Facilitation of motor evoked potentials in ischemic stroke patients: prognostic value and neurophysiologic correlations

Cue responsiveness as a measure of emerging language ability in aphasia

Background: Prior research suggests that initial aphasia severity, lesion size, and lesion location are the most salient factors in predicting recovery outcomes. While these factors provide important prognostic information, information that is individualized and readily available to clinicians is limited. Deficits in naming are common to all aphasia types and are routinely targeted in aphasia assessment and treatment, with cues provided to facilitate lexical retrieval.Objectives: In this study, we examine aphasia recovery factors that are readily available to clinicians, examining whether a person's ability to improve naming with cues, indicating "stimulability," will be predictive of future word retrieval.Methods: Ten participants with aphasia following a left-hemisphere stroke participated in initial assessment, seven of whom met criteria for longitudinal assessment. Stroke and early clinical recovery data were collected for all participants. At four timepoints over one year we evaluated longitudinal participants' naming ability and measured the proportion of successful lexical retrieval with the presentation of phonemic, feature, and sentence cues.Results: For all participants, multiple descriptive factors regarding recovery, including lesion information, information from the acute inpatient timeframe, and communication opportunities, were examined. For individuals followed longitudinally, naming stimulability did not consistently predict naming accuracy at the subsequent assessment timepoint. Individuals' attempts at naming emerged as a metric related to future naming performance warranting further evaluation.Conclusions: Multiple factors related to recovery must be considered when providing prognostic information. Naming stimulability and attempts at naming provide some information regarding future performance, but are not consistently reliable across timepoints.

Multimodal magnetic resonance imaging correlates of motor outcome after stroke using machine learning

This study applied machine learning regression to predict motor function after stroke based on multimodal magnetic resonance imaging. Fifty-four stroke patients, who underwent T1 weighted, diffusion tensor, and resting state functional magnetic resonance imaging were retrospectively included. The kernel rigid regression machine algorithm was applied to gray and white matter maps in T1 weighted, fractional anisotropy and mean diffusivity maps in diffusion tensor, and two motor-related independent component analysis maps in resting state functional magnetic resonance imaging to predict Fugl-Meyer motor assessment scores with the covariate as the onset duration after stroke. The results were validated using the leave-one-subject-out cross-validation method. This study is the first to apply machine learning in this area using multimodal magnetic resonance imaging data, which constitutes the main novelty. Multimodal magnetic resonance imaging correctly predicted the Fugl-Meyer motor assessment score in 72 % of cases with a normalized mean squared error of 5.93 (p value = 0.0020). The ipsilesional premotor, periventricular, and contralesional cerebellar areas were shown to be of relatively high importance in the prediction. Machine learning using multimodal magnetic resonance imaging data after a stroke may predict motor outcome.

Detecting poststroke cortical motor maps with biphasic single- and monophasic paired-pulse TMS

BACKGROUND

Residual corticospinal connections are the precondition for poststroke motor recovery and necessary for targeted interventions. In severely affected patients, standard transcranial magnetic stimulation (TMS) may lead to false negative findings.

OBJECTIVE

Detecting the cortical representation of paralyzed forearm muscles by applying different stimulation techniques and maps beyond the hotspot.

METHODS

In seventeen chronic stroke patients with severe motor deficits, navigated biphasic single (SP) and monophasic paired-pulse (PP) TMS was applied at 100% stimulator output to an extended cortical area in the ipsilesional hemisphere, while recording surface EMG of the extensor carpi radialis muscle.

RESULTS

In eleven patients, residual connectivity to the paralyzed forearm was detected with either mapping technique (five SP and PP, four PP only, two SP only). In five patients, connections originated from non-primary motor areas.

CONCLUSION

These results could be instrumental for identifying candidates and stimulation targets for novel neuromodulation interventions in the context of neurorehabilitation.

Detection and Predictive Value of Fractional Anisotropy Changes of the Corticospinal Tract in the Acute Phase of a Stroke

BACKGROUND AND PURPOSE

A decrease in fractional anisotropy (FA) of the ipsilesional corticospinal tract (CST) distal to stroke lesions in the subacute (eg, 30 days) and chronic phase has been correlated with poor motor outcomes, but it is unclear whether FA values obtained within the acute stroke phase (here defined as 80 hours after onset) can predict later outcome.

