for chronic pain. For decades, SCS maximized overlap between stimulation-induced paresthesias and the patient's painful areas. Recently developed SCS paradigms relieve pain at sub-perceptible amplitudes, yet little is known about the neural response to these new waveforms or their analgesic mechanisms of action. Therefore, in this study, we investigated the neural response to multiple forms of paresthesia-free SCS.Approach.We used computational modeling to investigate the neurophysiological effects and the plausibility of commonly proposed mechanisms of three paresthesia-free SCS paradigms: burst, 1 kHz, and 10 kHz SCS. Specifically, in C- and Aβ-fibers, we investigated the effects of different SCS waveforms on spike timing and activation thresholds, as well as how stochastic ion channel gating affects the response of dorsal column axons. Finally, we characterized membrane polarization of superficial dorsal horn neurons.Main results.We found that none of the SCS waveforms activate nor modulate spike timing in C-fibers. Spike timing was modulated in Aβ-fibers only at suprathreshold amplitudes. Ion channel stochasticity had little effect on Aβ-fiber activation thresholds but produced heterogeneous spike timings at suprathreshold amplitudes. Finally, local cells were preferentially polarized in their axon terminals, and the magnitude of this polarization was dependent on cellular morphology and position relative to the stimulation electrodes.Significance.Overall, the mechanisms of action of subparesthetic SCS remain unclear. Our results suggest that no SCS waveforms directly activate C-fibers, and modulation of spike timing is unlikely at subthreshold amplitudes. We conclude that potential subthreshold neuromodulatory effects of SCS on local cells are likely to be presynaptic in nature, as axons are preferentially depolarized during SCS.

Novel Pulsed Ultrahigh-frequency Spinal Cord Stimulation Inhibits Mechanical Hypersensitivity and Brain Neuronal Activity in Rats after Nerve Injury

BACKGROUND

Spinal cord stimulation (SCS) is an important pain treatment modality. This study hypothesized that a novel pulsed ultrahigh-frequency spinal cord stimulation (pUHF-SCS) could safely and effectively inhibit spared nerve injury-induced neuropathic pain in rats.

METHODS

Epidural pUHF-SCS (± 3V, 2-Hz pulses comprising 500-kHz biphasic sinewaves) was implanted at the thoracic vertebrae (T9 to T11). Local field brain potentials after hind paw stimulation were recorded. Analgesia was evaluated by von Frey-evoked allodynia and acetone-induced cold allodynia.

RESULTS

The mechanical withdrawal threshold of the injured paw was 0.91 ± 0.28 g lower than that of the sham surgery (24.9 ± 1.2 g). Applying 5-, 10-, or 20-min pUHF-SCS five times every 2 days significantly increased the paw withdrawal threshold to 13.3 ± 6.5, 18.5 ± 3.6, and 21.0 ± 2.8 g at 5 h post-SCS, respectively (P = 0.0002, < 0.0001, and < 0.0001; n = 6 per group) and to 6.1 ± 2.5, 8.2 ± 2.7, and 14.3 ± 5.9 g on the second day, respectively (P = 0.123, 0.013, and < 0.0001). Acetone-induced paw response numbers decreased from pre-SCS (41 ± 12) to 24 ± 12 and 28 ± 10 (P = 0.006 and 0.027; n = 9) at 1 and 5 h after three rounds of 20-min pUHF-SCS, respectively. The areas under the curve from the C component of the evoked potentials at the left primary somatosensory and anterior cingulate cortices were significantly decreased from pre-SCS (101.3 ± 58.3 and 86.9 ± 25.5, respectively) to 39.7 ± 40.3 and 36.3 ± 20.7 (P = 0.021, and 0.003; n = 5) at 60 min post-SCS, respectively. The intensity thresholds for pUHF-SCS to induce brain and sciatic nerve activations were much higher than the therapeutic intensities and thresholds of conventional low-frequency SCS.

CONCLUSIONS

Pulsed ultrahigh-frequency spinal cord stimulation inhibited neuropathic pain-related behavior and paw stimulation evoked brain activation through mechanisms distinct from low-frequency SCS.

EDITOR’S PERSPECTIVE

Blockage of pain by electrical spinal cord stimulation

BACKGROUND

Electrical spinal cord stimulation (SCS) is an alternative to conventional medication for chronic pain relief. Several hypotheses exist concerning the neurophysiological, vascular, and neurochemical mechanism behind SCS.

METHODS

The excitation and blockade effects of the three common SCS waveforms (tonic, burst, and high-frequency stimulation) on the nerve fibers bypassing the region of the electrodes are analyzed in a computational study. The simulations are based on the model of Hodgkin and Huxley which is fitted to spike durations of 1 ms.

RESULTS

SCS is a FDA approved technique for pain relief, but the mechanisms of action are still under investigation. The first element in the chain of mechanisms is the generation and the block of spikes in nerve fibers close to the stimulating electrode. For these "primary fibers" computer simulations showed that conventional SCS generates sharply synchronized spikes whereas the spread of the spiking times by burst stimulation is expected to cause the suppression of paresthesia. This rather uniform spread of spiking times (in comparison to tonic stimulation) is a consequence of more pulses (5 vs. 1), longer pulses, and increasing intensities within each train of 5 pulses.

CONCLUSIONS

High-frequency stimulation can block the conduction of spikes but the distance of the fiber to the lead is a critical factor.

