EMG for assessing the recovery of voluntary movement after acute spinal cord injury in man

Passive activity enhances residual control ability in patients with complete spinal cord injury

JOURNAL/nrgr/04.03/01300535-202508000-00024/figure1/v/2024-09-30T120553Z/r/image-tiff Patients with complete spinal cord injury retain the potential for volitional muscle activity in muscles located below the spinal injury level. However, because of prolonged inactivity, initial attempts to activate these muscles may not effectively engage any of the remaining neurons in the descending pathway. A previous study unexpectedly found that a brief clinical round of passive activity significantly increased volitional muscle activation, as measured by surface electromyography. In this study, we further explored the effect of passive activity on surface electromyographic signals during volitional control tasks among individuals with complete spinal cord injury. Eleven patients with chronic complete thoracic spinal cord injury were recruited. Surface electromyography data from eight major leg muscles were acquired and compared before and after the passive activity protocol. The results indicated that the passive activity led to an increased number of activated volitional muscles and an increased frequency of activation. Although the cumulative root mean square of surface electromyography amplitude for volitional control of movement showed a slight increase after passive activity, the difference was not statistically significant. These findings suggest that brief passive activity may enhance the ability to initiate volitional muscle activity during surface electromyography tasks and underscore the potential of passive activity for improving residual motor control among patients with motor complete spinal cord injury.

Enhanced inhibitory input to triceps brachii in humans with spinal cord injury

Most individuals with cervical spinal cord injury (SCI) show increased muscle weakness in the elbow extensor compared to elbow flexor muscles. Although this is a well-known functional deficit, the underlying neural mechanisms remain poorly understood. To address this question, we measured the suppression of voluntary electromyographic activity (svEMG; a measurement thought to reflect changes in intracortical inhibition) by applying low-intensity transcranial magnetic stimulation over the arm representation of the primary motor cortex during 10% of isometric maximal voluntary contraction (MVC) into elbow flexion or extension in individuals with and without chronic cervical SCI. We found that the svEMG latency and duration were not different between the biceps and triceps brachii in controls but prolonged in the triceps in individuals with SCI. The svEMG area was larger in the triceps compared to the biceps in both groups and further increased in SCI participants, suggesting a pronounced intracortical inhibitory input during elbow extension. A negative correlation was found between svEMG area and MVCs indicating that control and SCI participants with lower svEMG area had larger MVCs. The svEMG area was not different between 5% and 30% of MVC, making it less probable that differences in muscle strength between groups contributed to our results. These findings support the existence of strong inhibitory input to corticospinal projections controlling elbow extensor compared to flexor muscles, which is more pronounced after chronic cervical SCI. KEY POINTS: After cervical spinal cord injury (SCI), people often recover function in elbow flexor, but much less in elbow extensor muscles. The neural mechanisms contributing to this difference remain unknown. We measured the suppression of voluntary electromyographic activity (svEMG) elicited through low-intensity transcranial magnetic stimulation of the primary motor cortex (assumed to reflect changes in intracortical inhibition) in the biceps and triceps muscles in controls and individuals with cervical chronic incomplete SCI. We found increased svEMG area in the triceps compared to the biceps in controls and SCI participants, with this measurement being even more pronounced in the triceps after SCI. The svEMG area correlated with maximal voluntary contraction values in both groups, suggesting the people with lesser inhibition had larger motor output. Our results support the presence of strong cortical inhibitory input to corticospinal projections controlling elbow extensor compared to elbow flexors muscles after cervical SCI.

Validity and reliability study of a novel surface electromyography sensor using a well-consolidated electromyography system in individuals with cervical spinal cord injury

STUDY DESIGN

Non-interventional, cross-sectional pilot study.

OBJECTIVES

To establish the validity and reliability of the BioStamp nPoint biosensor (Medidata Solutions, New York, NY, USA [formerly MC10, Inc.]) for measuring electromyography in individuals with cervical spinal cord injury (SCI) by comparing the surface electromyography (sEMG) metrics with the Trigno wireless electromyography system (Delsys, Natick, MA, USA).

SETTING

Participants were recruited from the Shirley Ryan AbilityLab registry.

METHODS

Individuals aged 18-70 years with cervical SCI were evaluated with the two biosensors to capture activity on upper-extremity muscles during two study sessions conducted over 2 days (day 1-consent alone; day 2-two data collections in same session). Time and frequency metrics were captured, and signal-to-noise ratio was determined for each muscle group. Test-retest reliability was determined using Pearson's correlation. Validation of the BioStamp nPoint system was based on Bland-Altmann analysis.

RESULTS

Among the 11 participants, 30.8% had subacute cervical injury at C5-C6; 53.8% were injured within 1 year of the study. Results from the test-retest reliability assessment revealed that most Pearson's correlations between the two sensory measurements were strong (≥0.50). The Bland-Altman analysis found values of the signal-to-noise ratio, frequency, and peak amplitude were within the level of agreement. Signal-to-noise ratios ranged from 7.06 to 22.1.

CONCLUSIONS

In most instances, the performance of the BioStamp nPoint sensors was moderately to strongly correlated with that of the Trigno sensors in all muscle groups tested. The BioStamp nPoint system is a valid and reliable approach to assess sEMG measures in individuals with cervical SCI.

SPONSORSHIP

The present study was supported by AbbVie Inc.

