Clinical Value of the Assessment of Changes in MEP Duration with Voluntary Contraction

Different descending pathways mediate early and late portions of lower limb responses to transcranial magnetic stimulation

Although recent studies in nonhuman primates have provided evidence that transcranial magnetic stimulation (TMS) activates cells within the reticular formation, it remains unclear whether descending brain stem projections contribute to the generation of TMS-induced motor evoked potentials (MEPs) in skeletal muscles. We compared MEPs in muscles with extensive direct corticomotoneuronal input (first dorsal interosseous) versus a prominent role in postural control (gastrocnemius) to determine whether the amplitudes of early and late MEPs were differentially modulated by cortical suppression. Suprathreshold TMS was applied with and without a preceding suprathreshold TMS pulse at two interstimulus intervals (50 and 80 ms). H reflexes in target muscles were also tested with and without TMS conditioning. Early and late gastrocnemius MEPs were differentially modulated by cortical inhibition, the amplitude of the early MEP being significantly reduced by cortical suppression and the late MEP facilitated. The amplitude of H reflexes in the gastrocnemius was reduced within the cortical silent period. Early MEPs in the first dorsal interosseous were also reduced during the silent period, but late MEPs were unaffected. Independent modulation of early and late MEPs in the gastrocnemius muscle supports the idea that the MEP is generated by multiple descending pathways. Suppression of the early MEP is consistent with transmission along the fast-conducting corticospinal tract, whereas facilitation of the late MEP suggests transmission along a corticofugal, potentially cortico-reticulospinal, pathway. Accordingly, differences in late MEP modulation between the first dorsal interosseous and gastrocnemius reflect an increased role of corticofugal pathways in the control of postural muscles.NEW & NOTEWORTHY Early and late portions of the response to transcranial magnetic stimulation (TMS) in a lower limb postural muscle are modulated independently by cortical suppression, late motor evoked potentials (MEPs) being facilitated during cortical inhibition. These results suggest a cortico-brain stem transmission pathway for late portions of the TMS-induced MEP.

Clinical neurophysiology of functional motor disorders: IFCN Handbook Chapter

Functional Motor Disorders are common and disabling. Clinical diagnosis has moved from one of exclusion of other causes for symptoms to one where positive clinical features on history and examination are used to make a "rule in" diagnosis wherever possible. Clinical neurophysiological assessments have developed increasing importance in assisting with this positive diagnosis, not being used simply to demonstrate normal sensory-motor pathways, but instead to demonstrate specific abnormalities that help to positively diagnose these disorders. Here we provide a practical review of these techniques, their application, interpretation and pitfalls. We also highlight particular areas where such tests are currently lacking in sensitivity and specificity, for example in people with functional dystonia and functional tic-like movements.

Cortical resting motor threshold difference in asleep-awake craniotomy for motor eloquent gliomas: WHO grading influences motor pathway excitability

Developing neurophysiological tools to predict WHO tumor grade can empower the treating teams for a better surgical decision-making process. A total of 38 patients with supratentorial diffuse gliomas underwent an asleep-awake-sedated craniotomies for tumor removal with intraoperative neuromonitoring. The resting motor threshold was calculated for different train stimulation paradigms during awake and asleep phases. Receiver operating characteristic analysis and Bayesian regression models were performed to analyze the prediction of tumor grading based on the resting motor threshold differences. Significant positive spearman correlations were observed between resting motor threshold excitability difference and WHO tumor grade for train stimulation paradigms of 5 (R = 0.54, P = 0.00063), 4 (R = 0.49, P = 0.002), 3 (R = 0.51, P = 0.001), and 2 pulses (R = 0.54, P = 0.0007). Kruskal-Wallis analysis of the median revealed a positive significant difference between the median of excitability difference and WHO tumor grade in all paradigms. Receiver operating characteristic analysis showed 3 mA difference as the best predictor of high-grade glioma across different patterns of motor pathway stimulation. Bayesian regression found that an excitability difference above 3 mA would indicate a 75.8% probability of a glioma being high grade. Our results suggest that cortical motor excitability difference between the asleep and awake phases in glioma surgery could correlate with tumor grade.

