Posts Tagged Transcranial magnetic stimulation

[WEB SITE] What effect does transcranial magnetic stimulation have on the brain?

The procedure facilitates reorganization of connections between neurons which could be useful for therapies

Date: June 5, 2018
Source: Ruhr-University Bochum
Summary:
Researchers have gained new insights on the question of how transcranial magnetic stimulation (TMS) effects functional interconnectivity of neurons. For visualization, they employed fluorescent dyes which provide information on the activity of neurons by light. Using this technique, they showed in an animal model that TMS predisposes neuronal connections in the visual cortex of the brain for processes of reorganization.
 
FULL STORY

Researchers of the Ruhr-Universität Bochum have gained new insights on the question of how transcranial magnetic stimulation (TMS) effects functional interconnectivity of neurons. For visualisation, they employed fluorescent dyes which provide information on the activity of neurons by light. Using this technique, they showed in an animal model that TMS predisposes neuronal connections in the visual cortex of the brain for processes of reorganisation.

TMS is being used as a treatment for a number of brain diseases such as depression, Alzheimer’s disease and schizophrenia, but there has been little research on how exactly TMS works. The team of associate professor Dr Dirk Jancke of the Optical Imaging Lab in Bochum describes its new discoveries in the journal Proceedings of the National Academy of Science (PNAS).

Examining the effects on cortical maps in the visual cortex

The researchers have investigated how TMS affects the organisation of so-called orientation maps in the visual part of the brain. Those maps are partly genetically determined and partly shaped by the interaction with our surroundings. In the visual cortex, for example, neurons respond to contrast edges of certain orientations, which typically constitute boundaries of objects. Neurons that preferably respond to edges of a specific orientation are closely grouped while clusters of neurons with other orientation preferences are gradually located further away, altogether forming a systematic map across all orientations.

The team employed high frequency TMS and compared the behaviour of neurons to visual stimuli with a specific angular orientation before and after the procedure. The result: After the magnetic stimulation the neurons responded more variable, that is, their preference for a particular orientation was less pronounced than before the TMS. “You could say that after the TMS the neurons were somewhat undecided and hence, potentially open to new tasks,” explains Dirk Jancke. “Therefore, we reasoned that the treatment provides us with a time window for the induction of plastic processes during which neurons can change their functional preference.”

A short visual training remodels the maps

The team then looked into the impact of a passive visual training after TMS treatment. 20-minutes of exposure to images of a specific angular orientation led to enlargement of those areas of the brain representing the trained orientation. “Thus, the map in the visual cortex has incorporated the bias in information content of the preceding visual stimulation by changing its layout within a short time,” says Jancke. “Such a procedure — that is a targeted sensory or motor training after TMS to modify the brain’s connectivity pattern — might be a useful approach to therapeutic interventions as well as for specific forms of sensory-motor training,” explains Dirk Jancke.

Methodological challenges

Transcranial magnetic stimulation is a non-invasive painless procedure: A solenoid is being positioned above the head and the brain area in question can be activated or inhibited by means of magnetic waves. So far little is known about the impact of the procedure on a cellular network level, because the strong magnetic field of the TMS superimposes signals that are used by researchers in order to monitor the neuronal effects of the TMS. The magnetic pulse interferes in particular with electrical measurement techniques, such as EEG. In addition, other procedures used in human participants, e.g. functional magnetic resonance imaging, are too slow or their spatial resolution is too low.

Dirk Jancke’s team used voltage dependent fluorescent dyes, embedded in the membranes of the neurons, in order to measure the brain’s activity after the TMS with high spatiotemporal resolution. As soon as a neuron’s activity is modulated, the dye molecules change emission intensity. Light signals therefore provide information about immediate changes in activity of groups of neurons.

Story Source: Materials provided by Ruhr-University BochumNote: Content may be edited for style and length.


Journal Reference:

  1. Vladislav Kozyrev, Robert Staadt, Ulf T. Eysel, Dirk Jancke. TMS-induced neuronal plasticity enables targeted remodeling of visual cortical mapsProceedings of the National Academy of Sciences, 2018; 201802798 DOI: 10.1073/pnas.1802798115

 

via What effect does transcranial magnetic stimulation have on the brain? The procedure facilitates reorganization of connections between neurons which could be useful for therapies — ScienceDaily

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[Abstract+References] Brain Plasticity and Modern Neurorehabilitation Technologies