Comparison Between the Efficacy of Spinal Cord Stimulation and of Endovascular Revascularization in the Treatment of Diabetic Foot Ulcers: A Retrospective Observational Study

OBJECTIVE

We aimed to compare the effects of spinal cord stimulation (SCS) with those of endovascular revascularization on the treatment of diabetic foot ulcers.

MATERIALS AND METHODS

A total of 104 patients with diabetic foot ulcers who met the inclusion criteria were retrospectively analyzed and classified to the SCS treatment group (n = 46) and endovascular revascularization treatment group (n = 46). The quality-of-life scores (Quality of Life Scale for Patients with Liver Cancer v2.0), visual pain analog scale score, lower limb skin temperature, lower limb arterial ultrasound results, and lower extremity electromyography results were analyzed to compare the efficacy of the two treatments for diabetic foot ulcers in the two groups before surgery and six months after surgery.

RESULTS

A total of 92 patients (men: 73.9%, mean age: 66.51 ± 11.67 years) completed the six-month postoperative follow-up period. The patients in the SCS treatment group had a higher quality-of-life score (25.54% vs 13.77%, p < 0.05), a larger reduction in pain scores (69.18% vs 37.21%, p < 0.05), and a larger reduction in foot temperature (18.56% vs 7.24%, p < 0.05) than those of the endovascular revascularization treatment group at six months after surgery. The degree of vasodilation in the lower limbs on color Doppler arterial ultrasound and the nerve conduction velocity were higher in the SCS treatment group than in the endovascular revascularization treatment group at six months after surgery (p < 0.05).

CONCLUSION

SCS was more effective than endovascular revascularization in improving quality of life, relieving pain, improving lower limb skin temperature, increasing lower limb blood flow, and improving nerve conduction in patients with diabetic foot ulcers at six months after surgery.

An optimization framework for targeted spinal cord stimulation

Objective. Spinal cord stimulation (SCS) is a common neurostimulation therapy to manage chronic pain. Technological advances have produced new neurostimulation systems with expanded capabilities in an attempt to improve the clinical outcomes associated with SCS. However, these expanded capabilities have dramatically increased the number of possible stimulation parameters and made it intractable to efficiently explore this large parameter space within the context of standard clinical programming procedures. Therefore, in this study, we developed an optimization approach to define the optimal current amplitudes or fractions across individual contacts in an SCS electrode array(s).Approach. We developed an analytic method using the Lagrange multiplier method along with smoothing approximations. To test our optimization framework, we used a hybrid computational modeling approach that consisted of a finite element method model and multi-compartment models of axons and cells within the spinal cord. Moreover, we extended our approach to multi-objective optimization to explore the trade-off between activating regions of interest (ROIs) and regions of avoidance (ROAs).Main results. For simple ROIs, our framework suggested optimized configurations that resembled simple bipolar configurations. However, when we considered multi-objective optimization, our framework suggested nontrivial stimulation configurations that could be selected from Pareto fronts to target multiple ROIs or avoid ROAs.Significance. We developed an optimization framework for targeted SCS. Our method is analytic, which allows for the fast calculation of optimal solutions. For the first time, we provided a multi-objective approach for selective SCS. Through this approach, we were able to show that novel configurations can provide neural recruitment profiles that are not possible with conventional stimulation configurations (e.g. bipolar stimulation). Most importantly, once integrated with computational models that account for sources of interpatient variability (e.g. anatomy, electrode placement), our optimization framework can be utilized to provide stimulation settings tailored to the needs of individual patients.

Computational modeling of dorsal root ganglion stimulation using an Injectrode

Objective

Minimally invasive neuromodulation therapies like the Injectrode, which is composed of a tightly wound polymer-coated platinum/iridium microcoil, offer a low-risk approach for administering electrical stimulation to the dorsal root ganglion (DRG). This flexible electrode is aimed to conform to the DRG. The stimulation occurs through a transcutaneous electrical stimulation (TES) patch, which subsequently transmits the stimulation to the Injectrode via a subcutaneous metal collector. However, effectiveness of stimulation relies on the specific geometrical configurations of the Injectrode-collector-patch system. Hence, there is a need to investigate which design parameters influence the activation of targeted neural structures.

Approach

We employed a hybrid computational modeling approach to analyze the impact of the Injectrode system design parameters on charge delivery and the neural response to stimulation. We constructed multiple finite element method models of DRG stimulation and multi-compartment models of DRG neurons. We simulated the neural responses using parameters based on prior acute preclinical experiments. Additionally, we developed multiple human-scale computational models of DRG stimulation to investigate how design parameters like Injectrode size and orientation influenced neural activation thresholds.

Main results

Our findings were in accordance with acute experimental measurements and indicated that the Injectrode system predominantly engages large-diameter afferents (Aβ-fibers). These activation thresholds were contingent upon the surface area of the Injectrode. As the charge density decreased due to increasing surface area, there was a corresponding expansion in the stimulation amplitude range before triggering any pain-related mechanoreceptor (Aδ-fibers) activity.

Significance

The Injectrode demonstrates potential as a viable technology for minimally invasive stimulation of the DRG. Our findings indicate that utilizing a larger surface area Injectrode enhances the therapeutic margin, effectively distinguishing the desired Aβ activation from the undesired Aδ-fiber activation.

