Reliability of intracortical and corticomotor excitability estimates obtained from the upper extremities in chronic stroke

The test-retest reliability of non-navigated transcranial magnetic stimulation (TMS) measures of corticospinal pathway excitability early after stroke

PURPOSE

Motor evoked potential (MEP) characteristics are potential biomarkers of whether rehabilitation interventions drive motor recovery after stroke. The test-retest reliability of Transcranial Magnetic Stimulation (TMS) measurements in sub-acute stroke remains unclear. This study aims to determine the test-retest reliability of upper limb MEP measures elicited by non-neuronavigated transcranial magnetic stimulation in sub-acute-stroke.

METHODS

In two identical data collection sessions, 1-3 days apart, TMS measures assessed: motor threshold (MT), amplitude, latency (MEP-L), silent period (SP), recruitment curve slope in the biceps brachii (BB), extensor carpi radialis (ECR), and abductor pollicis brevis (APB) muscles of paretic and non-paretic upper limbs. Test-retest reliability was calculated using the intra-class correlation coefficient (ICC) and 95% confidence intervals (CI). Acceptable reliability was set at a lower 95% CI of 0.70 or above. The limits of agreement (LOA) and smallest detectable change (SDC) were calculated.

RESULTS

30 participants with sub-acute stroke were included (av 36 days post stroke) reliability was variable between poor to good for the different MEP characteristics. The SDC values differed across muscles and MEP characteristics in both paretic and less paretic limbs.

CONCLUSIONS

The present findings indicate there is limited evidence for acceptable test-retest reliability of non-navigated TMS outcomes when using the appropriate 95% CI for ICC, SDC and LOA values.

CLINICAL TRIAL REGISTRATION

Current Controlled Trials: ISCRT 19090862, http://www.controlled-trials.com.

Reliability of a method to assess corticomotor excitability of lower limb muscles using a normalized EMG motor thresholding procedure

Given the importance of determining intervention-induced neuroplastic changes with lower extremity functional tasks, a reliable transcranial magnetic stimulation (TMS) methodology for proximal lower extremity muscles is needed. A pre-set fixed voltage value is typically used as the criterion for identifying a motor evoked potential (MEP) during the motor thresholding procedure. However, the fixed voltage value becomes problematic when the procedure is applied to proximal lower extremity muscles where active contractions are required. We sought to establish the reliability of a method measuring corticomotor excitability of gluteus maximus and vastus lateralis using normalized electromyography (EMG) as the criterion for identifying MEPs during the motor thresholding procedure. The active motor threshold for each muscle was determined using the lowest stimulator intensity required to elicit 5 MEPs that exceeded 20% maximal voluntary isometric contraction from 10 stimulations. TMS data were obtained from 10 participants on 2 separate days and compared using random-effect intra-class correlation coefficients (ICCs). Slopes from two input-output curve fitting methods as well as the maximum MEP of gluteus maximus and vastus lateralis were found to exhibit good to excellent reliability (ICCs ranging from 0.75 to 0.99). The described TMS method using EMG-normalized criteria for motor thresholding produced reliable results utilizing a relatively low number of TMS pulses.

Test-Retest Reliability and Agreement of Single Pulse Transcranial Magnetic Stimulation (TMS) for Measuring Activity in Motor Cortex in Patients With Acute Ischemic Stroke

Background

Transcranial magnetic stimulation (TMS) is often used to examine neurophysiology. We aimed to investigate the inter-rater reliability and agreement of single pulse TMS in hospitalised acute ischemic stroke patients.

Methods

Thirty-one patients with first-time acute ischemic stroke (median age 72 (IQR 64-75), 35% females) underwent TMS motor threshold (MT) assessment in 4 muscles bilaterally, conducted by 1 of 2 physiotherapists. Test-retest reliability was evaluated using a two-way random effects model (2,1) absolute agreement-type Interclass Correlation Coefficient (ICC). Standard Error of Measurement (SEM) and Smallest Detectable Change (SDC) were used to evaluate agreement.