Motor potentials evoked by transcranial magnetic stimulation: interpreting a simple measure of a complex system

Transcranial magnetic stimulation (TMS) is a non-invasive technique that is increasingly used to study the human brain. One of the principal outcome measures is the motor-evoked potential (MEP) elicited in a muscle following TMS over the primary motor cortex (M1), where it is used to estimate changes in corticospinal excitability. However, multiple elements play a role in MEP generation, so even apparently simple measures such as peak-to-peak amplitude have a complex interpretation. Here, we summarize what is currently known regarding the neural pathways and circuits that contribute to the MEP and discuss the factors that should be considered when interpreting MEP amplitude measured at rest in the context of motor processing and patients with neurological conditions. In the last part of this work, we also discuss how emerging technological approaches can be combined with TMS to improve our understanding of neural substrates that can influence MEPs. Overall, this review aims to highlight the capabilities and limitations of TMS that are important to recognize when attempting to disentangle sources that contribute to the physiological state-related changes in corticomotor excitability.

Developmental models of motor-evoked potential features by transcranial magnetic stimulation across age groups from childhood to adulthood

To derive the maturation of neurophysiological processes from childhood to adulthood reflected by the change of motor-evoked potential (MEP) features. 38 participants were recruited from four groups (age mean in years [SD in months], number (males)): children (7.3 [4.2], 7(4)), preadolescents (10.3 [6.9], 10(5)), adolescents (15.3 [9.8], 11(5)), and adults (26.9 [46.2], 10(5)). The navigated transcranial magnetic stimulation was performed on both hemispheres at seven stimulation intensity (SI) levels from sub- to supra-threshold and targeted to the representative cortical area of abductor pollicis brevis muscle. MEPs were measured from three hand- and two forearm-muscles. The input-output (I/O) curves of MEP features across age groups were constructed using linear mixed-effect models. Age and SI significantly affected MEP features, whereas the stimulated side had a minor impact. MEP size and duration increased from childhood to adulthood. MEP onset- and peak-latency dropped in adolescence, particularly in hand muscles. Children had the smallest MEPs with the highest polyphasia, whereas I/O curves were similar among preadolescents, adolescents, and adults. This study illustrates some of the changing patterns of MEP features across the ages, suggesting developing patterns of neurophysiological processes activated by TMS, and to motivate studies with larger sample size.

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.

Double-target neural circuit-magnetic stimulation improves motor function in spinal cord injury by attenuating astrocyte activation

Multi-target neural circuit-magnetic stimulation has been clinically shown to improve rehabilitation of lower limb motor function after spinal cord injury. However, the precise underlying mechanism remains unclear. In this study, we performed double-target neural circuit-magnetic stimulation on the left motor cortex and bilateral L5 nerve root for 3 successive weeks in a rat model of incomplete spinal cord injury caused by compression at T10. Results showed that in the injured spinal cord, the expression of the astrocyte marker glial fibrillary acidic protein and inflammatory factors interleukin 1β, interleukin-6, and tumor necrosis factor-α had decreased, whereas that of neuronal survival marker microtubule-associated protein 2 and synaptic plasticity markers postsynaptic densification protein 95 and synaptophysin protein had increased. Additionally, neural signaling of the descending corticospinal tract was markedly improved and rat locomotor function recovered significantly. These findings suggest that double-target neural circuit-magnetic stimulation improves rat motor function by attenuating astrocyte activation, thus providing a theoretical basis for application of double-target neural circuit-magnetic stimulation in the clinical treatment of spinal cord injury.

