Impaired facilitation of motor evoked potentials in incomplete spinal cord injury

Advanced Assessment of the Upper Limb in Tetraplegia: A Three-Tiered Approach to Characterizing Paralysis

Background: More than half of all individuals who sustain a spinal cord injury (SCI) experience some degree of impairment in the upper limb. Functional use of the arm and hand is of paramount importance to these individuals. Fortunately, the number of clinical trials and advanced interventions targeting upper limb function are increasing, generating optimism for improved recovery and restoration after SCI. New interventions for restoring function and improving recovery require more detailed examination of the motor capacities of the upper limb. Objectives: The purpose of this article is to introduce a three-tiered approach to evaluating motor function, with specific attention to the characteristics of weak and fully paralyzed muscles during acute rehabilitation. The three tiers include (1) evaluation of voluntary strength via manual muscle testing, (2) evaluation of lower motor neuron integrity in upper motor neuron-paralyzed muscles using surface electrical stimulation, and (3) evaluation of latent motor responses in paralyzed muscles that exhibit a strong response to electrical stimulation, using surface electromyographic recording electrodes. These characteristics contribute important information that can be utilized to mitigate potential secondary conditions such as contractures and identify effective interventions such as activity-based interventions or reconstructive procedures. Our goal is to encourage frontline clinicians - occupational and physical therapists who are experts in muscle assessment - to consider a more in-depth analysis of paralysis after SCI. Conclusion: Given the rapid advancements in SCI research and clinical interventions, it is critical that methods of evaluation and classification evolve. The success or failure of these interventions may depend on the specific characteristics identified in our three-tiered assessment. Without this assessment, the physiological starting point for each individual is unknown, adding significant variability in the outcomes of these interventions.

Modulation of corticospinal input to the legs by arm and leg cycling in people with incomplete spinal cord injury

The spinal cervico-lumbar interaction during rhythmic movements in humans has recently been studied; however, the role of arm movements in modulating the corticospinal drive to the legs is not well understood. The goals of this study were to investigate the effect of active rhythmic arm movements on the corticospinal drive to the legs (study 1) and assess the effect of simultaneous arm and leg training on the corticospinal pathway after incomplete spinal cord injury (iSCI) (study 2). In study 1, neurologically intact (NI) participants or participants with iSCI performed combinations of stationary and rhythmic cycling of the arms and legs while motor evoked potentials (MEPs) were recorded from the vastus lateralis (VL) muscle. In the NI group, arm cycling alone could facilitate the VL MEP amplitude, suggesting that dynamic arm movements strongly modulate the corticospinal pathway to the legs. No significant difference in VL MEP between conditions was found in participants with iSCI. In study 2, participants with iSCI underwent 12 wk of electrical stimulation-assisted cycling training: one group performed simultaneous arm and leg (A&L) cycling and the other legs-only cycling. MEPs in the tibialis anterior (TA) muscle were compared before and after training. After training, only the A&L group had a significantly larger TA MEP, suggesting increased excitability in the corticospinal pathway. The findings demonstrate the importance of arm movements in modulating the corticospinal drive to the legs and suggest that active engagement of the arms in lower limb rehabilitation may produce better neural regulation and restoration of function.NEW & NOTEWORTHY This study aimed to demonstrate the importance of arm movements in modulating the corticospinal drive to the legs. It provides direct evidence in humans that active movement of the arms could facilitate corticospinal transmission to the legs and, for the first time, shows that facilitation is absent after spinal cord injury. Active engagement of the arms in lower limb rehabilitation increased the excitability of the corticospinal pathway and may produce more effective improvement in leg function.

Longitudinal estimation of intramuscular Tibialis Anterior coherence during subacute spinal cord injury: relationship with neurophysiological, functional and clinical outcome measures

Background

Estimation of surface intramuscular coherence has been used to indirectly assess pyramidal tract activity following spinal cord injury (SCI), especially within the 15-30 Hz bandwidth. However, change in higher frequency (>40 Hz) muscle coherence during SCI has not been characterised. Thus, the objective of this study was to identify change of high and low frequency intramuscular Tibialis Anterior (TA) coherence during incomplete subacute SCI.

