Intracortical excitability in patients with relapsing–remitting and secondary progressive multiple sclerosis

Motor Evoked Potential-A Pilot Study Looking at Reliability and Clinical Correlations in Multiple Sclerosis

PURPOSE

Multiple sclerosis (MS) is a clinically heterogeneous disease. Biomarkers that can assess pathological processes that are unseen with conventional imaging remain an unmet need in MS disease management. Motor evoked potentials (MEPs) could be such a biomarker. To determine and follow longitudinal MEP reliability and correlations with clinical measures in MS patients.

METHODS

This is a single-center study in alemtuzumab-treated MS patients to evaluate temporal reliability of MEPs, identify MEP minimum detectible differences, and explore correlations with existing clinical scales. Ten MS patients recently treated with alemtuzumab were evaluated every 6 months over 3 years. Clinical evaluations consisted of expanded disability status scale, timed 25-foot walk, 6-minute walk, and nine-hole peg test. MEPs were measured twice, 2 weeks apart, every 6 months.

RESULTS

Eight patients completed all 3 years of study. The intraclass correlation coefficient for MEP parameters ranged from 0.76 to 0.98. TA latency and amplitude with facilitation significantly and strongly correlated with all clinical measures, whereas the MEP duration modestly correlated. Biceps latency with facilitation significantly and moderately correlated with 9-hole peg test. Longitudinal correlations demonstrated good predictive values for either clinical deterioration or improvement.

CONCLUSIONS

MEPs have excellent intrapatient and intrarater reliability, and TA MEPs significantly and strongly correlated with expanded disability status scale, 6-minute walk, and timed 25-foot walk, whereas biceps MEPs significantly and moderately correlated with nine-hole peg test. Further studies using larger cohorts of MS patients are indicated.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov, Identifier: NCT02623946.

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.

Assessment of Motor Evoked Potentials in Multiple Sclerosis

Transcranial magnetic stimulation (TMS) is a noninvasive technique mainly used for the assessment of corticospinal tract integrity and excitability of the primary motor cortices. Motor evoked potentials (MEPs) play a pivotal role in TMS studies. TMS clinical guidelines, concerning the use and interpretation of MEPs in diagnosing and monitoring corticospinal tract integrity in people with multiple sclerosis (pwMS), were established almost ten years ago and refer mainly to the use of TMS implementation; this comprises the magnetic stimulator connected to a standard EMG unit, with the positioning of the coil performed by using the external landmarks on the head. The aim of the present work was to conduct a narrative literature review on the MEP assessment and outcome measures in clinical and research settings, assessed by TMS Methodological characteristics of different TMS system implementations (TMS without navigation, line-navigated TMS and e-field-navigated TMS); these were discussed in the context of mapping the corticospinal tract integrity in MS. An MEP assessment of two case reports, by using an e-field-navigated TMS, was presented; the results of the correspondence between the e-field-navigated TMS with MRI, and the EDSS classifications were presented. Practical and technical guiding principles for the improvement of TMS studies in MEP assessment for MS are discussed, suggesting the use of e-field TMS assessment in the sense that it can improve the accuracy of corticospinal tract integrity testing by providing a more objective correspondence of the neurophysiological (e-field-navigated TMS) and clinical (Expanded Disability Status Scale-EDSS) classifications.

How is neuromuscular fatigability affected by perceived fatigue and disability in people with multiple sclerosis?

Whereas fatigue is recognized to be the main complaint of patients with multiple sclerosis (PwMS), its etiology, and particularly the role of resistance to fatigability and its interplay with disability level, remains unclear. The purposes of this review were to (i) clarify the relationship between fatigue/disability and neuromuscular performance in PwMS and (ii) review the corticospinal and muscular mechanisms of voluntary muscle contraction that are altered by multiple sclerosis, and how they may be influenced by disability level or fatigue. Neuromuscular function at rest and during exercise are more susceptible to impairement, due to deficits in voluntary activation, when the disability is greater. Fatigue level is related to resistance to fatigability but not to neuromuscular function at rest. Neurophysiological parameters related to signal transmission such as central motor conduction time, motor evoked potentials amplitude and latency are affected by disability and fatigue levels but their relative role in the impaired production of torque remain unclear. Nonetheless, cortical reorganization represents the most likely explanation for the heightened fatigability during exercise for highly fatigued and/or disabled PwMS. Further research is needed to decipher how the fatigue and disability could influence fatigability for an ecological task, especially at the corticospinal level.

People with multiple sclerosis have reduced TMS-evoked motor cortical output compared with healthy individuals during fatiguing submaximal contractions

People with multiple sclerosis (PwMS) typically experience greater levels of exercise-induced fatigue compared with healthy individuals. Therefore, this study examined performance fatigability in PwMS when executing a prolonged submaximal contraction. Nine PwMS (38 ± 7 yr, 6 females) and nine healthy controls (35 ± 6 yr, 4 females) performed an elbow flexion at 15% maximal voluntary contraction (MVC) for 26 min. MVCs were performed every 2 min during, and following, the contraction to determine if maximal force was impaired by the low-intensity contraction. Single-pulse transcranial magnetic stimulation (TMS) was delivered to the primary motor cortex with a circular coil during each MVC and during the submaximal contraction. Superimposed and resting twitches were calculated from elbow flexion torque, whereas motor-evoked potentials were calculated from biceps brachii electromyography. Ratings of perceived exertion (RPE) were obtained before each MVC. During the fatiguing contraction protocol, the MS group exhibited a reduced MVC torque compared with the healthy control group (P = 0.044), which aligned with group differences in biceps brachii EMG activity (P = 0.022) and superimposed twitch amplitude (P = 0.016). Fatigue-related decrements in MVC torque (P = 0.044) and biceps brachii EMG activity (P = 0.043) demonstrated in the MS group persisted throughout recovery. However, MS did not affect the RPE during the fatigue task. These findings suggest that PwMS may have greater levels of performance fatigability due to decreased voluntary drive from the motor cortex, which is not associated with greater ratings of perceived exertion.NEW & NOTEWORTHY By combining TMS and motor nerve stimulation during a low-intensity exercise task, we were able to uncover the contribution that different levels of the CNS have during fatiguing exercise in PwMS. Our findings are novel and revealed that PwMS experienced decreased voluntary drive from the motor cortex during a low-intensity sustained fatiguing task that was associated with heightened levels of performance fatigability.

Multiple sclerosis: Inflammation, autoimmunity and plasticity

In recent years, experimental studies have clarified that immune system influences the functioning of the central nervous system (CNS) in both physiologic and pathologic conditions. The neuro-immune crosstalk plays a crucial role in neuronal development and may be critically involved in mediating CNS response to neuronal damage. Multiple sclerosis (MS) represents a good model to investigate how the immune system regulates neuronal activity. Accordingly, a growing body of evidence has demonstrated that increased levels of pro-inflammatory mediators may significantly impact synaptic mechanisms, influencing overall neuronal excitability and synaptic plasticity expression. In this chapter, we provide an overview of preclinical data and clinical studies exploring synaptic functioning noninvasively with transcranial magnetic stimulation (TMS) in patients with MS. Moreover, we examine how inflammation-driven synaptic dysfunction could affect synaptic plasticity expression, negatively influencing the MS course. Contrasting CSF inflammation together with pharmacologic enhancement of synaptic plasticity and application of noninvasive brain stimulation, alone or in combination with rehabilitative treatments, could improve the clinical compensation and prevent the accumulating deterioration in MS.