A systematic review of computational models for the design of spinal cord stimulation therapies: from neural circuits to patient-specific simulations

Seventy years ago, Hodgkin and Huxley published the first mathematical model to describe action potential generation, laying the foundation for modern computational neuroscience. Since then, the field has evolved enormously, with studies spanning from basic neuroscience to clinical applications for neuromodulation. Computer models of neuromodulation have evolved in complexity and personalization, advancing clinical practice and novel neurostimulation therapies, such as spinal cord stimulation. Spinal cord stimulation is a therapy widely used to treat chronic pain, with rapidly expanding indications, such as restoring motor function. In general, simulations contributed dramatically to improve lead designs, stimulation configurations, waveform parameters and programming procedures and provided insight into potential mechanisms of action of electrical stimulation. Although the implementation of neural models are relentlessly increasing in number and complexity, it is reasonable to ask whether this observed increase in complexity is necessary for improved accuracy and, ultimately, for clinical efficacy. With this aim, we performed a systematic literature review and a qualitative meta-synthesis of the evolution of computational models, with a focus on complexity, personalization and the use of medical imaging to capture realistic anatomy. Our review showed that increased model complexity and personalization improved both mechanistic and translational studies. More specifically, the use of medical imaging enabled the development of patient-specific models that can help to transform clinical practice in spinal cord stimulation. Finally, we combined our results to provide clear guidelines for standardization and expansion of computational models for spinal cord stimulation.

Remote targeted electrical stimulation

Objective:The ability to generate electric fields in specific targets remotely would transform manipulations of processes that rest on electrical signaling.Approach:This article shows that focal electric fields are generated from distance by combining two orthogonal, remotely applied energies-magnetic and focused ultrasonic fields. The effect derives from the Lorentz force equation applied to magnetic and ultrasonic fields.Main results:We elicited this effect using standard hardware and confirmed that the generated electric fields align with the Lorentz equation. The effect significantly and safely modulated human peripheral nerves and deep brain regions of non-human primates.Significance:This approach opens a new set of applications in which electric fields are generated at high spatiotemporal resolution within intact biological tissues or materials, thus circumventing the limitations of traditional electrode-based procedures.

Neurotechnology for Pain

Neurotechnologies for treating pain rely on electrical stimulation of the central or peripheral nervous system to disrupt or block pain signaling and have been commercialized to treat a variety of pain conditions. While their adoption is accelerating, neurotechnologies are still frequently viewed as a last resort, after many other treatment options have been explored. We review the pain conditions commonly treated with electrical stimulation, as well as the specific neurotechnologies used for treating those conditions. We identify barriers to adoption, including a limited understanding of mechanisms of action, inconsistent efficacy across patients, and challenges related to selectivity of stimulation and off-target side effects. We describe design improvements that have recently been implemented, as well as some cutting-edge technologies that may address the limitations of existing neurotechnologies. Addressing these challenges will accelerate adoption and change neurotechnologies from last-line to first-line treatments for people living with chronic pain.

Spinal Cord Stimulation in Chronic Low Back Pain Syndrome: Mechanisms of Modulation, Technical Features and Clinical Application

Chronic low-back pain (CLBP) is a common disease with several negative consequences on the quality of life, work and activity ability and increased costs to the health-care system. When pharmacological, psychological, physical and occupational therapies or surgery fail to reduce CLBP, patients may be a candidate for Spinal Cord Stimulation (SCS). SCS consists of the transcutaneous or surgical implantation of different types of electrodes in the epidural space; electrodes are then connected to an Implanted Pulse Generator (IPG) that generates stimulating currents. Through spinal and supraspinal mechanisms based on the “gate control theory for pain transmission”, SCS reduces symptoms of CLBP in the almost totality of well-selected patients and its effect lasts up to eight years in around 75% of patients. However, the evidence in favor of SCS still remains weak, mainly due to poor trial methodology and design. This narrative review is mainly addressed to those professionals that may encounter patients with CLBP failing conventional treatments. For this reason, we report the mechanisms of pain relief during SCS, the technical features and some clinical considerations about the application of SCS in patients with CLBP.

Cyclic Voltammetry Study of Noble Metals and Their Alloys for Use in Implantable Electrodes

Innovation in the application and miniaturization of implantable electrodes has caused a spike in new electrode material research; however, few robust studies are available that compare different metal electrodes in biologically relevant media. Herein, cyclic voltammetry has been employed to compare platinum, palladium, and gold-based electrodes' potentiometric scans and their corresponding charge storage capacities (CSCs). Ten different noble metals and alloys in these families were tested under pseudophysiological conditions in phosphate-buffered saline (pH 7.4) at 37 °C. Charge storage capacity values (mC/cm²) were calculated for the oxide reduction, hydrogen adsorption, hydrogen desorption, and oxide formation peaks. Five scan rates spanning 2 orders of magnitude (10, 50, 100, 500, and 1000 mV/s) in both sparged and aerated environments were evaluated. Materials have been ranked by their charge storage capacities, reversibility, and trends discussed. Palladium-based alloys outperformed platinum-based alloys in the sparged condition and were ranked equally as high in the aerated condition. The Paliney 1100 (Pd-Re) alloy gave the highest observed calculated CSC value of 0.64 ± 0.02 mC/cm² in the aerated condition, demonstrating 73 ± 5% reversibility. Trends between metal electrode families elicited in this study can afford valuable insight into future engineering of high performing implantable electrode materials.