Methods

Fifteen healthy subjects and 22 subjects with motor incomplete SCI (American Spinal Injury Association Impairment Scale, AIS, C or D grade) were recruited and tested during 4 sessions performed at 2-week intervals up to 8 months after SCI. Intramuscular TA coherence estimation was calculated within the 10–60 Hz bandwidth during controlled maximal isometric and isokinetic foot dorsiflexion. Maximal voluntary dorsiflexion torque, gait function measured with the WISCI II scale, and TA motor evoked potentials (MEP) were recorded.

Results

During subacute SCI, significant improvement in total lower limb manual muscle score, TA muscle strength and gait function were observed. No change in TA MEP amplitude was identified. Significant increase in TA coherence was detected in the 40–60 Hz, but not the 15–30 Hz bandwidth. The spasticity syndrome was associated with lower 15-30 Hz TA coherence during maximal isometric dorsiflexion and higher 10–60 Hz coherence during fast isokinetic movement (p < 0.05).

Conclusions

Longitudinal estimation of neurophysiological and clinical measures during subacute SCI suggest that estimation of TA muscle coherence during controlled movement provides indirect information regarding adaptive and maladaptive motor control mechanisms during neurorehabilitation.

Optical Imaging of the Motor Cortex Following Antidromic Activation of the Corticospinal Tract after Spinal Cord Injury

Spinal cord injury (SCI) disrupts neuronal networks of ascending and descending tracts at the site of injury, leading to a loss of motor function. Restoration and new circuit formation are important components of the recovery process, which involves collateral sprouting of injured and uninjured fibers. The present study was conducted to determine cortical responses to antidromic stimulation of the corticospinal tracts, to compare changes in the reorganization of neural pathways within normal and spinal cord-injured rats, and to elucidate differences in spatiotemporal activity patterns of the natural progression and reorganization of neural pathways in normal and SCI animals using optical imaging. Optical signals were recorded from the motor cortex in response to electrical stimulation of the ventral horn of the L1 spinal cord. Motor evoked potentials (MEPs) were evaluated to demonstrate endogenous recovery of physiological functions after SCI. A significantly shorter N1 peak latency and broader activation in the MEP optical recordings were observed at 4 weeks after SCI, compared to 1 week after SCI. Spatiotemporal patterns in the cerebral cortex differed depending on functional recovery. In the present study, optical imaging was found to be useful in revealing functional changes and may reflect conditions of reorganization and/or changes in surviving neurons after SCI.

Complete rat spinal cord transection as a faithful model of spinal cord injury for translational cell transplantation

Spinal cord injury (SCI) results in neural loss and consequently motor and sensory impairment below the injury. There are currently no effective therapies for the treatment of traumatic SCI in humans. Various animal models have been developed to mimic human SCI. Widely used animal models of SCI are complete or partial transection or experimental contusion and compression, with both bearing controversy as to which one more appropriately reproduces the human SCI functional consequences. Here we present in details the widely used procedure of complete spinal cord transection as a faithful animal model to investigate neural and functional repair of the damaged tissue by exogenous human transplanted cells. This injury model offers the advantage of complete damage to a spinal cord at a defined place and time, is relatively simple to standardize and is highly reproducible.

Clinical neurophysiology in the prognosis and monitoring of traumatic spinal cord injury

Preclinical studies for the repair of spinal cord injury (SCI) and potential therapies for accessing the inherent plasticity of the central nervous system (CNS) to promote recovery of function are currently moving into the translational stage. These emerging clinical trials of therapeutic interventions for the repair of SCI require improved assessment techniques and quantitative outcome measures to supplement the American Spinal Injuries Association (ASIA) Impairment Scales. This chapter attempts to identify those electrophysiological techniques that show the most promise for provision of objective and quantitative measures of sensory, motor, and autonomic function in SCI. Reviewed are: (1) somatosensory evoked potentials, including dermatomal somatosensory evoked potentials, and the electrical perceptual threshold as tests of the dorsal (posterior) column pathway; (2) laser evoked potentials and contact heat evoked potentials as tests of the anterior spinothalamic tract; (3) motor evoked potentials in limb muscles, in response to transcranial magnetic stimulation of the motor cortex as tests of the corticospinal tract, and the application of the technique to assessment of trunk and sphincter muscles; and (4) the sympathetic skin response as a test of spinal cord access to the sympathetic chain.