Results

Reliability, SEM, and SDC of TMS was found to be moderate in right opponens pollicis (0.78 [CI 95% 0.55-0.89], SEM: 4.51, SDC: 12.51), good in right vastus medialis and tibial anterior (0.88 [CI 95% 0.72-0.96], SEM: 2.89, SDC: 8.01 and 0.88 [CI 95% 0.76-0.94], SEM: 2.88, SDC: 7.98 respectively), and excellent in right and left biceps brachii (0.98 [CI 95% 0.96-0.99], SEM: 1.79 SDC: 4.96, and 0.94 [CI 95% 0.89-0.97], SEM: 2.17 SDC: 6.01), opponens pollicis (0.92 [CI 95% 0.83-0.96], SEM: 2.68 SDC: 8.26, vastus medialis (0.92 [CI 95% 0.84-0.96], SEM: 2.87 SDC: 7.95), and tibial anterior (0.93 [CI 95% 0.86-0.96], SEM: 2.51 SDC: 6.95).

Conclusion

The TMS demonstrated moderate to excellent inter-rater reliability confirming the ability of these measures to reliably discriminate between individuals in the current study sample. Improvements of less than 4.96 to 12.51 could be a result of measurement error and may therefore not be considered a true change.

Corticomotor excitability of gluteus maximus and hip extensor strength: The influence of sex

PURPOSE

To compare hip extensor strength and corticomotor excitability (CME) of gluteus maximus (GM) between males and females. A secondary purpose was to determine if CME of GM is predictive of hip extensor strength.

METHOD

Thirty-two healthy individuals participated (15 males and 17 females). CME of GM was assessed using the input-output curve (IOC) procedure acquired from transcranial magnetic stimulation (average slope). Hip extensor strength was measured by a dynamometer during a maximal voluntary isometric contraction. Independent t-tests were used to compare CME of GM and peak hip extensor torque between males and females. Linear regression analysis was used to determine whether peak hip extensor torque was predicted by CME of GM.

RESULT

Compared to males, females demonstrate lower peak hip extensor torque (4.42 ± 1.11 vs. 6.15 ± 1.72 Nm/kg/m2, p < 0.01) and lower CME of GM (1.36 ± 1.07 vs. 2.67 ± 1.30, p < 0.01). CME of GM was a significant predictor of peak hip extensor torque for males and females combined (r² = 0.36, p < 0.001).

CONCLUSION

Our findings support the premise that corticomotor excitability plays a role in the ability of a muscle to generate torque.

Investigating the Intervention Parameters of Endogenous Paired Associative Stimulation (ePAS)

Advances in our understanding of neural plasticity have prompted the emergence of neuromodulatory interventions, which modulate corticomotor excitability (CME) and hold potential for accelerating stroke recovery. Endogenous paired associative stimulation (ePAS) involves the repeated pairing of a single pulse of peripheral electrical stimulation (PES) with endogenous movement-related cortical potentials (MRCPs), which are derived from electroencephalography. However, little is known about the optimal parameters for its delivery. A factorial design with repeated measures delivered four different versions of ePAS, in which PES intensities and movement type were manipulated. Linear mixed models were employed to assess interaction effects between PES intensity (suprathreshold (Hi) and motor threshold (Lo)) and movement type (Voluntary and Imagined) on CME. ePAS interventions significantly increased CME compared to control interventions, except in the case of Lo-Voluntary ePAS. There was an overall main effect for the Hi-Voluntary ePAS intervention immediately post-intervention (p = 0.002), with a sub-additive interaction effect at 30 min’ post-intervention (p = 0.042). Hi-Imagined and Lo-Imagined ePAS significantly increased CME for 30 min post-intervention (p = 0.038 and p = 0.043 respectively). The effects of the two PES intensities were not significantly different. CME was significantly greater after performing imagined movements, compared to voluntary movements, with motor threshold PES (Lo) 15 min post-intervention (p = 0.012). This study supports previous research investigating Lo-Imagined ePAS and extends those findings by illustrating that ePAS interventions that deliver suprathreshold intensities during voluntary or imagined movements (Hi-Voluntary and Hi-Imagined) also increase CME. Importantly, our findings indicate that stimulation intensity and movement type interact in ePAS interventions. Factorial designs are an efficient way to explore the effects of manipulating the parameters of neuromodulatory interventions. Further research is required to ensure that these parameters are appropriately refined to maximise intervention efficacy for people with stroke and to support translation into clinical practice.