Neuromodulation devices for heart failure

Autonomic imbalance with a sympathetic dominance is acknowledged to be a critical determinant of the pathophysiology of chronic heart failure with reduced ejection fraction (HFrEF), regardless of the etiology. Consequently, therapeutic interventions directly targeting the cardiac autonomic nervous system, generally referred to as neuromodulation strategies, have gained increasing interest and have been intensively studied at both the pre-clinical level and the clinical level. This review will focus on device-based neuromodulation in the setting of HFrEF. It will first provide some general principles about electrical neuromodulation and discuss specifically the complex issue of dose-response with this therapeutic approach. The paper will thereafter summarize the rationale, the pre-clinical and the clinical data, as well as the future prospectives of the three most studied form of device-based neuromodulation in HFrEF. These include cervical vagal nerve stimulation (cVNS), baroreflex activation therapy (BAT), and spinal cord stimulation (SCS). BAT has been approved by the Food and Drug Administration for use in patients with HfrEF, while the other two approaches are still considered investigational; VNS is currently being investigated in a large phase III Study.

Poststroke arm and hand paresis: should we target the cervical spinal cord?

Despite advances in understanding of corticospinal motor control and stroke pathophysiology, current rehabilitation therapies for poststroke upper limb paresis have limited efficacy at the level of impairment. To address this problem, we make the conceptual case for a new treatment approach. We first summarize current understanding of motor control deficits in the arm and hand after stroke and their shared physiological mechanisms with spinal cord injury (SCI). We then review studies of spinal cord stimulation (SCS) for recovery of locomotion after SCI, which provide convincing evidence for enhancement of residual corticospinal function. By extrapolation, we argue for using cervical SCS to restore upper limb motor control after stroke.

Burst Spinal Cord Stimulation in the Management of Chronic Pain: Current Perspectives

Over the last several decades, opioid diversion, misuse, and over-prescription have run rampant in the United States. Spinal cord stimulation (SCS) has been FDA approved for treatment for a primary indication of neuropathic limb pain that is resistant to more conservative medical therapy. The disorders qualified for treatment include neuropathic, post-surgical, post-amputation, osteodegenerative, and pain related to vascular disease. Some of the most frequently cited conditions for treatment of SCS include failed back surgery syndrome, complex regional pain syndrome (CRPS) Type I and Type II, and post-herpetic neuralgias. Developments in SCS systems have led to the differentiation between the delivered electromechanical waveform patterns, including tonic, burst, and high-frequency. Burst SCS mitigates traditional paresthesia due to expedited action potential and offers improved pain relief. Burst SCS has been shown in available studies to be non-inferior to the traditional SCS, which can cause pain paresthesia in patients who already have chronic pain. Burst SCS does not seem to cause or need the paresthesia seen in traditional SCS, making SCS not tolerable to patients.

Moreover, some studies suggest that burst SCS may decrease opioid consumption in patients with chronic pain. This can make burst SCS an extremely useful tool in the battle against chronic pain and the raging opioid epidemic. As of now, more research needs to be performed to further delineate the effectiveness and long-term safety of this device.

Dorsal Root Ganglion Stimulation for Chronic Pain: Hypothesized Mechanisms of Action

Dorsal root ganglion stimulation (DRGS) is a neuromodulation therapy for chronic pain that is refractory to conventional medical management. Currently, the mechanisms of action of DRGS-induced pain relief are unknown, precluding both our understanding of why DRGS fails to provide pain relief to some patients and the design of neurostimulation technologies that directly target these mechanisms to maximize pain relief in all patients. Due to the heterogeneity of sensory neurons in the dorsal root ganglion (DRG), the analgesic mechanisms could be attributed to the modulation of one or many cell types within the DRG and the numerous brain regions that process sensory information. Here, we summarize the leading hypotheses of the mechanisms of DRGS-induced analgesia, and propose areas of future study that will be vital to improving the clinical implementation of DRGS. PERSPECTIVE: This article synthesizes the evidence supporting the current hypotheses of the mechanisms of action of DRGS for chronic pain and suggests avenues for future interdisciplinary research which will be critical to fully elucidate the analgesic mechanisms of the therapy.

The role of endogenous opioid neuropeptides in neurostimulation-driven analgesia

Due to the prevalence of chronic pain worldwide, there is an urgent need to improve pain management strategies. While opioid drugs have long been used to treat chronic pain, their use is severely limited by adverse effects and abuse liability. Neurostimulation techniques have emerged as a promising option for chronic pain that is refractory to other treatments. While different neurostimulation strategies have been applied to many neural structures implicated in pain processing, there is variability in efficacy between patients, underscoring the need to optimize neurostimulation techniques for use in pain management. This optimization requires a deeper understanding of the mechanisms underlying neurostimulation-induced pain relief. Here, we discuss the most commonly used neurostimulation techniques for treating chronic pain. We present evidence that neurostimulation-induced analgesia is in part driven by the release of endogenous opioids and that this endogenous opioid release is a common endpoint between different methods of neurostimulation. Finally, we introduce technological and clinical innovations that are being explored to optimize neurostimulation techniques for the treatment of pain, including multidisciplinary efforts between neuroscience research and clinical treatment that may refine the efficacy of neurostimulation based on its underlying mechanisms.

