Electrical low-frequency stimulation induces central neuroplastic changes of pain processing in man

Large-scale analysis of neural activity and connectivity from high-density electroencephalographic data

INTRODUCTION

High-density electroencephalography (hdEEG) is a technique used for the characterization of the neural activity and connectivity in the human brain. The analysis of EEG data involves several steps, including signal pre-processing, head modelling, source localization and activity/connectivity quantification. Visual check of the analysis steps is often necessary, making the process time- and resource-consuming and, therefore, not feasible for large datasets.

FINDINGS

Here we present the Noninvasive Electrophysiology Toolbox (NET), an open-source software for large-scale analysis of hdEEG data, running on the cross-platform MATLAB environment. NET combines all the tools required for a complete hdEEG analysis workflow, from raw signals to final measured values. By relying on reconstructed neural signals in the brain, NET can perform traditional analyses of time-locked neural responses, as well as more advanced functional connectivity and brain mapping analyses. The extracted quantitative neural data can be exported to provide broad compatibility with other software.

CONCLUSIONS

NET is freely available (https://github.com/bind-group-kul/net) under the GNU public license for non-commercial use and open-source development, together with a graphical user interface (GUI) and a user tutorial. While NET can be used interactively with the GUI, it is primarily aimed at unsupervised automation to process large hdEEG datasets efficiently. Its implementation creates indeed a highly customizable program suitable for analysis automation and tight integration into existing workflows.

Evaluation of Washout Periods After Dorsal Root Ganglion Stimulation Trial

OBJECTIVE

Dorsal root ganglion stimulation (DRG-S) is a novel therapy to treat chronic pain. It has shown efficacy when delivered intermittently, suggesting a delayed washout effect exists. To measure the washout period, and to determine whether there are differences in washout times among different types of treated pain, we measured the time for pain to return at the end of the patients' one-week DRG stimulation trials.

MATERIALS AND METHODS

Patients who completed a successful DRG-S trial were included. The times until 25% (t25) and 90% (t90) of baseline pain level returned were recorded. The patients were divided into neuropathic, nociceptive, and mixed pain groups for subgroup comparison. t25 and t90 were plotted in the entire cohort and subgroups using reverse Kaplan-Meier plots (failure curves) and compared using a log-rank test.

RESULTS

In total, 29 consecutive patients were included. Median t25 and t90 times were 7.1 and 19.5 hours, respectively. Median (interquartile range) times were longest for the nociceptive pain group (n = 17) and shortest for the neuropathic pain group (n = 6), with the mixed-pain group (n = 6) in between (t25: 7.1 [1.7-19.4], 3.40 [1.4-8.4], and 5.7 [0.8-17.6]; t90, 22.0 [10.7-71.0], 7.6 [3.6-19.8], and 20.9 [14.2-31.2], respectively). t90 times differed significantly by pain type (p = 0.040).

CONCLUSIONS

This study showed a prolonged washout period after cessation of DRG-S therapy. Washout times vary according to pain type. The observed effects are possibly due to long-term depression of pain signaling and could allow the implementation of alternative stimulation strategies with DRG-S. Further investigations evaluating DRG-S washout times are warranted.

Pain-related evoked potentials with concentric surface electrodes in patients and healthy subjects: a systematic review

Pain-related evoked potentials with concentric surface electrodes (PREP with CE) have been increasingly used in the diagnostics of polyneuropathies as well as in pain research. However, the study results are partly inconsistent regarding their utility to distinguish between normal and abnormal findings. The present systematic review aimed to summarise and compare study results, where PREP with CE were used in healthy subjects or patients and to identify possible influencing factors. We found 36 research articles, of which 21 investigated disorders in patients compared to healthy controls, while the other 15 focussed on basic research in healthy subjects. Patients with polyneuropathies showed the most consistent PREP results with similar prolonged latencies and reduced amplitude values. Findings in other patient groups or in healthy subjects were more heterogeneous. There was evidence for an influence by age and height as well as by central effects like emotions, which should be considered in further studies. Further systematic research analysing PREP results depending on individual and disease-specific factors is needed to develop optimal normative values.