METHODS

Fifty-eight patients underwent an assessment of motor impairment in the acute phase and at 3 months using the upper extremity Fugl-Meyer assessment. FA values, obtained within 80 hours after stroke onset, were determined in 2 regions of interest: cerebral peduncle and a stretch of the CST caudal to each stroke lesion (nearest-5-slices).

RESULTS

The FA laterality index for the cerebral peduncle-regions of interest was a poor predictor of 3-month outcome (R(2)=0.044; P=0.137), whereas the slope over the FA laterality index of the nearest-5-slices showed a relatively weak but significant prediction (R(2)=0.11; P=0.022) with the affected side having lower FA values. Initial upper extremity Fugl-Meyer (R(2)=0.69; P<0.001) and the weighted CST lesion load (R(2)=0.71; P<0.001) were strong predictors of 3-month outcome. In multivariate analyses, controlling for initial upper extremity Fugl-Meyer, weighted CST lesion load, and days-of-therapy, neither the FA laterality index of the cerebral peduncle nor the slope over the FA laterality index of the nearest-5-slices significantly contributed to the prediction of 86% of the variance in the upper extremity Fugl-Meyer at 3 months.

CONCLUSIONS

FA reductions of the CST can be detected near the ischemic lesion in the acute stroke phase, but offer minimal predictive value to motor outcomes at 3 months.

Corticospinal tract lesion load: An imaging biomarker for stroke motor outcomes

OBJECTIVE

The aim of this work was to investigate whether an imaging measure of corticospinal tract (CST) injury in the acute phase can predict motor outcome at 3 months in comparison to clinical assessment of initial motor impairment.

METHODS

A two-site prospective cohort study followed up a group of first-ever ischemic stroke patients using the Upper-Extremity Fugl-Meyer (UE-FM) Scale to measure motor impairment in the acute phase and at 3 months. A weighted CST lesion load (wCST-LL) was calculated by overlaying the patient's lesion map on magnetic resonance imaging with a probabilistic CST constructed from healthy control subjects. Regression models were fit to assess the predictive value of wCST-LL and compared with initial motor impairment.

RESULTS

Seventy-six patients (37 from cohort 1 and 39 from cohort 2) completed the study. wCST-LL as well as assessment of motor impairment (UE-FM) in the acute phase correlated with motor impairment (UE-FM) at 3 months in both cohort 1 (R(2)  = 0.69 vs. R(2)  = 0.67; p = 0.43) and cohort 2 (R(2)  = 0.69 vs. R(2)  = 0.62; p = 0.25). In the severely impaired subgroup (defined as UE-FM ≤ 10 at baseline), wCST-LL correlated with outcomes significantly better than clinical assessment (R(2)  = 0.47 vs. R(2)  = 0.11; p = 0.03). In the nonseverely impaired subgroup, stroke patients recovered approximately 70% of their maximal recovery potential. All stroke patients in both cohorts had poor motor outcomes at 3 months (defined as UE-FM ≤ 25) when wCST-LL was ≥ 7.0 cc (positive predictive value was 100%).

INTERPRETATION

wCST-LL, an imaging biomarker determined in the acute phase, can predict poststroke motor outcomes at 3 months, especially in patients with severe impairment at baseline.

Neural Plasticity as a Basis for Motor Learning and Neurorehabilitation

Skilled action is the end-product of learning processes that can improve several aspects of motor control such as strategic movement organisation, perceptual–motor associations, or muscle commands for basic components of sequentially evolving, complex movements. Experimental studies in healthy participants using functional imaging and transcranial magnetic stimulation have identified separable processes that form cortical motor representations and that assist this formation of representations. These processes capitalise on use-dependent plasticity and changes in cortical excitability before and after practice. In terms of neural circuits, motor learning manifests measurably via structures that support transient phenomena, such as attentive error monitoring, or through continued activation of brain structures that support control processes still adapting. Specifically, movement guidance engages the dorsal premotor and parietal cortex along the intraparietal sulcus in addition to the supplementary motor area and the anterior cerebellum. Movement conception based on explicit experience of the movement task involves the inferior premotor cortex. Evidence in patients recovering from brain lesions such as stroke, suggests that similar principles hold for neurorehabilitation as well. The challenging issue is to what degree altered motor strategies afford improvement in function through relearning and neural plasticity.