Motor improvements enabled by spinal cord stimulation combined with physical training after spinal cord injury: review of experimental evidence in animals and humans

Electrical spinal cord stimulation (SCS) has been gaining momentum as a potential therapy for motor paralysis in consequence of spinal cord injury (SCI). Specifically, recent studies combining SCS with activity-based training have reported unprecedented improvements in motor function in people with chronic SCI that persist even without stimulation. In this work, we first provide an overview of the critical scientific advancements that have led to the current uses of SCS in neurorehabilitation: e.g. the understanding that SCS activates dormant spinal circuits below the lesion by recruiting large-to-medium diameter sensory afferents within the posterior roots. We discuss how this led to the standardization of implant position which resulted in consistent observations by independent clinical studies that SCS in combination with physical training promotes improvements in motor performance and neurorecovery. While all reported participants were able to move previously paralyzed limbs from day 1, recovery of more complex motor functions was gradual, and the timeframe for first observations was proportional to the task complexity. Interestingly, individuals with SCI classified as AIS B and C regained motor function in paralyzed joints even without stimulation, but not individuals with motor and sensory complete SCI (AIS A). Experiments in animal models of SCI investigating the potential mechanisms underpinning this neurorecovery suggest a synaptic reorganization of cortico-reticulo-spinal circuits that correlate with improvements in voluntary motor control. Future experiments in humans and animal models of paralysis will be critical to understand the potential and limits for functional improvements in people with different types, levels, timeframes, and severities of SCI.

A Call to Action Toward Optimizing the Electrical Dose Received by Neural Targets in Spinal Cord Stimulation Therapy for Neuropathic Pain

Spinal cord stimulation has seen unprecedented growth in new technology in the 50 years since the first subdural implant. As we continue to grow our understanding of spinal cord stimulation and relevant mechanisms of action, novel questions arise as to electrical dosing optimization. Programming adjustment — dose titration — is often a process of trial and error that can be time-consuming and frustrating for both patient and clinician. In this report, we review the current preclinical and clinical knowledge base in order to provide insights that may be helpful in developing more rational approaches to spinal cord stimulation dosing. We also provide key conclusions that may help in directing future research into electrical dosing, given the advent of newer waveforms outside traditional programming parameters.

Neuromodulation using ultra low frequency current waveform reversibly blocks axonal conduction and chronic pain

Chronic pain remains a leading cause of disability worldwide, and there is still a clinical reliance on opioids despite the medical side effects associated with their use and societal impacts associated with their abuse. An alternative approach is the use of electrical neuromodulation to produce analgesia. Direct current can block action potential propagation but leads to tissue damage if maintained. We have developed a form of ultra low frequency (ULF) biphasic current and studied its effects. In anesthetized rats, this waveform produced a rapidly developing and completely reversible conduction block in >85% of spinal sensory nerve fibers excited by peripheral stimulation. Sustained ULF currents at lower amplitudes led to a slower onset but reversible conduction block. Similar changes were seen in an animal model of neuropathic pain, where ULF waveforms blocked sensory neuron ectopic activity, known to be an important driver of clinical neuropathic pain. Using a computational model, we showed that prolonged ULF currents could induce accumulation of extracellular potassium, accounting for the slowly developing block observed in rats. Last, we tested the analgesic effects of epidural ULF currents in 20 subjects with chronic leg and back pain. Pain ratings improved by 90% after 2 weeks. One week after explanting the electrodes, pain ratings reverted to 72% of pretreatment screening value. We conclude that epidural spinal ULF neuromodulation represents a promising therapy for treating chronic pain.

Electrical epidural stimulation of the cervical spinal cord: implications for spinal respiratory neuroplasticity after spinal cord injury

Traumatic cervical spinal cord injury (cSCI) can lead to damage of bulbospinal pathways to the respiratory motor nuclei and consequent life-threatening respiratory insufficiency due to respiratory muscle paralysis/paresis. Reports of electrical epidural stimulation (EES) of the lumbosacral spinal cord to enable locomotor function after SCI are encouraging, with some evidence of facilitating neural plasticity. Here, we detail the development and success of EES in recovering locomotor function, with consideration of stimulation parameters and safety measures to develop effective EES protocols. EES is just beginning to be applied in other motor, sensory, and autonomic systems; however, there has only been moderate success in preclinical studies aimed at improving breathing function after cSCI. Thus, we explore the rationale for applying EES to the cervical spinal cord, targeting the phrenic motor nucleus for the restoration of breathing. We also suggest cellular/molecular mechanisms by which EES may induce respiratory plasticity, including a brief examination of sex-related differences in these mechanisms. Finally, we suggest that more attention be paid to the effects of specific electrical parameters that have been used in the development of EES protocols and how that can impact the safety and efficacy for those receiving this therapy. Ultimately, we aim to inform readers about the potential benefits of EES in the phrenic motor system and encourage future studies in this area.

Somatic inhibition by microscopic magnetic stimulation

Electric currents can produce quick, reversible control of neural activity. Externally applied electric currents have been used in inhibiting certain ganglion cells in clinical practices. Via electromagnetic induction, a miniature-sized magnetic coil could provide focal stimulation to the ganglion neurons. Here we report that high-frequency stimulation with the miniature coil could reversibly block ganglion cell activity in marine mollusk Aplysia californica, regardless the firing frequency of the neurons, or concentration of potassium ions around the ganglion neurons. Presence of the ganglion sheath has minimal impact on the inhibitory effects of the coil. The inhibitory effect was local to the soma, and was sufficient in blocking the neuron's functional output. Biophysical modeling confirmed that the miniature coil induced a sufficient electric field in the vicinity of the targeted soma. Using a multi-compartment model of Aplysia ganglion neuron, we found that the high-frequency magnetic stimuli altered the ion channel dynamics that were essential for the sustained firing of action potentials in the soma. Results from this study produces several critical insights to further developing the miniature coil technology for neural control by targeting ganglion cells. The miniature coil provides an interesting neural modulation strategy in clinical applications and laboratory research.

