Duration and reliability of the silent period in individuals with spinal cord injury

Weak grip strength among persons with multiple sclerosis having minimal disability is not related to agility or integrity of the corticospinal tract

INTRODUCTION

Mobility impairment is common in multiple sclerosis (MS); however, agility has received less attention. Agility requires strength and neuromuscular coordination to elicit controlled propulsive rapid whole-body movement. Grip strength is a common method to assess whole body force production, but also reflects neuromuscular integrity and global brain health. Impaired agility may be linked to loss of neuromuscular integrity (reflected by grip strength or corticospinal excitability).

OBJECTIVES

We aimed to determine whether grip strength would be associated with agility and transcranial magnetic stimulation (TMS)-based indices of corticospinal excitability and inhibition in persons with MS having low disability. We hypothesized that low grip strength would predict impaired agility and reflect low corticospinal excitability.

METHODS

We recruited 34 persons with relapsing MS (27 females; median [range] age 45.5 [21.0-65.0] years) and mild disability (median [range] Expanded Disability Status Scale 2.0 [0-3.0]), as well as a convenience sample of age- and sex-matched apparently healthy controls. Agility was tested by measuring hop length during bipedal hopping on an instrumented walkway. Grip strength was measured using a calibrated dynamometer. Corticospinal excitability and inhibition were examined using TMS-based motor evoked potential (MEP) and corticospinal silent period (CSP) recruitment curves, respectively.

RESULTS

MS participants had significantly lower grip strength than controls independent of sex. Females with and without MS had weaker grip strength than males. There were no statistically significant sex or group differences in agility. After controlling for sex, weaker grip strength was associated with shorter hop length in controls only (r = 0.645, p < .05). Grip strength did not significantly predict agility in persons with MS, nor was grip strength predicted by corticospinal excitability or inhibition.

CONCLUSIONS

In persons with MS having low disability, grip strength (normalized to body mass) was reduced despite having intact agility and walking performance. Grip strength was not associated with corticospinal excitability or inhibition, suggesting peripheral neuromuscular function, low physical activity or fitness, or other psychosocial factors may be related to weakness. Low grip strength is a putative indicator of early neuromuscular aging in persons with MS having mild disability and normal mobility.

Corpus Callosum-Mediated Interhemispheric Interactions in Cervical Spondylotic Myelopathy

PURPOSE

The corpus callosum is crucial for interhemispheric interactions in the motor control of limb functions. Human and animal studies suggested spinal cord pathologies may induce cortical reorganization in sensorimotor areas. We investigate participation of the corpus callosum in executions of a simple motor task in patients with cervical spondylotic myelopathy (CSM) using transcranial magnetic stimulation.

METHODS

Twenty patients with CSM with various MRI grades of severity of cord compression were compared with 19 normal controls. Ipsilateral silent period, contralateral silent period, central motor conduction time, and transcallosal conduction time (TCT) were determined.

RESULTS

In both upper and lower limbs, TCTs were significantly increased for patients with CSM than normal controls ( p < 0.001 for all), without side-to-side differences. Ipsilateral silent period and contralateral silent period durations were significantly increased bilaterally for upper limbs in comparison to controls ( p < 0.01 for all), without side-to-side differences. There were no significant correlations of TCT with central motor conduction time nor severity of CSM for both upper and lower limbs ( p > 0.05 for all) bilaterally.

CONCLUSIONS

Previous transcranial magnetic stimulation studies show increased motor cortex excitability in CSM; hence, increased TCTs observed bilaterally may be a compensatory mechanism for effective unidirectional and uniplanar execution of muscle activation in the distal limb muscles. Lack of correlation of TCTs with severity of CSM or central motor conduction time may be in keeping with a preexistent role of the corpus callosum as a predominantly inhibitory pathway for counteracting redundant movements resulting from increased motor cortex excitability occurring after spinal cord lesions.

Corticospinal inhibition investigated in relation to upper extremity motor function in cervical spinal cord injury

OBJECTIVE

Corticospinal inhibitory mechanisms are relevant to functional recovery but remain poorly understood after spinal cord injury (SCI). Post-injury characteristics of contralateral silent period (CSP), a measure of corticospinal inhibition evaluated using transcranial magnetic stimulation (TMS), is inconsistent in literature. We envisioned that investigating CSP across muscles with varying degrees of weakness may be a reasonable approach to resolve inconsistencies and elucidate the relevance of corticospinal inhibition for upper extremity function following SCI.