Instrument validity and reliability of a choice response time test for subjects with incomplete spinal cord injury: relationship with function

OBJECTIVES

To investigate instrument validity and reliability of a choice response time (CRT) test for the lower extremities in subjects with incomplete spinal cord injury (iSCI). CRT in subjects with iSCI is hypothesized to be increased because of, for example, muscle weakness or increased corticospinal conduction velocity.

DESIGN

Case-control study.

SETTING

Spinal cord injury center of a university hospital in Switzerland.

PARTICIPANTS

Patients with iSCI (N=28; mean age, 51y; 57% men; neurologic level, C3-L5; median time since injury, 148d) compared with age-matched controls (50% men) to assess instrument validity by comparing CRTs. A subgroup of subjects with iSCI (n = 9) and controls (n = 13) to determine reliability.

INTERVENTION

Not applicable.

MAIN OUTCOME MEASURES

CRTs of the lower extremity were assessed and divided into reaction and movement times. Additionally, subjects with iSCI were tested for lower-extremity muscle strength, gait capacity and mobility, independence, history of falls, and fear of falling.

RESULTS

CRTs in the control group (mean ± SD, 517 ± 71 ms) were significantly faster than those in the iSCI group (743 ± 177 ms; P<.001). Retest reliability was high in controls (intraclass correlation coefficient [ICC]>.98) and subjects with iSCI (ICC>.93). In subjects with iSCI, there were moderate to good correlations between CRT and several functional outcome measures, but not with reported number of falls.

CONCLUSION

Lower-extremity CRT testing appears reliable in healthy controls and ambulating subjects with iSCI.

Multi-limb acquisition of motor evoked potentials and its application in spinal cord injury

The motor evoked potential (MEP) is an electrical response of peripheral neuro-muscular pathways to stimulation of the motor cortex. MEPs provide objective assessment of electrical conduction through the associated neural pathways, and therefore detect disruption due to a nervous system injury such as spinal cord injury (SCI). In our studies of SCI, we developed a novel, multi-channel set-up for MEP acquisition in rat models. Unlike existing electrophysiological systems for SCI assessment, the set-up allows for multi-channel MEP acquisition from all limbs of rats and enables longitudinal monitoring of injury and treatment for in vivo models of experimental SCI. The article describes the development of the set-up and discusses its capabilities to acquire MEPs in rat models of SCI. We demonstrate its use for MEP acquisition under two types of anesthesia as well as a range of cortical stimulation parameters, identifying parameters yielding consistent and reliable MEPs. To validate our set-up, MEPs were recorded from a group of 10 rats before and after contusive SCI. Upon contusion with moderate severity (12.5mm impact height), MEP amplitude decreased by 91.36±6.03%. A corresponding decline of 93.8±11.4% was seen in the motor behavioral score (BBB), a gold standard in rodent models of SCI.

Voluntary ankle flexor activity and adaptive coactivation gain is decreased by spasticity during subacute spinal cord injury

Although spasticity has been defined as an increase in velocity-dependent stretch reflexes and muscle hypertonia during passive movement, the measurement of flexor muscle paresis may better characterize the negative impact of this syndrome on residual motor function following incomplete spinal cord injury (iSCI). In this longitudinal study Tibialis Anterior (TA) muscle paresis produced by a loss in maximal voluntary contraction during dorsiflexion and ankle flexor muscle coactivation during ramp-and-hold controlled plantarflexion was measured in ten patients during subacute iSCI. Tibialis Anterior activity was measured at approximately two-week intervals between 3-5 months following iSCI in subjects with or without spasticity, characterized by lower-limb muscle hypertonia and/or involuntary spasms. Following iSCI, maximal voluntary contraction ankle flexor activity was lower than that recorded from healthy subjects, and was further attenuated by the presence of spasticity. Furthermore the initially high percentage value of TA coactivation increased at 75% but not at 25% maximal voluntary torque (MVT), reflected by an increase in TA coactivation gain (75%/25% MVT) from 2.5+/-0.4 to 7.5+/-1.9, well above the control level of 2.9+/-0.2. In contrast contraction-dependent TA coactivation gain decreased from 2.4+/-0.3 to 1.4+/-0.1 during spasticity. In conclusion the adaptive increase in TA coactivation gain observed in this pilot study during subacute iSCI was also sensitive to the presence of spasticity. The successful early diagnosis and treatment of spasticity would be expected to further preserve and promote adaptive motor function during subacute iSCI neurorehabilitation.