Electronics with shape actuation for minimally invasive spinal cord stimulation

Spinal cord stimulation is one of the oldest and most established neuromodulation therapies. However, today, clinicians need to choose between bulky paddle-type devices, requiring invasive surgery under general anesthetic, and percutaneous lead-type devices, which can be implanted via simple needle puncture under local anesthetic but offer clinical drawbacks when compared with paddle devices. By applying photo- and soft lithography fabrication, we have developed a device that features thin, flexible electronics and integrated fluidic channels. This device can be rolled up into the shape of a standard percutaneous needle then implanted on the site of interest before being expanded in situ, unfurling into its paddle-type conformation. The device and implantation procedure have been validated in vitro and on human cadaver models. This device paves the way for shape-changing bioelectronic devices that offer a large footprint for sensing or stimulation but are implanted in patients percutaneously in a minimally invasive fashion.

Quantitative Sensory Testing of Spinal Cord and Dorsal Root Ganglion Stimulation in Chronic Pain Patients

BACKGROUND/OBJECTIVES

The physiological mechanisms underlying the pain-modulatory effects of clinical neurostimulation therapies, such as spinal cord stimulation (SCS) and dorsal root ganglion stimulation (DRGS), are only partially understood. In this pilot prospective study, we used patient-reported outcomes (PROs) and quantitative sensory testing (QST) to investigate the physiological effects and possible mechanisms of action of SCS and DRGS therapies.

MATERIALS AND METHODS

We tested 16 chronic pain patients selected for SCS and DRGS therapy, before and after treatment. PROs included pain intensity, pain-related symptoms (e.g., pain interference, pain coping, sleep interference) and disability, and general health status. QST included assessments of vibration detection theshold (VDT), pressure pain threshold (PPT) and tolerance (PPToL), temporal summation (TS), and conditioned pain modulation (CPM), at the most painful site.

RESULTS

Following treatment, all participants reported significant improvements in PROs (e.g., reduced pain intensity [p < 0.001], pain-related functional impairment [or pain interference] and disability [p = 0.001 for both]; better pain coping [p = 0.03], sleep [p = 0.002]), and overall health [p = 0.005]). QST showed a significant treatment-induced increase in PPT (p = 0.002) and PPToL (p = 0.011), and a significant reduction in TS (p = 0.033) at the most painful site, but showed no effects on VDT and CPM. We detected possible associations between a few QST measures and a few PROs. Notably, higher TS was associated with increased pain interference scores at pre-treatment (r = 0.772, p = 0.009), and a reduction in TS was associated with the reduction in pain interference (r = 0.669, p = 0.034) and pain disability (r = 0.690, p = 0.027) scores with treatment.

CONCLUSIONS

Our preliminary findings suggest significant clinical and therapeutic benefits associated with SCS and DRGS therapies, and the possible ability of these therapies to modulate pain processing within the central nervous system. Replication of our pilot findings in future, larger studies is necessary to characterize the physiological mechanisms of SCS and DRGS therapies.

Finding Optimal Neuromodulation for Chronic Pain: Waves, Bursts, and Beyond

Background

Spinal cord stimulation (SCS) has emerged as state-of-the-art evidence-based treatment for chronic intractable pain related to spinal and peripheral nerve disorders. Traditionally delivered as steady-state, paraesthesia-producing electrical stimulation, newer technology has augmented the SCS option and outcome in the last decade.

Objective

To present an overview of the traditional and newer SCS waveforms.

Materials and Methods

We present a short literature review of SCS waveforms in reference to newer waveforms and describing paraesthesia-free, high frequency, and burst stimulation methods as well as advances in waveform paradigms and programming modalities. Pertinent literature was reviewed, especially in the context of evolution in the waveforms of SCS and stimulation parameters.

Results

Conventional tonic SCS remains one of the most utilized and clinically validated SCS waveforms. Newer waveforms such as burst stimulation, high-frequency stimulation, and the sub-perception SCS have emerged in the last decades with favorable results with no or minimal paraesthesia, including in cases otherwise intractable to conventional tonic SCS. The recent evolution and experience of closed-loop SCS is promising and appealing. The experience and validation of the newer SCS waveforms, however, remain limited but optimistic.

Conclusions

Advances in SCS device technology and waveforms have improved patient outcomes, leading to its increased utilization of SCS for chronic pain. These improvements and the development of closed-loop SCS have been increasingly promising development and foster a clinical translation of improved pain relief as the years of research and clinical study beyond conventional SCS waveform come to fruition.

Biophysics of Temporal Interference Stimulation

Temporal interference (TI) is a non-invasive neurostimulation technique that utilizes high-frequency external electric fields to stimulate deep neuronal structures without affecting superficial, off-target structures. TI represents a potential breakthrough for treating conditions, such as Parkinson's disease and chronic pain. However, early clinical work on TI stimulation was met with mixed outcomes challenging its fundamental mechanisms and applications. Here, we apply established physics to study the mechanisms of TI with the goal of optimizing it for clinical use. We argue that TI stimulation cannot work via passive membrane filtering, as previously hypothesized. Instead, TI stimulation requires an ion-channel mediated signal rectification process. Unfortunately, this mechanism is also responsible for high-frequency conduction block in off-target tissues, thus challenging clinical applications of TI. In consequence, we propose a set of experimental controls that should be performed in future experiments to refine our understanding and practice of TI stimulation. A record of this paper's transparent peer review process is included in the Supplemental Information.