Corticomotor Excitability of Gluteus Maximus Is Associated with Hip Biomechanics During a Single-Leg Drop-Jump

The purpose of this study was to determine the association between corticomotor excitability (CME) of gluteus maximus (GM) and hip biomechanics during a single-leg drop-jump task. Thirty-two healthy individuals participated. The slope of the input-output curve (IOC) obtained from transcranial magnetic stimulation was used to assess CME of GM. The average hip extensor moment and peak hip flexion angle during the stance phase of the drop jump task was calculated. The slope of the IOC of GM was found to be a predictor of the average hip extensor moment (r² = 0.18, p = 0.016) and peak hip flexion angle (r² = 0.20, p = 0.01). Our results demonstrate that greater functional use of the hip was associated with enhanced descending neural drive of GM.

A Systematic Review of Paired Associative Stimulation (PAS) to Modulate Lower Limb Corticomotor Excitability: Implications for Stimulation Parameter Selection and Experimental Design

Non-invasive neuromodulatory interventions have the potential to influence neural plasticity and augment motor rehabilitation in people with stroke. Paired associative stimulation (PAS) involves the repeated pairing of single pulses of electrical stimulation to a peripheral nerve and single pulses of transcranial magnetic stimulation over the contralateral primary motor cortex. Efficacy of PAS in the lower limb of healthy and stroke populations has not been systematically appraised. Optimal protocols including stimulation parameter settings have yet to be determined. This systematic review (a) examines the efficacy of PAS on lower limb corticomotor excitability in healthy and stroke populations and (b) evaluates the stimulation parameters employed. Five databases were searched for randomized, non-randomized, and pre-post experimental studies evaluating lower limb PAS in healthy and stroke populations. Two independent reviewers identified eligible studies and assessed methodological quality using a modified Downs and Blacks Tool and the TMS Checklist. Intervention stimulation parameters and TMS measurement details were also extracted and compared. Twelve articles, comprising 24 experiments, met the inclusion criteria. Four articles evaluated PAS in people with stroke. Following a single session of PAS, 21 experiments reported modulation of corticomotor excitability, lasting up to 60 min; however, the research lacked methodological rigor. Intervention stimulation parameters were highly variable across experiments, and whilst these appeared to influence efficacy, variations in the intervention and outcome assessment methods hindered the ability to draw conclusions about optimal parameters. Lower limb PAS research requires further investigation before considering its translation into clinical practice. Eight key recommendations serve as guide for enhancing future research in the field.

Transcranial Magnetic Stimulation Measures in the Elderly: Reliability, Smallest Detectable Change and the Potential Influence of Lifestyle Habits

Background: Transcranial magnetic stimulation (TMS) is a non-invasive technique that can be used to evaluate cortical function and corticospinal pathway in normal and pathological aging. Yet, the metrologic properties of TMS-related measurements is still limited in the aging population.

Objectives: The aim of this cross-sectional study was to document the reliability and smallest detectable change of TMS measurements among community-dwelling seniors. A secondary objective was to test if TMS measurements differ between elders based on lifestyle, medical and socio-demographic factors.

Methods: Motor evoked potentials (MEPs) elicited by single-pulse TMS were recorded in the first dorsal interosseous (FDI) in 26 elderly individuals (mean age = 70 ± 3.8 years). Resting motor threshold (rMT), MEP amplitudes and contralateral silent period (cSP) were measured on two separate occasions (1-week interval), and the standard error of the measurement (SEM_eas), intraclass correlation coefficient (ICC), and smallest detectable change in an individual (SDC_indv) were calculated. Lifestyle, medical and socio-demographic factors were collected using questionnaires. TMS-related outcomes were compared using independent sample t-test based on the presence of chronic health diseases, chronic medication intake, obesity, history of smoking, physical activity levels, gender, and level of education.