Predictors of upper limb recovery after stroke: a systematic review and meta-analysis

OBJECTIVE

To systematically review and summarize the current available literature on prognostic variables relating to upper limb recovery following stroke. To identify which, if any variables predict upper limb recovery following stroke.

DATA SOURCES

We completed searches in MEDLINE, EMBASE, AMED, CINAHL and Cochrane CENTRAL databases. Searches were completed in November 2010.

REVIEW METHODS

Studies were included if predictor variables were measured at baseline and linked to an outcome of upper limb recovery at a future time point. Exclusion criteria included predictor variables relating to response to treatment and outcome measurements of very specific upper limb impairments such as spasticity or pain. Two independent reviewers completed data extraction and assessed study quality.

RESULTS

Fifty-eight studies met the inclusion criteria. Predictor variables which have been considered within these studies include; age, sex, lesion site, initial motor impairment, motor-evoked potentials and somatosensory-evoked potentials. Initial measures of upper limb impairment and function were found to be the most significant predictors of upper limb recovery; odds ratio 14.84 (95% confidence intervals (CI) 9.08-24.25) and 38.62 (95% CI 8.40-177.53), respectively.

CONCLUSIONS

Interpretation of these results is complicated by methodological factors including variations in study populations, upper limb motor outcome scales, timing of baseline and outcome assessments and predictors selected. The most important predictive factors for upper limb recovery following stroke appears to the initial severity of motor impairment or function.

Corticospinal tract integrity correlates with knee extensor weakness in chronic stroke survivors

OBJECTIVE

Muscle weakness develops rapidly after stroke, adversely affecting motor performance, and contributing to reduced functional ability. While the contributions of structural and functional alterations in skeletal muscle to post-stroke weakness have been well described, the relationship between motor pathway integrity, measured using both radiological and electrophysiological techniques, and post-stroke muscle weakness is not clear. This study sought to determine the role of corticospinal tract (CST) integrity on knee extensor weakness in chronic stroke survivors.

METHODS

Knee extensor strength and activation testing were performed at 90° of knee flexion using an interpolated triplet technique. CST integrity was evaluated using data obtained from Diffusion Tensor Imaging and transcranial magnetic stimulation.

RESULTS

Recordings in nine stroke subjects indicated substantial knee extensor weakness and activation deficits in the paretic legs of the stroke survivors. Regression analysis revealed that asymmetry in CST integrity was strongly related to between-leg differences in knee strength.

CONCLUSIONS

The results of this study suggest a strong link between CST integrity and lower extremity strength, and add to the growing evidence of substantial knee extensor weakness and activation impairments in stroke survivors.

SIGNIFICANCE

The findings from this study further our understanding of the anatomical and neurophysiological contributions to motor impairments after stroke, which may benefit clinicians and researchers in the field of stroke rehabilitation.

TMS modeling toolbox for realistic simulation

Transcranial magnetic stimulation (TMS) is a technique for brain stimulation using rapidly changing magnetic fields generated by coils. It has been established as an effective stimulation technique to treat patients suffering from damaged brain functions. Although TMS is known to be painless and noninvasive, it can also be harmful to the brain by incorrect focusing and excessive stimulation which might result in seizure. Therefore there is ongoing research effort to elucidate and better understand the effect and mechanism of TMS. Lately Boundary element method (BEM) and Finite element method (FEM) have been used to simulate the electromagnetic phenomenon of TMS. However, there is a lack of general tools to generate the models of TMS due to some difficulties in realistic modeling of the human head and TMS coils. In this study, we have developed a toolbox through which one can generate high-resolution FE TMS models. The toolbox allows creating FE models of the head with isotropic and anisotropic electrical conductivities in five different tissues of the head and the coils in 3D. The generated TMS model is importable to FE software packages such as ANSYS for further and efficient electromagnetic analysis. We present a set of demonstrative results of realistic simulation of TMS with our toolbox.

