Mechanisms of Dorsal Root Ganglion Stimulation in Pain Suppression: A Computational Modeling Analysis

An in silico analysis of neuromodulation for pain relief: Determining the role of classical electrodynamics

There has been ongoing debate about the efficacy and mechanism of action of neuromodulation devices in pain relief applications. It has recently been suggested that both issues may be resolved if electromagnetic theory is incorporated into the understanding and application of this technology, and we therefore undertook an in silico analysis to further explore this idea. We created a CAD replication of a standard neuromodulation electrode array with a generic linear 3/6 mm 8-contact lead, developed a parameterized algorithmic model for the pulse delivered by the device and assigned material properties to biologic media to accurately reflect their electromagnetic properties. We then created a physical simulation of the device's output both in air and in the biophysical environment. The simulations confirmed the presence of an electromagnetic field (EM field). Variations in programming of the device affected the strength of the EM field by orders of magnitude. The biologic media all absorbed the EM field, an effect which was particularly pronounced in cerebrospinal fluid and muscle. We discuss the implications of all these findings in relation to the literature. We suggest that knowledge of electromagnetic theory and its application within the biophysical space is required for the optimal use of neuromodulation devices in pain relief applications.

Comparison of the efficacy of spinal cord stimulation and dorsal root ganglion stimulation in the treatment of painful diabetic peripheral neuropathy: a prospective, cohort-controlled study

Objective

The aim of this study was to compare the clinical outcomes of spinal cord stimulation (SCS) and dorsal root ganglion stimulation (DRG-S) in the treatment of painful diabetic peripheral neuropathy (PDPN).

Methods

In this prospective cohort study, 55 patients received dorsal column spinal cord stimulation (SCS group) and 51 patients received dorsal root spinal cord stimulation (DRG-S group). The primary outcome was a Numerical Rating Scale (NRS) remission rate of ≥50%, and secondary outcomes included the effects of SCS and DRG-S on quality of life scores (EQ-5D-3L), nerve conduction velocity, and HbA1c, respectively.

Results

The percentage of NRS remission rate ≥ 50% at 6 months was 80.43 vs. 79.55%, OR (95% CI): 1.06 (0.38-2.97) in the SCS and DRG-S groups, respectively, and the percentage of VAS remission rate ≥ 50% at 12 months was 79.07 vs. 80.95%, OR (95% CI): 0.89 (0.31-2.58). Compared with baseline, there were significant improvements in EQ-5D and EQ-VAS at 6 and 12 months (p < 0.05), but there was no difference in improvement between the SCS and DRG-S groups (p > 0.05). Nerve conduction velocities of the common peroneal, peroneal, superficial peroneal, and tibial nerves were significantly improved at 6 and 12 months compared with the preoperative period in both the SCS and PND groups (p < 0.05). However, at 6 and 12 months, there was no difference in HbA1c between the two groups (p > 0.05).

Conclusion

Both SCS and DRG-S significantly improved pain, quality of life, and lower extremity nerve conduction velocity in patients with PDPN, and there was no difference between the two treatments at 12 months.

Case report: Dorsal root ganglion stimulator lead fracture

BACKGROUND

One of the unique advances in neuromodulation for chronic pain has been spinal cord stimulators (SCS) and dorsal root ganglion stimulators (DRG-S). These devices have aided in conditions such as neuropathic pain, complex regional pain syndromes, failed back surgery, and peripheral neuropathies. With these benefits, however, complications from implantable stimulators have included lead fractures and migration. The authors reviewed a lead migration, kinking, and subsequent fracture event involving a patient with complex regional pain syndrome (CRPS) II, who was treated with a DRG-S.

CASE PRESENTATION

The case report follows this patient, from their past medical history to assessment of appropriate qualifications for neuromodulation, to successful surgical placement, to follow-up care. The authors further monitored assessment of inefficacy of pain relief, and identification of lead migration and kinking through imaging. In the process of removal, due to lead stress, lead fracturing occurred. After lead removal, the leads were fully replaced, and the patient was followed up and experienced improved pain relief.

CONCLUSION

The case report assesses probable mechanisms of lead fracture and considerations for physicians for future assessment and triage of neuromodulation efficacy.

Mechanisms of Action of Dorsal Root Ganglion Stimulation

The dorsal root ganglion (DRG) serves as a pivotal site for managing chronic pain through dorsal root ganglion stimulation (DRG-S). In recent years, the DRG-S has emerged as an attractive modality in the armamentarium of neuromodulation therapy due to its accessibility and efficacy in alleviating chronic pain refractory to conventional treatments. Despite its therapeutic advantages, the precise mechanisms underlying DRG-S-induced analgesia remain elusive, attributed in part to the diverse sensory neuron population within the DRG and its modulation of both peripheral and central sensory processing pathways. Emerging evidence suggests that DRG-S may alleviate pain by several mechanisms, including the reduction of nociceptive signals at the T-junction of sensory neurons, modulation of pain gating pathways within the dorsal horn, and regulation of neuronal excitability within the DRG itself. However, elucidating the full extent of DRG-S mechanisms necessitates further exploration, particularly regarding its supraspinal effects and its interactions with cognitive and affective networks. Understanding these mechanisms is crucial for optimizing neurostimulation technologies and improving clinical outcomes of DRG-S for chronic pain management. This review provides a comprehensive overview of the DRG anatomy, mechanisms of action of the DRG-S, and its significance in neuromodulation therapy for chronic pain.

Burst Spinal Cord Stimulation as Compared With L2 Dorsal Root Ganglion Stimulation in Pain Relief for Nonoperated Discogenic Low Back Pain: Analysis of Two Prospective Studies

INTRODUCTION

Chronic discogenic low back pain (CD-LBP) is caused by degenerated disks marked by neural and vascular ingrowth. Spinal cord stimulation (SCS) has been shown to be effective for pain relief in patients who are not responsive to conventional treatments. Previously, the pain-relieving effect of two variations of SCS has been evaluated in CD-LBP: Burst SCS and L2 dorsal root ganglion stimulation (DRGS). The aim of this study is to compare the effectivity in pain relief and pain experience of Burst SCS with that of conventional L2 DRGS in patients with CD-LBP.

MATERIALS AND METHODS

Subjects were implanted with either Burst SCS (n = 14) or L2 DRGS with conventional stimulation (n = 15). Patients completed the numeric pain rating score (NRS) for back pain and Oswestry disability index (ODI) and EuroQoL 5D (EQ-5D) questionnaires at baseline, and at three, six, and 12 months after implantation. Data were compared between time points and between groups.

RESULTS

Both Burst SCS and L2 DRGS significantly decreased NRS, ODI, and EQ-5D scores as compared with baseline. L2 DRGS resulted in significantly lower NRS scores at 12 months and significantly increased EQ-5D scores at six and 12 months.

CONCLUSIONS

Both L2 DRGS and Burst SCS resulted in reduction of pain and disability, and increased quality of life in patients with CD-LBP. L2 DRGS provided significantly increased pain relief and improvement in quality of life when compared with Burst SCS.

CLINICAL TRIAL REGISTRATION

The clinical trial registration numbers for the study are NCT03958604 and NL54405.091.15.

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.

[Neurosurgical treatment of postherpetic neuralgia]

Postherpetic neuralgia is a chronic and debilitating condition that can occur following an episode of herpes zoster (shingles). It is characterized by severe, persistent pain in the area where the shingles rash occurred. While various treatment approaches exist, including medications and non-invasive therapies, some cases of postherpetic neuralgia may require neurosurgical intervention. Neurosurgical treatment options for postherpetic neuralgia aim to alleviate the pain by targeting the affected nerves or neural pathways. One common approach is spinal cord stimulation (SCS). In SCS, electrodes are implanted along the spinal cord, and electrical impulses are delivered to interfere with the transmission of pain signals. This technique can modulate pain perception and significantly reduce the intensity and frequency of postherpetic neuralgia symptoms. Neurosurgical treatment of postherpetic neuralgia is typically considered when conservative measures have failed to provide sufficient relief. However, it is crucial for patients to undergo a comprehensive evaluation and consultation with a neurosurgeon to determine the most appropriate treatment approach based on their specific condition and medical history. The risks, benefits, and potential outcomes of neurosurgical interventions should be carefully discussed between the patient and their healthcare provider to make an informed decision.

