Timea Hodics

Cerebral Autoregulation of Blood Velocity and Volumetric Flow During Steady-State Changes in Arterial Pressure

The validity of using transcranial Doppler measurement of cerebral blood flow velocity (CBFV) to assess cerebral autoregulation (CA) still is a concern. This study measured CBFV in the middle cerebral artery using transcranial Doppler and volumetric cerebral blood flow (CBF) in the internal carotid artery (ICA) using color-coded duplex ultrasonography to assess CA during steady-state changes in mean arterial pressure (MAP). Twenty-one healthy adults participated. MAP was changed stepwise by intravenous infusion of sodium nitroprusside and phenylephrine. Changes in CBFV, CBF, cerebrovascular resistance (CVR=MAP/CBF), or cerebrovascular resistance index (CVRi=MAP/CBFV) were measured to assess CA by linear regression analysis. The relationship between changes in ICA diameter and MAP was assessed. All values were normalized as percentage changes from baseline. Drug-induced changes in MAP were from −26% to 31%. Changes in CBFV and CVRi in response to MAP were linear, and the regression slopes were similar between middle cerebral artery and ICA. However, CBF in ICA remained unchanged despite large changes in MAP. Consistently, a steeper slope of changes in CVR relative to CVRi was observed (0.991 versus 0.804; P<0.05). The ICA diameter changed inversely in response to MAP (r=−0.418; P<0.05). These findings indicate that CA can be assessed with transcranial Doppler measurements of CBFV and CVRi in middle cerebral artery. However, it is likely to be underestimated when compared with the measurements of CBF and CVR in ICA. The inverse relationship between changes in ICA diameter and MAP suggests that large cerebral arteries are involved in CA.

Wolf Motor Function Test for Characterizing Moderate to Severe Hemiparesis in Stroke Patients

OBJECTIVE To extend the applicability of the Wolf Motor Function Test (WMFT) to describe the residual functional abilities of moderate to severely affected stroke patients. DESIGN Data were collected as part of 2 double-blind, sham-controlled, randomized interventional studies: the Transcranial Direct Current Stimulation (tDCS) in Chronic Stroke Recovery and the tDCS Enhanced Stroke Recovery and Cortical Reorganization. Stroke patients were evaluated with the upper extremity Fugl-Meyer (UFM) and the WMFT in the same setting before treatment. SETTING University inpatient rehabilitation and outpatient clinic. PARTICIPANTS Stroke patients (N=32) with moderate to severe hemiparesis enrolled in the tDCS in Chronic Stroke Recovery and the tDCS Enhanced Stroke Recovery and Cortical Reorganization studies. INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES WMFT scores were calculated using (1) median performance times and (2) a new calculation using the mean rate of performance. We compared the distribution of values from the 2 methods and examined the WMFT-UFM correlation for the traditional and the new calculation. RESULTS WMFT rate values were more evenly distributed across their range than median WMFT time scores. The association between the WMFT rate and UFM was as good as the association between the median WMFT time scores and UFM (Spearman ρ, .84 vs -.79). CONCLUSIONS The new WMFT mean rate of performance is valid and a more sensitive measure in describing the functional activities of the moderate to severely affected upper extremity of stroke subjects and avoids the pitfalls of the median WMFT time calculations.

Functional near-infrared spectroscopy maps cortical plasticity underlying altered motor performance induced by transcranial direct current stimulation

Abstract. Transcranial direct current stimulation (tDCS) of the human sensorimotor cortex during physical rehabilitation induces plasticity in the injured brain that improves motor performance. Bi-hemispheric tDCS is a noninvasive technique that modulates cortical activation by delivering weak current through a pair of anodal–cathodal (excitation–suppression) electrodes, placed on the scalp and centered over the primary motor cortex of each hemisphere. To quantify tDCS-induced plasticity during motor performance, sensorimotor cortical activity was mapped during an event-related, wrist flexion task by functional near-infrared spectroscopy (fNIRS) before, during, and after applying both possible bi-hemispheric tDCS montages in eight healthy adults. Additionally, torque applied to a lever device during isometric wrist flexion and surface electromyography measurements of major muscle group activity in both arms were acquired concurrently with fNIRS. This multiparameter approach found that hemispheric suppression contralateral to wrist flexion changed resting-state connectivity from intra-hemispheric to inter-hemispheric and increased flexion speed (p<0.05). Conversely, exciting this hemisphere increased opposing muscle output resulting in a decrease in speed but an increase in accuracy (p<0.05 for both). The findings of this work suggest that tDCS with fNIRS and concurrent multimotor measurements can provide insights into how neuroplasticity changes muscle output, which could find future use in guiding motor rehabilitation.

