Differential effects of subcutaneous electrical stimulation (SQS) and transcutaneous electrical nerve stimulation (TENS) in rodent models of chronic neuropathic or inflammatory pain

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.

Transcutaneous Electric Nerve Stimulation in Animal Model Studies: From Neural Mechanisms to Biological Effects for Analgesia

OBJECTIVE

This systematic and meta-analysis review evaluated the transcutaneous electrical nerve stimulation (TENS)-induced action mechanisms for animal analgesia.

MATERIALS AND METHODS

Two independent investigators identified relevant articles published until February 2021 through a literature review, and a random-effects meta-analysis was performed to synthesize the results.

RESULTS

Of the 6984 studies found in the data base search, 53 full-text articles were selected and used in the systematic review. Most studies used Sprague Dawley rats (66.03%). High-frequency TENS was applied to at least one group in 47 studies, and most applications were performed for 20 minutes (64.15%). Mechanical hyperalgesia was analyzed as the primary outcome in 52.83% of the studies and thermal hyperalgesia in 23.07% of studies using a heated surface. More than 50% of the studies showed a low risk of bias on allocation concealment, random housing, selective outcome reporting, and acclimatization before the behavioral tests. Blinding was not performed in only one study and random outcome assessment in another study; acclimatization before the behavioral tests was not performed in just one study. Many studies had an uncertain risk of bias. Meta-analyses indicated no difference between low-frequency and high-frequency TENS with variations among the pain models.

CONCLUSIONS

This systematic review and meta-analysis suggests that TENS has presented a substantial scientific foundation for its hypoalgesic effect in preclinical studies for analgesia.

Basic mechanisms of peripheral nerve injury and treatment via electrical stimulation

Previous studies on the mechanisms of peripheral nerve injury (PNI) have mainly focused on the pathophysiological changes within a single injury site. However, recent studies have indicated that within the central nervous system, PNI can lead to changes in both injury sites and target organs at the cellular and molecular levels. Therefore, the basic mechanisms of PNI have not been comprehensively understood. Although electrical stimulation was found to promote axonal regeneration and functional rehabilitation after PNI, as well as to alleviate neuropathic pain, the specific mechanisms of successful PNI treatment are unclear. We summarize and discuss the basic mechanisms of PNI and of treatment via electrical stimulation. After PNI, activity in the central nervous system (spinal cord) is altered, which can limit regeneration of the damaged nerve. For example, cell apoptosis and synaptic stripping in the anterior horn of the spinal cord can reduce the speed of nerve regeneration. The pathological changes in the posterior horn of the spinal cord can modulate sensory abnormalities after PNI. This can be observed in cases of ectopic discharge of the dorsal root ganglion leading to increased pain signal transmission. The injured site of the peripheral nerve is also an important factor affecting post-PNI repair. After PNI, the proximal end of the injured site sends out axial buds to innervate both the skin and muscle at the injury site. A slow speed of axon regeneration leads to low nerve regeneration. Therefore, it can take a long time for the proximal nerve to reinnervate the skin and muscle at the injured site. From the perspective of target organs, long-term denervation can cause atrophy of the corresponding skeletal muscle, which leads to abnormal sensory perception and hyperalgesia, and finally, the loss of target organ function. The mechanisms underlying the use of electrical stimulation to treat PNI include the inhibition of synaptic stripping, addressing the excessive excitability of the dorsal root ganglion, alleviating neuropathic pain, improving neurological function, and accelerating nerve regeneration. Electrical stimulation of target organs can reduce the atrophy of denervated skeletal muscle and promote the recovery of sensory function. Findings from the included studies confirm that after PNI, a series of physiological and pathological changes occur in the spinal cord, injury site, and target organs, leading to dysfunction. Electrical stimulation may address the pathophysiological changes mentioned above, thus promoting nerve regeneration and ameliorating dysfunction.

Transcutaneous Electrical Nerve Stimulation in Rodent Models of Neuropathic Pain: A Meta-Analysis

Transcutaneous electrical nerve stimulation (TENS) is a non-invasive therapeutic intervention that is typically used for many years to treat chronic pain in patients who are refractory to pain medications. However, evidence of the efficacy of TENS treatment for neuropathic pain is lacking in humans. To further understand the efficacy of TENS under various intervention conditions and illuminate the current circumstance and future research directions, we systematically reviewed animal studies investigating the efficacy of TENS in relieving pain in neuropathic pain rodent models. We searched the Cochrane Library, EMBASE, MEDLINE (via PubMed), and Web of Science and identified 11 studies. Two meta-analyses were performed. The first meta-analysis showed that a single TENS treatment was capable of temporarily ameliorating neuropathic pain when compared to control groups with a significant effect (standardized mean difference: 1.54; 95% CI: 0.65, 2.42; p = 0.0007; I² = 58%). Significant temporary alleviation in neuropathic pain intensity was also observed in the meta-analysis of repetitive TENS (standardized mean difference: 0.85; 95% CI: 0.31, 1.40; p = 0.002; I² = 75%). Subgroup analysis showed no effect of the timing of the application of TENS (test for subgroup difference, p = 0.47). Leave-one-out sensitivity analyses suggested that no single study had an outsized effect on the pooled estimates, which may partly prove the robustness of these findings. Other stratified analyses were prevented by the insufficient number of included studies. Overall, current data suggest that TENS might be a promising therapy to ameliorate neuropathic pain. However, the high risk of bias in the included studies suggests that cautions must be considered when interpreting these findings and it is not reasonable to directly generalize the results obtained from animal studies to clinical practice. Future studies should pay more attention to improving the quality of study design and reporting, thereby facilitating the understanding of mechanisms underlying TENS treatment, reducing more potentially unsuccessful clinical trials, and optimizing the efficacy of TENS for people with neuropathic pain.