A computational model of surface electromyography signal alterations after spinal cord injury

Objective. Spinal cord injury (SCI) can cause significant impairment and disability with an impact on the quality of life for individuals with SCI and their caregivers. Surface electromyography (sEMG) is a sensitive and non-invasive technique to measure muscle activity and has demonstrated great potential in capturing neuromuscular changes resulting from SCI. The mechanisms of the sEMG signal characteristic changes due to SCI are multi-faceted and difficult to studyin vivo. In this study, we utilized well-established computational models to characterize changes in sEMG signal after SCI and identify sEMG features that are sensitive and specific to different aspects of the SCI.Approach. Starting from existing models for motor neuron pool organization and motor unit action potential generation for healthy neuromuscular systems, we implemented scenarios to model damages to upper motor neurons, lower motor neurons, and the number of muscle fibers within each motor unit. After simulating sEMG signals from each scenario, we extracted time and frequency domain features and investigated the impact of SCI disruptions on sEMG features using the Kendall Rank Correlation analysis.Main results. The commonly used amplitude-based sEMG features (such as mean absolute values and root mean square) cannot differentiate between injury scenarios, but a broader set of features (including autoregression and cepstrum coefficients) provides greater specificity to the type of damage present.Significance. We introduce a novel approach to mechanistically relate sEMG features (often underused in SCI research) to different types of neuromuscular alterations that may occur after SCI. This work contributes to the further understanding and utilization of sEMG in clinical applications, which will ultimately improve patient outcomes after SCI.

Testing a Novel Wearable Device for Motor Recovery of the Elbow Extensor Triceps Brachii in Chronic Spinal Cord Injury

After corticospinal tract damage, reticulospinal connections to motoneurons strengthen preferentially to flexor muscles. This could contribute to the disproportionately poor recovery of extensors often seen after spinal cord injury (SCI) and stroke. In this study, we paired electrical stimulation over the triceps muscle with auditory clicks, using a wearable device to deliver stimuli over a prolonged period of time. Healthy human volunteers wore the stimulation device for ∼6 h and a variety of electrophysiological assessments were used to measure changes in triceps motor output. In contrast to previous results in the biceps muscle, paired stimulation: (1) did not increase the StartReact effect; (2) did not decrease the suppression of responses to transcranial magnetic brain stimulation (TMS) following a loud sound; (3) did not enhance muscle responses elicited by a TMS coil oriented to induce anterior-posterior current. In a second study, chronic cervical SCI survivors wore the stimulation device for ∼4 h every day for four weeks; this was compared with a four-week period without wearing the device. Functional and electrophysiological assessments were repeated at week 0, week 4, and week 8. No significant changes were observed in electrophysiological assessments after paired stimulation. Functional measurements such as maximal force and variability and speed of trajectories made during a planar reaching task also remained unchanged. Our results suggest that the triceps muscle shows less potential for plasticity than biceps; pairing clicks with muscle stimulation does not seem beneficial in enhancing triceps recovery after SCI.

Paired pulse transcranial magnetic stimulation in the assessment of biceps voluntary activation in individuals with tetraplegia

After spinal cord injury (SCI), motoneuron death occurs at and around the level of injury which induces changes in function and organization throughout the nervous system, including cortical changes. Muscle affected by SCI may consist of both innervated (accessible to voluntary drive) and denervated (inaccessible to voluntary drive) muscle fibers. Voluntary activation measured with transcranial magnetic stimulation (VATMS) can quantify voluntary cortical/subcortical drive to muscle but is limited by technical challenges including suboptimal stimulation of target muscle relative to its antagonist. The motor evoked potential (MEP) in the biceps compared to the triceps (i.e., MEP ratio) may be a key parameter in the measurement of biceps VATMS after SCI. We used paired pulse TMS, which can inhibit or facilitate MEPs, to determine whether the MEP ratio affects VATMS in individuals with tetraplegia. Ten individuals with tetraplegia following cervical SCI and ten non-impaired individuals completed single pulse and paired pulse VATMS protocols. Paired pulse stimulation was delivered at 1.5, 10, and 30 ms inter-stimulus intervals (ISI). In both the SCI and non-impaired groups, the main effect of the stimulation pulse (paired pulse compared to single pulse) on VATMS was not significant in the linear mixed-effects models. In both groups for the stimulation parameters we tested, the MEP ratio was not modulated across all effort levels and did not affect VATMS. Linearity of the voluntary moment and superimposed twitch moment relation was lower in SCI participants compared to non-impaired. Poor linearity in the SCI group limits interpretation of VATMS. Future work is needed to address methodological issues that limit clinical application of VATMS.

The assessment of biceps voluntary activation with transcranial magnetic stimulation in individuals with tetraplegia

BACKGROUND: Assessment of voluntary activation is useful in the study of neuromuscular impairments, particularly after spinal cord injury (SCI). Measurement of voluntary activation with transcranial magnetic stimulation (VATMS) is limited by technical challenges, including the difficulty in preferential stimulation of cortical neurons projecting to the target muscle and minimal stimulation of antagonists. Thus, the motor evoked potential (MEP) response to TMS in the target muscle compared to its antagonist may be an important parameter in the assessment of VATMS. OBJECTIVE: The purpose of this study was to evaluate the effect of isometric elbow flexion angle on two metrics in individuals with tetraplegia following SCI: 1) the ratio of biceps/triceps MEP amplitude across a range of voluntary efforts, and 2) VATMS. METHODS: Ten individuals with tetraplegia and ten nonimpaired individuals were recruited to participate in three sessions wherein VATMS was assessed at 45°, 90°, and 120° of isometric elbow flexion. RESULTS: In SCI participants, the biceps/triceps MEP ratio was not modulated by elbow angle. In nonimpaired participants, the biceps/triceps MEP ratio was greater in the more flexed elbow angle (120° flexion) compared to 90° during contractions of 50% and 75% MVC, but VATMS was not different. VATMS assessed in the more extended elbow angle (45° flexion) was lower relative to 90° elbow flexion; this effect was dependent on the biceps/triceps MEP ratio. In both groups, VATMS was sensitive to the linearity of the voluntary moment and superimposed twitch relationship, regardless of elbow angle. Linearity was lower in SCI relative to nonimpaired participants. CONCLUSIONS: Increasing the MEP ratio via elbow angle did not enable estimation of VATMS in SCI participants. VATMS may not be a viable approach to assess neuromuscular function in individuals with tetraplegia.