Results: rMT and cSP measures were the most reliable outcomes, with the lowest SEM_eas and highest ICCs, whereas MEP amplitude-related measures were less reliable. SDC_indv levels were generally high, even for rMT (7.29 %MSO) and cSP (43.16–50.84 ms) measures. Although not systematically significant, results pointed toward a higher corticospinal excitability in elderly individuals who were regularly active, who had no chronic medical conditions and who did not take any medication.

Conclusion: Even though SDC_indv levels were relatively high, these results show that rMT and cSP are the most reliable outcomes to investigate age-related changes in the corticomotor system and suggest that the influence of factors such as lifestyle habits and medications on TMS measures should be investigated further.

Individual recovery profiles of manual dexterity, and relation to corticospinal lesion load and excitability after stroke -a longitudinal pilot study

OBJECTIVES

In this longitudinal pilot study, we investigated how manual dexterity recovery was related to corticospinal tract (CST) injury and excitability, in six patients undergoing conventional rehabilitation.

METHODS

Key components of manual dexterity, namely finger force control, finger tapping rate and independence of finger movements, were quantified. Structural MRI was obtained to calculate CST lesion load. CST excitability was assessed by measuring rest motor threshold (RMT) and the amplitude of motor evoked potentials (MEPs) using transcranial magnetic stimulation (TMS). Measurements were obtained at two weeks, three and six months post-stroke.

RESULTS

At six months post-stroke, complete recovery of hand gross motor impairment (i.e., maximal Fugl-Meyer score for hand) had occurred in three patients and four patients had recovered ability to accurately control finger force. However, tapping rate and independence of finger movements remained impaired in all six patients at six months. Recovery in hand gross motor impairment and finger force control occurred in patients with smaller CST lesion load and almost complete recovery of CST excitability, although RMT or MEP size remained slightly altered in the stroke-affected hemisphere compared to the unaffected hemisphere. The two patients with poorest recovery showed persistent absence of MEPs and greatest structural injury to CST.

DISCUSSION

The findings support good motor recovery being overall correlated with smaller CST lesion, and with almost complete recovery of CST excitability. However, impairment of manual dexterity persisted despite recovery in gross hand movements and grasping abilities, suggesting involvement of additional brain structures for fine manual tasks.

Corticomotor Excitability Effects of Peripheral Nerve Electrical Stimulation to the Paretic Arm in Stroke

OBJECTIVE

This study aimed to examine the corticomotor excitability changes after peripheral nerve electrical stimulation (PNS) on the stroke-impaired arm.

DESIGN

This randomized cross-over study included 32 subjects with chronic stroke. They received a 1-hr PNS or placebo PNS by random order to the ulnar and radial nerves of the paretic arm in separate sessions. The primary outcome was excitability of the corticospinal projections for the contralateral first dorsal interosseous hand muscle in terms of slope of the recruitment curve, peak motor-evoked potential amplitude, and duration of the cortical silent period, measured with transcranial magnetic stimulation on both cerebral hemispheres. Seventeen of the subjects were measured for their paretic hand dexterity (using Purdue Pegboard Test) and pinch strength as secondary outcomes.

RESULTS

Peripheral nerve electrical stimulation, but not placebo PNS, increased recruitment curve slope and peak motor-evoked potential amplitude in both the lesioned and nonlesioned hemispheres and lengthened the cortical silent period duration in the nonlesioned hemisphere. Peripheral nerve electrical stimulation significantly improved hand dexterity scores compared with placebo PNS. Pinch strength was not changed by the interventions.

CONCLUSIONS

A 1-hr PNS alone to the paretic arm could increase corticomotor excitability in both hemispheres, together with hand dexterity improvement in people presented with mild-to-moderate motor impairment in the paretic upper limb after stroke.

TO CLAIM CME CREDITS

Complete the self-assessment activity and evaluation online at http://www.physiatry.org/JournalCME CME OBJECTIVES: Upon completion of this article, the readers should be able to: (1) Understand the effects of a stroke on the corticomotor excitability of the lesioned and intact hemisphere; (2) Describe the effect of peripheral nerve electrical stimulation on the lesioned and intact hemisphere corticomotor excitability following stroke; and (3) Understand the effect of peripheral nerve electrical stimulation on dexterity of the paretic hand following stroke.