Postural support requirements preferentially modulate late components of the gastrocnemius response to transcranial magnetic stimulation

Mounting evidence suggests that motor evoked potentials (MEPs) recorded in upper limb muscles with postural support roles following transcranial magnetic stimulation receive contributions from both corticospinal and non-corticospinal descending pathways. We tested the hypothesis that neural structures responsible for regulating upright balance are involved in transmitting late portions of TMS-induced MEPs in a lower limb muscle. MEPs were recorded in the medial gastrocnemius muscles of each leg, while participants supported their upright posture in five postural conditions that required different levels of support from the target muscles. We observed that early and late portions of the MEP were modulated independently, with early MEP amplitude being reduced when high levels of postural support were required from a target muscle. Independent modulation of early and late MEPs by altered postural demand suggests largely separable transmission of each part of the MEP. The early component of the MEP is likely generated by fast-conducting corticospinal pathways, whereas the later component may be primarily transmitted along a polysynaptic cortico-reticulospinal pathway.

Does the reticulospinal tract mediate adaptation to resistance training in humans?

Resistance training increases volitional force-producing capacity, and it is widely accepted that such an increase is partly underpinned by adaptations in the central nervous system, particularly in the early phases of training. Despite this, the neural substrate(s) responsible for mediating adaptation remains largely unknown. Most studies have focused on the corticospinal tract, the main descending pathway controlling movement in humans, with equivocal findings. It is possible that neural adaptation to resistance training is mediated by other structures; one such candidate is the reticulospinal tract. The aim of this narrative mini-review is to articulate the potential of the reticulospinal tract to underpin adaptations in muscle strength. Specifically, we 1) discuss why the structure and function of the reticulospinal tract implicate it as a potential site for adaptation; 2) review the animal and human literature that supports the idea of the reticulospinal tract as an important neural substrate underpinning adaptation to resistance training; and 3) examine the potential methodological options to assess the reticulospinal tract in humans.

Evaluating the Effects of 5-Hz Repetitive Transcranial Magnetic Stimulation With and Without Wrist-Ankle Acupuncture on Improving Spasticity and Motor Function in Children With Cerebral Palsy: A Randomized Controlled Trial

Objective: The goal of this study is to explore the effect of wrist-ankle acupuncture combined with 5-Hz repetitive transcranial magnetic stimulation (rTMS) on improving spastic state and motor function of children with spastic cerebral palsy by measuring electrophysiological parameters and behaviors.

Methods: Twenty-five children with spastic cerebral palsy were enrolled in a single-blind and randomized controlled trial. The control group received 20 sessions of 5-Hz rTMS over the affected hemisphere with 1,000 pulses. The experimental group was given wrist-ankle acupuncture on the basis of the control group. Gross motor function measure (GMFM-66), muscle tension, and electrophysiological parameters of the two groups were assessed at baseline and after intervention.

Results: After treatment, the GMFM-66 scores in the same groups were significantly improved (p < 0.001). Besides, the R-value of soleus, gastrocnemius, and hamstring muscle decreased (p < 0.05), and the results showed a trend of shortening MEP latency, increasing amplitude and duration (p < 0.05). Compared to the controlled group, the experimental group displayed more excellent changes in the GMFM-66 scores and motor evoked potential (MEP) latency. The statistical results showed that the increase of GMFM-66 score and the shortening of MEP latency in the experimental group were greater than that in the control group (p < 0.05). However, no significant differences were found in the assessment of muscle tension, amplitude, and duration of MEPs between two groups (p > 0.05).

Conclusion: Wrist-ankle acupuncture combined with 5-Hz rTMS is optimal to improve gross motor function and enhance the conductivity of corticospinal tract in children with cerebral palsy but cannot highlight its clinical superiority in improving spasticity.

Clinical Trial Registration: [http://www.chictr.org.cn/index.aspx], identifier [chictr2000039495].