Non-invasive brain stimulation to assess neurophysiologic underpinnings of lower limb motor impairment in multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is a neuroinflammatory disease resulting in axonal demyelination and an amalgamation of symptoms which commonly result in decreased quality of life due to mobility dysfunction and limited participation in meaningful activities.

NEW METHOD

The use of non-invasive brain stimulation (NIBS) techniques, specifically transcranial magnetic and transcranial direct current stimulation, have been essential in understanding the pathophysiological decrements related to disease progression, particularly with regard to motor impairments. Although the research in this area has primarily focused on the upper extremities, new interest has arisen in understanding the neurophysiological underpinnings of lower limb impairment. Therefore, the purpose of this review is to: first, provide an overview of common NIBS techniques used to explore sensorimotor neurophysiology; second, summarize lower limb neuromuscular and mobility impairments typically observed in PwMS; third, review the current knowledge regarding interactions between TMS-assessed neurophysiology and lower limb impairments in PwMS; and fourth, provide recommendations for future NIBS studies based on current gaps in the literature.

RESULTS

PwMS exhibit reduced excitability and increased inhibitory neurophysiologic function which has been related to disease severity and lower limb motor impairments. Comparison with existing methods: Moreover, promising results indicate that the use of repetitive stimulation and transcranial direct current stimulation may prime neural adaptability and prove useful as a therapeutic tool in ameliorating lower limb impairments.

CONCLUSIONS

While these studies are both informative and promising, additional studies are necessary to be conclusive. As such, studies assessing objective measures of lower limb impairments associated with neurophysiological adaptations need further evaluation.

Transcranial magnetic stimulation (TMS) and repetitive TMS in multiple sclerosis

Multiple sclerosis (MS) is the most well-known autoimmune disorder of the central nervous system, and constitutes a major cause of disability, especially in young individuals. A wide array of pharmacological treatments is available, but they have often been proven to be ineffective in ameliorating disease symptomatology or slowing disease progress. As such, non-invasive and non-pharmacological techniques have been gaining more ground. Transcranial magnetic stimulation (TMS) utilizes the electric field generated by a magnetic coil to stimulate neurons and has been applied, usually paired with electroencephalography, to study the underlying pathophysiology of MS, and in repetitive trains, in the form of repetitive transcranial magnetic stimulation (rTMS), to induce long-lasting changes in neuronal circuits. In this review, we present the available literature on the application of TMS and rTMS in the context of MS, with an emphasis on its therapeutic potential on various clinical aspects, while also naming the ongoing trials, whose results are anticipated in the future.

A cross-sectional comparison of performance, neurophysiological and MRI outcomes of responders and non-responders to fampridine treatment in multiple sclerosis - An explorative study

OBJECTIVE

To compare baseline physical and cognitive performance, neurophysiological, and magnetic resonance imaging (MRI) outcomes and examinetheir interrelationship inparticipants with Multiple Sclerosis (MS), already established aseither responder or non-responder to Fampridine treatment, andto examine associationswiththe expanded disability status scale (EDSS) and 12-item MS walking scale (MSWS-12).

METHODS

Baseline data from an explorative longitudinal observational study were analyzed. Participants underwent the Timed 25-Foot Walk Test (T25FW), Six Spot Step Test (SSST), Nine-Hole Peg Test, Five Times Sit-to-Stand Test, Symbol Digit Modalities Test (SDMT), neurophysiological testing, including central motor conduction time (CMCT), peripheral motor conduction time (PMCT), motor evoked potential (MEP) amplitudesand electroneuronographyof the lower extremities, and brain MRI (brain volume, number and volume of T2-weighted lesions and lesion load normalized to brain volume).

RESULTS

41 responders and 8 non-responders were examined. There were no intergroup differences inphysical performance, cognitive, neurophysiological, andMRI outcomes (p > 0.05).CMCT was associated withT25FW, SSST, EDSS, and MSWS-12,(p < 0.05). SDMT was associated with the number and volume of T2-weighted lesions, and lesion load normalized to brain volume (p < 0.05).

CONCLUSION

No differences were identified between responders and non-responders to Fampridine treatment regarding physical and cognitive performance, neurophysiological or MRI outcomes. The results call for cautious interpretation and further large-scale studies are needed to expand ourunderstanding of underlying mechanisms discriminating Fampridine responders and non-responders.CMCT may be used as a marker of disability and walking impairment, while SDMT was associated with white matter lesions estimated by MRI. ClinicalTrials.gov identifier: NCT03401307.

Neurophysiological impairments in multiple sclerosis-Central and peripheral motor pathways

A systematic review of the literature was conducted comparing neurophysiological outcomes in persons with multiple sclerosis (PwMS) to healthy controls (HC), in studies of the central nervous system (CNS) function comprising motor evoked potentials (MEP) elicited by transcranial magnetic stimulation (TMS) and in studies of the peripheral nervous system (PNS) function comprising electroneuronography (ENG) outcomes elicited by peripheral nerve stimulation. Studies comparing neuromuscular function, assessed during maximal voluntary contraction (MVC) of muscle, were included if they reported muscle strength along with muscle activation by use of electromyography (EMG) and/or interpolated twitch technique (ITT). Studies investigating CNS function showed prolonged central motor conduction times, asymmetry of nerve conduction motor pathways, and prolonged latencies in PwMS when compared to HC. Resting motor threshold, amplitude, and cortical silent periods showed conflicting results. CNS findings generally correlated with disabilities. Studies of PNS function showed near significant prolongation in motor latency of the median nerve, reduced nerve conduction velocities in the tibial and peroneal nerves, and decreased compound muscle action potential amplitudes of the tibial nerve in PwMS. ENG findings did not correlate with clinical severity of disabilities. Studies of neuromuscular function showed lower voluntary muscle activation and increased central fatigue in PwMS, whereas EMG showed divergent muscle activation (ie, EMG amplitude) during MVC. When comparing the existing literature on neurophysiological motor examinations in PwMS and HC, consistent and substantial impairments of CNS function were seen in PwMS, whereas impairments of the PNS were less pronounced and inconsistent. In addition, impairments in muscle activation were observed in PwMS.