LEVEL

Advanced ACCREDITATION: The Association of Academic Physiatrists is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.The Association of Academic Physiatrists designates this Journal-based CME activity for a maximum of 0.75 AMA PRA Category 1 Credit(s)™. Physicians should only claim credit commensurate with the extent of their participation in the activity.

Interhemispheric Inhibition Measurement Reliability in Stroke: A Pilot Study

OBJECTIVE

Reliable transcranial magnetic stimulation (TMS) measures for probing corticomotor excitability are important when assessing the physiological effects of noninvasive brain stimulation. The primary objective of this study was to examine test-retest reliability of an interhemispheric inhibition (IHI) index measurement in stroke.

MATERIALS AND METHODS

Ten subjects with chronic stroke (≥6 months) completed two IHI testing sessions per week for three weeks (six testing sessions total). A single investigator measured IHI in the contra-to-ipsilesional primary motor cortex direction and in the opposite direction using bilateral paired-pulse TMS. Weekly sessions were separated by 24 hours with a 1-week washout period separating testing weeks. To determine if motor-evoked potential (MEP) quantification method affected measurement reliability, IHI indices computed from both MEP amplitude and area responses were found. Reliability was assessed with two-way, mixed intraclass correlation coefficients (ICC_(3,k) ). Standard error of measurement and minimal detectable difference statistics were also determined.

RESULTS

With the exception of the initial testing week, IHI indices measured in the contra-to-ipsilesional hemisphere direction demonstrated moderate to excellent reliability (ICC = 0.725-0.913). Ipsi-to-contralesional IHI indices depicted poor or invalid reliability estimates throughout the three-week testing duration (ICC= -1.153-0.105). The overlap of ICC 95% confidence intervals suggested that IHI indices using MEP amplitude vs. area measures did not differ with respect to reliability.

CONCLUSIONS

IHI indices demonstrated varying magnitudes of reliability irrespective of MEP quantification method. Several strategies for improving IHI index measurement reliability are discussed.

The reliability of repeated TMS measures in older adults and in patients with subacute and chronic stroke

The reliability of transcranial magnetic stimulation (TMS) measures in healthy older adults and stroke patients has been insufficiently characterized. We determined whether common TMS measures could reliably evaluate change in individuals and in groups using the smallest detectable change (SDC), or could tell subjects apart using the intraclass correlation coefficient (ICC). We used a single-rater test-retest design in older healthy, subacute stroke, and chronic stroke subjects. At twice daily sessions on two consecutive days, we recorded resting motor threshold, test stimulus intensity, recruitment curves, short-interval intracortical inhibition, and facilitation, and long-interval intracortical inhibition. Using variances estimated from a random effects model, we calculated the SDC and ICC for each TMS measure. For all TMS measures in all groups, SDCs for single subjects were large; only with modest group sizes did the SDCs become low. Thus, while these TMS measures cannot be reliably used as a biomarker to detect individual change, they can reliably detect change exceeding measurement noise in moderate-sized groups. For several of the TMS measures, ICCs were universally high, suggesting that they can reliably discriminate between subjects. TMS measures should be used based on their reliability in particular contexts. More work establishing their validity, responsiveness, and clinical relevance is still needed.

Effect of a Gluten-Free Diet on Cortical Excitability in Adults with Celiac Disease

INTRODUCTION

An imbalance between excitatory and inhibitory synaptic excitability was observed in de novo patients with celiac disease (CD) in a previous study with Transcranial Magnetic Stimulation (TMS), suggesting a subclinical involvement of GABAergic and glutamatergic neurotransmission in asymptomatic patients. The aim of this investigation was to monitor the eventual changes in the same cohort of patients, evaluated after a period of gluten-free diet.

METHODS

Patients were re-evaluated after a median period of 16 months during which an adequate gluten-free diet was maintained. Clinical, cognitive and neuropsychiatric assessment was repeated, as well as cortical excitability by means of single- and paired-pulse TMS from the first dorsal interosseous muscle of the dominant hand.

RESULTS

Compared to baseline, patients showed a significant decrease of the median resting motor threshold (from 35% to 33%, p<0.01). The other single-pulse (cortical silent period, motor evoked potentials latency and amplitude, central motor conduction time) and paired-pulse TMS measures (intracortical inhibition and intracortical facilitation) did not change significantly after the follow-up period. Antibodies were still present in 7 subjects.