The Impact of Dorsal Root Ganglion Stimulation on Pain Levels and Functionality in Patients With Chronic Postsurgical Knee Pain

BACKGROUND

Chronic postsurgical pain is a considerable source of disabling neuropathic pain. Rates of knee replacement surgeries are increasing, and many patients report chronic postsurgical pain in their wake. When conventional therapies prove ineffective, neuromodulation options such as dorsal root ganglion stimulation (DRGS) may be used. However, little is known about the effect of DRGS on improvements in quantitative functional outcome parameters.

MATERIALS AND METHODS

In a prospective observational study at two pain centers, patients with chronic postsurgical knee pain underwent implantation with a DRGS system after an interdisciplinary multimodal pain program. Ratings of pain, mood, quality of life, and function were captured at baseline and through 12 months of treatment. Quantitative measures (range of motion, walking distance, and pain medication usage) were also recorded.

RESULTS

Visual analog scale ratings of pain decreased from 8.6 to 3.0 (p < 0.0001; N = 11), and other pain measures agreed. Quality of life on the 36-Item Short Form Health Survey questionnaire improved from 69.3 to 87.6 (p < 0.0001), whereas the improvement in depression ratings was nonsignificant. International Knee Documentation Committee questionnaire ratings of function improved from 27.7 to 51.7 (p < 0.0001), which aligned with other functional measures. On average, knee range of motion improved by 24.5°, and walking distance dramatically increased from 125 meters to 1481. Cessation of opioids, antidepressants, and/or anticonvulsants was achieved by 73% of participants.

CONCLUSIONS

Both subjective-based questionnaire and quantitative examination-based variables were in broad agreement on the value of DRGS in improving functionality and chronic postsurgical pain in the knee. Although this finding is limited by the small sample size, this intervention may have utility in the many cases in which pain becomes problematic after orthopedic knee surgery.

Spinal cord stimulation for visceral pain associated with medullary sponge kidney

Chronic pain is a common reason for which people in the USA seek medical care. It is linked to opioid consumption, anxiety and a reduction in quality of life. Over the past 50 years, spinal cord stimulation (SCS) has evolved as a safe and efficacious treatment for chronic pain etiologies. The authors present the first known case of SCS for pain due to medullary sponge kidney disease. This report adds to the growing body of literature supporting the use of SCS for treating visceral organ pain, while also highlighting the utility of ventral lead placement for treating visceral pain. As SCS utilization increases, it is expected that there will be a decrease in opioid consumption, and this will help us contain the opioid epidemic.

Current Neurostimulation Therapies for Chronic Pain Conditions

PURPOSE OF REVIEW

Neurostimulation treatment options have become more commonly used for chronic pain conditions refractory to these options. In this review, we characterize current neurostimulation therapies for chronic pain conditions and provide an analysis of their effectiveness and clinical adoption. This manuscript will inform clinicians of treatment options for chronic pain.

RECENT FINDINGS

Non-invasive neurostimulation includes transcranial direct current stimulation and repetitive transcranial magnetic stimulation, while more invasive options include spinal cord stimulation (SCS), peripheral nerve stimulation (PNS), dorsal root ganglion stimulation, motor cortex stimulation, and deep brain stimulation. Developments in transcranial direct current stimulation, repetitive transcranial magnetic stimulation, spinal cord stimulation, and peripheral nerve stimulation render these modalities most promising for the alleviating chronic pain. Neurostimulation for chronic pain involves non-invasive and invasive modalities with varying efficacy. Well-designed randomized controlled trials are required to delineate the outcomes of neurostimulatory modalities more precisely.

Treatment of pain in length-dependent peripheral neuropathy with the use of spinal cord stimulation: a systematic review

BACKGROUND

Chronic intractable pain from peripheral neuropathy is a debilitating condition that might not respond to conventional medical management and pharmacotherapy. The primary objective of this systematic review was to assess change (or reduction) in pain intensity in patients with length-dependent peripheral neuropathy after spinal cord stimulation (SCS) therapy.

METHODS

This systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. The primary outcome was change (or reduction) in pain intensity after 12 months of SCS therapy compared with baseline in participants with length-dependent peripheral neuropathy. Secondary outcomes included change in pain intensity after 6 months and change in opioid consumption after 12 months. The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) guidelines were used to appraise the quality of evidence.

RESULTS

Nineteen studies consisting of 376 participants who underwent SCS implantation met the inclusion criteria. Qualitative synthesis revealed that all eligible studies reported a significant improvement in pain intensity after 12 months of SCS therapy as compared with baseline. Mean differences with 95% confidence intervals were calculated for 4 studies, all of which achieved the minimal clinically important difference for change in pain intensity at 12 months. The GRADE quality of evidence for this outcome was appraised as very low quality.

CONCLUSION

This systematic review highlights that SCS could lead to significant improvement in pain intensity for length-dependent peripheral neuropathy, although future well-powered randomized controlled trials are warranted to increase the certainty of evidence in this finding.

STUDY REGISTRATION

PROSPERO (https://www.crd.york.ac.uk/PROSPERO/) ID: CRD42022377572.

Neuromodulation for Peripheral Nerve Regeneration: Systematic Review of Mechanisms and In Vivo Highlights

While denervation can occur with aging, peripheral nerve injuries are debilitating and often leads to a loss of function and neuropathic pain. Although injured peripheral nerves can regenerate and reinnervate their targets, this process is slow and directionless. There is some evidence supporting the use of neuromodulation to enhance the regeneration of peripheral nerves. This systematic review reported on the underlying mechanisms that allow neuromodulation to aid peripheral nerve regeneration and highlighted important in vivo studies that demonstrate its efficacy. Studies were identified from PubMed (inception through September 2022) and the results were synthesized qualitatively. Included studies were required to contain content related to peripheral nerve regeneration and some form of neuromodulation. Studies reporting in vivo highlights were subject to a risk of bias assessment using the Cochrane Risk of Bias tool. The results of 52 studies indicate that neuromodulation enhances natural peripheral nerve regeneration processes, but still requires other interventions (e.g., conduits) to control the direction of reinnervation. Additional human studies are warranted to verify the applicability of animal studies and to determine how neuromodulation can be optimized for the greatest functional restoration.

Narrative review of current neuromodulation modalities for spinal cord injury

Neuromodulation is a developing field of medicine that includes a vast array of minimally invasive and non-invasive therapies including transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), vagus nerve stimulation (VNS), peripheral nerve stimulation, and spinal cord stimulation (SCS). Although the current literature surrounding the use of neuromodulation in managing chronic pain is abundant, there is an insufficient amount of evidence specifically regarding neuromodulation in patients with spinal cord injury (SCI). Given the pain and functional deficits that these patients face, that are not amenable to other forms conservative therapy, the purpose of this narrative review is to examine and assess the use of various neuromodulation modalities to manage pain and restore function in the SCI population. Currently, high-frequency spinal cord stimulation (HF-SCS) and burst spinal cord stimulation (B-SCS) have been shown to have the most promising effect in improving pain intensity and frequency. Additionally, dorsal root ganglion stimulation (DRG-S) and TMS have been shown to effectively increase motor responses and improve limb strength. Although these modalities carry the potential to enhance overall functionality and improve a patient's degree of disability, there is a lack of long-term, randomized-controlled trials in the current space. Additional research is warranted to further support the clinical use of these emerging modalities to provide improved pain management, increased level of function, and ultimately an overall better quality of life in the SCI population.