Multiple sclerosis-related fatigue: the role of impaired corticospinal responses and heightened exercise fatigability

It is unclear whether motor fatigability and perceived fatigue share a common pathophysiology in people with multiple sclerosis (PwMS). This cross-sectional investigation explored the relationship between the mechanisms of motor fatigability from cycling and fatigue severity in PwMS. Thirteen highly fatigued (HF) and thirteen nonfatigued (LF) PwMS and thirteen healthy controls (CON) completed a step test until volitional exhaustion on an innovative cycle ergometer. Neuromuscular evaluations involving femoral nerve electrical stimulation and transcranial magnetic stimulation were performed every 3 min throughout cycling. One-way ANOVA at baseline and exhaustion uncovered evidence of consistently smaller motor evoked potential (MEP) amplitudes (P = 0.011) and prolonged MEP latencies (P = 0.041) in HF as well as a greater decline in maximal voluntary contraction force (HF: 63 ± 13%; LF: 75 ± 13%; CON: 73 ± 11% of pre; P = 0.037) and potentiated twitch force (HF: 35 ± 13%; LF: 50 ± 16%; CON: 47 ± 17% of pre; P = 0.049) in HF at volitional exhaustion. Hierarchical regression determined that fatigue severity on the Fatigue Severity Scale was predicted by prolonged MEP latencies (change in r² = 0.389), elevated peripheral muscle fatigability (change in r² = 0.183), and depressive symptoms (change in r² = 0.213). These findings indicate that MS-related fatigue is distinguished by disrupted corticospinal responsiveness, which could suggest progressive pathology, but fatigability from whole body exercise and depressive symptoms also influence perceptions of fatigue in PwMS.NEW & NOTEWORTHY The etiology of fatigability from whole body exercise was examined for the first time to accurately elucidate the relationship between fatigue and fatigability in multiple sclerosis (MS). Compromised corticospinal responsiveness predicted fatigue severity, providing a novel, objective indicator of fatigue in MS. Although the impaired corticomotor transmission did not aggravate muscle activation in this group of people with multiple sclerosis (PwMS) of lower disability, heightened muscle fatigability was seen to contribute to perceptions of fatigue in PwMS.

Can Operant Conditioning of EMG-Evoked Responses Help to Target Corticospinal Plasticity for Improving Motor Function in People With Multiple Sclerosis?

Corticospinal pathway and its function are essential in motor control and motor rehabilitation. Multiple sclerosis (MS) causes damage to the brain and descending connections, and often diminishes corticospinal function. In people with MS, neural plasticity is available, although it does not necessarily remain stable over the course of disease progress. Thus, inducing plasticity to the corticospinal pathway so as to improve its function may lead to motor control improvements, which impact one's mobility, health, and wellness. In order to harness plasticity in people with MS, over the past two decades, non-invasive brain stimulation techniques have been examined for addressing common symptoms, such as cognitive deficits, fatigue, and spasticity. While these methods appear promising, when it comes to motor rehabilitation, just inducing plasticity or having a capacity for it does not guarantee generation of better motor functions. Targeting plasticity to a key pathway, such as the corticospinal pathway, could change what limits one's motor control and improve function. One of such neural training methods is operant conditioning of the motor-evoked potential that aims to train the behavior of the corticospinal-motoneuron pathway. Through up-conditioning training, the person learns to produce the rewarded neuronal behavior/state of increased corticospinal excitability, and through iterative training, the rewarded behavior/state becomes one's habitual, daily motor behavior. This minireview introduces operant conditioning approach for people with MS. Guiding beneficial CNS plasticity on top of continuous disease progress may help to prolong the duration of maintained motor function and quality of life in people living with MS.

Inflammation and Corticospinal Functioning in Multiple Sclerosis: A TMS Perspective

Transcranial magnetic stimulation (TMS) has been employed in multiple sclerosis (MS) to assess the integrity of the corticospinal tract and the corpus callosum and to explore some physiological properties of the motor cortex. Specific alterations of TMS measures have been strongly associated to different pathophysiological mechanisms, particularly to demyelination and neuronal loss. Moreover, TMS has contributed to investigate the neurophysiological basis of MS symptoms, particularly those not completely explained by conventional structural damage, such as fatigue. However, variability existing between studies suggests that alternative mechanisms should be involved. Knowledge of MS pathophysiology has been enriched by experimental studies in animal models (i.e., experimental autoimmune encephalomyelitis) demonstrating that inflammation alters synaptic transmission, promoting hyperexcitability and neuronal damage. Accordingly, TMS studies have demonstrated an imbalance between cortical excitation and inhibition in MS. In particular, cerebrospinal fluid concentrations of different proinflammatory and anti-inflammatory molecules have been associated to corticospinal hyperexcitability, highlighting that inflammatory synaptopathy may represent a key pathophysiological mechanism in MS. In this perspective article, we discuss whether corticospinal excitability alterations assessed with TMS in MS patients could be useful to explain the pathophysiological correlates and their relationships with specific MS clinical characteristics and symptoms. Furthermore, we discuss evidence indicating that, in MS patients, inflammatory synaptopathy could be present since the early phases, could specifically characterize relapses, and could progressively increase during the disease course.

Early diagnosis of secondary progressive multiple sclerosis: focus on fluid and neurophysiological biomarkers

BACKGROUND AND AIMS

Most patients with multiple sclerosis presenting with a relapsing-remitting disease course at diagnosis transition to secondary progressive multiple sclerosis (SPMS) 1-2 decades after onset. SPMS is characterized by predominant neurodegeneration and atrophy. These pathogenic hallmarks result in unsatisfactory treatment response in SPMS patients. Therefore, early diagnosis of SPMS is necessary for prompt treatment decisions. The aim of this review was to assess neurophysiological and fluid biomarkers that have the potential to monitor disease progression and support early SPMS diagnosis.

METHODS

We performed a systematic review of studies that analyzed the role of neurophysiological techniques and fluid biomarkers in supporting SPMS diagnosis using the preferred reporting items for systematic reviews and meta-analyses statement.

RESULTS

From our initial search, we selected 24 relevant articles on neurophysiological biomarkers and 55 articles on fluid biomarkers.

CONCLUSION

To date, no neurophysiological or fluid biomarker is sufficiently validated to support the early diagnosis of SPMS. Neurophysiological measurements, including short interval intracortical inhibition and somatosensory temporal discrimination threshold, and the neurofilament light chain fluid biomarker seem to be the most promising. Cross-sectional studies on an adequate number of patients followed by longitudinal studies are needed to confirm the diagnostic and prognostic value of these biomarkers. A combination of neurophysiological and fluid biomarkers may be more sensitive in detecting SPMS conversion.

Probing Context-Dependent Modulations of Ipsilateral Premotor-Motor Connectivity in Relapsing-Remitting Multiple Sclerosis

Objective: We employed dual-site TMS to test whether ipsilateral functional premotor-motor connectivity is altered in relapsing-remitting Multiple Sclerosis (RR-MS) and is related to central fatigue.