Reduced Muscle Activity of the Upper Extremity in Individuals with Spinal Cord Injuries

Compromised physical ability due to musculoskeletal impairment among spinal cord injury (SCI) patients is known to negatively affect their quality of life. It is essential to comprehensively understand the muscle strength of the upper extremity among patients with SCI to enhance muscle function and capacity to engage in an active lifestyle. The objective of this study was to evaluate the muscle strength of 15 upper extremity muscles among patients with SCI and compare the relative weakness of individual muscles to the control group. Seven male patients with SCI with ASIA impairment scale D and E and 33 males in the control group participated in this study. Each participant performed maximal voluntary contraction of individual muscles, and the electromyography data were recorded. The results showed that the majority of the upper extremity muscles (12 out of 15) showed considerable weakness (24 to 53%) relative to the control group. Furthermore, the relative strength (ranking) of individual muscles among 15 upper extremity muscles was different between patients with SCI and the control group. This information would be useful to the selective strengthening of specific muscles as an intensive rehabilitation effort and prevent overuse and adverse injuries due to excessive muscle training.

Quantitative electrophysiological assessments as predictive markers of lower limb motor recovery after spinal cord injury: a pilot study with an adaptive trial design

STUDY DESIGN

Observational, cohort study.

OBJECTIVES

(1) Determine the feasibility and relevance of assessing corticospinal, sensory, and spinal pathways early after traumatic spinal cord injury (SCI) in a rehabilitation setting. (2) Validate whether electrophysiological and magnetic resonance imaging (MRI) measures taken early after SCI could identify preserved neural pathways, which could then guide therapy.

SETTING

Intensive functional rehabilitation hospital (IFR).

METHODS

Five individuals with traumatic SCI and eight controls were recruited. The lower extremity motor score (LEMS), electrical perceptual threshold (EPT) at the S2 dermatome, soleus (SOL) H-reflex, and motor evoked potentials (MEPs) in the tibialis anterior (TA) muscle were assessed during the stay in IFR and in the chronic stage (>6 months post-SCI). Control participants were only assessed once. Feasibility criteria included the absence of adverse events, adequate experimental session duration, and complete dataset gathering. The relationship between electrophysiological data collected in IFR and LEMS in the chronic phase was studied. The admission MRI was used to calculate the maximal spinal cord compression (MSCC).

RESULTS

No adverse events occurred, but a complete dataset could not be collected for all subjects due to set-up configuration limitations and time constraints. EPT measured at IFR correlated with LEMS in the chronic phases (r = -0.67), whereas SOL H/M ratio, H latency, MEPs and MSCC did not.

CONCLUSIONS

Adjustments are necessary to implement electrophysiological assessments in an IFR setting. Combining MRI and electrophysiological measures may lead to better assessment of neuronal deficits early after SCI.

Validity and Reliability of Surface Electromyography Features in Lower Extremity Muscle Contraction in Healthy and Spinal Cord-Injured Participants

Background: Spinal cord injury (SCI) has a significant impact on motor control and active force generation. Quantifying muscle activation following SCI may help indicate the degree of motor impairment and predict the efficacy of rehabilitative interventions. In healthy persons, muscle activation is typically quantified by electromyographic (EMG) signal amplitude measures. However, in SCI, these measures may not reflect voluntary effort, and therefore other nonamplitude-based features should be considered. Objectives: The purpose of this study was to assess the correlation of time-domain EMG features with the exerted joint torque (validity) and their test-retest repeatability (reliability), which may contribute to characterizing muscle activation following SCI. Methods: Surface EMG (SEMG) and torque were measured while nine uninjured participants and four participants with SCI performed isometric contractions of tibialis anterior (TA) and soleus (SOL). Data collection was repeated at a subsequent session for comparison across days. Validity and test-retest reliability of features were assessed by Spearman and intraclass correlation (ICC) of linear regression coefficients. Results: In healthy participants, SEMG features correlated well with torque (TA: ρ > 0.92; SOL: ρ > 0.94) and showed high reliability (ICC_mean = 0.90; range, 0.72-0.99). In an SCI case series, SEMG features also correlated well with torque (TA: ρ > 0.86; SOL: ρ > 0.86), and time-domain features appeared no less repeatable than amplitude-based measures. Conclusion: Time-domain SEMG features are valid and reliable measures of lower extremity muscle activity in healthy participants and may be valid measures of sublesional muscle activity following SCI. These features could be used to gauge motor impairment and progression of rehabilitative interventions or in controlling assistive technologies.

Electrical epidural stimulation of the cervical spinal cord: implications for spinal respiratory neuroplasticity after spinal cord injury

Traumatic cervical spinal cord injury (cSCI) can lead to damage of bulbospinal pathways to the respiratory motor nuclei and consequent life-threatening respiratory insufficiency due to respiratory muscle paralysis/paresis. Reports of electrical epidural stimulation (EES) of the lumbosacral spinal cord to enable locomotor function after SCI are encouraging, with some evidence of facilitating neural plasticity. Here, we detail the development and success of EES in recovering locomotor function, with consideration of stimulation parameters and safety measures to develop effective EES protocols. EES is just beginning to be applied in other motor, sensory, and autonomic systems; however, there has only been moderate success in preclinical studies aimed at improving breathing function after cSCI. Thus, we explore the rationale for applying EES to the cervical spinal cord, targeting the phrenic motor nucleus for the restoration of breathing. We also suggest cellular/molecular mechanisms by which EES may induce respiratory plasticity, including a brief examination of sex-related differences in these mechanisms. Finally, we suggest that more attention be paid to the effects of specific electrical parameters that have been used in the development of EES protocols and how that can impact the safety and efficacy for those receiving this therapy. Ultimately, we aim to inform readers about the potential benefits of EES in the phrenic motor system and encourage future studies in this area.