Biomarker Optimization of Spinal Cord Stimulation Therapies

OBJECTIVES

We are in the process of designing and testing an intradural stimulation device that will shorten the distance between the location of the electrode array and the targeted neural tissue, thus improving the efficacy of electrical current delivery. Identifying a biomarker that accurately reflects the response to this intervention is highly valued because of the potential to optimize interventional parameters or predict a response before it is clinically measurable. In this report, we summarize the findings pertaining to the study of biomarkers so that we and others will have an up-to-date reference that critically evaluates the current approaches and select one or several for testing during the development of our device.

MATERIALS AND METHODS

We have conducted a broad survey of the existing literature to catalogue the biomarkers that could be coupled to intradural spinal cord stimulation. We describe in detail some of the most promising biomarkers, existing limitations, and suitability to managing chronic pain.

RESULTS

Chronic, intractable pain is an all-encompassing condition that is incurable. Many treatments for managing chronic pain are nonspecific in action and intermittently administered; therefore, patients are particularly susceptible to large fluctuations in pain control over the course of a day. The absence of a reliable biomarker challenges assessment of therapeutic efficacy and contributes to either incomplete and inconsistent pain relief or, alternatively, intolerable side effects. Fluctuations in metabolites or inflammatory markers, signals captured during dynamic imaging, and genomics will likely have a role in governing how a device is modulated.

CONCLUSIONS

Efforts to identify one or more biomarkers are well underway with some preliminary evidence supporting their efficacy. This has far-reaching implications, including improved outcomes, fewer adverse events, harmonization of treatment and individuals, performance gains, and cost savings. We anticipate that novel biomarkers will be used widely to manage chronic pain.

The Effect of Clinically Controllable Factors on Neural Activation During Dorsal Root Ganglion Stimulation

OBJECTIVE

Dorsal root ganglion stimulation (DRGS) is an effective therapy for chronic pain, though its mechanisms of action are unknown. Currently, we do not understand how clinically controllable parameters (e.g., electrode position, stimulus pulse width) affect the direct neural response to DRGS. Therefore, the goal of this study was to utilize a computational modeling approach to characterize how varying clinically controllable parameters changed neural activation profiles during DRGS.

MATERIALS AND METHODS

We coupled a finite element model of a human L5 DRG to multicompartment models of primary sensory neurons (i.e., Aα-, Aβ-, Aδ-, and C-neurons). We calculated the stimulation amplitudes necessary to elicit one or more action potentials in each neuron, and examined how neural activation profiles were affected by varying clinically controllable parameters.

RESULTS

In general, DRGS predominantly activated large myelinated Aα- and Aβ-neurons. Shifting the electrode more than 2 mm away from the ganglion abolished most DRGS-induced neural activation. Increasing the stimulus pulse width to 500 μs or greater increased the number of activated Aδ-neurons, while shorter pulse widths typically only activated Aα- and Aβ-neurons. Placing a cathode near a nerve root, or an anode near the ganglion body, maximized Aβ-mechanoreceptor activation. Guarded active contact configurations did not activate more Aβ-mechanoreceptors than conventional bipolar configurations.

CONCLUSIONS

Our results suggest that DRGS applied with stimulation parameters within typical clinical ranges predominantly activates Aβ-mechanoreceptors. In general, varying clinically controllable parameters affects the number of Aβ-mechanoreceptors activated, although longer pulse widths can increase Aδ-neuron activation. Our data support several Neuromodulation Appropriateness Consensus Committee guidelines on the clinical implementation of DRGS.

Anatomical and technical factors affecting the neural response to epidural spinal cord stimulation

OBJECTIVE

Spinal cord stimulation (SCS) is a common neurostimulation therapy to treat chronic pain. Computational models represent a valuable tool to study the potential mechanisms of action of SCS and to optimize the design and implementation of SCS technologies. However, it is imperative that these computational models include the appropriate level of detail to accurately predict the neural response to SCS and to correlate model predictions with clinical outcomes. Therefore, the goal of this study was to investigate several anatomic and technical factors that may affect model-based predictions of neural activation during thoracic SCS.

APPROACH

We developed computational models that consisted of detailed finite element models of the lower thoracic spinal cord, surrounding tissues, and implanted SCS electrode arrays. We positioned multicompartment models of sensory axons within the spinal cord to calculate the activation threshold for each sensory axon. We then investigated how activation thresholds changed as a function of several anatomical variables (e.g. spine geometry, dorsal rootlet anatomy), stimulation type (i.e. voltage-controlled vs. current-controlled), electrode impedance, lead position, lead type, and electrical properties of surrounding tissues (e.g. dura conductivity, frequency-dependent conductivity).

MAIN RESULTS

Several anatomic and modeling factors produced significant percent differences or errors in activation thresholds. Rostrocaudal positioning of the cathode with respect to the vertebrae had a large effect (up to 32%) on activation thresholds. Variability in electrode impedance produced significant changes in activation thresholds for voltage-controlled stimulation (38% to 51%), but had little effect on activation thresholds for current-controlled stimulation (less than 13%). Changing the dura conductivity also produced significant differences in activation thresholds.