Methods: Twelve patients with RR-MS and 12 healthy controls performed a visually cued Pinch-NoPinch task with their right hand. During the reaction time (RT) period of Pinch and No-Pinch trials, single-site TMS was applied to the left primary motor cortex (M1) or dual-site TMS was applied to the ipsilateral dorsal premotor cortex (PMd) and to M1. We traced context-dependent changes of corticospinal excitability and premotor–motor connectivity by measuring Motor-Evoked Potentials (MEPs) in the right first dorsal interosseus muscle. Central fatigue was evaluated with the Fatigue Scale for Motor and Cognitive Functions (FSMS).

Results: In both groups, single-pulse TMS revealed a consistent increase in mean MEP amplitude during the Reaction Time (RT) period relative to a resting condition. Task-related corticospinal facilitation increased toward the end of the RT period in Pinch trials, while it decreased in No-Pinch trials. Again, this modulation of MEP facilitation by trial type was comparable in patients and controls. Dual-site TMS showed no significant effect of a conditioning PMd pulse on ipsilateral corticospinal excitability during the RT period in either group. However, patients showed a trend toward a relative attenuation in functional PMd-M1 connectivity at the end of the RT period in No-Pinch trials, which correlated positively with the severity of motor fatigue (r = 0.69; p = 0.007).

Conclusions: Dynamic regulation of corticospinal excitability and ipsilateral PMd-M1 connectivity is preserved in patients with RR-MS. MS-related fatigue scales positively with an attenuation of premotor-to-motor functional connectivity during cued motor inhibition.

Significance: The temporal, context-dependent modulation of ipsilateral premotor-motor connectivity, as revealed by dual-site TMS of ipsilateral PMd and M1, constitutes a promising neurophysiological marker of fatigue in MS.

Cortical Excitability Measures May Predict Clinical Response to Fampridine in Patients with Multiple Sclerosis and Gait Impairment

Background: Most multiple sclerosis (MS) patients will develop walking limitations during the disease. Sustained-release oral fampridine is the only approved drug that will improve gait in a subset of MS patients. Objectives: (1) Evaluate fampridine cortical excitability effect in MS patients with gait disability. (2) Investigate whether cortical excitability changes can predict the therapeutic response to fampridine. Method: This prospective observational study enrolled 20 adult patients with MS and gait impairment planned to receive fampridine 10 mg twice daily for two consecutive weeks. Exclusion criteria included: Recent relapse (<3 months), modification of disease modifying drugs (<6 months), or Expanded Disability Status Scale (EDSS) score >7. Neurological examination, timed 25-foot walk test (T25wt), EDSS, and cortical excitability studies were performed upon inclusion and 14 days after initiation of fampridine. Results: After treatment, the mean improvement of T25wt (ΔT25wt) was 4.9 s. Significant enhancement of intra-cortical facilitation was observed (139% versus 241%, p = 0.01) following treatment. A positive correlation was found between baseline resting motor threshold (rMT) and both EDSS (r = 0.57; p < 0.01) and ΔT25wt (r = 0.57, p = 0.01). rMT above 52% of the maximal stimulator output was found to be a good predictor of a favorable response to fampridine (accuracy: 75%). Discussion: Fampridine was found to have a significant modulatory effect on the cerebral cortex, demonstrated by an increase in excitatory intracortical processes as unveiled by paired-pulse transcranial magnetic stimulation. rMT could be useful in selecting patients likely to experience a favorable response to fampridine.

Beyond rehabilitation in MS: Insights from non-invasive brain stimulation

Although the number of disease-modifying treatments for people with multiple sclerosis (pwMS) has meaningfully increased in the past years, targeting repair or compensation for central nervous system damage associated with the disease process remains an important clinical goal. With this aim, neurorehabilitation is a powerful approach targeting central nervous system plasticity. Another driver of brain plasticity is non-invasive brain stimulation (NIBS), receiving recent attention in neurology, particularly for its potential synergy with neurorehabilitation and as add-on treatment for several neurological conditions, from pain to fatigue to sensorimotor and cognitive deficits. In this review, we will resume the evidence exploring the neurobiological basis of NIBS and its applications to MS-related conditions.

Operant Up-Conditioning of the Tibialis Anterior Motor-Evoked Potential in Multiple Sclerosis: Feasibility Case Studies

Damage to the corticospinal pathway often results in weak dorsiflexion of the ankle, thereby limiting the mobility of people with multiple sclerosis (MS). Thus, strengthening corticospinal connectivity may improve locomotion. Here, we investigated the feasibility of tibialis anterior (TA) motor-evoked potential (MEP) operant conditioning and whether it can enhance corticospinal excitability and alleviate locomotor problems in people with chronic stable MS. The protocol consisted of 6 baseline and 24 up-conditioning sessions over 10 weeks. In all sessions, TA MEPs were elicited at 10% above active threshold while the sitting subject provided 30–35% maximum voluntary contraction (MVC) level of TA background EMG. During baseline sessions, MEPs were simply measured. During conditioning trials of the conditioning sessions, the subject was encouraged to increase MEP and was given immediate feedback indicating whether MEP size was above a criterion. In 3/4 subjects, TA MEP increased 32–75%, MVC increased 28–52%, locomotor EMG modulation improved in multiple leg muscles, and foot drop became less severe. In one of them, MEP and MVC increases were maintained throughout 3 years of extensive follow-up sessions. These initial results support a therapeutic possibility of MEP operant conditioning for improving locomotion in people with MS or other CNS disorders, such as spinal cord injury and stroke.

Cortical Excitability and Interhemispheric Connectivity in Early Relapsing-Remitting Multiple Sclerosis Studied With TMS-EEG

Evoked potentials (EPs) are well established in clinical practice for diagnosis and prognosis in multiple sclerosis (MS). However, their value is limited to the assessment of their respective functional systems. Here, we used transcranial magnetic stimulation (TMS) coupled with electroencephalography (TMS-EEG) to investigate cortical excitability and spatiotemporal dynamics of TMS-evoked neural activity in MS patients. Thirteen patients with early relapsing–remitting MS (RRMS) with a median Expanded Disability Status Scale (EDSS) of 1.0 (range 0–2.5) and 16 age- and gender-matched healthy controls received single-pulse TMS of left and right primary motor cortex (L-M1 and R-M1), respectively. Resting motor threshold for L-M1 and R-M1 was increased in MS patients. Latencies and amplitudes of N45, P70, N100, P180, and N280 TMS-evoked EEG potentials (TEPs) were not different between groups, except a significantly increased amplitude of the N280 TEP in the MS group, both for L-M1 and R-M1 stimulation. Interhemispheric signal propagation (ISP), estimated from the area under the curve of TEPs in the non-stimulated vs. stimulated M1, also did not differ between groups. In summary, findings show that ISP and TEPs were preserved in early-stage RRMS, except for an exaggerated N280 amplitude. Our findings indicate that TMS-EEG is feasible in testing excitability and connectivity in cortical neural networks in MS patients, complementary to conventional EPs. However, relevance and pathophysiological correlates of the enhanced N280 will need further study.