Properties of the surface electromyogram following traumatic spinal cord injury: a scoping review

Traumatic spinal cord injury (SCI) disrupts spinal and supraspinal pathways, and this process is reflected in changes in surface electromyography (sEMG). sEMG is an informative complement to current clinical testing and can capture the residual motor command in great detail-including in muscles below the level of injury with seemingly absent motor activities. In this comprehensive review, we sought to describe how the sEMG properties are changed after SCI. We conducted a systematic literature search followed by a narrative review focusing on sEMG analysis techniques and signal properties post-SCI. We found that early reports were mostly focused on the qualitative analysis of sEMG patterns and evolved to semi-quantitative scores and a more detailed amplitude-based quantification. Nonetheless, recent studies are still constrained to an amplitude-based analysis of the sEMG, and there are opportunities to more broadly characterize the time- and frequency-domain properties of the signal as well as to take fuller advantage of high-density EMG techniques. We recommend the incorporation of a broader range of signal properties into the neurophysiological assessment post-SCI and the development of a greater understanding of the relation between these sEMG properties and underlying physiology. Enhanced sEMG analysis could contribute to a more complete description of the effects of SCI on upper and lower motor neuron function and their interactions, and also assist in understanding the mechanisms of change following neuromodulation or exercise therapy.

Distinct Corticospinal and Reticulospinal Contributions to Voluntary Control of Elbow Flexor and Extensor Muscles in Humans with Tetraplegia

Humans with cervical spinal cord injury (SCI) often recover voluntary control of elbow flexors and, to a much lesser extent, elbow extensor muscles. The neural mechanisms underlying this asymmetrical recovery remain unknown. Anatomical and physiological evidence in animals and humans indicates that corticospinal and reticulospinal pathways differentially control elbow flexor and extensor motoneurons; therefore, it is possible that reorganization in these pathways contributes to the asymmetrical recovery of elbow muscles after SCI. To test this hypothesis, we examined motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation over the arm representation of the primary motor cortex, maximal voluntary contractions, the StartReact response (a shortening in reaction time evoked by a startling stimulus), and the effect of an acoustic startle cue on MEPs elicited by cervicomedullary stimulation (CMEPs) on biceps and triceps brachii in males and females with and without chronic cervical incomplete SCI. We found that SCI participants showed similar MEPs and maximal voluntary contractions in biceps but smaller responses in triceps compared with controls, suggesting reduced corticospinal inputs to elbow extensors. The StartReact and CMEP facilitation was larger in biceps but similar to controls in triceps, suggesting enhanced reticulospinal inputs to elbow flexors. These findings support the hypothesis that the recovery of biceps after cervical SCI results, at least in part, from increased reticulospinal inputs and that the lack of these extra inputs combined with the loss of corticospinal drive contribute to the pronounced weakness found in triceps.SIGNIFICANCE STATEMENT Although a number of individuals with cervical incomplete spinal cord injury show limited functional recovery of elbow extensors compared with elbow flexor muscles, to date, the neural mechanisms underlying this asymmetrical recovery remain unknown. Here, we provide for the first time evidence for increased reticulospinal inputs to biceps but not triceps brachii and loss of corticospinal drive to triceps brachii in humans with tetraplegia. We propose that this reorganization in descending control contributes to the asymmetrical recovery between elbow flexor and extensor muscles after cervical spinal cord injury.

Neurophysiological Characterization of a Non-Human Primate Model of Traumatic Spinal Cord Injury Utilizing Fine-Wire EMG Electrodes

This study aims to characterize traumatic spinal cord injury (TSCI) neurophysiologically using an intramuscular fine-wire electromyography (EMG) electrode pair. EMG data were collected from an agonist-antagonist pair of tail muscles of Macaca fasicularis, pre- and post-lesion, and for a treatment and control group. The EMG signals were decomposed into multi-resolution subsets using wavelet transforms (WT), then the relative power (RP) was calculated for each individual reconstructed EMG sub-band. Linear mixed models were developed to test three hypotheses: (i) asymmetrical volitional activity of left and right side tail muscles (ii) the effect of the experimental TSCI on the frequency content of the EMG signal, (iii) and the effect of an experimental treatment. The results from the electrode pair data suggested that there is asymmetry in the EMG response of the left and right side muscles (p-value < 0.001). This is consistent with the construct of limb dominance. The results also suggest that the lesion resulted in clear changes in the EMG frequency distribution in the post-lesion period with a significant increment in the low-frequency sub-bands (D4, D6, and A6) of the left and right side, also a significant reduction in the high-frequency sub-bands (D1 and D2) of the right side (p-value < 0.001). The preliminary results suggest that using the RP of the EMG data, the fine-wire intramuscular EMG electrode pair are a suitable method of monitoring and measuring treatment effects of experimental treatments for spinal cord injury (SCI).

Superconditioning TMS unmasks latent voluntary innervation in MND - A case report

Motor neuron disease (MND) includes both ALS and Progressive Muscular Atrophy (PMA) as variants. Abnormalities in brain excitability and upper motor neuron (UMN) function are characteristic of ALS, but by definition are absent in PMA. Transcranial magnetic stimulation (TMS) may be useful in demonstrating UMN pathology, but loss of muscle responsiveness with disease progression limits its usefulness in later stages of MND. We have developed a novel form of TMS comprised of 4 stimulating pulses that can enhance MEPs in target muscles already responding to traditional TMS inputs, in some cases even restoring MEPs in target muscles rendered unresponsive by the disease. An example of restored MEPs in response to this superconditioning TMS pattern (TMSsc) in a person with PMA is described, along with an unexpected finding. Despite a prolonged (> 5 year) history of movement paralysis in his right tibialis anterior (TA), immediately after cessation of TMSsc delivery the subject could now easily contract and relax this muscle; the presence of a latent pathway for voluntary innervation of his right TA was revealed. This modulation of central motor functional connectivity in response to TMSsc suggests a further, clinically-significant benefit of this form of noninvasive brain stimulation beyond its ability to enhance MEPs to traditional TMS inputs.

Characterization of Volitional Electromyographic Signals in the Lower Extremity After Motor Complete Spinal Cord Injury

BACKGROUND

Previous studies have demonstrated the presence of intact axons across a spinal cord lesion, even in those clinically diagnosed with complete spinal cord injury (SCI). These axons may allow volitional motor signals to be transmitted through the injury, even in the absence of visible muscle contraction.

