Functional electrical stimulation therapy for upper extremity rehabilitation following spinal cord injury: a pilot study

STUDY DESIGN

Pilot study.

OBJECTIVES

To examine if functional electrical stimulation therapy (FEST) improves neuromuscular factors underlying upper limb function in individuals with SCI.

SETTING

A tertiary spinal cord rehabilitation center specialized in spinal cord injury care in Canada.

METHODS

We examined 29 muscles from 4 individuals living with chronic, cervical, and incomplete SCI. The analysis was focused on the changes in muscle activation, as well as on how the treatment could change the ability to control a given muscle or on how multiple muscles would be coordinated during volitional efforts.

RESULTS

There was evidence of gains in muscle strength, activation, and median frequency after the FEST. Gains in muscle activation indicated the activation of a greater number of motor units and gains in muscle median frequency the involvement of higher threshold, faster motor units. In some individuals, these changes were smaller but accompanied by increased control over muscle contraction, evident in a greater ability to sustain a volitional contraction, reduce the co-contraction of antagonist muscles, and provide cortical drive.

CONCLUSIONS

FEST increases muscle strength and activation. Enhanced control of muscle contraction, reduced co-contraction of antagonist muscles, and a greater presence of cortical drive were some of the findings supporting the effects of FEST at the sensory-motor integration level.

Electrical stimulation for the treatment of spinal cord injuries: A review of the cellular and molecular mechanisms that drive functional improvements

Spinal cord injury (SCI) is a devastating condition that causes severe loss of motor, sensory and autonomic functions. Additionally, many individuals experience chronic neuropathic pain that is often refractory to interventions. While treatment options to improve outcomes for individuals with SCI remain limited, significant research efforts in the field of electrical stimulation have made promising advancements. Epidural electrical stimulation, peripheral nerve stimulation, and functional electrical stimulation have shown promising improvements for individuals with SCI, ranging from complete weight-bearing locomotion to the recovery of sexual function. Despite this, there is a paucity of mechanistic understanding, limiting our ability to optimize stimulation devices and parameters, or utilize combinatorial treatments to maximize efficacy. This review provides a background into SCI pathophysiology and electrical stimulation methods, before exploring cellular and molecular mechanisms suggested in the literature. We highlight several key mechanisms that contribute to functional improvements from electrical stimulation, identify gaps in current knowledge and highlight potential research avenues for future studies.

Phenotypes of Motor Deficit and Pain after Experimental Spinal Cord Injury

Motor disability is a common outcome of spinal cord injury (SCI). The recovery of motor function after injury depends on the severity of neurotrauma; motor deficit can be reversible, at least partially, due to the innate tissue capability to recover, which, however, deteriorates with age. Pain is often a comorbidity of injury, although its prediction remains poor. It is largely unknown whether pain can attend motor dysfunction. Here, we implemented SCI for modelling severe and moderate neurotrauma and monitored SCI rats for up to 5 months post-injury to determine the profiles of both motor deficit and nociceptive sensitivity. Our data showed that motor dysfunction remained persistent after a moderate SCI in older animals (5-month-old); however, there were two populations among young SCI rats (1 month-old) whose motor deficit either declined or exacerbated even more over 4–5 weeks after identical injury. All young SCI rats displayed changed nociceptive sensitivity in thermal and mechanical modalities. The regression analysis of the changes revealed a population trend with respect to hyper- or hyposensitivity/motor deficit. Together, our data describe the phenotypes of motor deficit and pain, the two severe complications of neurotrauma. Our findings also suggest the predictability of motor dysfunction and pain syndromes following SCI that can be a hallmark for long-term rehabilitation and recovery after injury.

Influence of Electrotherapy with Task-Oriented Training on Spasticity, Hand Function, Upper Limb Function, and Activities of Daily Living in Patients with Subacute Stroke: A Double-Blinded, Randomized, Controlled Trial