A reduced somatosensory gating response in individuals with multiple sclerosis is related to walking impairment

When identical stimuli are presented in rapid temporal succession, neural responses to the second stimulation are often weaker than those observed for the first. This phenomenon is termed sensory gating and is believed to be an adaptive feature that helps prevent higher-order cortical centers from being flooded with unnecessary information. Recently, sensory gating in the somatosensory system has been linked to deficits in tactile discrimination. Additionally, studies have linked poor tactile discrimination with impaired walking and balance in individuals with multiple sclerosis (MS). In this study, we examine the neural basis of somatosensory gating in patients with MS and healthy controls and assess the relationship between somatosensory gating and walking performance. We used magnetoencephalography to record neural responses to paired-pulse electrical stimulation applied to the right posterior tibial nerve. All participants also walked across a digital mat, which recorded their spatiotemporal gait kinematics. Our results showed the amplitude of the response to the second stimulation was sharply reduced only in controls, resulting in a significantly reduced somatosensory gating in the patients with MS. No group differences were observed in the amplitude of the response to the first stimulation nor the latency of the neural response to either the first or second stimulation. Interestingly, the altered somatosensory gating responses were correlated with aberrant spatiotemporal gait kinematics in the patients with MS. These results suggest that inhibitory GABA circuits may be altered in patients with MS, which impacts somatosensory gating and contributes to the motor performance deficits seen in these patients.NEW & NOTEWORTHY We aimed to determine whether somatosensory gating in patients with multiple sclerosis (MS) differed compared with healthy controls and whether a relationship exists between somatosensory gating and walking performance. We found reduced somatosensory gating responses in patients with MS, and these altered somatosensory gating responses were correlated with the mobility impairments. These novel findings show that somatosensory gating is impaired in patients with MS and is related to the mobility impairments seen in these patients.

Altered sensorimotor cortical oscillations in individuals with multiple sclerosis suggests a faulty internal model

Multiple sclerosis (MS) is a demyelinating disease that results in a broad array of symptoms, including impaired motor performance. How such demyelination of fibers affects the inherent neurophysiological activity in motor circuits, however, remains largely unknown. Potentially, the movement errors associated with MS may be due to imperfections in the internal model used to make predictions of the motor output that will meet the task demands. Prior magnetoencephalographic (MEG) and electroencephalographic brain imaging experiments have established that the beta (15-30 Hz) oscillatory activity in the sensorimotor cortices is related to the control of movement. Specifically, it has been suggested that the strength of the post-movement beta rebound may indicate the certainty of the internal model. In this study, we used MEG to evaluate the neural oscillatory activity in the sensorimotor cortices of individuals with MS and healthy individuals during a goal-directed isometric knee force task. Our results showed no difference between the individuals with MS and healthy individuals in the beta activity during the planning and execution stages of movement. However, we did find that individuals with MS exhibited a weaker post-movement beta rebound in the pre/postcentral gyri relative to healthy controls. Additionally, we found that the behavioral performance of individuals with MS was aberrant, and related to the strength of the post-movement beta rebound. These results suggest that the internal model may be faulty in individuals with MS. Hum Brain Mapp 38:4009-4018, 2017. © 2017 Wiley Periodicals, Inc.

Cortical excitability changes over time in progressive multiple sclerosis

In 25 patients with progressive forms of multiple sclerosis (MS), motor cortex excitability was longitudinally studied over one year by means of transcranial magnetic stimulation (TMS). The following TMS parameters were considered: resting and active motor thresholds (MTs), input-output curve, short-interval intracortical inhibition (SICI), and intracortical facilitation. Clinical evaluation was based on the Expanded Disability Status Scale (EDSS). In the 16 patients not receiving disease-modifying drugs, the EDSS score worsened, resting MT increased, and SICI decreased. By contrast, no clinical for neurophysiological changes were found over time in the nine patients receiving immunomodulatory therapy. The natural course of progressive MS appears to be associated with a decline in cortical excitability of both pyramidal neurons and inhibitory circuits. This pilot study based on a small sample suggests that disease-modifying drugs may allow cortical excitability to remain stable, even in patients with progressive MS.

Cortical and spinal excitability in patients with multiple sclerosis and spasticity after oromucosal cannabinoid spray

BACKGROUND

Delta-9-tetrahydrocannabinol and cannabidiol (THC:CBD) oromucosal spray (Sativex®) has been recently approved for the management of treatment-resistant multiple sclerosis (MS) spasticity. Although the symptomatic relief of Sativex® on MS-spasticity has been consistently demonstrated, the pathogenetic implications remain unclear and the few electrophysiological studies performed to address this topic yielded controversial results. We therefore aimed to investigate the mechanisms underpinning the modulation of spastic hypertonia by Sativex®, at both central and spinal levels, through an extensive neurophysiological battery in patients with MS.

METHODS

Nineteen MS patients with treatment-resistant spasticity were recruited. Before and after 4weeks of treatment with Sativex® patients were clinically assessed with the Modified Ashworth Scale (MAS) and underwent a large neurophysiological protocol targeting measures of excitability and inhibition at both cortical [e.g., intracortical facilitation (ICF), short (SICI) and long (LICI) intracortical inhibition, cortical silent period (CSP)] and spinal level [e.g., H-reflex, H/M ratio and recovery curve of the H-reflex (HRC)]. A group of 19 healthy subjects served as controls.

RESULTS

A significant reduction of the MAS score after 4weeks of Sativex® treatment was detected. Before treatment, an increase in the late facilitatory phase of HRC was recorded in patients compared to the control group, that normalised post treatment. At central level, SICI and LICI were significantly higher in patients compared to healthy subjects. After therapy, a significant strengthening of inhibition (e.g. reduced LICI) and a non-significant facilitation (e.g. marginally increased ICF) occurred, suggesting a modulatory effect of Sativex® on different pathways, predominantly of inhibitory type. Sativex® treatment was well tolerated, with only 3 patients complaining about dizziness and bitter taste in their mouth.

DISCUSSION

Our results confirm the clinical benefit of Sativex® on spastic hypertonia and demonstrate that it might modulate both cortical and spinal circuits, arguably in terms of both excitation and inhibition. We suggest that the clinical benefit was likely related to a net increase of inhibition at cortical level that, in turn, might have influenced spinal excitability.

Synaptopathy connects inflammation and neurodegeneration in multiple sclerosis

Multiple sclerosis (MS) has long been regarded as a chronic inflammatory disease of the white matter that leads to demyelination and eventually to neurodegeneration. In the past decade, several aspects of MS pathogenesis have been challenged, and degenerative changes of the grey matter, which are independent of demyelination, have become a topic of interest. CNS inflammation in MS and experimental autoimmune encephalomyelitis (EAE; a disease model used to study MS in rodents) causes a marked imbalance between GABAergic and glutamatergic transmission, and a loss of synapses, all of which leads to a diffuse 'synaptopathy'. Altered synaptic transmission can occur early in MS and EAE, independently of demyelination and axonal loss, and subsequently causes excitotoxic damage. Inflammation-driven synaptic abnormalities are emerging as a prominent pathogenic mechanism in MS-importantly, they are potentially reversible and, therefore, represent attractive therapeutic targets. In this Review, we focus on the connection between inflammation and synaptopathy in MS and EAE, which sheds light not only on the pathophysiology of MS but also on that of primary neurodegenerative disorders in which inflammatory processes contribute to disease progression.