The Predictive Value of Transcutaneous Electrical Nerve Stimulation for Patient Selection in Peripheral Nerve Field Stimulation for Chronic Low Back Pain: A Prospective Study

OBJECTIVE

Peripheral nerve field stimulation (PNFS) is an effective alternative treatment for patients with chronic low back pain. Transcutaneous electrical nerve stimulation (TENS) is frequently used in pain therapy. Aim of this prospective study was to examine the predictive value of TENS for later PNFS treatment.

MATERIALS AND METHODS

Between 2014 and 2019, a prospective cohort study of 41 patients with chronic lumbar pain was conducted. Pain intensity (NRS) was assessed before and after TENS use, preoperatively/postoperatively and in the follow-up after three and six months, SF12v2 questionnaires with physical (PCS) and mental component summary (MCS) scores, and Oswestry disability index (ODI) questionnaire at baseline as well as three and six months after PNFS implantation. Implantation of the PNFS-system with two percutaneous leads was performed after four to seven days of positive testing. Statistical analysis was performed using depending t-test, ANOVA, and Spearman correlation.

RESULTS

The cohort consisted of 41 patients (19 females, 22 males) with a median age of 60.5 years (IQR^25-75 52-67). Two patients were lost to follow-up. After positive PNFS testing a pulse generator (IPG) was implanted in 15 patients with positive TENS effect and 15 patients without TENS effect. Leads were explanted in nine patients after negative PNFS trial phase. TENS positive patients showed significant correlation to a positive effect in the PNFS trial phase in NRS reduction (p = 0.042) indicating that TENS responders will also respond to PNFS (94% patients). After three and six months follow-up median NRS and SF12v2 (PCS) improved significantly in both cohorts, SF12v2 (MCS) and ODI only in the TENS positive cohort, respectively.

CONCLUSION

TENS can be predictive for patient selection in PNFS, as TENS positive patients showed significant correlation with a positive PNFS trial period. Therefore, TENS positive patients might be justifiable to be directly implanted with leads and IPG. TENS positive patients further tend to show a better improvement in the follow-up.

Inhibition of chemokine CX3CL1 in spinal cord mediates the electroacupuncture-induced suppression of inflammatory pain

Purpose

Chemokine CX3CL1 and its receptor CX3CR1 in the lumbar spinal cord play crucial roles in pain processing. Electroacupuncture (EA) is recognized as an alternative therapy in pain treatment due to its efficacy and safety. However, the analgesic mechanism of EA remains unclear. The aim of this study was to investigate whether EA suppressed complete Freund’s adjuvant (CFA)-induced pain via modulating CX3CL1-CX3CR1 pathway.

Materials and methods

Inflammatory pain was induced by intraplantar injection of CFA to the left hind paw of Sprague-Dawley rats. EA with 2 Hz for 30 mins was given to bilateral Zusanli acupoints (ST36) on the first and third day after CFA injection. Mechanical allodynia and thermal hyperalgesia were tested with von Frey tests and Hargreaves tests, respectively. The expressions of CX3CL1, CX3CR1 and p38 mitogen-activated protein kinase (MAPK) were quantified with Western blots. The release of IL-1β, IL-6 and TNF-α were evaluated with ELISA. Recombinant CX3CL1 or control IgG were then injected through intrathecal catheters in the EA-treated CFA model rats. The behavioral tests, p38 MAPK activation and cytokine release were then evaluated.

Results

EA significantly inhibited inflammatory pain induced by CFA for 3 days. Meanwhile, EA downregulated the expression of CX3CL1 but not CX3CR1 in the lumbar spinal cord of the CFA rats. Besides, activation of p38 MAPK and the release of pain-related cytokines (IL-1β, IL-6 and TNF-α) were inhibited by EA. Intrathecal injection of CX3CL1 largely reversed the analgesic effect of EA treatment and re-activated p38 MAPK signaling, and resulted in pro-inflammatory cytokines increase in acupuncture-treated rats.

Conclusion

Our findings indicate that EA alleviates inflammatory pain via modulating CX3CL1 signaling in lumbar spinal cord, revealing a potential mechanism of anti-nociception of EA in inflammatory pain.