Abstract

In recent decades, interest in studies on basic and applied aspects of how the nervous system functions has been growing rapidly around the world. The recovery of lost functions rests on processes of neuroplasticity, which is determined by the ability of the brain to transform its structures in response to injury. The effects of both routine and state-of-the-art neurorehabilitation technologies are ensured by synaptic plasticity— long-term potentiation and long-term depression, which influence learning and the preservation of new knowledge and skills obtained during rehabilitation. The introduction of new methods of neuroimaging, neurophysiology, and mathematical statistics have powerfully stimulated the development of the neuroplasticity doctrine. It has become clear that the main role in the recovery of injured functions is played by the reorganization of cortical nets and not by tissue reparation as such. The Research Center of Neurology has accumulated significant experience in the use of innovative treatment methods based on modern neurorehabilitation principles. Some of them are used for acute stroke; among other things, their effectiveness and safety have been shown with regard to patients in intensive care units (cyclic robotic mechanotherapy) and patients with severe motor deficit and an associated somatic pathology (stimulation of plantar support zones). Opportunities to assess neuroplasticity under various rehabilitation methods using fMRI and navigated transcranial magnetic stimulation (TMS) are revealed. The center also studies the fundamentals of consciousness using original neuroimaging and neurophysiological protocols for the sake of its recovery. The center is actively introducing its data into the practice of domestic clinics specializing in recovery medicine and neurorehabilitation.

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[Review] Current evidence on transcranial magnetic stimulation and its potential usefulness in post-stroke neurorehabilitation: Opening new doors to the treatment of cerebrovascular disease – Full Text

Abstract

Introduction

Repetitive transcranial magnetic stimulation (rTMS) is a therapeutic reality in post-stroke rehabilitation. It has a neuroprotective effect on the modulation of neuroplasticity, improving the brain’s capacity to retrain neural circuits and promoting restoration and acquisition of new compensatory skills.

Development

We conducted a literature search on PubMed and also gathered the latest books, clinical practice guidelines, and recommendations published by the most prominent scientific societies concerning the therapeutic use of rTMS in the rehabilitation of stroke patients. The criteria of the International Federation of Clinical Neurophysiology (2014) were followed regarding the inclusion of all evidence and recommendations.

Conclusions

Identifying stroke patients who are eligible for rTMS is essential to accelerate their recovery. rTMS has proven to be safe and effective for treating stroke complications. Functional brain activity can be optimised by applying excitatory or inhibitory electromagnetic pulses to the hemisphere ipsilateral or contralateral to the lesion, respectively, as well as at the level of the transcallosal pathway to regulate interhemispheric communication. Different studies of rTMS in these patients have resulted in improvements in motor disorders, aphasia, dysarthria, oropharyngeal dysphagia, depression, and perceptual-cognitive deficits. However, further well-designed randomised controlled clinical trials with larger sample size are needed to recommend with a higher level of evidence, proper implementation of rTMS use in stroke subjects on a widespread basis.

Introduction

Stroke patients should receive early neurorehabilitation after convalescence. For many years, researchers have aimed to identify new therapeutic targets to hasten recovery from stroke. However, we continue to lack a universally accepted, approved pharmacological therapy for these patients.1234 ;  5 After stroke, organisational changes in brain interneuronal activity in the affected area and the surrounding healthy tissue may on occasion promote functional recovery. Neurorehabilitation may help achieve this aim. Unfortunately, there are also occasions when neural reorganisation is suboptimal; in these cases, the problem persists and becomes chronic. In this context, transcranial magnetic stimulation (TMS) emerged as a tool for studying the brain and has been used since the mid-1980s to treat certain neuropsychiatric disorders. Neurorehabilitation is based on the idea that the brain is a dynamic entity able to adapt to internal and external homeostatic changes. This adaptive capacity, called neuroplasticity, is also present in patients with acquired brain injuries. The degree of recovery and the functional prognosis of these patients depend on the extent of neuroplastic changes.12345 ;  6 When performed by experienced physicians, TMS is a safe, non-invasive technique which enables the organisation of these neural changes (Fig. 1). The technique’s applications are expanding rapidly.12345678 ;  9

Modern TMS device.

Figure 1.

Modern TMS device.

We present the results of a literature review of the most relevant articles, manuals, and clinical practice guidelines addressing TMS (background information, diagnostic and therapeutic uses, and especially its usefulness for stroke neurorehabilitation) and published between 1985 (when the technique was first used) and 2015.