Remapping in the ipsilesional motor cortex after VR-based training: a pilot fMRI study

In a single case longitudinal study, a 70 year old female subject who has had a subcortical stroke 8 years prior, was tested three times in fMRI using an interactive MRI-compatible VR environment. The subject performed sequential finger movements with her right (unaffected) hand. Her hand motion (recorded with the data glove) animated either the ipsilateral (corresponding) or contralateral (mirrored) virtual hand model. In a visual feedback control condition, the virtual hand models were replaced with ellipsoids. In between the second and third session, the patient participated in an intensive, two-week long VR-based training of her affected upper extremity. When comparing activation in the mirrored versus the non-mirrored virtual visual feedback condition, no significant activation was noted in motor or premotor areas in the baseline 1 or baseline 2 sessions. However, increased activation in the ipsilesional motor cortex occurred as a result of training, despite the absence of active involvement of the ipsilesional motor cortex in this condition. The left motor cortex was also recruited in this condition (though weaker) despite the subtracted out ellipsoid condition (in which subjects also moved their hand). Thus, the contralateral (mirrored) visual feedback may have had a facilitory effect bilaterally. These findings might have some important implications for the development of novel therapies in the acute phase, when paresis and the potential for neural remapping are greatest.

Wallerian Degeneration of the Corticofugal Tracts in Chronic Stroke: A Pilot Study Relating Diffusion Tensor Imaging, Transcranial Magnetic Stimulation, and Hand Function

Objective. This study tested the hypothesis that degeneration of the corticofugal tracts (CFTs) is related to poor functional outcome in the upper limb after stroke. Methods. The authors used diffusion tensor imaging to determine the degree of white matter integrity of the CFT (FAAH/UH) in chronic stroke patients and controls. The functional integrity of the corticospinal pathway was examined using transcranial magnetic stimulation. Recruitment curves and silent period duration were measured. The relationship between FA AH/UH and functional outcome of the upper limb was also assessed using a battery of upper limb function tests. Results. In patients, FA AH/UH correlated positively with the slope of recruitment curves (RC AH/UH) and grip strength. FAAH/UH also correlated negatively with the silent period duration (SPAH/UH). According to the FA AH/UH, patients were also classified into groups with minimal or extensive Wallerian degeneration (WD). Patients with more extensive WD had poorer grip strength, dexterity, and range of movement. Conclusions. The authors' results suggest that structural and functional integrity of the CFT is essential to maintain function of the upper limb after stroke.

Outcome prediction in acute monohemispheric stroke via magnetoencephalography

BACKGROUND

Following an ischemic stroke a highly variable clinical outcome is commonly evident despite similar onset symptoms as well as lesion characteristics. The aim of this study was to identify indexes providing early prediction of functional recovery, in addition to clinical severity and lesion dimension at onset of stroke.

METHODS

In 32 patients, magnetoencephalographic (MEG) parameters collected in the acute phase (<10 days from symptoms onset, T0) from affected (AH) and unaffected (UH) hemispheres at rest and evoked by sensory stimuli were evaluated in association with the clinical outcome in a stabilized phase (T1, median 7.8 months) classified with three levels: worsening, partial and full recovery.

RESULTS

Multiple multinomial logistic regression indicated AH gamma and UH delta band powers able to prognosticate clinical outcome at T1. After inclusion in this analysis, lesion volume had the strongest predictive ability, and UH delta band power remained as a predictive factor with a measurable cut-off, maximizing both sensitivity and specificity of the prediction: a patient with UH delta below cut-off would recover to some extent; a patient with UH delta above cut-off would have a probability of about 70% to worsen.

CONCLUSIONS

MEG UH delta and AH gamma band powers were found to provide useful information about long-term outcome prognosis. Only the increase of delta band activity in the unaffected hemisphere contains information about the outcome in addition to the lesion volume.