Motor evoked potential latency and duration from tibialis anterior in individuals with chronic stroke

Ipsilateral motor pathways from the contralesional hemisphere to the paretic limbs may be upregulated to compensate for impaired function after stroke. Onset latency and duration of motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) provide insight into compensatory pathways but have been understudied in the lower limb. This study assessed MEP onset latency and duration in the lower limb after stroke, and compared ipsilateral and contralateral MEPs in the paretic and non-paretic limb. We hypothesized that: (1) onset latency would be longer for ipsilateral than contralateral MEPs and longer for the paretic than the non-paretic limb, and (2) duration would be shorter for ipsilateral than contralateral MEPs and longer for the paretic than the non-paretic limb. Data were collected as a part of a pre-test of a randomized controlled trial. TMS was applied to the ipsilateral and contralateral hemisphere of the paretic and non-paretic limb. MEP onset latency and duration were calculated from the tibialis anterior. Thirty-five participants with chronic stroke were included in the final analysis. Onset latency was longer in the paretic than the non-paretic limb (~ 6.0 ms) and longer after ipsilateral than contralateral stimulation (~ 1.8 ms). Duration was longer in the paretic than the non-paretic limb (~ 9.2 ms) and longer after contralateral than ipsilateral stimulation (~ 5.2 ms). Ipsilateral MEPs may be elicited through ipsilateral pathways with fewer fibers with a higher activation threshold and/or greater spinal branching. MEPs from the paretic limb may reflect slower central motor conduction, peripheral changes, or changes in motor pathway.

Mapping Responses of Lumbar Paravertebral Muscles to Single-Pulse Cortical TMS Using High-Density Surface Electromyography

Motor evoked potential (MEP), which was elicited by transcranial magnetic stimulation (TMS), has been widely used to detect corticospinal projection from TMS cortical site to trunk muscles. It can help to find the stimulation hotspot in the scalp. However, it fails to precisely describe coordinated activities of trunk muscle groups with only single-channel myoelectric signal. In this study, we aimed to use high-density surface electromyography (sEMG) to explore the effect of cortical TMS on lumbar paravertebral muscles in healthy subjects. The cortical site at 1 cm anterior and 4 cm lateral to vertex was chosen to simulate using a single-pulse TMS with different intensities and forward-bending angles. A high-density electrode array (45 channels) was placed on the surface of lumbar paravertebral muscles to record sEMG signals during a TMS experiment. MEP signals elicited by TMS were extracted from 45-channel recordings and one topographic map of the MEP amplitudes with six spatial features was constructed at each sampling point. The results showed TMS could successfully evoke an oval area with high intensity in the MEP topographic map, while this area mainly located in ipsilateral side of the TMS site. Intensity features related to the high intensity area rose significantly with TMS intensity and forward-bending angle increasing, but location features showed no change. The optimal stimulation parameters were 80% of maximum stimulator output (MSO) for TMS intensity and 30/60 degree for forward-bending angle. This study provided a potentially effective mapping tool to explore the hotspot for transcranial stimulation on trunk muscles.

Response preparation involves a release of intracortical inhibition in task-irrelevant muscles

Action preparation involves widespread modulation of motor system excitability, but the precise mechanisms are unknown. In this study, we investigated whether intracortical inhibition changes in task-irrelevant muscle representations during action preparation. We used transcranial magnetic stimulation (TMS) combined with electromyography in healthy human adults to measure motor-evoked potentials (MEPs) and cortical silent periods (CSPs) in task-irrelevant muscles during the preparatory period of simple delayed response tasks. In experiment 1, participants responded with the left index finger in one task condition and the right index finger in another task condition, whereas MEPs and CSPs were measured from the contralateral nonresponding and tonically contracted index finger. During experiment 2, participants responded with the right pinky finger whereas MEPs and CSPs were measured from the tonically contracted left index finger. In both experiments, MEPs and CSPs were compared between the task preparatory period and a resting intertrial baseline. The CSP duration during response preparation decreased from baseline in every case. A laterality difference was also observed in experiment 1, with a greater CSP reduction during the preparation of left finger responses compared to right finger responses. Despite reductions in CSP duration, consistent with a release of intracortical inhibition, MEP amplitudes were smaller during action preparation when accounting for background levels of muscle activity, consistent with earlier studies that reported decreased corticospinal excitability. These findings indicate that intracortical inhibition associated with task-irrelevant muscles is transiently released during action preparation and implicate a novel mechanism for the controlled and coordinated release of motor cortex inhibition.NEW & NOTEWORTHY In this study, we observed the first evidence of a release of intracortical inhibition in task-irrelevant muscle representations during response preparation. We applied transcranial magnetic stimulation to elicit cortical silent periods in task-irrelevant muscles during response preparation, and observed a consistent decrease in the silent period duration relative to a resting baseline. These findings address the question of whether cortical mechanisms underlie widespread modulation in motor excitability during response preparation.

