High-frequency transcutaneous electrical nerve stimulation reduces pain and cardio-respiratory parameters in an animal model of acute pain: participation of peripheral serotonin

Literature Review: Mechanism, Indications, and Clinical Efficacy of Peripheral Nerve Stimulators in Lower Extremity Pain

PURPOSE OF REVIEW

Lower extremity pain is deemed by Center for Disease Control and Prevention (CDC) to be a significant source of chronic pain in adults. If not appropriately managed, patients are subjected to risks of prolonged musculoskeletal dysfunction, disruption to quality of life, and elevated healthcare expenditures. Peripheral nerve stimulation (PNS) has shown great potential in recent years demonstrating efficacy in multiple diagnoses ranging from acute post-surgical pain to complex regional pain syndrome (CRPS). This study seeks to delineate efficacy of peripheral neuromodulation in the context of chronic lower extremity pain.

RECENT FINDINGS

Prevailing clinical studies demonstrate evidence levels ranging from II to V (Oxford Centre of Level of Evidence) in lower limb PNS, attaining positive outcomes in pain scores, opioid use, and quality of life measures. Nerves most frequently targeted are the sciatic and femoral nerves with post-amputation pain and CRPS most commonly investigated for efficacy. PNS is a promising therapeutic modality demonstrated to be effective for a variety of nociceptive and neuropathic pain conditions in the lower extremity. PNS offers chronic pain physicians a powerful tool in the multi-modal management of lower limb chronic pain.

Relating Spinal Injury-Induced Neuropathic Pain and Spontaneous Afferent Activity to Sleep and Respiratory Dysfunction

Abstract Spinal cord injury (SCI) can induce dysfunction in a multitude of neural circuits including those that lead to impaired sleep, respiratory dysfunction, and neuropathic pain. We used a lower thoracic rodent contusion SCI model of neuropathic pain that has been shown to associate with increased spontaneous activity in primary afferents and hindlimb mechanosensory stimulus hypersensitivity. Here we paired capture of these variables with chronic capture of three state sleep and respiration to more broadly understand SCI-induced physiological dysfunction and to assess possible interrelations. Noncontact electric field sensors were embedded into home cages to non-invasively capture the temporal evolution of sleep and respiration changes for six weeks after SCI in naturally behaving mice. Hindlimb mechanosensitivity was assessed weekly, and terminal experiments measured primary afferent spontaneous activity in situ from intact lumbar dorsal root ganglia (DRG). We observed that SCI led to increased spontaneous primary afferent activity (both firing rate and the number of spontaneously active DRGs) that correlated with increased respiratory rate variability and measures of sleep fragmentation. This is the first study to measure and link sleep dysfunction and variability in respiratory rate in a SCI model of neuropathic pain, and thereby provide broader insight into the magnitude of overall stress burden initiated by neural circuit dysfunction after SCI.

Multimodal non-invasive non-pharmacological therapies for chronic pain: mechanisms and progress

BACKGROUND

Chronic pain conditions impose significant burdens worldwide. Pharmacological treatments like opioids have limitations. Non-invasive non-pharmacological therapies (NINPT) encompass diverse interventions including physical, psychological, complementary and alternative approaches, and other innovative techniques that provide analgesic options for chronic pain without medications.

MAIN BODY

This review elucidates the mechanisms of major NINPT modalities and synthesizes evidence for their clinical potential across chronic pain populations. NINPT leverages peripheral, spinal, and supraspinal mechanisms to restore normal pain processing and limit central sensitization. However, heterogeneity in treatment protocols and individual responses warrants optimization through precision medicine approaches.

CONCLUSION

Future adoption of NINPT requires addressing limitations in standardization and accessibility as well as synergistic combination with emerging therapies. Overall, this review highlights the promise of NINPT as a valuable complementary option ready for integration into contemporary pain medicine paradigms to improve patient care and outcomes.

C-low threshold mechanoreceptor activation becomes sufficient to trigger affective pain in spinal cord-injured mice in association with increased respiratory rates

The mechanisms of neuropathic pain after spinal cord injury (SCI) are not fully understood. In addition to the plasticity that occurs within the injured spinal cord, peripheral processes, such as hyperactivity of primary nociceptors, are critical to the expression of pain after SCI. In adult rats, truncal stimulation within the tuning range of C-low threshold mechanoreceptors (C-LTMRs) contributes to pain hypersensitivity and elevates respiratory rates (RRs) after SCI. This suggests that C-LTMRs, which normally encode pleasant, affiliative touch, undergo plasticity to transmit pain sensation following injury. Because tyrosine hydroxylase (TH) expression is a specific marker of C-LTMRs, in the periphery, here we used TH-Cre adult mice to investigate more specifically the involvement of C-LTMRs in at-level pain after thoracic contusion SCI. Using a modified light-dark chamber conditioned place aversion (CPA) paradigm, we assessed chamber preferences and transitions between chambers at baseline, and in response to mechanical and optogenetic stimulation of C-LTMRs. In parallel, at baseline and select post-surgical timepoints, mice underwent non-contact RR recordings and von Frey assessment of mechanical hypersensitivity. The results showed that SCI mice avoided the chamber associated with C-LTMR stimulation, an effect that was more pronounced with optical stimulation. They also displayed elevated RRs at rest and during CPA training sessions. Importantly, these changes were restricted to chronic post-surgery timepoints, when hindpaw mechanical hypersensitivity was also evident. Together, these results suggest that C-LTMR afferent plasticity, coexisting with potentially facilitatory changes in breathing, drives at-level affective pain following SCI in adult mice.