The effects of electrotherapy with task-oriented training on upper limb function in subacute stroke patients are unclear. This study investigated the influence of transcutaneous electrical nerve stimulation (TENS) with task-oriented training on spasticity, hand function, upper limb function, and activities of daily living in patients with subacute stroke. Forty-eight patients with subacute stroke were randomly assigned to either the TENS group (n = 22) or the placebo-TENS group (n = 21). High-frequency (100 Hz) TENS with below-motor threshold intensity or placebo-TENS was applied for 30 min/day, five times a week, for 4 weeks. The two groups also received task-oriented training after TENS. The Modified Ashworth Scale (MAS), Jebsen–Taylor Hand Function Test (JTHFT), Manual Function Test (MFT), and Modified Barthel Index (MBI) were used to assess spasticity, hand function, upper limb function, and activities of daily living, respectively. There was a significant time–group interaction with the MFT (p = 0.003). The TENS group showed significantly improved MAS (p = 0.003), JTHFT (p < 0.001), MFT (p < 0.001), and MBI (p < 0.001) scores after the intervention. The placebo-TENS group showed significantly improved JTHFT (p < 0.001), MFT (p = 0.001), and MBI scores (p < 0.001). There was a significant correlation between the MFT and MBI scores (p = 0.025). These results suggest that electrotherapy with task-oriented training can be used to improve upper limb function in patients with subacute stroke.

Electrode Materials for Chronic Electrical Microstimulation

Electrical microstimulation has enabled partial restoration of vision, hearing, movement, somatosensation, as well as improving organ functions by electrically modulating neural activities. However, chronic microstimulation is faced with numerous challenges. The implantation of an electrode array into the neural tissue triggers an inflammatory response, which can be exacerbated by the delivery of electrical currents. Meanwhile, prolonged stimulation may lead to electrode material degradation., which can be accelerated by the hostile inflammatory environment. Both material degradation and adverse tissue reactions can compromise stimulation performance over time. For stable chronic electrical stimulation, an ideal microelectrode must present 1) high charge injection limit, to efficiently deliver charge without exceeding safety limits for both tissue and electrodes, 2) small size, to gain high spatial selectivity, 3) excellent biocompatibility that ensures tissue health immediately next to the device, and 4) stable in vivo electrochemical properties over the application period. In this review, the challenges in chronic microstimulation are described in detail. To aid material scientists interested in neural stimulation research, the in vitro and in vivo testing methods are introduced for assessing stimulation functionality and longevity and a detailed overview of recent advances in electrode material research and device fabrication for improving chronic microstimulation performance is provided.

Insights on the Potential Mechanisms of Action of Functional Electrical Stimulation Therapy in Combination With Task-Specific Training: A Scoping Review

OBJECTIVES

This scoping review was undertaken to synthetize and appraise the literature on the potential mechanisms of action of functional electrical stimulation therapy in combination with task-specific training (FEST + TST) in the rehabilitation following stroke, spinal cord injury, traumatic brain injury, or multiple sclerosis.

MATERIALS AND METHODS

The literature search was performed using multiple databases (including APA, PsycInfo, Medline, PubMed, EMBASE, CCRCT, and Cochrane Database of Systematic Reviews) from 1946 to June 2020. The literature search used the following terms: (spinal cord injury, paraplegia, tetraplegia, quadriplegia, stroke, multiple sclerosis, traumatic brain injury, or acquired brain injury) AND (functional electrical stimulation or FES). The search included clinical and preclinical studies without limits to language.

RESULTS

Of the 8209 titles retrieved from the primary search, 57 publications fulfilled the inclusion and exclusion criteria for this scoping review. While most publications were clinical studies (n = 50), there were only seven preclinical studies using animal models. The results of this review suggest that FEST + TST can result in multiple effects on different elements from the muscle to the cerebral cortex. However, most studies were focused on the muscle changes after FEST + TST.

CONCLUSIONS

The results of this scoping review suggest that FEST + TST can result in multiple effects on different elements of the neuromuscular system, while most research studies were focused on the muscle changes after FEST + TST. Despite the efficacy of the FEST + TST in the neurorehabilitation after CNS injury or disease, the results of this review underline an important knowledge gap with regards to the actual mechanism of action of FEST + TST.