Neck rotation modulates motor-evoked potential duration of proximal muscle cortical representations in healthy adults

Transcranial magnetic stimulation (TMS) produces motor-evoked potentials (MEP) used to infer changes in corticomotor excitability. In humans, neck rotation can probe reticulospinal input on corticomotor output. This study investigated the effect of neck rotation on MEP duration in a proximal and distal upper limb muscle and compared responses between rest and preactivation. Single-pulse TMS to motor cortex was used to evoke MEPs at two stimulus intensities in 18 healthy adults (20-40 years). Surface electromyography recorded MEPs from the non-dominant biceps brachii (BB) and first dorsal interosseous (FDI). Participants were seated with the target muscle at rest or 10% preactivated, and head rotated ipsilateral, contralateral, or in neutral position. The primary outcome was MEP tail, defined as the mean difference in MEP duration between active and rest trials. Secondary outcomes were MEP duration and amplitude. MEP tail was modulated by neck rotation in the proximal BB (P = 0.03) but not distal FDI (P > 0.19), with shorter duration during ipsilateral or contralateral rotation relative to neutral. In a neutral neck position, MEP duration was prolonged by muscle preactivation and higher TMS intensities in the FDI and BB (P < 0.03). Neck rotation attenuated the prolongation of MEP duration during preactivation in the BB, but not the FDI. Neck rotation had no effect on MEP amplitude for either muscle (P > 0.05). Modulation of the late portion of the MEP by rotation of the neck could indicate subcortical projections to alpha-motoneuron pools are stronger in proximal than distal upper limb muscles. These findings may have relevance for using MEP duration as a neural biomarker in neurological diseases.

Transcranial Magnetic Stimulation as a Potential Biomarker in Multiple Sclerosis: A Systematic Review with Recommendations for Future Research

Multiple sclerosis (MS) is a demyelinating disorder of the central nervous system. Disease progression is variable and unpredictable, warranting the development of biomarkers of disease status. Transcranial magnetic stimulation (TMS) is a noninvasive method used to study the human motor system, which has shown potential in MS research. However, few reviews have summarized the use of TMS combined with clinical measures of MS and no work has comprehensively assessed study quality. This review explored the viability of TMS as a biomarker in studies of MS examining disease severity, cognitive impairment, motor impairment, or fatigue. Methodological quality and risk of bias were evaluated in studies meeting selection criteria. After screening 1603 records, 30 were included for review. All studies showed high risk of bias, attributed largely to issues surrounding sample size justification, experimenter blinding, and failure to account for key potential confounding variables. Central motor conduction time and motor-evoked potentials were the most commonly used TMS techniques and showed relationships with disease severity, motor impairment, and fatigue. Short-latency afferent inhibition was the only outcome related to cognitive impairment. Although there is insufficient evidence for TMS in clinical assessments of MS, this review serves as a template to inform future research.