Inhibition of cortical somatosensory processing during and after low frequency peripheral nerve stimulation in humans

OBJECTIVE

Transcutaneous low-frequency stimulation (LFS) elicits long-term depression-like effects on human pain perception. However, the neural mechanisms underlying LFS are poorly understood. We investigated cortical activation changes occurring during LFS and if changes were associated with reduced nociceptive processing and increased amplitude of spontaneous cortical oscillations post-treatment.

METHODS

LFS was applied to the radial nerve of 25 healthy volunteers over two sessions using active (1 Hz) or sham (0.02 Hz) frequencies. Changes in resting electroencephalography (EEG) and laser-evoked potentials (LEPs) were investigated before and after LFS. Somatosensory-evoked potentials were recorded during LFS and source analysis was carried out.

RESULTS

Ipsilateral midcingulate and operculo-insular cortex source activity declined linearly during LFS. Active LFS was associated with attenuated long-latency LEP amplitude in ipsilateral frontocentral electrodes and increased resting alpha (8-12 Hz) and beta (16-24 Hz) band power in electrodes overlying operculo-insular, sensorimotor and frontal cortical regions. Reduced ipsilateral operculo-insular cortex source activity during LFS correlated with a smaller post-treatment alpha-band power increase.

CONCLUSIONS

LFS attenuated somatosensory processing both during and after stimulation.

SIGNIFICANCE

Results further our understanding of the attenuation of somatosensory processing both during and after LFS.

Peripheral Electrical Stimulation Modulates Cortical Beta-Band Activity

Low-frequency peripheral electrical stimulation using a matrix electrode (PEMS) modulates spinal nociceptive pathways. However, the effects of this intervention on cortical oscillatory activity have not been assessed yet. The aim of this study was to investigate the effects of low-frequency PEMS (4 Hz) on cortical oscillatory activity in different brain states in healthy pain-free participants. In experiment 1, PEMS was compared to sham stimulation. In experiment 2, motor imagery (MI) was used to modulate the sensorimotor brain state. PEMS was applied either during MI-induced oscillatory desynchronization (concurrent PEMS) or after MI (delayed PEMS) in a cross-over design. For both experiments, PEMS was applied on the left forearm and resting-state electroencephalography (EEG) was recording before and after each stimulation condition. Experiment 1 showed a significant decrease of global resting-state beta power after PEMS compared to sham (p = 0.016), with a median change from baseline of −16% for PEMS and −0.54% for sham. A cluster-based permutation test showed a significant difference in resting-state beta power comparing pre- and post-PEMS (p = 0.018) that was most pronounced over bilateral central and left frontal sensors. Experiment 2 did not identify a significant difference in the change from baseline of global EEG power for concurrent PEMS compared to delayed PEMS. Two cluster-based permutation tests suggested that frontal beta power may be increased following both concurrent and delayed PEMS. This study provides novel evidence for supraspinal effects of low-frequency PEMS and an initial indication that the presence of a cognitive task such as MI may influence the effects of PEMS on beta activity. Chronic pain has been associated with changes in beta activity, in particular an increase of beta power in frontal regions. Thus, brain state-dependent PEMS may offer a novel approach to the treatment of chronic pain. However, further studies are warranted to investigate optimal stimulation conditions to achieve a reduction of pain.

Treatment of chronic migraine with transcutaneous stimulation of the auricular branch of the vagal nerve (auricular t-VNS): a randomized, monocentric clinical trial

Background

Aim of the study was assessment of efficacy and safety of transcutaneous stimulation of the auricular branch of the vagal nerve (t-VNS) in the treatment of chronic migraine.

Methods

A monocentric, randomized, controlled, double-blind study was conducted. After one month of baseline, chronic migraine patients were randomized to receive 25 Hz or 1 Hz stimulation of the sensory vagal area at the left ear by a handhold battery driven stimulator for 4 h/day during 3 months. Headache days per 28 days were compared between baseline and the last month of treatment and the number of days with acute medication was recorded The Headache Impact Test (HIT-6) and the Migraine Disability Assessment (MIDAS) questionnaires were used to assess headache-related disability.

Results

Of 46 randomized patients, 40 finished the study (per protocol). In the per protocol analysis, patients in the 1 Hz group had a significantly larger reduction in headache days per 28 days than patients in the 25 Hz group (−7.0 ± 4.6 vs. −3.3 ± 5.4 days, p = 0.035). 29.4 % of the patients in the 1 Hz group had a ≥50 % reduction in headache days vs. 13.3 % in the 25 Hz group. HIT-6 and MIDAS scores were significantly improved in both groups, without group differences. There were no serious treatment-related adverse events.