OBJECTIVE

To demonstrate the presence of volitional electromyographic (EMG) activity below the lesion in motor complete SCI and to characterize this activity to determine its value for potential use as a neuroprosthetic command source.

METHODS

Twenty-four subjects with complete (AIS A or B), chronic, cervical SCI were tested for the presence of volitional below-injury EMG activity. Surface electrodes recorded from 8 to 12 locations of each lower limb, while participants were asked to attempt specific movements of the lower extremity in response to visual and audio cues. EMG trials were ranked through visual inspection, and were scored using an amplitude threshold algorithm to identify channels of interest with volitional motor unit activity.

RESULTS

Significant below-injury muscle activity was identified through visual inspection in 16 of 24 participants, and visual inspection rankings were well correlated to the algorithm scoring.

CONCLUSIONS

The surface EMG protocol utilized here is relatively simple and noninvasive, ideal for a clinical screening tool. The majority of subjects tested were able to produce a volitional EMG signal below their injury level, and the algorithm developed allows automatic identification of signals of interest. The presence of this volitional activity in the lower extremity could provide an innovative new command signal source for implanted neuroprostheses or other assistive technology.

Traumatic spinal cord injury

Traumatic spinal cord injury (SCI) has devastating consequences for the physical, social and vocational well-being of patients. The demographic of SCIs is shifting such that an increasing proportion of older individuals are being affected. Pathophysiologically, the initial mechanical trauma (the primary injury) permeabilizes neurons and glia and initiates a secondary injury cascade that leads to progressive cell death and spinal cord damage over the subsequent weeks. Over time, the lesion remodels and is composed of cystic cavitations and a glial scar, both of which potently inhibit regeneration. Several animal models and complementary behavioural tests of SCI have been developed to mimic this pathological process and form the basis for the development of preclinical and translational neuroprotective and neuroregenerative strategies. Diagnosis requires a thorough patient history, standardized neurological physical examination and radiographic imaging of the spinal cord. Following diagnosis, several interventions need to be rapidly applied, including haemodynamic monitoring in the intensive care unit, early surgical decompression, blood pressure augmentation and, potentially, the administration of methylprednisolone. Managing the complications of SCI, such as bowel and bladder dysfunction, the formation of pressure sores and infections, is key to address all facets of the patient's injury experience.

Direct optical activation of skeletal muscle fibres efficiently controls muscle contraction and attenuates denervation atrophy

Neural prostheses can restore meaningful function to paralysed muscles by electrically stimulating innervating motor axons, but fail when muscles are completely denervated, as seen in amyotrophic lateral sclerosis, or after a peripheral nerve or spinal cord injury. Here we show that channelrhodopsin-2 is expressed within the sarcolemma and T-tubules of skeletal muscle fibres in transgenic mice. This expression pattern allows for optical control of muscle contraction with comparable forces to nerve stimulation. Force can be controlled by varying light pulse intensity, duration or frequency. Light-stimulated muscle fibres depolarize proportionally to light intensity and duration. Denervated triceps surae muscles transcutaneously stimulated optically on a daily basis for 10 days show a significant attenuation in atrophy resulting in significantly greater contractile forces compared with chronically denervated muscles. Together, this study shows that channelrhodopsin-2/H134R can be used to restore function to permanently denervated muscles and reduce pathophysiological changes associated with denervation pathologies.

Reorganization of corticospinal tract fibers after spinal cord injury in adult macaques

Previous studies have shown that sprouting of corticospinal tract (CST) fibers after spinal cord injury (SCI) contributes to recovery of motor functions. However, the neuroanatomical mechanism underlying the functional recovery through sprouting CST fibers remains unclear. Here we investigated the pattern of reorganization of CST fibers below the lesion site after SCI in adult macaques. Unilateral lesions were made at the level between the C7 and the C8 segment. The extent of spontaneous recovery of manual dexterity was assessed with a reaching/grasping task. The impaired dexterous manual movements were gradually recovered after SCI. When anterograde tract tracing with biotinylated dextran amine was performed to identify the intraspinal reinnervation of sprouting CST fibers, it was found that the laminar distribution of CST fibers was changed. The sprouting CST fibers extended preferentially into lamia IX where the spinal motor neuron pool was located, to innervate the motor neurons directly. Instead, few, if any, CST fibers were distributed in the dorsal laminae. The present results indicate that CST fibers below the lesion site after SCI in macaques are reorganized in conjunction with the recovery of dexterous manual movements.

Spinal cord regeneration

Three theories of regeneration dominate neuroscience today, all purporting to explain why the adult central nervous system (CNS) cannot regenerate. One theory proposes that Nogo, a molecule expressed by myelin, prevents axonal growth. The second theory emphasizes the role of glial scars. The third theory proposes that chondroitin sulfate proteoglycans (CSPGs) prevent axon growth. Blockade of Nogo, CSPG, and their receptors indeed can stop axon growth in vitro and improve functional recovery in animal spinal cord injury (SCI) models. These therapies also increase sprouting of surviving axons and plasticity. However, many investigators have reported regenerating spinal tracts without eliminating Nogo, glial scar, or CSPG. For example, many motor and sensory axons grow spontaneously in contused spinal cords, crossing gliotic tissue and white matter surrounding the injury site. Sensory axons grow long distances in injured dorsal columns after peripheral nerve lesions. Cell transplants and treatments that increase cAMP and neurotrophins stimulate motor and sensory axons to cross glial scars and to grow long distances in white matter. Genetic studies deleting all members of the Nogo family and even the Nogo receptor do not always improve regeneration in mice. A recent study reported that suppressing the phosphatase and tensin homolog (PTEN) gene promotes prolific corticospinal tract regeneration. These findings cannot be explained by the current theories proposing that Nogo and glial scars prevent regeneration. Spinal axons clearly can and will grow through glial scars and Nogo-expressing tissue under some circumstances. The observation that deleting PTEN allows corticospinal tract regeneration indicates that the PTEN/AKT/mTOR pathway regulates axonal growth. Finally, many other factors stimulate spinal axonal growth, including conditioning lesions, cAMP, glycogen synthetase kinase inhibition, and neurotrophins. To explain these disparate regenerative phenomena, I propose that the spinal cord has evolved regenerative mechanisms that are normally suppressed by multiple extrinsic and intrinsic factors but can be activated by injury, mediated by the PTEN/AKT/mTOR, cAMP, and GSK3b pathways, to stimulate neural growth and proliferation.