Thoracic dorsal root ganglion stimulation reduces acute myocardial ischemia induced ventricular arrhythmias

Background

Dorsal root ganglion stimulation (DRGS) may serve as a novel neuromodulation strategy to reduce cardiac sympathoexcitation and ventricular excitability.

Objective

In this pre-clinical study, we investigated the effectiveness of DRGS on reducing ventricular arrhythmias and modulating cardiac sympathetic hyperactivity caused by myocardial ischemia.

Methods

Twenty-three Yorkshire pigs were randomized to two groups, which was control LAD ischemia-reperfusion (CONTROL) or LAD ischemia-reperfusion + DRGS (DRGS) group. In the DRGS group (n = 10), high-frequency stimulation (1 kHz) at the second thoracic level (T2) was initiated 30 min before ischemia and continued throughout 1 h of ischemia and 2 h of reperfusion. Cardiac electrophysiological mapping and Ventricular Arrhythmia Score (VAS) were assessed, along with evaluation of cFos expression and apoptosis in the T2 spinal cord and DRG.

Results

DRGS decreased the magnitude of activation recovery interval (ARI) shortening in the ischemic region (CONTROL: -201 ± 9.8 ms, DRGS: -170 ± 9.4 ms, p = 0.0373) and decreased global dispersion of repolarization (DOR) at 30 min of myocardial ischemia (CONTROL: 9546 ± 763 ms2, DRGS: 6491 ± 636 ms2, p = 0.0076). DRGS also decreased ventricular arrhythmias (VAS-CONTROL: 8.9 ± 1.1, DRGS: 6.3 ± 1.0, p = 0.038). Immunohistochemistry studies showed that DRGS decreased % cFos with NeuN expression in the T2 spinal cord (p = 0.048) and the number of apoptotic cells in the DRG (p = 0.0084).

Conclusion

DRGS reduced the burden of myocardial ischemia-induced cardiac sympathoexcitation and has a potential to be a novel treatment option to reduce arrhythmogenesis.

The Effectiveness of Various Types of Electrical Stimulation of the Spinal Cord for Chronic Pain in Patients with Postherpetic Neuralgia: A Literature Review

Introduction

Postherpetic neuralgia (PHN) is a severe condition that remains a challenge to treat. Spinal cord stimulation (SCS) is used in cases of insufficient efficacy of conservative treatment. However, in contrast to many other neuropathic pain syndromes, there is a huge problem in reaching long-term stable pain relief in patients with PHN using conventional tonic SCS. The objective of this article was to present a review of the current management strategies of PHN, their efficacy, and safety.

Materials and Methods

We searched for articles containing the keywords "spinal cord stimulation AND postherpetic neuralgia," "high-frequency stimulation AND postherpetic neuralgia," "burst stimulation AND postherpetic neuralgia" and "dorsal root ganglion stimulation AND postherpetic neuralgia" in Pubmed, Web of Science, and Scopus databases. The search was limited to human studies published in the English language. There were no publication period limitations. Bibliographies and references of selected publications on neurostimulation for PHN were further manually screened. The full text of each article was studied once the abstract was analyzed by the searching reviewer and found appropriate. The initial search yielded 115 articles. Initial screening based on abstract and title allowed us to exclude 29 articles (letters, editorials, and conference abstracts). The full-text analysis allowed us to exclude another 74 articles (fundamental research articles, research utilizing animal subjects, and systemic and nonsystemic reviews) and results of PHN treatment presented with other conditions, leaving 12 articles for the final bibliography.

Results

12 articles reporting on the treatment of 134 patients with PHN were analyzed, with a disproportionally large amount of traditional SCS treatment than that to alternative SCS: DRGS (13 patients), burst SCS (1 patient), and high-frequency SCS (2 patients). Long-term pain relief was achieved in 91 patients (67.9%). The mean VAS score improvement was 61.4% with a mean follow-up time of 12.85 months. Although the number of patients in alternative SCS studies was very limited, almost all of them showed good responses to therapy with more than 50% VAS improvement and reduction of analgesic dosage. The article contains a review analysis of 12 articles concerning the current methods of treatment for postherpetic neuralgia including conservative treatment, spinal cord stimulation, and novel neuromodulation strategies. Available information on the pathophysiology of PHN and the effect or stimulation on its course, together with a number of technical nuances concerning various types of neurostimulation are also elucidated in this article. A number of alternative invasive treatments of PHN are also discussed.

Conclusions

Spinal cord stimulation is an established treatment option for patients with pharmacologically resistant PHN. High-frequency stimulation, burst stimulation, and dorsal root ganglion stimulation are promising options in the management of PHN due to the absence of paresthesias which can be painful for patients with PHN. But more research is still required to recommend the widespread use of these new methods.

Dorsal root ganglia control nociceptive input to the central nervous system

Accumulating observations suggest that peripheral somatosensory ganglia may regulate nociceptive transmission, yet direct evidence is sparse. Here, in experiments on rats and mice, we show that the peripheral afferent nociceptive information undergoes dynamic filtering within the dorsal root ganglion (DRG) and suggest that this filtering occurs at the axonal bifurcations (t-junctions). Using synchronous in vivo electrophysiological recordings from the peripheral and central processes of sensory neurons (in the spinal nerve and dorsal root), ganglionic transplantation of GABAergic progenitor cells, and optogenetics, we demonstrate existence of tonic and dynamic filtering of action potentials traveling through the DRG. Filtering induced by focal application of GABA or optogenetic GABA release from the DRG-transplanted GABAergic progenitor cells was specific to nociceptive fibers. Light-sheet imaging and computer modeling demonstrated that, compared to other somatosensory fiber types, nociceptors have shorter stem axons, making somatic control over t-junctional filtering more efficient. Optogenetically induced GABA release within DRG from the transplanted GABAergic cells enhanced filtering and alleviated hypersensitivity to noxious stimulation produced by chronic inflammation and neuropathic injury in vivo. These findings support "gating" of pain information by DRGs and suggest new therapeutic approaches for pain relief.

Lumbar trans-spinal direct current stimulation: A modeling-experimental approach to dorsal root ganglia stimulation

Introduction

The excitability of spinal motor neurons (MN) can be altered through subthreshold currents, such as transcutaneous spinal direct-current stimulation (tsDCS). Current evidence shows that tsDCS can interfere with ascending somatosensory pathways and lower motor neurons' (LMN) excitability, which points to its therapeutic potential for repairing altered spinal responses. We aim to define the best tsDCS montage for maximizing the electric field (E-field) in the lumbar spinal cord (L-SC) by computer modeling; and to apply this montage to measure the effect on LMN excitability and somatosensory evoked potentials (SSEP).

Methods

A human volume conductor model was obtained from an available database. The E-field distribution was calculated considering three different electrode settings aiming at maximizing the field at L-SC and right dorsal root ganglia (DRG). The best electrode setting was then selected and applied in a blind crossover pseudo-randomized study including 14 subjects. tsDCS was delivered for 15 min (cathodal vs. sham) over L2 vertebra level (4 mA, 144 mC/cm²), and its effect on F-waves, H-reflex (including homosynaptic depression, HD) and SSEPs was investigated in the lower limbs.

Results

All simulated montages showed higher current density and E-field magnitudes between the electrodes (>0.15 V/m), with a major longitudinal component and with rostral-caudal direction. The induced E-field involved the sensory ganglia and was maximum in the right T8-left L2 montage, which was the one selected for the experimental protocol. We disclosed a statistically significant increase of the H-reflex amplitude at 0.1 Hz, after cathodal tsDCS (c-tsDCS) on both sides. No other significant change was observed.