A Randomized Controlled Trial of Subcutaneous Nerve Stimulation for Back Pain Due to Failed Back Surgery Syndrome: The SubQStim Study

Objectives

To compare the effectiveness of peripheral nerve stimulation utilizing a subcutaneous lead implant technique—subcutaneous nerve stimulation (SQS) plus optimized medical management (SQS + OMM arm) vs. optimized medical management alone (OMM arm) in patients with back pain due to failed back surgery syndrome.

Patients and Methods

Patients were recruited from 21 centers, in Europe, Israel, and Australia. Eligible patients were randomized (1:1) to SQS + OMM or OMM arms. Those in the SQS arm were implanted with a neurostimulator and up to two subcutaneous percutaneous cylindrical leads in the area of pain. Patients were evaluated pre‐randomization and at one, three, six, and nine months post‐randomization. The primary endpoint was the proportion of subjects with a ≥50% reduction in back pain intensity (“responder”) from baseline to nine months. Secondary outcomes included proportion of responders with a ≥50% reduction in back pain intensity at six months and ≥30% reduction at nine months, and the mean change from baseline in back pain intensity at six and nine months between the two arms.

Results

Due to the slow rate of recruitment, the study was terminated early with 116 subjects randomized. A total of 33.9% (19/56, missing: n = 20 [36%]) of subjects in the SQS + OMM arm and 1.7% (1/60, missing: n = 24 [40%]) in the OMM arm were responders at Month 9 (p < 0.0001). Secondary objectives showed a significant difference in favor of SQS + OMM arm.

Conclusion

The results indicate that the addition of SQS to OMM is more effective than OMM alone in relieving low back pain at up to nine months.

Review of Recent Advances in Peripheral Nerve Stimulation (PNS)

Peripheral nerve stimulation (PNS) for the treatment of chronic pain has become an increasingly important field in the arena of neuromodulation, given the ongoing advances in electrical neuromodulation technology since 1999 permitting minimally invasive approaches using an percutaneous approach as opposed to implantable systems. Our review aims to provide clinicians with the recent advances and studies in the field, with specific emphasis on clinical data and indications that have been accumulated over the last several years. In addition, we aim to address key basic science studies to further emphasize the importance of translational research outcomes driving clinical management.

Peripheral neuromodulation: a review

Peripheral nerve stimulation (PNS) is likely the most diverse and rapidly expanding area of neuromodulation. Its expansion has become possible due to both technological and clinical advances in pain medicine. The first implantable systems were surgically placed. However, it is currently commonplace to use percutaneous leads, as this approach has become instrumental in its expansion. The first percutaneous peripheral nerve stimulators were reported in 1999. Cylindrical leads were implanted to stimulate the greater occipital nerve to manage intractable headache. It has been expanded into other individual nerves or nerve plexuses to treat neuropathic, visceral, cardiac, abdominal, low back and facial pain. The use of PNS in modulating organ function in treatment of syndromes such as epilepsy, incontinence and obesity with vagal, tibial and gastric stimulation is under extensive investigation. New technologies that allow easier and safer electrode placement are expected to further expand the uses of PNS. A noninvasive stimulation will open this treatment modality to more clinicians of varying backgrounds.

External Noninvasive Peripheral Nerve Stimulation Treatment of Neuropathic Pain: A Prospective Audit

BACKGROUND

Peripheral nerve stimulation (PNS) is a neuromodulation technique in which electrical current is applied to the peripheral nerves to ameliorate chronic pain through preferential activation of myelinated fibres, inducing long-term depression of synaptic efficacy. External noninvasive peripheral nerve stimulation (EN-PNS) is a novel and simple form of PNS that involves stimulation via an external nerve-mapping probe that is placed on the skin and connected to a power source.

OBJECTIVES

We aimed to assess the clinical utility of EN-PNS in patients with refractory neuropathic pains referred to a tertiary pain treatment center.

METHODS

We undertook a prospective audit of EN-PNS. Patients with a diagnosis of either complex regional pain syndrome or neuropathic pain after peripheral nerve injury who met inclusion criteria were included. Participants completed three stages of the audit: stage 1, six weekly outpatient treatment sessions; stage 2, six-week equipment home loan; stage 3, six weeks of no EN-PNS treatment. The primary outcome was the average post-treatment instantaneous pain intensity during the last week in stage 2 compared with baseline (11-point numerical rating scale).

RESULTS

EN-PNS provided significant short-term pain relief (n = 20 patients, average reduction of 2.8 numerical rating scale points, 95% CI 1.6-4.0, p < 0.001, intention-to-treat analysis). Eight patients (40%) improved in several outcome parameters ("responders"), including quality of life and function.

CONCLUSION

In this first prospective report on the use of EN-PNS in neuropathic pain, this technology provided significant clinical benefit for some patients. Controlled studies are required to confirm our results and the place of EN-PNS in future neuromodulation treatment algorithms. Given the refractory nature of these conditions, these results are encouraging.