Altering spinal cord excitability enables voluntary movements after chronic complete paralysis in humans

Previously, we reported that one individual who had a motor complete, but sensory incomplete spinal cord injury regained voluntary movement after 7 months of epidural stimulation and stand training. We presumed that the residual sensory pathways were critical in this recovery. However, we now report in three more individuals voluntary movement occurred with epidural stimulation immediately after implant even in two who were diagnosed with a motor and sensory complete lesion. We demonstrate that neuromodulating the spinal circuitry with epidural stimulation, enables completely paralysed individuals to process conceptual, auditory and visual input to regain relatively fine voluntary control of paralysed muscles. We show that neuromodulation of the sub-threshold motor state of excitability of the lumbosacral spinal networks was the key to recovery of intentional movement in four of four individuals diagnosed as having complete paralysis of the legs. We have uncovered a fundamentally new intervention strategy that can dramatically affect recovery of voluntary movement in individuals with complete paralysis even years after injury.

A novel command signal for motor neuroprosthetic control

BACKGROUND

Neuroprostheses can restore functions such as hand grasp or standing to individuals with spinal cord injury (SCI) using electrical stimulation to elicit movements in paralyzed muscles. Implanted neuroprostheses currently use electromyographic (EMG) activity from muscles above the lesion that remain under volitional control as a command input. Systems in development use a networked approach and will allow for restoration of multiple functions but will require additional command signals to control the system, especially in individuals with high-level tetraplegia.

OBJECTIVE

The objective of this study was to investigate the feasibility of using muscles innervated below the injury level as command sources for a neuroprosthesis. Recent anatomical and physiological studies have demonstrated the presence of intact axons across the lesion, even in those diagnosed with a clinically complete SCI; hence, EMG activity may be present in muscles with no sign of movement.

METHODS

Twelve participants with motor complete SCI were enrolled and EMG was recorded with surface electrodes from 8 muscles below the knee in each leg.

RESULTS

Significant activity was evident in 89% of the 192 muscles studied during attempted movements of the foot and lower limb. At least 2 muscles from each participant were identified as potential command signals for a neuroprosthesis based on 2-state, threshold classification.

CONCLUSIONS

Results suggest that voluntary activity is present and recordable in below lesion muscles even after clinically complete SCI.

Functional evaluation of paraplegic monkeys (Macaca mulatta) over fourteen months post-lesion

We report on the neurological and neurophysiological findings obtained from two adult Macaca mulatta sustaining complete spinal cord transections at T8-T9. We performed periodic neurological exams, recorded motor evoked potentials (MEPs) following transcranial magnetic stimulation (TMS), and recorded electromyograms (EMGs) during the execution of a lower limb motor test. The main observations were: (1) the spinal shock period lasted less than a week; tendon, cutaneous and withdrawal reflexes were uneven in range and occurrence, and Babinski's sign was not observed; (2) a protracted functional lesion in the tibial and common peroneal nerves appeared bilaterally early in the post-lesional period; (3) MEPs were elicited by TMS in the quadriceps muscle of both monkeys; they were recorded as early as the 5th week after lesion in one of the monkeys, and they persisted throughout the post-lesional period in both monkeys; and (4) motor unit action potentials in the quadriceps muscle recorded by EMG were simultaneous with attempts to perform intentional lower limb movements from post-lesion month 11 to 13.5 in both monkeys. The last two sets of observations argue in favor of a partial cortico-spinal functional gain and suggest that spinal cord regeneration can occur after complete spinal cord injury in primates.

Neurophysiological characterization of motor recovery in acute spinal cord injury

Study Design

Prospective cohort study

Objective

This study was designed to neurophysiologically characterize motor control recovery after spinal cord injury (SCI).

Setting

University of Louisville, Louisville, Kentucky, USA.

Material

Eleven acute SCI admissions and five non-injured subjects were recruited for this study.

Methods

The American Spinal Injury Association Impairment Scale (AIS) was used to categorize injury level and severity at onset. Multi-muscle surface EMG (sEMG) recording protocol of reflex and volitional motor tasks was initially performed between the day of injury and 11 days post onset (6.4 ± 3.6, mean ± SD days). Follow-up recordings were performed for up to 17 months after injury. Initial AIS distribution was: 4 AIS-A; 2 AIS-C; 5 AIS-D. Multi-muscle activation patterns were quantified from the sEMG amplitudes of selected muscles using a vector-based calculation that produces values for Magnitude and Similarity of SCI test-subject patterns to those produced by non-injured subjects.

Results

In SCI subjects, overall sEMG amplitudes were lower after SCI. Prime mover muscle voluntary recruitment was slower and multi-muscle patterns were disrupted by SCI. Recovery occurred in 9 of the 11 showing an increase in sEMG amplitudes, more rapid prime mover muscle recruitment rates and the progressive normalization of the multi-muscle activation patterns. The rate of increase was highly individualized, differing over time by limb and proximal or distal joint within each subject and across the SCI group.

Conclusions

Recovery of voluntary motor function can be quantitatively tracked using neurophysiological methods in the domains of time and multi-muscle motor unit activation.