Enriching neural stem cell and anti-inflammatory glial phenotypes with electrical stimulation after traumatic brain injury in male rats

Traumatic brain injury (TBI) by an external physical impact results in compromised brain function via undesired neuronal death. Following the injury, resident and peripheral immune cells, astrocytes, and neural stem cells (NSCs) cooperatively contribute to the recovery of the neuronal function after TBI. However, excessive pro‐inflammatory responses of immune cells, and the disappearance of endogenous NSCs at the injury site during the acute phase of TBI, can exacerbate TBI progression leading to incomplete healing. Therefore, positive outcomes may depend on early interventions to control the injury‐associated cellular milieu in the early phase of injury. Here, we explore electrical stimulation (ES) of the injury site in a rodent model (male Sprague–Dawley rats) to investigate its overall effect on the constituent brain cell phenotype and composition during the acute phase of TBI. Our data showed that a brief ES for 1 hr on day 2 of TBI promoted anti‐inflammatory phenotypes of microglia as assessed by CD206 expression and increased the population of NSCs and Nestin⁺ astrocytes at 7 days post‐TBI. Also, ES effectively increased the number of viable neurons when compared to the unstimulated control group. Given the salience of microglia and neural stem cells for healing after TBI, our results strongly support the potential benefit of the therapeutic use of ES during the acute phase of TBI to regulate neuroinflammation and to enhance neuroregeneration.

Analgesic Tolerance Development during Repetitive Electric Stimulations Is Associated with Changes in the Expression of Activated Microglia in Rats with Osteoarthritis

Electric stimulation is used for managing osteoarthritic (OA) pain; however, little is known about the development of analgesic tolerance during repeated stimulations and the relation of spinal microglia with OA pain. We investigated the changes in the analgesic effects of repeated electric stimulations and the relation between the development of analgesic tolerance and spinal microglial expression in rats with OA. To induce OA, monosodium iodoacetate was injected into the synovial space of the right knee joint of the rats (n = 185). Repeated high frequency, low frequency, or sham transcutaneous electric nerve stimulation (TENS) was performed to the ipsilateral knee joint for 20 min in rats with OA (n = 45). Minocycline or minocycline plus TENS (HF, LF, or sham) was treated in OA rats with repeated TENS-induced tolerance (n = 135). Immunohistochemistry of the microglia in the L3–L5 spinal segments was performed. Knee joint pain during passive movement of the knee joint were quantified using the knee-bend score and the proportion of activated microglia was calculated as primary variables. Paw withdrawal threshold (hypersensitivity to mechanical stimuli) was assessed and the resting and activated microglia were counted as secondary variables. Repeated applications decreased the analgesic effect of TENS on OA pain and failed to reduce the expression of activated microglia in the spinal cord. However, spinal microglial inhibition by minocycline restored the analgesic effect of TENS on OA pain in TENS-tolerant OA rats. TENS combined with minocycline treatment improved knee joint pain and mechanical hypersensitivity in TENS-tolerant OA rats, and inhibited the expression of activated microglia in the spinal cord. These results suggest a possible relationship between repetitive electric stimulation-induced analgesic tolerance for OA pain control and changes in microglia activation.

Electroconvulsive stimulation attenuates chronic neuroinflammation

Electroconvulsive therapy is highly effective in resistant depression by unknown mechanisms. Microglial toxicity was suggested to mediate depression and plays key roles in neuroinflammatory and degenerative diseases, where there is critical shortage in therapies. We examined the effects of electroconvulsive seizures (ECS) on chronic neuroinflammation and microglial neurotoxicity. Electric brain stimulation inducing full tonic-clonic seizures during chronic relapsing-progressive experimental autoimmune encephalomyelitis (EAE) reduced spinal immune cell infiltration, reduced myelin and axonal loss, and prevented clinical deterioration. Using the transfer EAE model, we examined the effect of ECS on systemic immune response in donor mice versus ECS effect on CNS innate immune activity in recipient mice. ECS did not affect encephalitogenicity of systemic T cells, but it targeted the CNS directly to inhibit T cell-induced neuroinflammation. In vivo and ex vivo assays indicated that ECS suppressed microglial neurotoxicity by reducing inducible NOS expression, nitric oxide, and reactive oxygen species (ROS) production, and by reducing CNS oxidative stress. Microglia from ECS-treated EAE mice expressed less T cell stimulatory and chemoattractant factors. Our findings indicate that electroconvulsive therapy targets the CNS innate immune system to reduce neuroinflammation by attenuating microglial neurotoxicity. These findings signify a potentially novel therapeutic approach for chronic neuroinflammatory, neuropsychiatric, and neurodegenerative diseases.

Transcutaneous Electrical Nerve Stimulation Reduces Knee Osteoarthritic Pain by Inhibiting Spinal Glial Cells in Rats

BACKGROUND

Transcutaneous electrical nerve stimulation (TENS) is commonly used for pain control. However, the effects of TENS on osteoarthritis (OA) pain and potential underlying mechanisms remain unclear.