Transcutaneous Electrical Nerve Stimulation: An Overview

Pain and its management hold a central place in health care. The pain associated with pregnancy and giving birth is unique in that it is a normal, physiologic phenomenon that is affected by cultural mores, personal experience, and internalized sensations. There are numerous nonpharmacologic tools available to treat discomfort during pregnancy and childbirth. Some methods of nonpharmacologic relief are underutilized, due to the lack of knowledge of the evidence. Childbirth educators, doulas, nurses, and midwives are a prime source of knowledge for birthing families to learn a variety of comfort techniques during pregnancy and labor. The purpose of this article is to discuss the use of transcutaneous electrical nerve stimulation (TENS) as a nonpharmacologic comfort technique.

Formalin-induced pain prolongs sub- to supra-second time estimation in rats

Background

Temporal estimation can be influenced by pain, which is a complex psychological and physiological phenomenon. However, the time range in which perception is most sensitive to pain remains unclear.

Methods

In the present study, we explored the effects of acute inflammatory pain on time perception in the sub- to supra-second (0.6–2.4-s) and supra-second (2–8-s) ranges in rats. Plantar formalin injection was used to induce acute inflammatory pain, and a temporal bisection task was used to measure time perception. Task test sessions were held for five consecutive days (one per day): the day before injection (baseline), immediately after injection, and the three post-injection days. The point of subjective equality (PSE, which reflects the subjective duration) and Weber fraction (which reflects temporal sensitivity) were calculated and analysed.

Results

In the 0.6–2.4-s range, the PSE was significantly lower, indicating prolonged subjective duration, in the formalin group relative to the saline group (p = 0.049) immediately after injection. Formalin-induced pain also tended to lengthened time perception in the 0.6–2.4-s range on post-injection days 2 (p = 0.06) and 3 (p = 0.054). In the 2–8-s range, formalin injection did not affect the PSE or Weber fraction.

Conclusions

The enhanced effect of pain on temporal perception in the sub- to supra-second range is observed in this study and this effect is attenuated with the prolongation of estimated time, even in rats.

Formalin-induced and neuropathic pain altered time estimation in a temporal bisection task in rats

Time perception is an important ability that is related closely to humans’ and animals’ daily activities. It can be distorted by various emotional states. In human studies, experimental pain has been shown to prolong the perception of time. However, related animal studies are lacking. In this study, we used a temporal bisection task to investigate how acute inflammatory pain (induced by hind-paw formalin injection) and chronic neuropathic pain [induced by spinal nerve ligation (SNL)] affected time perception in rats. Rats were trained to recognize “short” (1200-ms) and “long” (2400-ms) anchor-duration pure tones and were rewarded for corresponding lever presses. During testing, rats perceived a series of intermediate-duration and anchor-duration pure tones, and selected levers corresponding to the “short” and “long” tones. After formalin injection, rats gave more “long” lever-press responses than after saline injection. The point of subjective equality after formalin injection also increased, suggesting that formalin-induced acute pain extended time perception. In contrast, rats that had undergone SNL gave fewer “long” lever-press responses compared with the sham surgery group. This animal study suggests that formalin-induced pain and neuropathic pain may have different effects on time perception.

Spontaneous and Stimulus-Evoked Respiratory Rate Elevation Corresponds to Development of Allodynia in Spinal Cord-Injured Rats

Respiratory complications frequently accompany spinal cord injury (SCI) and slowed breathing has been shown to mitigate pain sensitivity. It is possible that elevated respiratory rates (RRs) signal the emergence of chronic pain after SCI. We previously validated the use of remote electric field sensors to noninvasively track breathing in freely behaving rodents. Here, we examined spontaneous (resting) and stimulus-evoked RRs as potential indices of mechanical hypersensitivity following SCI. Adult male Long-Evans rats received a lower thoracic hemisection or contusion SCI, or sham surgery, and underwent weekly assessments of mechanical and thermal sensitivity using the von Frey and Hargreaves tests, respectively. Resting RRs were recorded with remote sensors prior to nociception assays as well as 1 day post-surgery. Evoked RRs were quantified weekly in response to at-level mechanical stimulation provided by a small brush at various stimulation speeds, including those corresponding to the distinct tuning properties of a sub-population of cutaneous afferents known as C-low threshold mechanoreceptors. SCI rats developed mechanical hypersensitivity, which peaked 2-3 weeks after SCI. Compared with at baseline, hemisection SCI rats showed significantly heightened resting RRs at 1 day and 7 days post-injury, and the latter predicted development of pain hypersensitivity. In contusion SCI rats, resting RR increases were less substantial but occurred at all weekly time-points. Increases in brush-evoked RR coincided with full expression of hypersensitivity at 14 (hemisection) or 21 (contusion) days after SCI, and these effects were restricted to the lowest brush speeds. Our results support the possibility that early changes in RR may convey pain information in rats.