DISCUSSION

In patients under a gluten-free diet, a global increase of cortical excitability was observed, suggesting a glutamate-mediated functional reorganization compensating for disease progression. We hypothesize that glutamate receptor activation, probably triggered by CD-related immune system dysregulation, might result in a long-lasting motor cortex hyperexcitability with increased excitatory post-synaptic potentials, probably related to phenomena of long-term plasticity. The impact of the gluten-free diet on subclinical neurological abnormalities needs to be further explored.

Effect of a Gluten-Free Diet on Cortical Excitability in Adults with Celiac Disease

INTRODUCTION

An imbalance between excitatory and inhibitory synaptic excitability was observed in de novo patients with celiac disease (CD) in a previous study with Transcranial Magnetic Stimulation (TMS), suggesting a subclinical involvement of GABAergic and glutamatergic neurotransmission in asymptomatic patients. The aim of this investigation was to monitor the eventual changes in the same cohort of patients, evaluated after a period of gluten-free diet.

METHODS

Patients were re-evaluated after a median period of 16 months during which an adequate gluten-free diet was maintained. Clinical, cognitive and neuropsychiatric assessment was repeated, as well as cortical excitability by means of single- and paired-pulse TMS from the first dorsal interosseous muscle of the dominant hand.

RESULTS

Compared to baseline, patients showed a significant decrease of the median resting motor threshold (from 35% to 33%, p<0.01). The other single-pulse (cortical silent period, motor evoked potentials latency and amplitude, central motor conduction time) and paired-pulse TMS measures (intracortical inhibition and intracortical facilitation) did not change significantly after the follow-up period. Antibodies were still present in 7 subjects.

DISCUSSION

In patients under a gluten-free diet, a global increase of cortical excitability was observed, suggesting a glutamate-mediated functional reorganization compensating for disease progression. We hypothesize that glutamate receptor activation, probably triggered by CD-related immune system dysregulation, might result in a long-lasting motor cortex hyperexcitability with increased excitatory post-synaptic potentials, probably related to phenomena of long-term plasticity. The impact of the gluten-free diet on subclinical neurological abnormalities needs to be further explored.

Reliability of transcranial magnetic stimulation induced corticomotor excitability measurements for a hand muscle in healthy and chronic stroke subjects

Transcranial magnetic stimulation (TMS) has been used to evaluate neuroplastic changes in the brain in clinical trials. The purpose of this study was to establish the test-retest reliability of 4 TMS measures of corticomotor excitability - (1) resting motor threshold, (2) slope of input-output curve, (3) peak motor evoked potential amplitude, and (4) cortical silent period duration for the corticospinal projections to the first dorsal interosseous of the contralateral hand. Fourteen healthy subjects (mean age 27.4 years) and 27 subjects with stroke-induced upper limb hemiparesis (mean age 61.3 years) completed 2 repeated sessions of assessment of 1 week apart. Good to excellent test-retest reliability of the TMS measurements was confirmed in the stroke subjects for both hemispheres with the ICC ≥ 0.88. Measurement reliability was good (ICC ≥ 0.75) for the 4 outcome measures in healthy subjects. Contrary to the similarity in standard error of measurements in both hemispheres for outcome measures (1) to (3) in the stroke subjects, that of the cortical silent period duration was larger in magnitude in the lesioned hemisphere. The test-retest reliability coefficients determined for the four corticomotor excitability measurements allowed the estimation of 95% minimal detectable changes of these outcome variables for the respective subject group in future clinical trials.

Definition dependent properties of the cortical silent period in upper-extremity muscles, a methodological study

BACKGROUND

To explore if stimulus-response (S-R) characteristics of the silent period (SP) after transcranial magnetic stimulation (TMS) are affected by changing the SP definition and by changing data presentation in healthy individuals. This information would be clinically relevant to predict motor recovery in patients with stroke using stimulus-response curves.