Discussion

Our results can suggest the c-tsDCS applied to the L-SC and DRG can modulate synaptic efficiency increasing lower motor neurons response to Ia fibers excitation. The possible implications of our findings for treating clinical conditions will be addressed in future studies.

Advances in Pain Medicine: a Review of New Technologies

Purpose of Review

This narrative review highlights the interventional musculoskeletal techniques that have evolved in recent years.

Recent Findings

The recent progress in pain medicine technologies presented here represents the ideal treatment of the pain patient which is to provide personalized care. Advances in pain physiology research and pain management technologies support each other concurrently.

Summary

As new technologies give rise to new perspectives and understanding of pain, new research inspires the development of new technologies

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.

Diagnostic Roots Radiofrequency Sensory Stimulation Looking for Symptomatic Injured Roots in Multiple Lumbar Stenosis

Objective

We present how to perform radiofrequency sensory stimulation (RFSS) and whether RFSS could be helpful in identifying symptomatic injured roots in multilevel lumbar stenosis.

Methods

Consecutive patients who underwent RFSS from 2010 to 2012 were enrolled. To identify pathologic lesions, RFSS was performed for suspicious roots, as determined using lumbar magnetic resonance imaging (MRI). The RFSS procedure resembled transforaminal root block. During RFSS of the suspicious root, patients could indicate whether stimulation induced their usual pain and/or sensory changes and could indicate whether the same leg area was affected. The number of possible symptomatic roots on MRI was evaluated before and after RFSS. Based on the RFSS results, we confirmed the presence of symptomatic nerve root(s) and performed surgical decompression. Surgical results, such as numeric rating scale (NRS) scores for low back pain (LBP) and leg pain (LP), and Oswestry disability index (ODI), were evaluated.

Results

Ten patients were enrolled in the study. Their mean age was 70.1±9.7 years. Clinically, NRS-LBP, NRS-LP, and ODI before surgery were 5.1%, 7.5%, and 53.2%, respectively. The mean number of suspicious roots was 2.6±0.8. After RFSS, the mean number of symptomatic roots was 1.6±1.0. On average, 1.4 lumbar segments were decompressed. The follow-up period was 35.3±12.8 months. At the last follow-up, NRS-LBP, NRS-LP, and ODI were 3.1%, 1.5%, and 35.3%, respectively. There was no recurrence or need for further surgical treatment for lumbar stenosis.

Conclusion

RFSS is a potentially helpful diagnostic tool for verifying and localizing symptomatic injured root lesions, particularly in patients with multilevel spinal stenosis.

Dorsal root ganglion stimulation produces differential effects on action potential propagation across a population of biophysically distinct C-neurons

Dorsal root ganglion stimulation (DRGS) is a neurostimulation therapy used to manage chronic pain that does not respond to conventional therapies. Unfortunately, not all patients receive sufficient pain relief from DRGS, leaving them with few other treatment options. Presently, our understanding of the mechanisms of action of DRGS is incomplete, preventing us from determining why some patients do not receive analgesia from the therapy. One hypothesis suggests that DRGS augments the filtering of action potentials (APs) at the T-junction of nociceptive C-neurons. To test this hypothesis, we utilized a computational modeling approach in which we developed a population of one thousand biophysically distinct C-neuron models which each produced electrophysiological characteristics (e.g., AP height, AP duration) reported in previous experimental studies. We used this population of model C-neurons to study how morphological and electrophysiological characteristics affected the propagation of APs through the T-junction. We found that trains of APs can propagate through the T-junction in the orthodromic direction at a higher frequency than in the antidromic direction due to the decrease in axonal diameter from the peripheral to spinal axon. Including slow outward conductances in the axonal compartments near the T-junction reduced following frequencies to ranges measured experimentally. We next used the population of C-neuron models to investigate how DRGS affected the orthodromic propagation of APs through the T-junction. Our data suggest that suprathreshold DRGS augmented the filtering of APs at the T-junction of some model C-neurons while increasing the activity of other model C-neurons. However, the stimulus pulse amplitudes required to induce activity in C-neurons (i.e., several mA) fell outside the range of stimulation pulse amplitudes used clinically (i.e., typically ≤1 mA). Furthermore, our data suggest that somatic GABA currents activated directly or indirectly by the DRGS pulse may produce diverse effects on orthodromic AP propagation in C-neurons. These data suggest DRGS may produce differential effects across a population of C-neurons and indicate that understanding how inherent biological variability affects a neuron's response to therapeutic electrical stimulation may be helpful in understanding its mechanisms of action.

Dorsal Root Ganglion Stimulation for Lower Extremity Neuropathic Pain Syndromes: An Evidence-Based Literature Review

Dorsal root ganglion stimulation (DRG-S) is a form of selective neuromodulation therapy that targets the dorsal root ganglion. DRG-S offers analgesia in a variety of chronic pain conditions and is approved for treatment of complex regional pain syndrome (CRPS) by the US Food and Drug Administration (FDA). There has been increasing utilization of DRG-S to treat various neuropathic pain syndromes of the lower extremity, although evidence remains limited to one randomized controlled trial and 39 observational studies. In this review, we appraised the current evidence for DRG-S in the treatment of lower extremity neuropathic pain using the Grading of Recommendations, Assessment, Development, and Evaluations (GRADE) criteria. The primary outcome was change in pain intensity after DRG-S compared to baseline. We stratified presentation of results based of type of neuropathy (CRPS, painful diabetic neuropathy, mononeuropathy, polyneuropathy) as well as location of neuropathy (hip, knee, foot). Future powered randomized controlled trials with homogeneous participants are warranted.

Stimulation of the dorsal root ganglion using an Injectrode®

Objective. The goal of this work was to compare afferent fiber recruitment by dorsal root ganglion (DRG) stimulation using an injectable polymer electrode (Injectrode^®) and a more traditional cylindrical metal electrode.Approach. We exposed the L6 and L7 DRG in four cats via a partial laminectomy or burr hole. We stimulated the DRG using an Injectrode or a stainless steel (SS) electrode using biphasic pulses at three different pulse widths (80, 150, 300μs) and pulse amplitudes spanning the range used for clinical DRG stimulation. We recorded antidromic evoked compound action potentials (ECAPs) in the sciatic, tibial, and common peroneal nerves using nerve cuffs. We calculated the conduction velocity of the ECAPs and determined the charge-thresholds and recruitment rates for ECAPs from Aα, Aβ, and Aδfibers. We also performed electrochemical impedance spectroscopy measurements for both electrode types.Main results. The ECAP thresholds for the Injectrode did not differ from the SS electrode across all primary afferents (Aα, Aβ, Aδ) and pulse widths; charge-thresholds increased with wider pulse widths. Thresholds for generating ECAPs from Aβfibers were 100.0 ± 32.3 nC using the SS electrode, and 90.9 ± 42.9 nC using the Injectrode. The ECAP thresholds from the Injectrode were consistent over several hours of stimulation. The rate of recruitment was similar between the Injectrodes and SS electrode and decreased with wider pulse widths.Significance. The Injectrode can effectively excite primary afferents when used for DRG stimulation within the range of parameters used for clinical DRG stimulation. The Injectrode can be implanted through minimally invasive techniques while achieving similar neural activation to conventional electrodes, making it an excellent candidate for future DRG stimulation and neuroprosthetic applications.

Dorsal root ganglion stimulation for treatment of central poststroke pain in the lower extremity after medullary infarction

ABSTRACT

We report a case of relief in central poststroke pain of the lower extremity by stimulation of the dorsal root ganglion (DRG). Central poststroke pain is a poorly understood and even more poorly managed condition that can greatly impact the quality of life. To the best of our knowledge, this investigation is the first to describe the successful treatment of previously intractable chronic pain caused by stroke using DRG stimulation. Noting the anatomical structures and the physiological function, the efficacy of DRG stimulation in central poststroke pain could be explained in a neurophysiological manner. This clinical observation successfully builds on the existing understanding around the pathophysiology of central pain and offers the possibility of nondrug therapy for the treatment of this often refractory chronic pain syndrome.