Electrophysiological outcomes after spinal cord injury

Electrophysiological measures can provide information that complements clinical assessments such as the American Spinal Injury Association sensory and motor scores in the evaluation of outcomes after spinal cord injury (SCI). The authors review and summarize the literature regarding tests that are most relevant to the study of SCI recovery--in particular, motor evoked potentials and somatosensory evoked potentials (SSEPs). In addition, they discuss the role of other tests, including F-wave nerve conductance tests and electromyography, sympathetic skin response, and the Hoffman reflex (H-reflex) test as well as the promise of dermatomal SSEPs and the electrical perceptual threshold test, newer quantitative tests of sensory function. It has been shown that motor evoked potential amplitudes improve with SCI recovery but latencies do not. Somatosensory evoked potentials are predictive of ambulatory capacity and hand function. Hoffman reflexes are present during spinal shock despite the loss of tendon reflexes, but their amplitudes increase with time after injury. Further, H-reflex modulation is reflective of changes in spinal excitability. While these tests have produced data that is congruent with clinical evaluations, they have yet to surpass clinical evaluations in predicting outcomes. Continuing research using these methodologies should yield a better understanding of the mechanisms behind SCI recovery and thus provide potentially greater predictive and evaluative power.

Ankle paresis in incomplete spinal cord injury: relation to corticospinal conductivity and ambulatory capacity

There is limited data on the relation of corticospinal tract conductivity to clinical measures in incomplete spinal cord injury. This study examined the relationship of muscle strength to corticospinal tract input assessed by motor evoked potentials (MEPs) during static and dynamic conditions and to gait. Dorsiflexor strength was established by manual muscle test, maximal voluntary contraction, and maximal movement velocity, the latter being acquired during auditory-paced ankle movements. MEPs were elicited during isometric contractions with constant or continuously increasing ankle joint torque. Gait was assessed by quantifying its speed and independence. Linear regression analyses showed that maximal movement velocity was related to the MEP latencies and amplitudes in the dynamic condition (R(2)(adj.) = 0.62) and to the MEP latencies in the static condition (R(2)(adj.) = 0.45). Maximal voluntary contraction was only related to the MEP latencies in the static (R(2)(adj.) = 0.45) and the dynamic condition (R(2)(adj.) = 0.21), whereas manual muscle test did not show any relationship to the MEPs. In incomplete spinal cord injury patients, the dynamic measure maximal movement velocity might be a useful clinical assessment of corticospinal tract function. Clinical studies on recovery and repair of corticospinal tract function in spinal lesions could substantially benefit from implementing dynamic measures in the clinical assessment protocol.

Vestibulospinal responses in motor incomplete spinal cord injury

OBJECTIVE

Postural instability limits ambulatory capacity in patients with spinal cord injury (SCI). Galvanic vestibular stimulation (GVS) was used to investigate the integrity of vestibulospinal pathways and related changes in postural responses in SCI.

METHODS

Binaural bipolar galvanic stimuli of 400 ms duration and 3 mA intensity were applied in 8 motor incomplete SCI and 8 control subjects who stood facing towards the left. EMG responses were recorded from the right soleus muscle and the trajectory of the centre of pressure (CoP) was measured with a force plate.

RESULTS

There was no difference in excitability and amplitude of the responses between the groups. However, the latency and duration of the medium latency EMG response and all CoP responses were significantly longer in the SCI group. Additionally, postural stability was reduced in the SCI group, as shown by a greater tendency to fall due to GVS.

CONCLUSIONS

Despite early EMG responses proving the basic connectivity of the direct vestibulospinal pathways, the delayed GVS responses suggest a vestibulospinal deficit in the SCI subjects.

SIGNIFICANCE

GVS can be applied in incomplete SCI to supplement the neurological examination by revealing changes in vestibulospinal responses and impairment of postural stability.