Sponsorship

NIH NINDS funded project #NS049954-01

Cauda equina repair in the rat: part 1. Stimulus-evoked EMG for identifying spinal nerves innervating intrinsic tail muscles

Cauda equina injuries may produce severe leg and pelvic floor dysfunction, for which no effective treatments exist. We are developing a rat cauda equina injury model to allow nerve root identification and surgical repair. One possible difficulty in implementing any repair strategy after trauma in humans involves the correct identification of proximal and distal ends of nerve roots separated by the injury. Two series of studies were carried out. In Series 1, we electrically stimulated segmental contributors to the dorsal and ventral caudales nerves in order to characterize the recruitment patterns of muscles controlling rat tail movements. In Series 2, we attempted to identify individual nerve roots forming the cauda equina by both level of origin and function (i.e., dorsal or ventral), based solely upon the recruitment patterns in response to electrical stimulation. For Series 1 studies, electrical stimulation of the segmental contributors showed that all nerve roots-from the sixth lumbar to the first coccygeal-contributed to recruitment of muscles found at the base of the tail. Intrinsic tail muscles lying more distally in the tail showed a more root-specific pattern of innervation. For Series 2, the rate of successful identification of an unknown nerve root as being ventral was very high (>95%), and only somewhat lower (approximately 80%) for dorsal roots. Correctly identifying the level of origin of that root was more difficult, but for ventral roots this rate still exceeded 90%. Using the rat cauda equina model, we have shown that stimulus-evoked EMG can be used to identify ventral nerve roots innervating tail muscles with a high degree of accuracy. These findings support the feasibility of using this conceptual approach for identifying and repairing damaged human cauda equina nerve roots based on stimulus-evoked recruitment of muscles in the leg and pelvic floor.

Accelerating axon growth to overcome limitations in functional recovery after peripheral nerve injury

OBJECTIVE

Injured peripheral nerves regenerate at very slow rates. Therefore, proximal injury sites such as the brachial plexus still present major challenges, and the outcomes of conventional treatments remain poor. This is in part attributable to a progressive decline in the Schwann cells' ability to provide a supportive milieu for the growth cone to extend and to find the appropriate target. These challenges are compounded by the often considerable delay of regeneration across the site of nerve laceration. Recently, low-frequency electrical stimulation (as brief as an hour) has shown promise, as it significantly accelerated regeneration in animal models through speeding of axon growth across the injury site.

METHODS

To test whether this might be a useful clinical tool, we carried out a randomized controlled trial in patients who had experienced substantial axonal loss in the median nerve owing to severe compression in the carpal tunnel. To further elucidate the potential mechanisms, we applied rolipram, a cyclic adenosine monophosphate agonist, to rats after axotomy of the femoral nerve.

RESULTS

We demonstrated that effects similar to those observed in animal studies could also be attained in humans. The mechanisms of action of electrical stimulation likely operate through up-regulation of neurotrophic factors and cyclic adenosine monophosphate. Indeed, the application of rolipram significantly accelerated nerve regeneration.

CONCLUSION

With new mechanistic insights into the influencing factors of peripheral nerve regeneration, the novel treatments described above could form part of an armament of synergistic therapies that could make a meaningful difference to patients with peripheral nerve injuries.

A guidance channel seeded with autologous Schwann cells for repair of cauda equina injury in a primate model

BACKGROUND/OBJECTIVE

To evaluate an implantable guidance channel (GC) seeded with autologous Schwann cells to promote regeneration of transected spinal nerve root axons in a primate model.

METHODS

Schwann cells were obtained from sural nerve segments of monkeys (Macaca fascicularis; cynomolgus). Cells were cultured, purified, and seeded into a PAN/PVC GC. Approximately 3 weeks later, monkeys underwent laminectomy and dural opening. Nerve roots of the L4 through L7 segments were identified visually. The threshold voltage needed to elicit hindlimb muscle electromyography (EMG) after stimulation of intact nerve roots was determined. Segments of 2 or 3 nerve roots (each approximately 8-15 mm in length) were excised. The GC containing Schwann cells was implanted between the proximal and distal stumps of these nerve roots and attached to the stumps with suture. Follow-up evaluation was conducted on 3 animals, with survival times of 9 to 14 months.

RESULTS

Upon reexposure of the implant site, subdural nerve root adhesions were noted in all 3 animals. Several of the implanted GC had collapsed and were characterized by thin strands of connective tissue attached to either end. In contrast, 3 of the 8 implanted GC were intact and had white, glossy cables entering and exiting the conduits. Electrical stimulation of the tissue cable in each of these 3 cases led to low-threshold evoked EMG responses, suggesting that muscles had been reinnervated by axons regenerating through the repair site and into the distal nerve stump. During harvesting of the GC implant, sharp transection led to spontaneous EMG in the same 3 roots showing a low threshold to electrical stimulation, whereas no EMG was seen when harvesting nerve roots with high thresholds to elicit EMG. Histology confirmed large numbers of myelinated axons at the midpoint of 2 GC judged to have reinnervated target muscles.

CONCLUSIONS

We found a modest rate of successful regeneration and muscle reinnervation after treatment of nerve root transection with a Schwann cell-seeded, implanted synthetic GC. Newer treatments, which include the use of absorbable polymers, neurotrophins, and antiscar agents, may further improve spinal nerve regeneration for repair of cauda equina injury.

Evaluation of parameters of serially monitored F-wave in acute cervical spinal cord injury

OBJECTIVE

In this study, we aimed to determine how physicians can evaluate the severity of acute traumatic spinal cord injury (SCI) and predict the prognosis of this injury using the relationships of changes in clinical features and electrophysiological examination results.

MATERIALS AND METHODS

Serial recordings of F-waves were performed on 20 consecutive cervical SCI patients. In 12 of the patients, changes in several parameters of F-waves which were elicited by median and ulnar nerve stimulations were examined by analyzing their relationships to clinical symptoms.

RESULTS

The maximum amplitude of the F-waves (F-max) elicited by median nerve stimulation was found to be the most reliable (statistically significant) parameter for distinguishing clinically improved patients from nonimproved patients for the prognosis in the early stages after trauma. Other parameters, including the incidence of F-waves and the mean F-wave amplitude both of which were elicited by median nerve stimulation, were somewhat helpful for predicting the prognosis. These parameters of F-waves evoked by ulnar nerve stimulation could be useful for several weeks post-trauma.