The Neurophysiologist Perspective into MS Plasticity

Multiple sclerosis (MS) is a frequent, highly debilitating inflammatory demyelinating disease, starting to manifest in early adulthood and presenting a wide variety of symptoms, which are often resistant to pharmacological treatments. Cortical dysfunctions have been demonstrated to be key components of MS condition, and plasticity of the corticospinal motor system is highly involved in major MS symptoms, such as fatigue, spasticity, or pain. Cortical dysfunction in MS can be studied with neurophysiological tools, such as electroencephalography (EEG) and related techniques (evoked potentials) or transcranial magnetic stimulation (TMS). These techniques are now widely used to provide essential elements of MS diagnosis and can also be used to modulate plasticity. Indeed, the recent development of non-invasive brain stimulation techniques able to induce cortical plasticity, such as repetitive TMS or transcranial direct current stimulation, has brought promising results as add-on treatments. In this review, we will focus on the use of these tools (EEG and TMS) to study plasticity in MS and on the major techniques used to modulate plasticity in MS.

Cerebral hypoperfusion: a new pathophysiologic concept in multiple sclerosis?

The exact pathogenesis of multiple sclerosis (MS) is incompletely understood. Although auto-immune responses have an important role in the development of hallmark focal demyelinating lesions, the underlying mechanism of axonal degeneration, the other key player in MS pathology and main determinant of long-term disability, remains unclear and corresponds poorly with inflammatory disease activity. Perfusion-weighted imaging studies have demonstrated that there is a widespread cerebral hypoperfusion in patients with MS, which is present from the early beginning to more advanced disease stages. This reduced cerebral blood flow (CBF) does not seems to be secondary to loss of axonal integrity with decreased metabolic demands but appears to be mediated by elevated levels of the potent vasospastic peptide endothelin-1 in the cerebral circulation. Evidence is evolving that cerebral hypoperfusion in MS is associated with chronic hypoxia, focal lesion formation, diffuse axonal degeneration, cognitive dysfunction, and fatigue. Restoring CBF may therefore emerge as a new therapeutic target in MS.

The use of transcranial magnetic stimulation in diagnosis, prognostication and treatment evaluation in multiple sclerosis

Despite advances in brain imaging which have revolutionised the diagnosis and monitoring of patients with Multiple Sclerosis (MS), current imaging techniques have limitations, including poor correlation with clinical disability and prognosis. There is growing evidence that electrophysiological techniques may provide complementary functional information which can aid in diagnosis, prognostication and perhaps even monitoring of treatment response in patients with MS. Transcranial magnetic stimulation (TMS) is an underutilised technique with potential to assist diagnosis, predict prognosis and provide an objective surrogate marker of clinical progress and treatment response. This review explores the existing body of evidence relating to the use of TMS in patients with MS, outlines the practical aspects and scope of TMS testing and reviews the current evidence relating to the use of TMS in diagnosis, disease classification, prognostication and response to symptomatic and disease-modifying therapies.

Sativex in the management of multiple sclerosis-related spasticity: role of the corticospinal modulation

Sativex is an emergent treatment option for spasticity in patients affected by multiple sclerosis (MS). This oromucosal spray, acting as a partial agonist at cannabinoid receptors, may modulate the balance between excitatory and inhibitory neurotransmitters, leading to muscle relaxation that is in turn responsible for spasticity improvement. Nevertheless, since the clinical assessment may not be sensitive enough to detect spasticity changes, other more objective tools should be tested to better define the real drug effect. The aim of our study was to investigate the role of Sativex in improving spasticity and related symptomatology in MS patients by means of an extensive neurophysiological assessment of sensory-motor circuits. To this end, 30 MS patients underwent a complete clinical and neurophysiological examination, including the following electrophysiological parameters: motor threshold, motor evoked potentials amplitude, intracortical excitability, sensory-motor integration, and H_max/M_max ratio. The same assessment was applied before and after one month of continuous treatment. Our data showed an increase of intracortical inhibition, a significant reduction of spinal excitability, and an improvement in spasticity and associated symptoms. Thus, we can speculate that Sativex could be effective in reducing spasticity by means of a double effect on intracortical and spinal excitability.

Abnormal control of orbicularis oculi reflex excitability in multiple sclerosis

Brain lesions in patients with multiple sclerosis may lead to abnormal excitability of brainstem reflex circuits because of impairment of descending control pathways. We hypothesized that such abnormality should show in the analysis of blink reflex responses in the form of asymmetries in response size. The study was done in 20 patients with relapsing-remitting multiple sclerosis and 12 matched healthy subjects. We identified first patients with latency abnormalities (AbLat). Then, we analyzed response size by calculating the R2c/R2 ratio to stimulation of either side and the mean area of the R2 responses obtained in the same side. Patients with significantly larger response size with respect to healthy subjects in at least one side were considered to have abnormal response excitability (AbEx). We also examined the blink reflex excitability recovery (BRER) and prepulse inhibition (BRIP) of either side in search for additional indices of asymmetry in response excitability. Neurophysiological data were correlated with MRI-determined brain lesion-load and volume. Eight patients were identified as AbLat (median Expanded Disability Status Scale–EDSS = 2.75) and 7 of them had ponto-medullary lesions. Nine patients were identified as AbEx (EDSS = 1.5) and only 2 of them, who also were AbLat, had ponto-medullary lesions. In AbEx patients, the abnormalities in response size were confined to one side, with a similar tendency in most variables (significantly asymmetric R1 amplitude, BRER index and BRIP percentage). AbEx patients had asymmetric distribution of hemispheral lesions, in contrast with the symmetric pattern observed in AbLat. The brainstem lesion load was significantly lower in AbEx than in AbLat patients (p = 0.04). Asymmetric abnormalities in blink reflex response excitability in patients with multiple sclerosis are associated with lesser disability and lower tissue loss than abnormalities in response latency. Testing response excitability could provide a reliable neurophysiological index of dysfunction in early stages of multiple sclerosis.

Normative data and long-term test-retest reliability of the triple stimulation technique (TST) in multiple sclerosis

OBJECTIVES

Transcranial magnetic stimulation is useful for the assessment of cortico-spinal tract integrity in multiple sclerosis (MS). An advanced approach is the triple stimulation technique (TST), utilizing a combination of central and peripheral stimuli, reducing individual response variability. Although TST measures have been implemented in longitudinal studies, basic methodological data on temporal properties of abnormal TST values in MS are sparse.

METHODS

Normative TST data were obtained from 48 healthy participants. Longitudinal measures were derived from 17 MS-patients (relapsing-remitting: N=10; clinically isolated syndrome: N=7) prior to, three and twelve months following therapy initiation. Intraclass correlations were used to examine test-retest reliability. Complementary, patient ambulation and cognition were assessed.