SIGNIFICANCE

This study demonstrates several anatomic and technical factors that can affect the neural response to SCS. These factors should be considered in clinical implementation and in future computational modeling studies of thoracic SCS.

Risk Factors and Survival Analysis of Spinal Cord Stimulator Explantation

OBJECTIVE

The treatment failure rate for spinal cord stimulators (SCS) remains unacceptably high, with reports of removal in up to 30% of patients. The purpose of this study is to perform survival and multivariate regression analyses of patients who have undergone SCS explantation in order to identify patient characteristics that may predict treatment failure.

MATERIALS AND METHODS

We identified 253 patients who underwent SCS placement using current procedural terminology codes in a private health insurance data base spanning 2003-2016. Patient demographics, opioid use, surgical indications, as well as comorbidities were noted. At least 6 months of continuous claims data before and after implantation were required for inclusion. Patients who underwent explantation were defined as those who underwent removal without replacement within 90 days and had at least 90 days of continuous insurance eligibility following removal. Those who underwent removal for infectious reasons were identified with corresponding diagnosis codes.

RESULTS

Of the 252 patients who met the inclusion criteria, 17 (6.7%) underwent SCS explantation. Median follow-up time was 2.0 years. Of those who had their system explanted, six patients (2.8%) had their systems removed for infection and 11 (4.3%) for noninfectious reasons. Bivariate analysis revealed that younger age and tobacco use were associated with an increased likelihood of explantation. The Cox proportional hazards analysis demonstrated that younger age, tobacco use, and the presence of "other" mental health disorders were predictive of explantation.

CONCLUSIONS

In a cohort of SCS patients from multiple institutions, this study demonstrates that explantation for noninfectious reasons is more likely in younger patients, tobacco users, and those with certain psychiatric conditions. With an estimated 10% of patients opting to have their devices removed within 5 years of implantation, refining the ability of clinicians to predict who will see benefit from SCS treatment remains necessary.

A computational outlook on neurostimulation

Efficient identification of effective neurostimulation strategies is critical due to the growing number of clinical applications and the increasing complexity of the corresponding technology. In consequence, investigators are encouraged to accelerate translational research of neurostimulation technologies and move quickly to clinical applications. However, this process is hampered by rigorous, but necessary, regulations and lack of a mechanistic understanding of the interactions between electric fields and neural circuits. Here we discuss how computational models have influenced the field of neurostimulation for pain and movement recovery, deep brain stimulation, and even device regulations. Finally, we propose our vision on how computational models will be key to accelerate clinical developments through mechanistic understanding.

Patient-Specific Analysis of Neural Activation During Spinal Cord Stimulation for Pain

OBJECTIVE

Despite the widespread use of spinal cord stimulation (SCS) for chronic pain management, its neuromodulatory effects remain poorly understood. Computational models provide a valuable tool to study SCS and its effects on axonal pathways within the spinal cord. However, these models must include sufficient detail to correlate model predictions with clinical effects, including patient-specific data. Therefore, the goal of this study was to investigate axonal activation at clinically relevant SCS parameters using a computer model that incorporated patient-specific anatomy and electrode locations.

METHODS

We developed a patient-specific computer model for a patient undergoing SCS to treat chronic pain. This computer model consisted of two main components: 1) finite element model of the extracellular voltages generated by SCS and 2) multicompartment cable models of axons in the spinal cord. To determine the potential significance of a patient-specific approach, we also performed simulations with standard canonical models of SCS. We used the computer models to estimate axonal activation at clinically measured sensory, comfort, and discomfort thresholds.

RESULTS

The patient-specific and canonical models predicted significantly different axonal activation. Relative to the canonical models, the patient-specific model predicted sensory threshold estimates that were more consistent with the corresponding clinical measurements. These results suggest that it is important to account for sources of interpatient variability (e.g., anatomy, electrode locations) in model-based analysis of SCS.

CONCLUSIONS

This study demonstrates the potential for patient-specific computer models to quantitatively describe the axonal response to SCS and to address scientific questions related to clinical SCS.

Evoked Potentials Recorded From the Spinal Cord During Neurostimulation for Pain: A Computational Modeling Study

OBJECTIVES

Spinal cord stimulation (SCS) for pain is typically implemented in an open-loop manner using parameters that remain largely unchanged. To improve the overall efficacy and consistency of SCS, one closed-loop approach proposes to use evoked compound action potentials (ECAPs) recorded from the SCS lead(s) as a feedback control signal to guide parameter selection. The goal of this study was to use a computational modeling approach to investigate the source of these ECAP recordings and technical and physiological factors that affect their composition.

METHODS

We developed a computational model that coupled a finite element model of lower thoracic SCS with multicompartment models of sensory axons within the spinal cord. We used a reciprocity-based approach to calculate SCS-induced ECAPs recorded from the SCS lead.

RESULTS

Our model ECAPs contained a triphasic, P1, N1, P2 morphology. The model P2-N1 amplitudes and conduction velocities agreed with previous experimental data from human subjects. Model results suggested that the ECAPs are dominated by the activation of axons with diameters 8.7-10.0 μm located in the dorsal aspect of the spinal cord. We also observed changes in the ECAP amplitude and shape due to the electrode location relative to the vertebrae and spinal cord.

CONCLUSION

Our modeling results suggest that clinically effective SCS relies on the activation of numerous axons within a narrow fiber diameter range and that several factors affect the composition of the ECAP recordings. These results can improve how we interpret and implement these recordings in a potential closed-loop approach to SCS.