Individual variability of cerebral autoregulation, posterior cerebral circulation and white matter hyperintensity

Cerebral autoregulation (CA) is a key mechanism to protect brain perfusion in the face of changes in arterial blood pressure, but little is known about individual variability of CA and its relationship to the presence of brain white matter hyperintensity (WMH) in older adults, a type of white matter lesion related to cerebral small vessel disease (SVD). This study demonstrated the presence of large individual variability of CA in healthy older adults during vasoactive drug‐induced changes in arterial pressure assessed at the internal carotid and vertebral arteries. We also observed, unexpectedly, that it was the ‘over‐’ rather than the ‘less‐reactive’ CA measured at the vertebral artery that was associated with WMH severity. These findings challenge the traditional concept of CA and suggest that the presence of cerebral SVD, manifested as WMH, is associated with posterior brain hypoperfusion during acute increase in arterial pressure.

Use of functional near-infrared spectroscopy to monitor cortical plasticity induced by transcranial direct current stimulation

Electrical stimulation of the human cortex in conjunction with physical rehabilitation has been a valuable approach in facilitating the plasticity of the injured brain. One such method is transcranial direct current stimulation (tDCS) which is a non-invasive method to elicit neural stimulation by delivering current through electrodes placed on the scalp. In order to better understand the effects tDCS has on cortical plasticity, neuroimaging techniques have been used pre and post tDCS stimulation. Recently, neuroimaging methods have discovered changes in resting state cortical hemodynamics after the application of tDCS on human subjects. However, analysis of the cortical hemodynamic activity for a physical task during and post tDCS stimulation has not been studied to our knowledge. A viable and sensitive neuroimaging method to map changes in cortical hemodynamics during activation is functional near-infrared spectroscopy (fNIRS). In this study, the cortical activity during an event-related, left wrist curl task was mapped with fNIRS before, during, and after tDCS stimulation on eight healthy adults. Along with the fNIRS optodes, two electrodes were placed over the sensorimotor hand areas of both brain hemispheres to apply tDCS. Changes were found in both resting state cortical connectivity and cortical activation patterns that occurred during and after tDCS. Additionally, changes to surface electromyography (sEMG) measurements of the wrist flexor and extensor of both arms during the wrist curl movement, acquired concurrently with fNIRS, were analyzed and related to the transient cortical plastic changes induced by tDCS.

Enhancing motor performance improvement by personalizing non-invasive cortical stimulation with concurrent functional near-infrared spectroscopy and multi-modal motor measurements

Transcranial direct current stimulation (tDCS) is a non-invasive cortical stimulation technique that can facilitate task specific plasticity that can improve motor performance. Current tDCS interventions uniformly apply a chosen electrode montage to a subject population without personalizing electrode placement for optimal motor gains. We propose a novel perturbation tDCS (ptDCS) paradigm for determining a personalized electrode montage in which tDCS intervention yields maximal motor performance improvements during stimulation. PtDCS was applied to ten healthy adults and five stroke patients with upper hemiparesis as they performed an isometric wrist flexion task with their non-dominant arm. Simultaneous recordings of torque applied to a stationary handle, muscle activity by electromyography (EMG), and cortical activity by functional near-infrared spectroscopy (fNIRS) during ptDCS helped interpret how cortical activity perturbations by any given electrode montage related to changes in muscle activity and task performance quantified by a Reaction Time (RT) X Error product. PtDCS enabled quantifying the effect on task performance of 20 different electrode pair montages placed over the sensorimotor cortex. Interestingly, the electrode montage maximizing performance in all healthy adults did not match any of the ones being explored in current literature as a means of improving the motor performance of stroke patients. Furthermore, the optimal montage was found to be different in each stroke patient and the resulting motor gains were very significant during stimulation. This study supports the notion that task-specific ptDCS optimization can lend itself to personalizing the rehabilitation of patients with brain injury.

Transcranial direct current stimulation in acute stroke patients.

I have read with great interest the article from Dr. Rossi et al about the lack of benefit of anodal transcranial direct current stimulation (tDCS) in acute stroke patients. [1] More intense rehabilitation results in better functional recovery after strokes. [2] The effectiveness of rehabilitative treatment might be enhanced through simultaneous application of brain stimulation, possibly through improved learning ability.[3] However, non-standardized occupational therapy is variable in its duration, frequency and even in the focus on recovery of the more affected arm versus teaching compensatory techniques. Timing and intensity of the rehabilitative therapy in relation to tDCS is crucial considering the limited length of stimulation effect and possible metaplastic effects interfering with the effect of the treatment at different time points after the stimulation ends.[4,5] If these parameters were not standardized and balanced between sham and anodal tDCS, it may have been difficult for this otherwise well done challenging study to show benefit. Furthermore, although enrollment time after stroke was admirably uniform, other rehabilitative treatments, medical interventions, (e.g anti-epileptic or stimulant medications) lesion location and size and genetically heterogeneous individual responsiveness to stimulation may alter the effectiveness of tDCS that can confound results in small parallel groups. Lastly, due to its wider area of cortical stimulation, tDCS may have unexpected clinical benefits in other domains[6]. If the aim was to investigate the broader utility of tDCS to accelerate stroke recovery as the title claims, instead of focusing on motor recovery, then reporting additional behavioral measures would have been helpful. The NIH stroke scale may not be sensitive enough to demonstrate all clinically important changes in these domains.