RESULTS

Patient TST parameters were abnormal, involving excellent test-retest reliability and stable mean values. Cognitive and motor performance improved.

CONCLUSIONS

Results are the first to show that abnormal TST values in MS, reflecting diagnostic utility, are highly reliable in a long-term follow-up. Methodological properties are adequate for a longitudinal implementation of TST. Parameters were insensitive to alterations in cognitive/motor functioning. Sensitivity may be verified in subgroups with different treatment regimes.

SIGNIFICANCE

Results provide new normative data, support diagnostic utility of TST measures in MS, and confirm their long-term robustness.

Oral fingolimod reduces glutamate-mediated intracortical excitability in relapsing-remitting multiple sclerosis

OBJECTIVE

Fingolimod is an effective disease modifying therapy for multiple sclerosis (MS). Beyond its main action on peripheral lymphocytes, several noteworthy side effects have been demonstrated in vitro, among which modulation of neural excitability. Our aim was to explore cortical excitability in vivo in patients treated with fingolimod 0.5mg/day.

METHODS

Paired-pulse TMS was applied on the left primary motor cortex in 13 patients affected by relapsing-remitting MS, the day before the first dose of fingolimod (T0) and 60days later (T1). Resting motor threshold, baseline motor evoked potentials, short interval intracortical inhibition (at 1, 3, 5ms) and intracortical facilitation (at 7, 9, 11 and 13ms) were estimated at T0 and T1.

RESULTS

Intracortical facilitation was reduced at T1, without any changes in short interval intracortical inhibition.

CONCLUSIONS

Fingolimod selectively reduced intracortical facilitation, which is mainly mediated by glutamate.

SIGNIFICANCE

This is the first in vivo confirmation of the effects of fingolimod on glutamatergic drive in treated humans. Our results suggest a novel neuromodulatory activity of fingolimod with potential effect on glutamate-mediated excitotoxicity in vivo, as already seen in animal models.

Clinical neurophysiology of multiple sclerosis

The availability of new treatments able to modify the natural course of multiple sclerosis (MS) has generated interest in paraclinical measures to monitor disease evolution. Among these, neurophysiologic measures, mainly evoked potentials (EPs), are used in the functional assessment of central sensorimotor and cognitive networks affected by MS. EP abnormalities may reveal subclinical lesions, objectivate the involvement of sensory and motor pathways in the presence of vague disturbances, and provide indications of the demyelinating nature of the disease process. However, their diagnostic value is much lower than that of magnetic resonance imaging, and is more sensitive to brain and cervical spinal cord lesions. The application of EPs in assessing disease severity and monitoring the evolution of nervous damage is more promising, thanks to their good correlation with disability in cross-sectional and longitudinal studies, and potential use as paraclinical endpoints in clinical trials. Recent evidence indicates that EPs performed early in the disease may help to predict a worse future progression in the long term. If confirmed, these data suggest the possible usefulness of EPs in the early identification of patients who are more likely to develop future disability, thus requiring more frequent monitoring or being potential candidates for more aggressive disease-modifying treatments.

Effects of prophylactic and therapeutic teriflunomide in transcranial magnetic stimulation-induced motor-evoked potentials in the dark agouti rat model of experimental autoimmune encephalomyelitis

Teriflunomide is a once-daily oral immunomodulatory agent recently approved in the United States for the treatment of relapsing multiple sclerosis (RMS). This study investigated neurophysiological deficits in descending spinal cord motor tracts during experimental autoimmune encephalomyelitis (EAE; a model of multiple sclerosis) and the functional effectiveness of prophylactic or therapeutic teriflunomide treatment in preventing the debilitating paralysis observed in this model. Relapsing-remitting EAE was induced in Dark Agouti rats using rat spinal cord homogenate. Animals were treated with oral teriflunomide (10 mg/kg daily) prophylactically, therapeutically, or with vehicle (control). Transcranial magnetic motor-evoked potentials were measured throughout the disease to provide quantitative assessment of the neurophysiological status of descending motor tracts. Axonal damage was quantified histologically by silver staining. Both prophylactic and therapeutic teriflunomide treatment significantly reduced maximum EAE disease scores (P < 0.0001 and P = 0.0001, respectively) compared with vehicle-treated rats. Electrophysiological recordings demonstrated that both teriflunomide treatment regimens prevented a delay in wave-form latency and a decrease in wave-form amplitude compared with that observed in vehicle-treated animals. A significant reduction in axonal loss was observed with both teriflunomide treatment regimens compared with vehicle (P < 0.0001 and P = 0.0014, respectively). The results of this study suggest that therapeutic teriflunomide can prevent the deficits observed in this animal model in descending spinal cord motor tracts. The mechanism behind reduced axonal loss and improved motor function may be primarily the reduced inflammation and consequent demyelination observed in these animals through the known effects of teriflunomide on impairing proliferation of stimulated T cells. These findings may have significant implications for patients with RMS.

Evidence of impaired brain activity balance after passive sensorimotor stimulation in multiple sclerosis

OBJECTIVES

Examination of sensorimotor activation alone in multiple sclerosis (MS) patients may not yield a comprehensive view of cerebral response to task stimulation. Additional information may be obtained by examining the negative BOLD response (deactivation). Aim of this work was to characterize activation and deactivation patterns during passive hand movements in MS patients.

METHODS

13 relapsing remitting-MS patients (RRMS), 18 secondary progressive-MS patients (SPMS) and 15 healthy controls (HC) underwent an fMRI study during passive right-hand movements. Activation and deactivation contrasts in the three groups were entered into ANOVA, age and gender corrected. Post-hoc analysis was performed with one-sample and two-sample t-tests. For each patient we obtained lesion volume (LV) from both T1- and T2-weighted images.

RESULTS

Activations showed a progressive extension to the ipsilateral brain hemisphere according to the group and the clinical form (HC<RRMS<SPMS). Significant deactivation of the ipsilateral cortical sensorimotor areas was reduced in both patient groups with respect to HC. Deactivation of posterior cortical areas belonging to the default mode network (DMN), was increased in RRMS, but not in SPMS, with respect to HC. The amount of activation in the contralateral sensorimotor cortex was significantly correlated with that of deactivation in the DMN in HC and RRMS, but not in SPMS. Both increased activation and decreased deactivation patterns correlated with LV.

CONCLUSION

In RRMS patients, increased cortical activation was associated with increased deactivation of the posterior cortex suggesting a greater resting-state activity in the DMN, probably aimed at facilitating sensorimotor circuit engagement during task performance. In SPMS the coupling between increased sensorimotor activation/increased DMN deactivation was not observed suggesting disorganization between anticorrelated functional networks as a consequence of a higher level of disconnection.