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.

Analgesic Effects of Tonic and Burst Dorsal Root Ganglion Stimulation in Rats With Painful Tibial Nerve Injury

OBJECTIVES

Dorsal root ganglion (DRG) stimulation is effective in treating chronic pain. While burst stimulation has been proven to enhance the therapeutic efficacy in spinal cord stimulation, currently only a tonic stimulation waveform is clinically used in DRG stimulation. We hypothesized that burst DRG stimulation might also produce analgesic effect in a preclinical neuropathic pain model. We evaluated both the therapeutic effects of burst DRG stimulation and the possible effects of DRG stimulation upon inflammation within the DRG in a preclinical neuropathic pain model.

MATERIALS AND METHODS

Rats received either a painful tibial nerve injury or sham surgery. Analgesic effects of DRG stimulation were evaluated by testing a battery of evoked pain-related behaviors as well as measuring the positive affective state associated with relief of spontaneous pain using conditioned place preference. Histological evidence for neuronal trauma or neuroinflammation was evaluated.

RESULTS

All of the waveforms tested (20 Hz-tonic, 20 Hz-burst, and 40 Hz-burst) have similar analgesic effects in sensory tests and conditioned place preference. Long-term DRG stimulation for two weeks does not change DRG expression of markers for nerve injury and neuroinflammation.

CONCLUSIONS

DRG stimulation using burst waveform might be also suitable for treating neuropathic pain.

Dorsal Root Ganglion Stimulation for Chronic Groin Pain: A Review

INTRODUCTION

Chronic neuropathic groin pain develops in a significant number of postsurgical patients; however, multiple etiologies have been identified, and this makes it a challenging condition to treat. While treatment often involves a multimodal approach, advancements in neuromodulation technology, particularly dorsal root ganglion (DRG) stimulation, have benefited patients plagued by chronic pain refractory to standard treatment modalities. Our goal was to provide a definitive source of information for interventional pain physicians regarding groin pain and the use of DRG stimulation for its treatment.

MATERIALS AND METHODS

In this narrative review, we provide an overview of groin pain and discuss potential pain generators. We also outline appropriate treatment options with particular interest on DRG stimulation. Lastly, we provide a narrative review of the published literature regarding DRG stimulation for chronic groin pain from a variety of etiologies.

CONCLUSION

DRG stimulation has emerged as an alternative neuromodulatory technique for patients with chronic groin pain. While previous studies suggest substantial sustained pain relief with DRG stimulation in this patient population, prospective randomized controlled studies are necessary before formal recommendations can be made.

The Dorsal Root Ganglion as a Novel Neuromodulatory Target to Evoke Strong and Reproducible Motor Responses in Chronic Motor Complete Spinal Cord Injury: A Case Series of Five Patients

OBJECTIVES

Current strategies for motor recovery after spinal cord injury (SCI) aim to facilitate motor performance through modulation of afferent input to the spinal cord using epidural electrical stimulation (EES). The dorsal root ganglion (DRG) itself, the first relay station of these afferent inputs, has not yet been targeted for this purpose. The current study aimed to determine whether DRG stimulation can facilitate clinically relevant motor response in motor complete SCI.

MATERIALS AND METHODS

Five patients with chronic motor complete SCI were implanted with DRG leads placed bilaterally on level L4 during five days. Based on personalized stimulation protocols, we aimed to evoke dynamic (phase 1) and isotonic (phase 2) motor responses in the bilateral quadriceps muscles. On days 1 and 5, EMG-measurements (root mean square [RMS] values) and clinical muscle force measurements (MRC scoring) were used to measure motor responses and their reproducibility.

RESULTS

In all patients, DRG-stimulation evoked significant phase 1 and phase 2 motor responses with an MRC ≥4 for all upper leg muscles (rectus femoris, vastus lateralis, vastus medialis, and biceps femoris) (p < 0.05 and p < 0.01, respectively), leading to a knee extension movement strong enough to facilitate assisted weight bearing. No significant differences in RMS values were observed between days 1 and 5 of the study, indicating that motor responses were reproducible.

CONCLUSION

The current paper provides first evidence that bilateral L4 DRG stimulation can evoke reproducible motor responses in the upper leg, sufficient for assisted weight bearing in patients with chronic motor complete SCI. As such, a new target for SCI treatment has surfaced, using existing stimulation devices, making the technique directly clinically accessible.

Dorsal root ganglion stimulation lead fractures: potential mechanisms and ways to avoid

Dorsal root ganglion stimulation (DRGS) therapy is a rapidly emerging tool being used by pain physicians in the treatment of chronic pain. Complex regional pain syndrome (CRPS), a debilitating disease whose mechanism is still has yet to be fully elucidated, is a common pathology targeted by DRGS therapy, often better results than traditional spinal cord stimulation. DRGS therapy, however, is not bereft of complications. Lead migration and fracture are two examples in particular that are among the most common of these complications. The authors report an unusual case of lost efficacy due to lead fractures in patients with CRPS treated with DRGS. The case report narrates identification, management and probable mechanism of DRGS lead fracture. The structural instability of DRGS leads can yield distressing symptoms at any point during the therapy, and physicians should be cognisant of the complications of DRGS therapy.

Analgesic dorsal root ganglionic field stimulation blocks conduction of afferent impulse trains selectively in nociceptive sensory afferents

Increased excitability of primary sensory neurons after peripheral nerve injury may cause hyperalgesia and allodynia. Dorsal root ganglion field stimulation (GFS) is effective in relieving clinical pain associated with nerve injury and neuropathic pain in animal models. However, its mechanism has not been determined. We examined effects of GFS on transmission of action potentials (APs) from the peripheral to central processes by in vivo single-unit recording from lumbar dorsal roots in sham injured rats and rats with tibial nerve injury (TNI) in fiber types defined by conduction velocity. Transmission of APs directly generated by GFS (20 Hz) in C-type units progressively abated over 20 seconds, whereas GFS-induced Aβ activity persisted unabated, while Aδ showed an intermediate pattern. Activity generated peripherally by electrical stimulation of the sciatic nerve and punctate mechanical stimulation of the receptive field (glabrous skin) was likewise fully blocked by GFS within 20 seconds in C-type units, whereas Aβ units were minimally affected and a subpopulation of Aδ units was blocked. After TNI, the threshold to induce AP firing by punctate mechanical stimulation (von Frey) was reduced, which was reversed to normal during GFS. These results also suggest that C-type fibers, not Aβ, mainly contribute to mechanical and thermal hypersensitivity (von Frey, brush, acetone) after injury. Ganglion field stimulation produces use-dependent blocking of afferent AP trains, consistent with enhanced filtering of APs at the sensory neuron T-junction, particularly in nociceptive units.

Conventional Dorsal Root Ganglion Stimulation in an Experimental Model of Painful Diabetic Peripheral Neuropathy: A Quantitative Immunocytochemical Analysis of Intracellular γ-Aminobutyric Acid in Dorsal Root Ganglion Neurons

Background and Objective

The sensory cell somata in the DRG contain all equipment necessary for extensive GABAergic signaling and are able to release GABA upon depolarization. With this study, we hypothesize that pain relief induced by conventional dorsal root ganglion stimulation (Con‐DRGS) in animals with experimental painful diabetic peripheral neuropathy is related to the release of GABA from DRG neurons. With use of quantitative immunocytochemistry, we hypothesize DRGS to result in a decreased intensity of intracellular GABA‐immunostaining in DRG somata.