Conclusion

Treatment of chronic migraine by t-VNS at 1 Hz was safe and effective. The mean reduction of headache days after 12 weeks of treatment exceeded that reported for other nerve stimulating procedures.

Application of low frequency and medium frequency currents in the management of acute and chronic pain-a narrative review

Trancutaneous electrical nerve stimulation (TENS) and interferential therapy (IFT) have been a regular line of treatment for various types of acute and chronic pain. This review aims to compile the latest literature in pain management using these modalities which use low-frequency and medium-frequency currents. The Cochrane Library, Scopus, PubMed, MEDLINE, and CINAHL were searched and studies were examined from their inception till October 2013. After title and abstract screening the relevant studies were included for this review. We found through this review that even though TENS and IFT are used in management of pain, there is limited amount of high quality research available in this area. Most of the studies lack methodological quality and have a low sample size.

Evidence of heterosynaptic LTD in the human nociceptive system: superficial skin neuromodulation using a matrix electrode reduces deep pain sensitivity

Long term depression (LTD) is a neuronal learning mechanism after low frequency stimulation (LFS). This study compares two types of electrodes (concentric vs. matrix) and stimulation frequencies (4 and 30 Hz) to examine homo- and heterosynaptic effects indirectly depicted from the somatosensory profile of healthy subjects. Both electrodes were compared in a prospective, randomized, controlled cross-over study using 4 Hz as the conditioning LFS compared to 30 Hz (intended sham condition). Quantitative sensory testing (QST) was used to examine 13 thermal and mechanical detection and pain thresholds. Sixteen healthy volunteers (10 women, age 31.0±12.7 years) were examined. Depending on the electrodes and frequencies used a divergent pattern of sensory minus signs occurred. Using LFS the concentric electrode increased thermal thresholds, while the matrix electrode rather increased mechanical including deep pain thresholds. Findings after cutaneous neuromodulation using LFS and a matrix electrode are consistent with the concept of heterosynaptic LTD in the human nociceptive system, where deep pain sensitivity was reduced after superficial stimulation of intraepidermal nerve fibres. Cutaneous neuromodulation using LFS and a matrix electrode may be a useful tool to influence deep pain sensitivity in a variety of chronic pain syndromes.

Neuromodulation and Palliative Medicine

The palliative care population is generally vulnerable to experiencing medication-induced adverse effects and drug–drug interactions. Neuromodulation may offer particular advantages over systemic medications in this population. Spinal cord stimulation and peripheral nerve stimulation have long been utilized in efforts to provide analgesia for various painful conditions. More recently, deep brain stimulation/motor cortex stimulation has anecdotally been utilized for certain intractable pain states. Although brain electrical stimulation has not been adequately trialed or in some cases even tried at all for management of a variety of symptoms, it is conceivable that in the future it may be a potential therapeutic option in efforts to palliate various severe refractory symptoms (eg, intractable pain, nausea, dyspnea, delirium).

Somatosensory abnormalities for painful and innocuous stimuli at the back and at a site distinct from the region of pain in chronic back pain patients

Chronic low back pain (CLBP) was shown to be associated with pathophysiological changes at several levels of the sensorimotor system. Changes in sensory thresholds have been reported but complete profiles of Quantitative Sensory Testing (QST) were only rarely obtained in CLBP patients. The aim of the present study was to investigate comprehensive QST profiles in CLBP at the painful site (back) and at a site distinct from their painful region (hand) and to compare these data with similar data in healthy controls. We found increased detection thresholds in CLBP patients compared to healthy controls for all innocuous stimuli at the back and extraterritorial to the painful region at the hand. Additionally, CLBP patients showed decreased pain thresholds at both sites. Importantly, there was no interaction between the investigated site and group, i.e. thresholds were changed both at the affected body site and for the site distinct from the painful region (hand). Our results demonstrate severe, widespread changes in somatosensory sensitivity in CLBP patients. These widespread changes point to alterations at higher levels of the neuraxis or/and to a vulnerability to nociceptive plasticity in CLBP patients.