Spinal myoclonus after spinal cord injury

BACKGROUND/OBJECTIVE

In the course of examining spinal motor function in many hundreds of people with traumatic spinal cord injury, we encountered 6 individuals who developed involuntary and rhythmic contractions in muscles of their legs. Although there are many reports of unusual muscle activation patterns associated with different forms of myoclonus, we believe that certain aspects of the patterns seen with these 6 subjects have not been previously reported. These patterns share many features with those associated with a spinal central pattern generator for walking.

METHODS

Subjects in this case series had a history of chronic injury to the cervical spinal cord, resulting in either complete (ASIA A; n = 4) or incomplete (ASIA D; n = 2) quadriplegia. We used multi-channel electromyography recordings of trunk and leg muscles of each subject to document muscle activation patterns associated with different postures and as influenced by a variety of sensory stimuli.

RESULTS

Involuntary contractions spanned multiple leg muscles bilaterally, sometimes including weak abdominal contractions. Contractions were smooth and graded and were highly reproducible in rate for a given subject (contraction rates were 0.3-0.5 Hz). These movements did not resemble the brief rapid contractions (ie, "jerks") ascribed to some forms of spinal myoclonus. For all subjects, the onset of involuntary muscle contraction was dependent upon hip angle; contractions did not occur unless the hips (and knees) were extended (ie, subjects were supine). In the 4 ASIA A subjects, contractions occurred simultaneously in all muscles (agonists and antagonists) bilaterally. In sharp contrast, contractions in the 2 ASIA D subjects were reciprocal between agonists and antagonists within a limb and alternated between limbs, such that movements in these 2 subjects looked just like repetitive stepping. Finally, each of the 6 subjects had a distinct pathology of their spinal cord, nerve roots, distal trunk, or thigh; in 4 of these subjects, treatment of the pathology eliminated the involuntary movements.

CONCLUSION

The timing, distribution, and reliance upon hip angle suggest that these movement patterns reflect some elements of a central pattern generator for stepping. Emergence of these movements in persons with chronic spinal cord injury is extremely rare and appears to depend upon a combination of the more rostrally placed injury and a pathologic process leading to a further enhancement of excitability in the caudal spinal cord.

Tendon reflexes for predicting movement recovery after acute spinal cord injury in humans

OBJECTIVE

Use the tendon reflex to examine spinal cord excitability after acute spinal cord injury (SCI), relating excitability findings to prognosis.

METHODS

We conducted repeated measures of reflex responses to mechanical taps at the patellar and Achilles tendons of the lower limbs, and the wrist flexor tendons of the upper limbs in persons with acute SCI, beginning as early as the day of injury. The single largest EMG response (peak-to-peak) for each site was recorded. Subjects were compared based on level of injury and final neurologic status of lower limb motor function (i.e. absence of any voluntary recruitment in a lower limb muscle: motor-complete; voluntary recruitment in 1 or more lower-limb muscles: motor-incomplete).

RESULTS

We studied 229 subjects with acute SCI. Persons with injury to the cervical or thoracic spinal cord and who were (or became) motor-incomplete showed large tendon responses, even at the time of initial evaluation. In combination with larger tendon response amplitudes, the presence of the 'crossed-adductor' response to patellar tendon taps at the acute stage was highly predictive of functional motor recovery following SCI. In marked contrast, tendon responses were small (e.g. < 0.1 mV) or absent in persons with acute, motor-complete injury (and which remained motor-complete), and the crossed-adductor response was never seen. Reflex amplitudes and the incidence of the crossed-adductor response increased somewhat over time in persons with motor-complete SCI, but did not approach the values seen in motor-incomplete subjects.

CONCLUSIONS

Taken together, tendon response amplitude and reflex spread were sensitive and specific indicators of preserved supraspinal control over lower limb musculature in subjects with acute SCI. A simple algorithm using these outcome measures predicted a 'motor-complete' status with 100% accuracy, and a motor-incomplete status with accuracy exceeding 91%.

Abductor hallucis for monitoring lower-limb recovery after spinal cord injury in man

STUDY DESIGN

Electromyogram (EMG) study on patients with acute spinal cord injury (SCI).

OBJECTIVES

We hypothesized that subjects with mild to moderate acute SCI would have a higher probability of recovering function in intrinsic muscles of the foot compared to more proximal lower-limb muscles, based on the relative density of corticospinal tract innervation to these different motoneuron pools.

SETTING

Miami and Syracuse, USA.

METHODS

We conducted repeated measures of EMG during voluntary contractions from lower-limb muscles in subjects with acute traumatic SCI. For this study, analysis was restricted to those subjects who had either no recruitment (ie 'motor-complete') or limited recruitment (ie 'motor-incomplete') in any lower-limb muscle of either leg during the initial evaluation, and all of whom had converted to a motor-incomplete status in one or both legs at the time of final evaluation. Recruitment of the abductor hallucis (AbH) muscle during contraction attempts was judged as being either 'present' or 'absent', based upon the presence or absence of EMG-based volitional motor unit recruitment.

RESULTS

A total of 70 subjects were included in this study. Of these, 58 had motor-incomplete injury at or rostral to the T10 vertebral level, and another 12 had injury caudal to T10. In the former group, the AbH muscle showed a recovery probability that was considerably higher than that of other lower-limb muscles. Quite the opposite pattern was seen in persons with injury caudal to T10. In these subjects, recruitment was more common in proximal muscles of the thigh (psoas and quadriceps), and least common in the AbH muscle.

DISCUSSION

For persons with SCI at or rostral to the T10 vertebral level, the AbH muscle proved to be an earlier and more sensitive indicator of lower-limb contraction recovery following acute SCI compared to other lower-limb muscles. Including this intrinsic muscle of the foot as part of a neurologic assessment of muscle function after SCI should increase the test's sensitivity to preserved (or restored) supraspinal motor influence over lower-limb motoneuron pools, and is recommended.