Materials and Methods

Female Sprague‐Dawley rats (n = 31) were injected with streptozotocin (STZ) in order to induce Diabetes Mellitus. Animals that developed neuropathic pain after four weeks (Von Frey) were implanted with a unilateral DRGS device at L4 (n = 14). Animals were then stimulated for 30 min with Con‐DRGS (20 Hz, pulse width = 0.2 msec, amplitude = 67% of motor threshold, n = 8) or Sham‐DRGS (n = 6), while pain behavior (von Frey) was measured. DRGs were then collected and immunostained for GABA, and a relation to size of sensory cell soma diameter (small: 12–26 μm, assumed to be C‐fiber related sensory neurons; medium: 26–40 μm, assumed to be Aδ related sensory neurons; and large: 40–54 μm, assumed to be Aβ related sensory neurons) was made.

Results

DRGS treated animals showed significant reductions in STZ‐induced mechanical hypersensitivity. No significant differences in GABA immunostaining intensity per sensory neuron cell soma type (small‐, medium‐, or large‐sized) were noted in DRGs of stimulated (Con‐DRGS) animals versus Sham animals. No differences in GABA immunostaining intensity per sensory cell soma type in ipsi‐ as compared to contralateral DRGs were observed.

Conclusion

Con‐DRGS does not affect the average intracellular GABA immunofluorescence staining intensity in DRG sensory neurons of those animals which showed significant pain reduction. Similarly, no soma size related changes in intracellular GABA immunofluorescence were observed following Con‐DRGS.

Review of complex regional pain syndrome and the role of the neuroimmune axis

Background

Complex regional pain syndrome (CRPS) is a progressive and painful disease of the extremities that is characterized by continuous pain inconsistent with the initial trauma. CRPS is caused by a multi-mechanism process that involves both the peripheral and central nervous system, with a prominent role of inflammation in CRPS pathophysiology. This review examines what is currently known about the CRPS inflammatory and pain mechanisms, as well as the possible impact of neurostimulation therapies on the neuroimmune axis of CRPS.

Study design

A narrative review of preclinical and clinical studies provided an overview of the pain and inflammatory mechanisms in CRPS and addressed the effect of neurostimulation on immunomodulation.

Methods

A systematic literature search was conducted based on the PRISMA guidelines between September 2015 to September 2020. Data sources included relevant literature identified through searches of PubMed, Embase and the Cochrane Database of Systematic Reviews.

Results

Sixteen preclinical and eight clinical studies were reviewed. Preclinical studies identified different mechanisms of pain development in the acute and chronic CRPS phases. Several preclinical and clinical studies investigating inflammatory mechanisms, autoimmunity, and genetic profiles in CRPS, supported a role of neuroinflammation in the pathophysiology of CRPS. The immunomodulatory effects of neurostimulation therapy is still unclear, despite clinical improvement in the CRPS patients.

Conclusions

Increasing evidence supports a role for inflammation and neuroinflammation in CRPS pathophysiology. Preliminary neurostimulation findings, together with the role of (neuro)inflammation in CRPS, seems to provide a compelling rationale for its use in CRPS pain treatment. The possible immunomodulatory effects of neurostimulation opens new therapeutic possibilities, however further research is needed to gain a better understanding of the working mechanisms.

Dorsal Root Ganglion (DRG) and Chronic Pain

Context

Chronic neuropathic pain is a common condition, and up to 11.9% of the population have been reported to suffer from uncontrolled neuropathic pain. Chronic pain leads to significant morbidity, lowered quality of life, and loss of workdays, and thus carries a significant price tag in healthcare costs and lost productivity. dorsal root ganglia (DRG) stimulation has been recently increasingly reported and shows promising results in the alleviation of chronic pain. This paper reviews the background of DRG stimulation, anatomical, and clinical consideration and reviews the clinical evidence to support its use.

Evidence Acquisition

The DRG span the length of the spinal cord and house the neurons responsible for sensation from the periphery. They may become irritated by direct compression or local inflammation. Glial cells in the DRG respond to nerve injury, producing inflammatory markers and contribute to the development of chronic pain, even after the resolution of the original insult. While the underlying mechanism is still being explored, recent studies explored the efficacy of DRG stimulation and neuromodulation for chronic pain treatment.

Results

Several reported cases and a small number of randomized trials were published in recent years, describing different methods of DRG stimulation and neuromodulation with promising results. Though evidence quality is mostly low, these results provide evidence to support the utilization of this technique.

Conclusions

Chronic neuropathic pain is a common condition and carries significant morbidity and impact on the quality of life. Recent evidence supports the use of DRG neuromodulation as an effective technique to control chronic pain. Though studies are still emerging, the evidence appears to support this technique. Further studies, including large randomized trials evaluating DRG modulation versus other interventional and non-interventional techniques, are needed to further elucidate the efficacy of this method. These studies are also likely to inform the patient selection and the course of treatment.

Neuromodulation for Chronic Pelvic Pain: A Single-Institution Experience With a Collaborative Team

BACKGROUND

Secondary to the complex care, involved specialty providers, and various etiologies, chronic pelvic pain patients do not receive holistic care.

OBJECTIVE

To compare our general and neuromodulation cohorts based on referrals, diagnosis, and therapy and describe our neuromodulation patients.

METHODS

A multidisciplinary team was established at our center. The intake coordinator assessed demographics and facilitated care of enrolled patients. Outcomes were compared using minimal clinical important difference of current Numerical Rating Scale (NRS) between patients with neuropathic pain who received neuromodulation and those who did not. The neuromodulation cohort completed outcome metrics at baseline and recent follow-up, including NRS score (best, worst, and current), Oswestry Disability Index (ODI), Beck Depression Inventory, and Pain Catastrophizing Scale.

RESULTS

Over 7 yr, 233 patients were referred to our consortium and 153 were enrolled. A total of 55 patients had neuropathic pain and 44 of those were managed medically. Eleven underwent neuromodulation. A total of 45.5% patients of the neuromodulation cohort were classified as responders by minimal clinically important difference compared to 26.6% responders in the control cohort at most recent follow-up (median 25 and 33 mo, respectively). Outcome measures revealed improvement in NRS at worst (P = .007) and best (P = .025), ODI (P = .014), and Pain Catastrophizing Scale Rumination (P = .043).

CONCLUSION

Eleven percent of patients were offered neuromodulation. There were more responders in the neuromodulation cohort than the conservatively managed neuropathic pain cohort. Neuromodulation patients showed significant improvement at 29 mo in NRS best and worst pain, disability, and rumination. We share our algorithm for patient management.

Peripherally Induced Reconditioning of the Central Nervous System: A Proposed Mechanistic Theory for Sustained Relief of Chronic Pain with Percutaneous Peripheral Nerve Stimulation

Peripheral nerve stimulation (PNS) is an effective tool for the treatment of chronic pain, although its efficacy and utilization have previously been significantly limited by technology. In recent years, purpose-built percutaneous PNS devices have been developed to overcome the limitations of conventional permanently implanted neurostimulation devices. Recent clinical evidence suggests clinically significant and sustained reductions in pain can persist well beyond the PNS treatment period, outcomes that have not previously been observed with conventional permanently implanted neurostimulation devices. This narrative review summarizes mechanistic processes that contribute to chronic pain, and the potential mechanisms by which selective large diameter afferent fiber activation may reverse these changes to induce a prolonged reduction in pain. The interplay of these mechanisms, supported by data in chronic pain states that have been effectively treated with percutaneous PNS, will also be discussed in support of a new theory of pain management in neuromodulation: Peripherally Induced Reconditioning of the Central Nervous System (CNS).

Dorsal Root Ganglion Stimulation for the Treatment of Non-Complex Regional Pain Syndrome Related Chronic Pain Syndromes: A Systematic Review

BACKGROUND

While the majority of indications and approvals for dorsal root ganglion stimulation (DRGS) are for the refractory management of complex regional pain syndrome (CRPS), emerging evidence has suggested that DRGS may be favorably used for a plethora of other chronic pain phenomena. Consequently, we aimed to characterize the use and efficacy of DRGS for these non-CRPS-related chronic pain syndromes.