METHODS

Different landmarks to define the SP onset and offset were used to construct S-R curves from the biceps brachii (BB) and abductor digiti minimi (ADM) muscles in 15 healthy participants using rectified versus non-rectified surface electromyography (EMG). A non-linear mixed model fit to a sigmoid Boltzmann function described the S-R characteristics. Differences between S-R characteristics were compared using paired sample t-tests. The Bonferroni correction was used to adjust for multiple testing.

RESULTS

For the BB, no differences in S-R characteristics were observed between different SP onset and offset markers, while there was no influence of data presentation either. For the ADM, no differences were observed between different SP onset markers, whereas both the SP offset marker "the first return of any EMG-activity" and presenting non-rectified data showed lower active motor thresholds and less steep slopes.

CONCLUSIONS

The use of different landmarks to define the SP offset as well as data presentation affect SP S-R characteristics of the ADM in healthy individuals.

Feasibility of automated analysis and inter-examiner variability of cortical silent period induced by transcranial magnetic stimulation

Cortical silent period (cSP) is a short interruption in electromyography (EMG) during active muscle contraction induced with transcranial magnetic stimulation (TMS). The cSP is a measure of cortical inhibition and is believed to represent inhibitory interneuron effects on excited motor cortical areas. Several pathological conditions and pharmacological manipulations induce changes to cSP duration indicating alterations in intracortical inhibition. At present, it is common to manually analyse the cSP duration from measured EMG. However, to avoid inter-examiner effects on cSP interpretation and detection, as well as to allow for quick measurement online, automated routine would be preferable. In this study, we evaluate the feasibility of a straight-forward cSP detection routine based on analysing the rectified first derivative of the EMG signal following TMS. Previously measured cSPs of 54 healthy subjects were reanalysed manually by two of the authors and using the automated routine. Furthermore, we recruited one subject for whom the cSPs were induced with several stimulation intensities, and those cSPs were analysed manually by two of the authors as well as using the automated routine. We found that cSPs were detected correctly by the automated cSP detection routine, and agreement with manually analysed subject-specific mean cSPs was excellent (ICC=0.992, p<0.001). The inter-examiner variability was similar to the variability between manual and automated analysis. Hence, we believe the introduced cSP detection routine would be feasible for online cSP detection, in such a way that is presently used to detect the motor evoked potentials.

Sensorimotor plasticity after music-supported therapy in chronic stroke patients revealed by transcranial magnetic stimulation

BACKGROUND

Several recently developed therapies targeting motor disabilities in stroke sufferers have shown to be more effective than standard neurorehabilitation approaches. In this context, several basic studies demonstrated that music training produces rapid neuroplastic changes in motor-related brain areas. Music-supported therapy has been recently developed as a new motor rehabilitation intervention.

METHODS AND RESULTS

In order to explore the plasticity effects of music-supported therapy, this therapeutic intervention was applied to twenty chronic stroke patients. Before and after the music-supported therapy, transcranial magnetic stimulation was applied for the assessment of excitability changes in the motor cortex and a 3D movement analyzer was used for the assessment of motor performance parameters such as velocity, acceleration and smoothness in a set of diadochokinetic movement tasks. Our results suggest that the music-supported therapy produces changes in cortical plasticity leading the improvement of the subjects' motor performance.

CONCLUSION

Our findings represent the first evidence of the neurophysiological changes induced by this therapy in chronic stroke patients, and their link with the amelioration of motor performance. Further studies are needed to confirm our observations.

Assessing brain plasticity across the lifespan with transcranial magnetic stimulation: why, how, and what is the ultimate goal?

Sustaining brain and cognitive function across the lifespan must be one of the main biomedical goals of the twenty-first century. We need to aim to prevent neuropsychiatric diseases and, thus, to identify and remediate brain and cognitive dysfunction before clinical symptoms manifest and disability develops. The brain undergoes a complex array of changes from developmental years into old age, putatively the underpinnings of changes in cognition and behavior throughout life. A functionally “normal” brain is a changing brain, a brain whose capacity and mechanisms of change are shifting appropriately from one time-point to another in a given individual's life. Therefore, assessing the mechanisms of brain plasticity across the lifespan is critical to gain insight into an individual's brain health. Indexing brain plasticity in humans is possible with transcranial magnetic stimulation (TMS), which, in combination with neuroimaging, provides a powerful tool for exploring local cortical and brain network plasticity. Here, we review investigations to date, summarize findings, and discuss some of the challenges that need to be solved to enhance the use of TMS measures of brain plasticity across all ages. Ultimately, TMS measures of plasticity can become the foundation for a brain health index (BHI) to enable objective correlates of an individual's brain health over time, assessment across diseases and disorders, and reliable evaluation of indicators of efficacy of future preventive and therapeutic interventions.