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.

Electro-analgesia for sheep husbandry practices: a review

Several sheep-husbandry practices such as mulesing, castration, ear-tagging and tail-docking are currently performed with no, or little, anaesthesia or analgesia. The potential for using electrotherapies to provide analgesia during and after these operations is examined in this review. The most common electrotherapy is transcutaneous electrical nerve stimulation (TENS). TENS is the application of an electrical current from electrodes placed on the skin. Analysis of a large number of trials in humans and in animal models indicates that TENS provides effective relief from acute and chronic pain, including pain associated with surgery. There is strong evidence now that TENS analgesia operates at the levels of the periphery, the spinal cord and in the brain. The mechanisms involve the autonomic nervous system, the opioid pathways and neurotransmitters involved in descending inhibitory pathways from the brain. Centrally operating pathways mean the current does not have to be applied near the injured site and there is evidence of sustained pain relief lasting hours, days or even weeks post-treatment, particularly after very high-frequency, randomly variable current applications. Treatment of sheep during painful operations with such a current has the potential to provide immediate and possibly sustained pain relief. Combining such a treatment with electro-immobilisation of the animal would be advantageous for sheep-husbandry operations, but there is considerable evidence that high-intensity currents producing tetanic contractions are aversive and probably painful for sheep. Investigations of the application and efficacy of electrotherapies for painful sheep operations should be undertaken.

Kinesio taping or just taping in shoulder subacromial impingement syndrome? A randomized, double-blind, placebo-controlled trial

OBJECTIVE

To verify effects of kinesio taping (KT) in shoulder subacromial impingement syndrome (SIS) when compared to sham taping applied in the same way with KT.

PATIENTS AND METHODS

Patients were randomized as group 1 (n = 21) KT group and group 2 (n = 20) sham-taping group. Taping was applied every three days, three times during the study period. We assessed all the patients at baseline, at the end of the taping period (12th day), and at one-month post-intervention. We assessed pain on the 100 mm visual analog scale (VAS). Shoulder range of motion (ROM), Constant Scores, and Nottingham Health Profile (NHP) scores were evaluated.

RESULTS

Of the 41 participants, 13 were males (32%) and 28 were females (68%). The mean age was 45 ± 15 years (range 20-65 years). We documented a significant decrease in VAS for nocturnal pain, and Constant Score in both groups. The KT group showed additional significant change in NHP pain and physical activity scores.

CONCLUSION

KT and sham taping generated similar results regarding pain and Constant Scores.

Key role of 5-HT3 receptors in the nucleus tractus solitarii in cardiovagal stress reactivity

Serotonin plays a modulatory role in central control of the autonomic nervous system (ANS). The nucleus tractus solitarii (NTS) in the medulla is an area of viscerosomatic integration innervated by both central and peripheral serotonergic fibers. Influences from different origins therefore trigger the release of serotonin into the NTS and exert multiple influences on the ANS. This major influence on the ANS is also mediated by activation of several receptors in the NTS. In particular, the NTS is the central zone with the highest density of serotonin₃ (5-HT₃) receptors. In this review, we present evidence that 5-HT₃ receptors in the NTS play a key role in one of the crucial homeostatic responses to acute and chronic stress: inhibitory modulation of the parasympathetic component of the ANS. The possible functional interactions of 5-HT₃ receptors with GABA_A and NK₁ receptors in the NTS are also discussed.

A single trial of transcutaneous electrical nerve stimulation reduces chronic neuropathic pain following median nerve injury in rats

Neuropathic pain is a devastating chronic condition and is often induced in the upper limb following nerve injury or damage. Various drugs or surgical methods have been used to manage neuropathic pain; however, these are frequently accompanied by undesirable side effects. Transcutaneous electrical nerve stimulation (TENS) is a safe and non-invasive intervention that has been used to alleviate different types of pain in the clinic, but it is unclear whether TENS can improve chronic neuropathic pain in the upper limb. Thus, the aim of this study was to investigate the effects of a single trial of TENS on chronic neuropathic pain following median nerve injury. Male rats weighing 200-250 g received median nerve-ligation of the right forearm, while the control group received only skin-incision without nerve-ligation. Neuropathic pain-behaviors, including mechanical, cold, and thermal allodynia, were measured for 4 weeks. After the development of chronic neuropathic pain, TENS (100 Hz, 200 µs, sub-motor threshold) or placebo-TENS (sham stimulation) was applied for 20 min to the ipsilateral or contralateral side. Neuropathic pain behavior was assessed before and after intervention. Median nerve-ligation significantly induced and maintained neuropathic pain in the ipsilateral side. TENS application to the ipsilateral side effectively attenuated the three forms of chronic neuropathic pain in the ipsilateral side compared to sham-treated rats (peripheral and central effects), while TENS application to contralateral side only reduced mechanical allodynia in the ipsilateral side (central effect). Our findings demonstrate that TENS can alleviate chronic neuropathic pain following median nerve injury.