Multimodal Evaluation of TMS - Induced Somatosensory Plasticity and Behavioral Recovery in Rats With Contusion Spinal Cord Injury

Introduction: Spinal cord injury (SCI) causes partial or complete damage to sensory and motor pathways and induces immediate changes in cortical function. Current rehabilitative strategies do not address this early alteration, therefore impacting the degree of neuroplasticity and subsequent recovery. The following study aims to test if a non-invasive brain stimulation technique such as repetitive transcranial magnetic stimulation (rTMS) is effective in promoting plasticity and rehabilitation, and can be used as an early intervention strategy in a rat model of SCI. Methods: A contusion SCI was induced at segment T9 in adult rats. An rTMS coil was positioned over the brain to deliver high frequency stimulation. Behavior, motor and sensory functions were tested in three groups: SCI rats that received high-frequency (20 Hz) rTMS within 10 min post-injury (acute-TMS; n = 7); SCI rats that received TMS starting 2 weeks post-injury (chronic-TMS; n = 5), and SCI rats that received sham TMS (no-TMS, n = 5). Locomotion was evaluated by the Basso, Beattie, and Bresnahan (BBB) and gridwalk tests. Motor evoked potentials (MEP) were recorded from the forepaw across all groups to measure integrity of motor pathways. Functional MRI (fMRI) responses to contralateral tactile hindlimb stimulation were measured in an 11.7T horizontal bore small-animal scanner. Results: The acute-TMS group demonstrated the fastest improvements in locomotor performance in both the BBB and gridwalk tests compared to chronic and no-TMS groups. MEP responses from forepaw showed significantly greater difference in the inter-peak latency between acute-TMS and no-TMS groups, suggesting increases in motor function. Finally, the acute-TMS group showed increased fMRI-evoked responses to hindlimb stimulation over the right and left hindlimb (LHL) primary somatosensory representations (S1), respectively; the chronic-TMS group showed moderate sensory responses in comparison, and the no-TMS group exhibited the lowest sensory responses to both hindlimbs. Conclusion: The results suggest that rTMS therapy beginning in the acute phase after SCI promotes neuroplasticity and is an effective rehabilitative approach in a rat model of SCI.

Voluntary Movement Takes Shape: The Link Between Movement Focusing and Sensory Input Gating

The aim of the study was to investigate the relationship between motor surround inhibition (mSI) and the modulation of somatosensory temporal discrimination threshold (STDT) induced by voluntary movement. Seventeen healthy volunteers participated in the study. To assess mSI, we delivered transcranial magnetic stimulation (TMS) single pulses to record motor evoked potentials (MEPs) from the right abductor digiti minimi (ADM; “surround muscle”) during brief right little finger flexion. mSI was expressed as the ratio of ADM MEP amplitude during movement to MEP amplitude at rest. We preliminarily measured STDT values by assessing the shortest interval at which subjects were able to recognize a pair of electric stimuli, delivered over the volar surface of the right little finger, as separate in time. We then evaluated the STDT by using the same motor task used for mSI. mSI and STDT modulation were evaluated at the same time points during movement. mSI and STDT modulation displayed similar time-dependent changes during index finger movement. In both cases, the modulation was maximally present at the onset of the movement and gradually vanished over about 200 ms. Our study provides the first neurophysiological evidence about the relationship between mSI and tactile-motor integration during movement execution.

Effects of posture and coactivation on corticomotor excitability of ankle muscles

BACKGROUND

The use of transcranial magnetic stimulation (TMS) to evaluate corticomotor excitability of lower limb (LL) muscles can provide insights about neuroplasticity mechanisms underlying LL rehabilitation. However, to date, a majority of TMS studies have focused on upper limb muscles. Posture-related activation is an important under-investigated factor influencing corticomotor excitability of LL muscles.

OBJECTIVE

The purpose of this study was to evaluate effects of posture and background activation on corticomotor excitability of ankle muscles.

METHODS

Fourteen young neurologically-unimpaired participants (26.1±4.1 years) completed the study. TMS-evoked motor evoked potentials (MEPs) were recorded from the tibialis anterior (TA) and soleus during 4 conditions - standing, standing coactivation, sitting, and sitting coactivation. TA and soleus MEP amplitudes were compared during: (1) standing versus sitting;(2) standing coactivation (standing while activating both TA and soleus) versus sitting coactivation; and (3) standing coactivation versus standing. For each comparison, background EMG for TA and soleus were matched. Trial-to-trial coefficient of variation of MEP amplitude and coil-positioning errors were additional dependent variables.