Changes in corticospinal function and ankle motor control during recovery from incomplete spinal cord injury

Little is known about the mechanisms that underlie motor recovery after incomplete spinal cord injury (iSCI) in humans. This study assessed changes in corticospinal tract (CST) function by measuring motor-evoked potentials (MEPs) and ankle motor control at 1, 3, and 6 months after acute iSCI. In 12 iSCI patients and matched controls, MEPs (evoked at 20% of maximal voluntary contraction [MVC]) were combined with a comprehensive ankle motor assessment protocol that measured ankle dorsiflexor strength (MVC, manual muscle testing, maximal movement velocity [MMV]), dexterity (the ability to accurately time ankle dorsiflexion movements) and gait (speed, walking aids). In the first 6 months after iSCI, all measures of muscle strength, gait and the MEP amplitudes significantly increased. The level of background electromyography (EMG) at 20% MVC remained stable, although absolute MVC increased. The MEP latencies were significantly delayed and remained unchanged during the first 6 months after iSCI. In addition, dexterity was preserved throughout rehabilitation. The percentage increase in MEP amplitude was significantly related only to the percentage improvement in MMV. The finding of unchanged CST conductivity, as assessed by MEP latencies in acute iSCI patients recovering motor function, is in accordance with previous studies in human SCI on this issue. The increased MEP facilitation at stable background EMG might indicate improved synchronization of the descending volley and/or responsiveness of motoneurons to supra-spinal input. The absence of a relationship between MEP amplitudes and recovery of ambulation and muscle strength implies that plastic changes in spinal neural circuits and preserved motor units might have contributed to the functional improvement.

The amplitude of lower leg motor evoked potentials is a reliable measure when controlled for torque and motor task

OBJECTIVES

Motor evoked potential (MEP) amplitudes have the disadvantage of a high variability when repeatedly assessed. This affects the reliability of MEP amplitude measurements taken during the course of motor incomplete spinal cord injury (iSCI). The study investigated the reliability of anterior tibial (TA) MEP measures controlled for dorsal flexion torque and motor task.

METHODS

TA MEPs were recorded at 10, 20, 40 and 60% of maximal voluntary contraction (MVC) during a static and dynamic (isometric increase of dorsal flexion torque) motor task. To determine reliability, 20 healthy and five chronic iSCI subjects were tested twice (> or =7 days) by the same investigator. Intraclass correlation coefficients (ICCs) were calculated. MEP amplitudes and latencies were compared between 20 healthy and 29 iSCI subjects.

RESULTS

The reliability of MEP amplitude was in general good (ICC > or = 0.52) and was highest during the static task at 40% MVC (ICC = 0.77). The increased facilitation by the dynamic motor task showed the best reliability at 20% MVC (ICC = 0.48). The reliability was good to excellent for MEP latency (0.46 < or = ICC < or = 0.81), MVC (ICC > or = 0.90) and for the TMS threshold required to evoke a MEP response (ICC > or = 0.77). The torque generated by the MEP response ()0.02 < or = ICC < or = 0.55) and the duration of the silent period (0.07 < or = ICC < or = 0.50) were not reliable. Both MEP amplitudes and latencies differed significantly between healthy and iSCI subjects.

CONCLUSIONS

Controlling for torque generation and motor task establishes a reliability of TA MEP amplitudes that is sufficient for longitudinal assessments in motor incomplete SCI.

Guidelines for the conduct of clinical trials for spinal cord injury (SCI) as developed by the ICCP panel: clinical trial outcome measures

An international panel reviewed the methodology for clinical trials of spinal cord injury (SCI), and provided recommendations for the valid conduct of future trials. This is the second of four papers. It examines clinical trial end points that have been used previously, reviews alternative outcome tools and identifies unmet needs for demonstrating the efficacy of an experimental intervention after SCI. The panel focused on outcome measures that are relevant to clinical trials of experimental cell-based and pharmaceutical drug treatments. Outcome measures are of three main classes: (1) those that provide an anatomical or neurological assessment for the connectivity of the spinal cord, (2) those that categorize a subject's functional ability to engage in activities of daily living, and (3) those that measure an individual's quality of life (QoL). The American Spinal Injury Association impairment scale forms the standard basis for measuring neurologic outcomes. Various electrophysiological measures and imaging tools are in development, which may provide more precise information on functional changes following treatment and/or the therapeutic action of experimental agents. When compared to appropriate controls, an improved functional outcome, in response to an experimental treatment, is the necessary goal of a clinical trial program. Several new functional outcome tools are being developed for measuring an individual's ability to engage in activities of daily living. Such clinical end points will need to be incorporated into Phase 2 and Phase 3 trials. QoL measures often do not correlate tightly with the above outcome tools, but may need to form part of Phase 3 trial measures.