MATERIALS AND METHODS

A systematic review of clinical studies demonstrating the use of DRGS for non-CRPS-related chronic pain syndromes. The literature search was performed using PubMed, Cochrane Library, and CINAHL plus across August and September 2020.

RESULTS

A total of 28 reports comprising 354 total patients were included in the analysis. Of the chronic pain syndromes presented, axial low back pain, chronic pelvic and groin pain, other peripheral neuropathies, and studies with multiple concomitant pain syndromes, a majority demonstrated >50% mean pain reduction at the time of last follow-up following DRGS. Physical function, quality of life (QOL), and lesser pain medication usage also were repeatedly reported to be significantly improved.

CONCLUSIONS

DRGS continues to lack supportive evidence from well designed, high level studies and recommendations from consensus committee experts. However, we present repeated and consistent evidence from lower level studies showing success with the use of DRGS for various non-CRPS chronic pain syndromes in reducing pain along with increasing function and QOL from one week to three years. Due to such low-level, high bias evidence, we strongly encourage the continuation of high-level studies in order to provide a stronger foundation for the use of DRGS in non-CRPS chronic pain patients. However, it may be reasonable and appropriate to evaluate patients for DRGS candidacy on a case-by-case basis particularly if they manifest focal pain syndromes refractory to noninterventional measures and may not be ideal candidates for other forms of neuromodulation.

Very Low Frequencies Maintain Pain Relief From Dorsal Root Ganglion Stimulation: An Evaluation of Dorsal Root Ganglion Neurostimulation Frequency Tapering

BACKGROUND

Dorsal root ganglion neurostimulation (DRG-S) is effective in treating various refractory chronic pain syndromes. In preclinical studies, DRG-S at very low frequencies (<5 Hz) reduces excitatory output in the superficial dorsal horn. Clinically, we have also observed the effectiveness of DRG-S at low frequencies. We conducted a case series to describe the effect of very low-frequency DRG-S stimulation on clinical outcomes.

MATERIALS AND METHODS

DRG-S for refractory low back pain was initiated at parameters consistent with published values. Thereafter, the stimulation frequency of DRG-S was reduced in a stepwise fashion to the lowest frequency that maintained pain relief. Pain intensity, disability, and general health status data were collected at baseline, prior to initiation of tapering, and at four weeks after each patient's lowest effective stimulation frequency was reached.

RESULTS

After device activation (N = 20), DRG-S frequency was tapered from 16 to 4 Hz over a 4- to 17-week period, reducing charge-per-second by nearly two-thirds. Even so, pain relief was maintained at more than 75%, with consistent findings in the other measures.

CONCLUSION

DRG-S may have utility in treating chronic pain at lower stimulation frequencies than previously recognized. We have previously theorized that the mechanism of action may involve preferential recruitment of low-threshold mechanoreceptor fibers via the endogenous opioid system. Of clinical relevance, lower frequency stimulation maintains DRG-S efficacy regarding improvements in pain, disability, and quality of life. It can extend battery life and may potentially lead to the development of smaller implantable pulse generators.

Dorsal Root Ganglion Stimulation Alleviates Pain-related Behaviors in Rats with Nerve Injury and Osteoarthritis

BACKGROUND

Dorsal root ganglion field stimulation is an analgesic neuromodulation approach in use clinically, but its mechanism is unknown as there is no validated animal model for this purpose. The authors hypothesized that ganglion stimulation is effective in reducing pain-like behaviors in preclinical chronic pain models.

METHODS

The authors provided ganglion stimulation or spinal cord stimulation to rats with traumatic neuropathy (tibial nerve injury), or osteoarthritis induced by intraarticular knee monosodium iodoacetate, or without injury (naïve). Analgesia was evaluated by testing a battery of pain-related reflexive, functional, and affective behaviors.

RESULTS

In rats with nerve injury, multilevel L4 and L5 ganglion stimulation decreased hypersensitivity to noxious mechanical stimulation more (area under curve, -1,447 ± 423 min × % response; n = 12) than single level ganglion stimulation at L4 ([-960 ± 251 min × % response; n = 8; P = 0.012] vs. L4 and L5), and L5 ([-676 ± 295 min × % response; n = 8; P < 0.0001] vs. L4 and L5). Spontaneous pain-like behavior, evaluated by conditioned place preference, responded to single L4 (Pretest [-93 ± 65 s] vs. Test [87 ± 82 s]; P = 0.002; n = 9), L5 (Pretest [-57 ± 36 s] vs. Test [137 ± 73 s]; P = 0.001; n = 8), and multilevel L4 and L5 (Pretest: -81 ± 68 s vs. Test: 90 ± 76 s; P = 0.003; n = 8) ganglion stimulation. In rats with osteoarthritis, multilevel L3 and L4 ganglion stimulation reduced sensitivity to knee motion more (-156 ± 28 min × points; n = 8) than L3 ([-94 ± 19 min × points in knee bend test; n = 7; P = 0.002] vs. L3 and L4) or L4 ([-71 ± 22 min × points; n = 7; P < 0.0001] vs. L3 and L4). Conditioned place preference during osteoarthritis revealed analgesic effectiveness for ganglion stimulation when delivered at L3 (Pretest [-78 ± 77 s] vs. Test [68 ± 136 s]; P = 0.048; n = 9), L4 (Pretest [-96 ± 51 s] vs. Test [73 ± 111 s]; P = 0.004; n = 9), and L3 and L4 (Pretest [-69 ± 52 s; n = 7] vs. Test [55 ± 140 s]; P = 0.022; n = 7).

CONCLUSIONS

Dorsal root ganglion stimulation is effective in neuropathic and osteoarthritic preclinical rat pain models with peripheral pathologic origins, demonstrating effectiveness of ganglion stimulation in a placebo-free setting and justifying this model as a suitable platform for mechanistic studies.

Dorsal Root Ganglion Stimulation for Chronic Postoperative Pain Following Thoracic Surgery: A Pilot Study

OBJECTIVES

Post-thoracotomy pain syndrome (PTPS) is defined as persistent pain following a thoracotomy and has an incidence of 21-61%. Dorsal root ganglion stimulation (DRG-S) is a form of neuromodulation that modulates pain signal transmission to the spinal cord. The aims of this study were to investigate the efficacy of DRG-S for the management of PTPS and to assess the role of thoracic paravertebral blocks (t-PVB) as a tool for prediction of success of DRG-S.

MATERIALS AND METHODS

In this prospective study, we included all patients undergoing thoracic surgery, with PTPS not responding to pharmacotherapy and treated with DRG-S from September 2018 to February 2019. t-PVB followed by a percutaneous DRG-S trial was performed on all patients. Pain intensity was assessed through a numeric rating scale (NRS) and Douleur Neuropathique en 4 Questions (DN4) at baseline, post-trial, at 14 days, 90 days, and at one year after DRG-S implantation. Data summarized as continuous variables were expressed as means and standard deviations (SDs), and categorical variables were expressed as raw numbers and percentages.

RESULTS

Four patients out of 51 who underwent thoracic surgery at our institution surveyed were included (mean age ± SD, 56 ± 16 years old). Mean NRS and DN4 were, respectively, 7.2 ± 0.96 SD and 8.2 ± 0.5 SD at baseline, 2.5 ± 0.6 SD and 3.2 ± 0.5 SD after t-PVB, 2.2 ± 0.5 SD and 2.2 ± 0.5 SD at 14 days, 90 days, and at one year after DRG-S implantation. No complications or side effects were reported.