OBJECTIVE

The objective of this study was to investigate the effect of TENS on OA pain treatment and underlying mechanisms related to glial cell inhibition.

DESIGN

This was an experimental study.

METHODS

OA was induced by injection of monosodium iodoacetate into the synovial space of the right knee joint of rats. High-frequency (HF) TENS (100 Hz), low-frequency (LF) TENS (4 Hz), or sham TENS was applied to the ipsilateral knee joint for 20 minutes. Paw withdrawal threshold (PWT), weight bearing, and knee bend score (KBS) were measured. Immunohistochemistry for microglia and astrocytes was performed with L3 to L5 spinal segment samples. To investigate the effects of glial inhibition on OA pain, minocycline, l-α-aminoadipate, or artificial cerebrospinal fluid was injected intrathecally, and PWT and KBS were measured.

RESULTS

Compared with sham TENS, both HF TENS and LF TENS significantly increased PWT, decreased KBS, and inhibited activated microglia in the L3 to L5 segments but did not decrease the total number of microglia, except in the L4 segment (HF TENS). Astrocyte expression was significantly decreased in the L3 to L5 segments following LF TENS and in the L3 segment following HF TENS. Compared with artificial cerebrospinal fluid, both minocycline and l-α-aminoadipate increased PWT and decreased KBS.

LIMITATIONS

These results cannot be generalized to humans.

CONCLUSIONS

TENS alleviates OA pain in rats by inhibiting activated microglia and reducing astrocyte expression in the spinal cord. Although these results may not be generalizable to chronic pain in patients with OA, within the limitation of the experimental animal model used in the present study, they suggest a possible mechanism and preclinical evidence supporting further experimentation or clinical use of TENS in humans.

Role of the BDNF-TrkB pathway in KCC2 regulation and rehabilitation following neuronal injury: A mini review

In most mature neurons, low levels of intracellular Cl^- concentrations ([Cl^-]_i) are maintained by channels and transporters, particularly the K⁺-Cl^- cotransporter 2 (KCC2), which is the only Cl^- extruder in most neurons. Recent studies have implicated KCC2 expression in the molecular mechanisms underlying neuronal disorders, such as spasticity, epilepsy and neuropathic pain. Alterations in KCC2 expression have been associated with brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB). The present review summarizes recent progress regarding the roles of Cl^- regulators in immature and mature neurons. Moreover, we focus on the role of KCC2 regulation via the BDNF-TrkB pathway in spinal cord injury and rehabilitation, as prior studies have shown that the BDNF-TrkB pathway can affect both the pathological development and functional amelioration of spinal cord injuries. Evidence suggests that rehabilitation using active exercise and mechanical stimulation can attenuate spasticity and neuropathic pain in animal models, likely due to the upregulation of KCC2 expression via the BDNF-TrkB pathway. Moreover, research suggests that such rehabilitation efforts may recover KCC2 expression without the use of exogenous BDNF.

Spinal cord stimulation postconditioning reduces microglial activation through down-regulation of ERK1/2 phosphorylation during spinal cord ischemic reperfusion in rabbits

Microglial activation plays a critical role in spinal cord ischemic reperfusion injury. Spinal cord stimulation preconditioning and postconditioning has shown spinal cord protection in ischemic reperfusion injury in animal studies. However, whether spinal cord stimulation could reduce microglial activation is still unclear. In this study, rabbits experienced 28-min infrarenal aorta occlusion and reperfusion for 8 h, 1, 3, and 7 days correspondingly. Immediately after reperfusion, rabbits received spinal cord stimulation of 2 or 50 Hz for 30 min and daily for a week. The results showed that spinal cord stimulation of 2 Hz reduced microglial activation. Microglial activation was accompanied with up-regulated p-ERK1/2, and microglial inhibition by 2 Hz spinal cord stimulation was associated with down-regulated p-ERK1/2. Spinal cord stimulation increased the expression of IL-1β. Our results revealed, for the first time, that spinal cord stimulation postconditioning suppresses microglial activation during spinal cord ischemic reperfusion by down-regulation of p-ERK1/2, which may be the protective mechanism of spinal cord stimulation.