RESULTS

No differences were observed in TA or soleus MEP amplitudes during standing versus sitting. Compared to sitting coactivation, larger MEPs were observed during standing coactivation for soleus but not TA. Compared to standing, the standing coactivation task demonstrated larger MEPs and reduced trial-to-trial MEP variability.

CONCLUSION

Our findings suggest that incorporation of measurements in standing in future TMS studies may provide novel insights into neural circuits controlling LL muscles. Standing and standing coactivation tasks may be beneficial for obtaining functionally-relevant neuroplasticity assessments of LL musculature.

Physiological processes influencing motor-evoked potential duration with voluntary contraction

Voluntary contraction leads to facilitation of motor-evoked potentials (MEPs) producing greater amplitude, shorter onset latency, and prolonged duration of the electromyography potential. Whereas hyperexcitability of spinal motoneurons and changes in descending corticospinal volleys have been proposed as putative mechanisms for changes in MEP amplitude and onset latency, a contribution of propriospinal interneurons, exerting modulatory effects on α-motoneurons, has been proposed as a potential explanation for prolongation of MEP duration. The aim of the present study is to gain further insight into the physiological processes underlying changes in MEP duration. Transcranial magnetic stimulation (TMS) studies were undertaken on 30 healthy controls, using a 90-mm circular coil, with MEPs recorded at rest and during facilitation, produced by contraction of abductor pollicis brevis. In the same experiment, short interval-intracortical inhibition (SICI) was recorded at rest. Facilitation resulted in a significant prolongation of MEP duration, which increased with stimulus intensity and was accompanied by an increase in MEP amplitude. The main effect (TMS intensity × activation state) was correlated with MEP duration (F = 10.9, P < 0.001), whereas TMS intensity (F = 30.5, P < 0.001) and activation state (F = 125.8, P < 0.001) in isolation were correlated with MEP amplitude. There was a significant inverse relationship between SICI and MEP duration at rest (R² = 0.141, P = 0.041) and during facilitation (R² = 0.340, P = 0.001). The present findings suggest that similar physiological processes mediate changes in the facilitated MEP duration and amplitude and that both cortical and nonpropriospinal spinal mechanisms contribute to changes in MEP duration.NEW & NOTEWORTHY Muscle contraction is associated with a significant increase in motor-evoked potential (MEP) duration and amplitude. Whereas the increase in MEP duration was linear, the amplitude increase exhibited a ceiling effect. Importantly, the MEP duration increase strongly correlated with short interval-intracortical inhibition, a biomarker of motor cortical function. This suggests that whereas similar physiological processes contribute to changes in facilitated MEP duration and amplitude, cortical mechanisms appear to contribute to MEP duration changes.

The utility of electrodiagnostic tests for the assessment of medically unexplained weakness and sensory deficit

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Medically unexplained symptoms (MUS) are challenge for electrodiagnostic testing.

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Weakness and sensory deficit can be manifestations of psychogenic disorders.

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This is a review of electrodiagnostic methods used for the assessment of MUS.

Patients with suspected medically unexplained symptoms or psychogenic disorders are frequently requested to undergo an EMG exam. However, the suspected diagnosis is not always told to the electromyography practitioner, who must be able to recognize such a condition to avoid false positive diagnosis without dismissing the possibility to uncover any true dysfunction. There are many clinical manoeuvers to assess the consistency of the patients’ reported weakness or sensory deficit. The electrodiagnostic practitioner should be aware of those clinical tricks and interpret the electrodiagnostic findings in the clinical context. There are many electrodiagnostic tests that the practitioner can use for the assessment of motor and sensory functions but these tests have also important drawbacks and limitations. Only after a good clinical evaluation would the practitioner be able to give his/her opinion on the clinical relevance of the electrodiagnostic findings. Here we review some of the tests that can help the practitioner to define the electrophysiological characteristics of a suspected functional disorder presenting with weakness or sensory deficit.