CONCLUSIONS

Our preliminary results show that DRG-S is an effective therapy for PTPS after thoracic surgery. In addition, thoracic paravertebral blocks performed prior to DRG-S correlated with a positive outcome with treatment.

A Review of Neuromodulation for Treatment of Complex Regional Pain Syndrome in Pediatric Patients and Novel Use of Dorsal Root Ganglion Stimulation in an Adolescent Patient With 30-Month Follow-Up

OBJECTIVES

Complex regional pain syndrome (CRPS) is a disorder in which pain and discomfort are out of proportion to the amount of tissue damage. While prevalence is 20.57 per 100,000 adults, it has not been studied in the U.S. pediatric population. Conservative treatment options include pharmacologic, behavioral, and psychological treatment. If these fail, invasive therapies such as sympathetic blocks and neuromodulation may be performed. The ACCURATE study demonstrated efficacy of dorsal root ganglion (DRG) stimulation for the treatment of refractory CRPS, but did not include pediatric patients and there are no reported cases of its use in the pediatric population. We review the use of neuromodulation for CRPS in pediatric patients and present a novel case using DRG stimulation in an adolescent patient.

MATERIALS AND METHODS

A literature search was performed for any studies of neuromodulation as treatment of pediatric CRPS. The literature is reviewed and the case of a 17-year-old female with CRPS treated with DRG stimulation is presented.

RESULTS

A 15-year-old female developed CRPS type I of the left ankle after a sports injury. She was unable to wear a shoe or venture outside the home due to pain, swelling, and allodynia. Conservative therapies, including medications, physical therapy, and lumbosacral sympathetic plexus blocks, failed. At age 17, she underwent DRG stimulator lead placement targeting the left L4 and L5 nerve roots. Her therapy provided 100% relief of pain within four days of placement and was sustained at 30-month follow-up.

CONCLUSIONS

DRG stimulation has been shown to significantly alter the symptomatology in patients with CRPS. This case suggests that DRG stimulation may be as effective for the treatment of refractory CRPS in pediatric patients as in adults.

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.

Three-Year Outcomes After Dorsal Root Ganglion Stimulation in the Treatment of Neuropathic Pain After Peripheral Nerve Injury of Upper and Lower Extremities

OBJECTIVES

Traumatic peripheral nerve injuries (PNI) often result in severe neuropathic pain which typically becomes chronic, is recalcitrant to common analgesics, and is associated with sleep disturbances, anxiety, and depression. Pharmacological treatments proven to be effective against neuropathic pain are not well tolerated due to side effects. Neuromodulative interventions such as peripheral nerve or spinal cord stimulation have generated mixed results and may be limited by reduced somatotopic specificity. Dorsal root ganglion (DRG) stimulation may be more effective in this etiology.

MATERIALS AND METHODS

Twenty-seven patients were trialed with a DRG neurostimulation system for PNI; trial success (defined as ≥50% pain relief) was 85%, and 23 patients received a permanent stimulator. However, 36-month outcome data was only available for 21 patients. Pain, quality of life, mental and physical function, and opioid usage were assessed at baseline and at 3-, 6-, 12-, 18-, 24-, and 36 months post-permanent implant. Implant-related complications were also documented.

RESULTS

Compared to baseline, we observed a significant pain relief (p < 0.001) at 3 (58%), 12 (66%), 18 (69%), 24 (71%), and 36 months (73%) in 21 patients (52.5 ± 14.2 years; 12 female), respectively. Mental and physical function showed immediate and sustained improvements. Participants reported improvements in quality of life. Opioid dosage reduced significantly (p < 0.001) at 3 (30%), 12 (93%), 18 (98%), 24 (99%), and 36 months (99%), and 20 of 21 patients were completely opioid-free after 36 months. There were five lead migrations and two electrode fractures (corrected by surgical intervention) and one wound infection (conservatively managed).

CONCLUSIONS

DRG neuromodulation appears to be a safe, effective, and durable option for treating neuropathic pain caused by PNI. The treatment allows cessation of often ineffective pharmacotherapy (including opioid misuse) and significantly improves quality of life.

The Pathways and Processes Underlying Spinal Transmission of Low Back Pain: Observations From Dorsal Root Ganglion Stimulation Treatment

BACKGROUND

Dorsal root ganglion stimulation (DRG-S) is a novel approach to treat chronic pain. Lead placement at L2 has been reported to be an effective treatment for axial low back pain (LBP) primarily of discogenic etiology. We have recently shown, in a diverse cohort including cases of multilevel instrumentation following extensive prior back surgeries, that DRG-S lead placement at T12 is another promising target. Local effects at the T12 DRG, alone, are insufficient to explain these results.

MATERIALS AND METHODS

We performed a literature review to explore the mechanisms of LBP relief with T12 DRG-S.

FINDINGS

Branches of individual spinal nerve roots innervate facet joints and posterior spinal structures, while the discs and anterior vertebrae are carried via L2, and converge in the dorsal horn (DH) of the spinal cord at T8-T9. The T12 nerve root contains cutaneous afferents from the low back and enters the DH of the spinal cord at T10. Low back Aδ and C-fibers then ascend via Lissauer's tract (LT) to T8-T9, converging with other low back afferents. DRG-S at T12, then, results in inhibition of the converged low back fibers via endorphin-mediated and GABAergic frequency-dependent mechanisms. Therefore, T12 lead placement may be the optimal location for DRG-S to treat LBP.

Neuromodulation of the Dorsal Root Ganglion for Chronic Postsurgical Pain

Objective

The objective of this study is to review the available evidence for dorsal root ganglion (DRG) stimulation for the treatment of complex regional pain syndrome type II (CRPS II; peripheral causalgia) associated with chronic neuropathic postsurgical pain (NPP).

Design

Available literature was identified through a search of the US National Library of Medicine’s Medline database, PubMed.gov. References from published articles also were reviewed for relevant citations.

Results

The data published to date support the use of DRG stimulation to treat chronic NPP of the groin, knee, and foot. NPP following procedures such as thoracotomy, hernia surgery, and knee replacement surgery were identified as some of the conditions for which DRG stimulation is likely to be effective.

Conclusion

DRG stimulation is known to be an effective treatment for focal neuropathic pain. Currently, NPP of the foot, groin, and knee all appear to be the conditions with the most clinical experience, backed by a limited but growing body of evidence. However, prospective studies lag behind real-world clinical experience and are needed to confirm these findings.

Unique Characteristics of the Dorsal Root Ganglion as a Target for Neuromodulation

Objective

The dorsal root ganglion (DRG) is a novel target for neuromodulation, and DRG stimulation is proving to be a viable option in the treatment of chronic intractable neuropathic pain. Although the overall principle of conventional spinal cord stimulation (SCS) and DRG stimulation—in which an electric field is applied to a neural target with the intent of affecting neural pathways to decrease pain perception—is similar, there are significant differences in the anatomy and physiology of the DRG that make it an ideal target for neuromodulation and may account for the superior outcomes observed in the treatment of certain chronic neuropathic pain states. This review highlights the anatomy of the DRG, its function in maintaining homeostasis and its role in neuropathic pain, and the unique value of DRG as a target in neuromodulation for pain.

Methods

A narrative literature review was performed.

Results

Overall, the DRG is a critical structure in sensory transduction and modulation, including pain transmission and the maintenance of persistent neuropathic pain states. Unique characteristics including selective somatic organization, specialized membrane characteristics, and accessible and consistent location make the DRG an ideal target for neuromodulation. Because DRG stimulation directly recruits the somata of primary sensory neurons and harnesses the filtering capacity of the pseudounipolar neural architecture, it is differentiated from SCS, peripheral nerve stimulation, and other neuromodulation options.

Conclusions

There are several advantages to targeting the DRG, including lower energy usage, more focused and posture-independent stimulation, reduced paresthesia, and improved clinical outcomes.