A qualitative review of the neurophysiological underpinnings of fatigue in multiple sclerosis

Fatigue is debilitating in multiple sclerosis (MS) and may have multiple causes. Recent investigations into objectively measurable correlates of fatigue have used transcranial magnetic stimulation (TMS) to examine a range of neurophysiological measures of neural excitability that may be altered in patients with MS. This qualitative review was conducted to test the hypothesis that changes in neural excitability are a contributing factor in MS-related fatigue. A search of the English language literature led to the compilation and synthesis of original research papers in which various aspects of neural excitability and neural transmission were measured using TMS in patients with MS. The resulting papers were classified into three categories of study relevant to fatigue: abnormalities in excitability and their correlation with self-reported fatigue; effects of exercise-induced fatigue on neural excitability; and effects of fatigue medications on neural excitability. Evidence of an association between fatigue and intracortical inhibition is both limited and conflicting, and no evidence suggests associations of fatigue with corticomotor excitability or neuronal conduction. Pharmacologically-induced changes in fatigue were found to correlate with changes in intracortical excitability. No conclusions could be drawn regarding neural excitability and exercise-induced fatigue, due to variability in study populations, outcome measures, and exercise protocols across different studies. Suggestions for future studies in this area are proposed with a view to identifying potentially modifiable factors contributing to fatigue in MS.

Superficial brain stimulation in multiple sclerosis

Central motor conduction time (CMCT) is the most frequently studied measure derived from transcranial magnetic stimulation (TMS) in multiple sclerosis (MS); it is abnormal in 57-93% of patients. Addition of the triple stimulation technique and combining motor with other evoked potentials (EPs) increases sensitivity. Cross-sectional correlations of TMS measures with clinical assessments of motor dysfunction or global disability are high. Longitudinally, CMCT is sensitive to both worsening and improvement of motor function, showing its potential to detect therapeutic responses. Moreover, combined multimodal EPs are valid quantitative predictors of the clinical course over periods ranging from 2 to 14 years. Measures of transcallosal connectivity (ipsilateral silent period and interhemispheric inhibition) are altered even in early MS, and yield complementary information on subclinical changes. Pathological brain plasticity in MS has been demonstrated by paired associative stimulation studies revealing a compensatory role of the ipsilateral motor and premotor areas. Central motor fatigue is associated with reduced motor EP amplitudes and increased cortical silent periods in normal controls, whereas patients with MS suffering from subjective fatigue show various abnormalities in cortical modulation of the motor system.

Corticospinal output during muscular fatigue differs in multiple sclerosis patients compared to healthy controls

Background:

In multiple sclerosis (MS), fatigue is a common and often disabling symptom. It has multiple causes with central motor fatigue playing an important role.

Objective:

The objective of this study was to analyse the central motor conduction changes in relation to muscle contraction force during muscle fatigue and recovery in MS patients compared to healthy controls.

Methods:

A total of 23 MS patients with fatigue and 13 healthy subjects were assessed during 2 minutes of fatiguing exercise of the abductor digiti minimi muscle of the hand and the subsequent 7 minutes of recovery. Central motor conduction was quantified by transcranial magnetic stimulation using the triple stimulation protocol and calculating a central conduction index (CCI).

Results:

Force declined to 36% of the pre-exercise level (SD 16%; p < 0.01) in MS patients and to 44% (SD 9%, p < 0.01) in healthy subjects (group differences, not statistically significant). The decline of the CCI was significantly less marked in patients (–20%, SD 26%, p < 0.05) than in healthy subjects (–57%, SD 15%, p < 0.05; group differences, p < 0.05). The decline of force and CCI were not correlated in either group.

Conclusions:

During a fatiguing exercise, the decline in central motor conduction is significantly less pronounced in MS patients than healthy subjects, although the reduction of force is similar.

Relating brain damage to brain plasticity in patients with multiple sclerosis

BACKGROUND

Failure of adaptive plasticity with increasing pathology is suggested to contribute to progression of disability in multiple sclerosis (MS). However, functional impairments can be reduced with practice, suggesting that brain plasticity is preserved even in patients with substantial damage.

OBJECTIVE

. Here, functional magnetic resonance imaging (fMRI) was used to probe systems-level mechanisms of brain plasticity associated with improvements in visuomotor performance in MS patients and related to measures of microstructural damage.

METHODS

23 MS patients and 12 healthy controls underwent brain fMRI during the first practice session of a visuomotor task (short-term practice) and after 2 weeks of daily practice with the same task (longer-term practice). Participants also underwent a structural brain MRI scan.

RESULTS

Patients performed more poorly than controls at baseline. Nonetheless, with practice, patients showed performance improvements similar to controls and independent of the extent of MRI measures of brain pathology. Different relationships between performance improvements and activations were found between groups: greater short-term improvements were associated with lower activation in the sensorimotor, posterior cingulate, and parahippocampal cortices for patients, whereas greater long-term improvements correlated with smaller activation reductions in the visual cortex of controls.

CONCLUSIONS

Brain plasticity for visuomotor practice is preserved in MS patients despite a high burden of cerebral pathology. Cognitive systems different from those acting in controls contribute to this plasticity in patients. These findings challenge the notion that increasing pathology is accompanied by an outright failure of adaptive plasticity, supporting a neuroscientific rationale for recovery-oriented strategies even in chronically disabled patients.

Cortical dysfunction underlies disability in multiple sclerosis

BACKGROUND

Gray matter atrophy has been implicated in the development of secondary progressive multiple sclerosis (SPMS). Cortical function may be assessed by transcranial magnetic stimulation (TMS). Determining whether cortical dysfunction was a feature of SPMS could be of pathophysiological significance.

OBJECTIVES

Consequently, novel paired-pulse threshold tracking TMS techniques were used to assess whether cortical dysfunction was a feature of SPMS.

METHODS

Cortical excitability studies were undertaken in 15 SPMS, 25 relapsing-remitting MS patients (RRMS) and 66 controls.

RESULTS

Short interval intracortical inhibition (SPMS 3.0 ± 2.1%; RRMS 12.8 ± 1.7%, p < 0.01; controls 10.5 ± 0.7%, p < 0.01) and motor evoked potential (MEP) amplitude (SPMS 11.5 ± 2.2%; RRMS 26.3 ± 3.6%, p <0.05; controls 24.7 ± 1.8%, p < 0.01) were reduced in SPMS, while intracortical facilitation (SPMS -5.2 ± 1.9%; RRMS -2.0 ± 1.4, p < 0.05; controls -0.9 ± 0.7, p < 0.01) and resting motor threshold were increased (SPMS 67.5 ± 4.5%; RRMS 56.0 ± 1.5%, p < 0.01; controls 59.0 ± 1.1%, p < 0.001). Further, central motor conduction time was prolonged in SPMS (9.1 ± 1.2 ms, p < 0.001) and RRMS (7.0 ± 0.9 ms, p < 0.05) patients compared with controls (5.5 ± 0.2 ms). The observed changes in cortical function correlated with the Expanded Disability Status Scale.

CONCLUSION

Together, these findings suggest that cortical dysfunction is associated with disability in MS, and documentation of such cortical dysfunction may serve to quantify disease severity in MS.