METHODS

We studied 27 adults with chronic C1-C8 SCI (age 48.8 ± 16.1 years, 3 females) and 16 able-bodied participants (age 33.2 ± 11.8 years, 9 females). CSP characteristics were assessed across biceps (muscle power = 3-5) and triceps (muscle power = 1-3) representing stronger and weaker muscles, respectively. We assessed functional abilities using the Capabilities of the Upper Extremity Test (CUE-T).

RESULTS

Participants with chronic SCI had prolonged CSPs for biceps but delayed and diminished CSPs for triceps compared to able-bodied participants. Early-onset CSPs for biceps and longer, deeper CSPs for triceps correlated with better CUE-T scores.

CONCLUSIONS

Corticospinal inhibition is pronounced for stronger biceps but diminished for weaker triceps muscle in SCI indicating innervation relative to the level of injury matters in the study of CSP.

SIGNIFICANCE

Nevertheless, corticospinal inhibition or CSP holds relevance for upper extremity function following SCI.

Electrospinography for non-invasively recording spinal sensorimotor networks in humans

Objective. Currently, few non-invasive measures exist for directly measuring spinal sensorimotor networks. Electrospinography (ESG) is one non-invasive method but is primarily used to measure evoked responses or for monitoring the spinal cord during surgery. Our objectives were to evaluate the feasibility of ESG to measure spinal sensorimotor networks by determining spatiotemporal and functional connectivity changes during single-joint movements at the spinal and cortical levels.Approach. We synchronously recorded electroencephalography (EEG), electromyography, and ESG in ten neurologically intact adults while performing one of three lower-limb tasks (no movement, plantar-flexion and knee flexion) in the prone position. A multi-pronged approach was applied for removing artifacts usingH∞filtering, artifact subspace reconstruction and independent component (IC) analysis. Next, data were segmented by task and ICs of EEG were clustered across participants. Within-participant analysis of ICs and ESG data was conducted, and ESG was characterized in the time and frequency domains. Generalized partial directed coherence analysis was performed within ICs and between ICs and ESG data by participant and task.Results.K-means clustering resulted in five clusters of ICs at Brodmann areas (BAs) 9, BA 8, BA 39, BA 4, and BA 22. Areas associated with motor planning, working memory, visual processing, movement, and attention, respectively. Time-frequency analysis of ESG data found localized changes during movement execution when compared to no movement. Lastly, we found bi-directional changes in functional connectivity (p < 0.05, adjusted for multiple comparisons) within IC's and between IC's and ESG sensors during movement when compared to the no movement condition.Significance. To our knowledge this is the first report using high density ESG for characterizing single joint lower limb movements. Our findings provide support that ESG contains information about efferent and afferent signaling in neurologically intact adults and suggests that we can utilize ESG to directly study the spinal cord.

Research progress on the application of transcranial magnetic stimulation in spinal cord injury rehabilitation: a narrative review

Traumatic or non-traumatic spinal cord injury (SCI) can lead to severe disability and complications. The incidence of SCI is high, and the rehabilitation cycle is long, which increases the economic burden on patients and the health care system. However, there is no practical method of SCI treatment. Recently, transcranial magnetic stimulation (TMS), a non-invasive brain stimulation technique, has been shown to induce changes in plasticity in specific areas of the brain by regulating the activity of neurons in the stimulation site and its functionally connected networks. TMS is a new potential method for the rehabilitation of SCI and its complications. In addition, TMS can detect the activity of neural circuits in the central nervous system and supplement the physiological evaluation of SCI severity. This review describes the pathophysiology of SCI as well as the basic principles and classification of TMS. We mainly focused on the latest research progress of TMS in the physiological evaluation of SCI as well as the treatment of motor dysfunction, neuropathic pain, spasticity, neurogenic bladder, respiratory dysfunction, and other complications. This review provides new ideas and future directions for SCI assessment and treatment.