Opposite, bidirectional shifts in excitation and inhibition in specific types of dorsal horn interneurons are associated with spasticity and pain post-SCI

Spasticity, a common complication after spinal cord injury (SCI), is frequently accompanied by chronic pain. The physiological origin of this pain (critical to its treatment) remains unknown, although spastic motor dysfunction has been related to the hyperexcitability of motoneurons and to changes in spinal sensory processing. Here we show that the pain mechanism involves changes in sensory circuits of the dorsal horn (DH) where nociceptive inputs integrate for pain processing. Spasticity is associated with the DH hyperexcitability resulting from an increase in excitation and disinhibition occurring in two respective types of sensory interneurons. In the tonic-firing inhibitory lamina II interneurons, glutamatergic drive was reduced while glycinergic inhibition was potentiated. In contrast, excitatory drive was boosted to the adapting-firing excitatory lamina II interneurons while GABAergic and glycinergic inhibition were reduced. Thus, increased activity of excitatory DH interneurons coupled with the reduced excitability of inhibitory DH interneurons post-SCI could provide a neurophysiological mechanism of central sensitization and chronic pain associated with spasticity.

Endocannabinoid activation of CB1 receptors contributes to long-lasting reversal of neuropathic pain by repetitive spinal cord stimulation

BACKGROUND

Spinal cord stimulation (SCS) has been shown to be effective in the management of certain neuropathic pain conditions, however, the underlying mechanisms are incompletely understood. In this study, we investigated repetitive SCS in a rodent neuropathic pain model, revealing long-lasting and incremental attenuation of hyperalgesia and a mechanism of action involving endocannabinoids.

METHOD

Animals were implanted with monopolar electrodes at the time of partial sciatic nerve injury. Dorsal columns at spinal segments T12/13 were stimulated 3 days later (early SCS), and again at day 7 (late SCS) using low-frequency parameters. Hypersensitivity to cutaneous mechanical stimuli was assessed using von Frey filaments. Pharmacological agents, selected to identify endocannabinoid and opioid involvement, were administered intraperitoneally, 10 min before SCS.

RESULTS

Early SCS caused partial reversal of mechanical hypersensitivity with corresponding changes in the biomarker of central sensitization, [phospho-Tyr¹⁴⁷² ]-GluN2B. The partial reversal of hyperalgesia by early SCS was amplified by co-administration of LY 2183240, an inhibitor of endocannabinoid reuptake/breakdown. This amplification was inhibited by a CB₁ R antagonist, AM251, but not by a CB₂ R antagonist, AM630. Early SCS-induced reversal of hyperalgesia was attenuated by naloxone, indicating a role for opioids. Late SCS resulted in an incremental level of reversal of hyperalgesia, which was inhibited by AM251, but not by CB₂ or opioid receptor antagonists.

CONCLUSION

The endocannabinoid system, and in particular the CB₁ R, plays a pivotal role in the long-lasting and incremental reversal of hyperalgesia induced by repetitive SCS in a neuropathic pain model.

SIGNIFICANCE

Alternative parameters for repetitive spinal cord stimulation (SCS) at 25/10 Hz elicit particularly long-lasting and incremental reversal of hyperalgesia in a neuropathic pain model through a mechanism involving endocannabinoids.

Preclinical models of muscle spasticity: valuable tools in the development of novel treatment for neurological diseases and conditions

Poor validity of preclinical animal models is one of the most commonly discussed explanations for the failures to develop novel drugs in general and in neuroscience in particular. However, there are several areas of neuroscience such as injury-induced spasticity where etiological factor can be adequately recreated and models can focus on specific pathophysiological mechanisms that likely contribute to spasticity syndrome in humans (such as motoneuron hyperexcitability and spinal hyperreflexia). Methods used to study spasticity in preclinical models are expected to have a high translational value (e.g., electromyogram (EMG)-based electrophysiological tools) and can efficiently assist clinical development programs. However, validation of these models is not complete yet. First, true predictive validity of these models is not established as clinically efficacious drugs have been used to reverse validate preclinical models while newly discovered mechanisms effective in preclinical models are yet to be fully explored in humans (e.g., 5-HT2C receptor inverse agonists, fatty acid amid hydrolase inhibitors). Second, further efforts need to be invested into cross-laboratory validation of study protocols and tools, adherence to the highest quality standards (blinding, randomization, pre-specified study endpoints, etc.), and systematic efforts to replicate key sets of data. These appear to be readily achievable tasks that will enable development not only of symptomatic but also of disease-modifying therapy of spasticity, an area that seems to be currently not in focus of research efforts.