Considerations for stimulus-response curves in stroke: an investigation comparing collection and analysis methods

Stimulus-response curves (SRCs) provide information about the neurophysiological strength of corticospinal connections. The method for measuring and/or drawing meaningful conclusions of SRCs has not been well established in the stroke population. This study sought to characterize the use of an abbreviated SRC in stroke by comparing two data collection and two analysis approaches. SRCs were obtained using transcranial magnetic stimulation (TMS) in 25 survivors of stroke. The SRC data were collected with TMS intensities referenced to either motor threshold (MT; 5% increments between 100% and 140% of MT) or stimulator output (5% increments between 30% and 100% output) with peak-to-peak motor evoked potential amplitudes determined and averaged for each intensity level. Data were modeled with a three-parameter sigmoid function and a linear regression and were analyzed with a mixed design analysis of variance. There was no difference in the accuracy of prediction (r2) between the different analyses. There were significant differences in the slope parameters depending on the analysis method, but this was not dependent on the SRC data collection method. A linear regression of the SRC accurately represents the slope of the modeled data and is strongly correlated with the calculated peak slope, yet is less computationally complex than the sigmoid function. This study demonstrates the use of a linear analysis approach for SRCs as outcome measures for stroke studies.

Reliability of TMS-related measures of tibialis anterior muscle in patients with chronic stroke and healthy subjects

A lack of normative data for transcranial magnetic stimulation (TMS)-related measures of the lower limb muscles in patients with stroke prevents us from understanding whether changes in TMS-related measures are induced by treatment or are due to their variability and/or the natural evolution of the disease. The purpose of this study was to determine the reliability of three TMS-related measures: motor threshold (MT), motor evoked potential latency (MEP Lat) and MEP amplitude (MEP Amp), linked to the corticospinal control of the tibialis anterior (TA) muscle in sixteen patients with chronic stroke and in sixteen aged-matched healthy subjects. Test-retest reliability was estimated using the intraclass correlation coefficient (ICC) with its 95% confidence interval (95% CI) and standard error of measurement (SEM). In healthy subjects the reliability of all the TMS-related measures yielded an ICC≥0.75. Similar reproducibility levels were found in patients with chronic stroke, with the exception of MEP Amp on the paretic side (ICC=0.38). These results suggest that the TMS-related measures investigated are reliable both in healthy subjects and, with the exception of MEP Amp on the paretic side, in patients with chronic stroke.

Reliability of transcranial magnetic stimulation for mapping swallowing musculature in the human motor cortex

OBJECTIVE

Although transcranial magnetic stimulation (TMS) has been widely used to study motor cortex organization and excitability, the reliability of this technique has not been thoroughly investigated. Furthermore, previous reports of TMS reliability have been restricted to upper limb musculature. We sought to determine the test-retest reliability for TMS mapping of motor representations for swallowing musculature.

METHODS

Twenty healthy volunteers were tested twice over two weeks using TMS to determine motor threshold, map area, map volume, maximal MEP site location and maximal MEP site size for the suprahyoid complex and pharyngeal musculature.

RESULTS

Good test-retest reliability was found in both swallowing muscle sites for the following test parameters: motor map area, maximal MEP site location: lateral coordinate, maximal MEP site size and motor threshold (ICC=0.76-0.98). Moderate reliability was observed for motor map volume and maximal MEP site location: anterior-posterior coordinate (ICC=0.68-0.74).

CONCLUSIONS

TMS assessments of motor representation size, location and excitability appear to be highly reproducible, although the reliability of these measures may vary according to the specific muscle under investigation.

SIGNIFICANCE

These works provide much needed psychometric data to validate the use of TMS to assess the cortical representation of swallowing musculature.