Clinical diagnostic utility of transcranial magnetic stimulation in neurological disorders. Updated report of an IFCN committee

The review provides a comprehensive update (previous report: Chen R, Cros D, Curra A, Di Lazzaro V, Lefaucheur JP, Magistris MR, et al. The clinical diagnostic utility of transcranial magnetic stimulation: report of an IFCN committee. Clin Neurophysiol 2008;119(3):504-32) on clinical diagnostic utility of transcranial magnetic stimulation (TMS) in neurological diseases. Most TMS measures rely on stimulation of motor cortex and recording of motor evoked potentials. Paired-pulse TMS techniques, incorporating conventional amplitude-based and threshold tracking, have established clinical utility in neurodegenerative, movement, episodic (epilepsy, migraines), chronic pain and functional diseases. Cortical hyperexcitability has emerged as a diagnostic aid in amyotrophic lateral sclerosis. Single-pulse TMS measures are of utility in stroke, and myelopathy even in the absence of radiological changes. Short-latency afferent inhibition, related to central cholinergic transmission, is reduced in Alzheimer's disease. The triple stimulation technique (TST) may enhance diagnostic utility of conventional TMS measures to detect upper motor neuron involvement. The recording of motor evoked potentials can be used to perform functional mapping of the motor cortex or in preoperative assessment of eloquent brain regions before surgical resection of brain tumors. TMS exhibits utility in assessing lumbosacral/cervical nerve root function, especially in demyelinating neuropathies, and may be of utility in localizing the site of facial nerve palsies. TMS measures also have high sensitivity in detecting subclinical corticospinal lesions in multiple sclerosis. Abnormalities in central motor conduction time or TST correlate with motor impairment and disability in MS. Cerebellar stimulation may detect lesions in the cerebellum or cerebello-dentato-thalamo-motor cortical pathways. Combining TMS with electroencephalography, provides a novel method to measure parameters altered in neurological disorders, including cortical excitability, effective connectivity, and response complexity.

Neurophysiologic evaluation of patients with cervical spondylotic myelopathy

Background

Cervical spondylotic myelopathy (CSM) is a neurodegenerative disease caused by repetitive spinal cord damage that has resulted in significant clinical morbidity. The clinical evaluation of signs and symptoms, as well as neuroimaging and several neurophysiological tests, are used to make the diagnosis.

Objectives

To investigate changes in the cutaneous silent period (CuSP), cortical silent period (CoSP), and H-reflex in CSM patients, and to correlate these tests with the Japanese Orthopedic Association (JOA) score and Nurick’s grading, as well as to determine the diagnostic value of each of them.

Methods

Twenty patients (14 males and 6 females) with CSM were clinically diagnosed and documented by magnetic resonance imaging (MRI), and they were paired with another 20 healthy volunteers (13 males and 7 females) as a control group. CuSP, CoSP, and H-reflex tests were performed on both groups.

Results

In CSM patients, CuSP latency and duration are substantially longer and shorter in CSM patients, respectively. The degree of changes in CuSP latency is well correlated with the severity of the disease. Further, CoSP duration is significantly shortened. The H-reflex parameters did not differ significantly between the patient and control groups.

Conclusion

The shortened CoSP’s duration and the prolonged CuSP's latency suggest malfunction of the inhibitory and excitatory circuits in the spinal cord. The CuSP is more sensitive and specific than the CoSP in the diagnosis of a patient with CSM.

Utility of transcranial magnetic stimulation in the assessment of spinal cord injury: Current status and future directions

Comprehensive assessment following traumatic spinal cord injury (SCI) is needed to improve prognostication, advance the understanding of the neurophysiology and better targeting of clinical interventions. The International Standards for Neurological Classification of Spinal Cord Injury is the most common clinical examination recommended for use after a SCI. In addition, there are over 30 clinical assessment tools spanning across different domains of the International Classification of Functioning, Disability, and Health that have been validated and recommended for use in SCI. Most of these tools are subjective in nature, have limited value in predicting neurologic recovery, and do not provide insights into neurophysiological mechanisms. Transcranial magnetic stimulation (TMS) is a non-invasive neurophysiology technique that can supplement the clinical assessment in the domain of body structure and function during acute and chronic stages of SCI. TMS offers a better insight into neurophysiology and help in better detection of residual corticomotor connectivity following SCI compared to clinical assessment alone. TMS-based motor evoked potential and silent period duration allow study of excitatory and inhibitory mechanisms following SCI. Changes in muscle representations in form of displacement of TMS-based motor map center of gravity or changes in the map area can capture neuroplastic changes resulting from SCI or following rehabilitation. Paired-pulse TMS measures help understand the compensatory reorganization of the cortical circuits following SCI. In combination with peripheral stimulation, TMS can be used to study central motor conduction time and modulation of spinal reflexes, which can be used for advanced diagnostic and treatment purposes. To strengthen the utility of TMS in SCI assessment, future studies will need to standardize the assessment protocols, address population-specific concerns, and establish the psychometric properties of TMS-based measurements in the SCI population.