Recovery from ischemic stroke: a translational research perspective for neurology

Ischemic stroke is the most frequent neurological disease, characterized by an age-related incidence and chronic disability in the majority of patients. A great challenge in acute stroke is to predict the degree to which a patient will eventually recover. Magnetic resonance imaging has revealed that treatment-induced reperfusion limits the extent of ischemic brain damage, thereby enabling rapid and profound recovery. Nevertheless, patients may retain deficits in motor, sensory or cognitive functions due to the residual lesion. Functional neuroimaging and transcranial magnetic stimulation have shown that recovery is associated with abnormal activation in the perilesional vicinity and in brain areas remote from the lesion. This is likely related to altered functional properties or morphological changes in both cerebral hemispheres. Recent neurorehabilitative strategies, including forced use, mental imagery and peripheral nerve or cortex stimulation, aim at modulating these functional networks. Accordingly, translational research has provided new vistas on the neurobiological mechanisms of recovery and opened future avenues for science-based pharmacological and neurophysiological training strategies in stroke.

Arm function after stroke: Neurophysiological correlates and recovery mechanisms assessed by transcranial magnetic stimulation

Transcranial Magnetic Stimulation has been used for over 20 years to investigate recovery of motor function in stroke patients. In particular, it has been used to quantify the extent of damage to the corticospinal output, reorganisation of the cortical representation of the affected body parts and excitability of intracortical and cortico-cortical circuitries in both hemispheres. In this review, we provide a detailed account of most of the published data with particular reference to methodological issues that affect their interpretation.

Transcranial magnetic stimulation and stroke: A computer-based human model study

This paper explores how transcranial magnetic stimulation (TMS) induced currents in the brain are perturbed by electrical and anatomical changes following a stroke in its chronic stage. Multiple MRI derived finite element head models were constructed and evaluated to address the effects that strokes can have on the induced stimulating TMS currents by comparing stroke models of various sizes and geometries to a healthy head model under a number of stimulation conditions. The TMS induced currents were significantly altered for stimulation proximal to the lesion site in all of the models analyzed. The current density distributions were modified in magnitude, location, and orientation such that the population of neural elements that are stimulated will be correspondingly altered. The current perturbations were minimized for conditions tested where the coil was far removed from the lesion site, including models of stimulation contralateral to the lesioned hemisphere. The present limitations of TMS to the peri-lesional cortex are explored, ultimately concluding that conventional clinical standards for stimulation are unreliable and potentially dangerous predictors of the site and degree of stimulation when TMS is applied proximal to infarction site.

Effects of carotid endarterectomy on motor evoked potentials elicited by transcranial magnetic stimulation

PURPOSE

To investigate the within 3 days effects of carotid endarterectomy (CEA) on functional status of the central motor system in patients with carotid stenosis by means of transcranial magnetic stimulation (TMS).

PATIENTS AND METHOD

We studied 30 consecutive patients, 20 males and 10 females with a mean age of 69.2+/-7.1 years, who underwent CEA for symptomatic carotid stenosis. All patients had suffered an ischemic attack 6 months prior to the operation. Two TMS studies, one before and one shortly after CEA were performed on both sides in each of the patients. Resting motor threshold, motor evoked potentials (MEP) amplitude at rest, MEP latency at rest and during contraction and silent period duration (SPD) were recorded and analyzed. Two groups of data were collected. Group 1 consisted of data from the operated side in all 30 patients. Group 2 consisted of data from the contralateral side and served as a control.

RESULTS

Motor resting thresholds were similar in the two groups. Intragroup pre and post CEA comparisons showed no difference in the operated group and significant increased threshold after CEA on the non-operated side. There was no significant difference of TMS intensity for maximal MEP in either side before or after CEA. Latency at rest and during voluntary contraction and amplitude at rest showed no significant differences between or within groups' comparisons. In group 1 SPD showed a statistically significant increase after CEA as opposed to baseline. In group 2 SPD showed a non significant increase after CEA.

CONCLUSION

In the absence of other MEP changes, our finding of prolonged SPD post-operatively suggests preferential influence of the inhibitory cortical circuits. The potential favorable effect of CEA in patients with hyperexcitability such as disabling spasticity after stroke should be further studied.