 

Development

The organisation of language in the brain

The left hemisphere of the brain is the anatomo-functional seat of language in 96% of right-handed and 70% of left-handed individuals. Language processing in the left hemisphere involves certain anatomical pathways for language comprehension, repetition, and production (Fig. 2). Positron emission tomography and functional magnetic resonance imaging (fMRI) studies conducted during multiple language tasks have shown brain activation not only in the main language centres (lesions to these areas may cause Broca aphasia, Wernicke aphasia, etc.) (Fig. 3) but also in many other locations, such as the thalamus (alertness), the basal ganglia (motor modulation), and the limbic system (affect and memory). Language is the perfect model for understanding how the central nervous system works as a whole.10 ;  11

Figure 2. The functional pathways involved in comprehension, repetition, and production of written, gesture, and spoken language, according to the Wernicke-Geschwind model. Within the left hemisphere, language organisation follows certain anatomical pathways for language comprehension, repetition, and production. Sounds are processed by the bilateral auditory cortex, in the superior temporal gyrus (primary auditory area), and decoded in the posterior area of the left temporal cortex (Wernicke area); the latter is connected to other cortical areas or networks which assign meaning to words. During reading, output from the primary visual area (bilaterally) travels to other parieto-occipital association areas for word and phrase recognition (especially the left fusiform gyrus, located in the inferior surface of the temporal lobe, where there is a key word recognition centre) and reaches the angular gyrus, which processes language-related visual and auditory information. In spontaneous language repetition and production, auditory information must travel through the arcuate fasciculus towards the left inferior frontal region (Broca area), which is responsible for language production; this area is also known to be involved in such other functions as action comprehension (mirror neurons). To produce written or spoken language, output from the Wernicke area, the Broca area, and nearby association areas must reach the primary motor cortex.10 ;  11
Adapted with permission from Bear et al.10

[…]

Continue —> Current evidence on transcranial magnetic stimulation and its potential usefulness in post-stroke neurorehabilitation: Opening new doors to the treatment of cerebrovascular disease

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[Abstract] Transcranial and spinal cord magnetic stimulation in treatment of spasticity: a literature review and meta-analysis

INTRODUCTION: Spasticity is associated with various diseases of the nervous system. Current treatments such as drug therapy, botulinum toxin injections, kinesitherapy, and physiotherapy are not sufficiently effective in a large number of patients. Transcranial magnetic stimulation (TMS) can be considered as an alternative method of treatment. The purpose of this article was to conduct a systematic review and meta-analysis of all available publications assessing the efficacy of repetitive TMS in treatment of spasticity.

EVIDENCE ACQUISITION: Search for articles was conducted in databases PubMed, Willey, and Google. Keywords included “TMS”, “spasticity”, “TMS and spasticity”, “non-invasive brain stimulation”, and “non-invasive spinal cord stimulation”. The difference in scores according to the Modified Ashworth Scale (MAS) for one joint before and after treatment was taken as the effect size.
EVIDENCE SYNTHESIS: We found 26 articles that examined the TMS efficacy in treatment of spasticity. Meta-analysis included 6 trials comprising 149 patients who underwent real stimulation or simulation. No statistically significant difference in the effect of real and simulated stimulation was found in stroke patients. In patients with spinal cord injury and spasticity, the mean effect size value and the 95% confidence interval were -0.80 and (-1.12, -0.49), respectively, in a group of real stimulation; in the case of simulated stimulation, these parameters were 0.15 and (-0.30, -0.00), respectively. Statistically significant differences between groups of real stimulation and simulation were demonstrated for using high-frequency repetitive TMS or iTBS mode for the M1 area of the spastic leg (P=0.0002).
CONCLUSIONS: According to the meta-analysis, the statistically significant effect of TMS in the form of reduced spasticity was demonstrated only for the developed due to lesions at the brain stem and spinal cord level. To clarify the amount of the antispasmodic effect of repetitive TMS at other lesion levels, in particular in patients with hemispheric stroke, further research is required.

via Transcranial and spinal cord magnetic stimulation in treatment of spasticity: a literature review and meta-analysis – European Journal of Physical and Rehabilitation Medicine 2018 February;54(1):75-84 – Minerva Medica – Journals

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[ARTICLE] Effects of High-Frequency Repetitive Transcranial Magnetic Stimulation Combined with Task-Oriented Mirror Therapy Training on Hand Rehabilitation of Acute Stroke Patients – Full Text PDF

BACKGROUND: Impairments of hand function make it difficult to perform daily life activities and to return to work. The aim of this study was to investigate the effect of high-frequency repetitive transcranial magnetic stimulation (HF-rTMS) combined with task-oriented mirror therapy (TOMT) on hand rehabilitation in acute stroke patients.
MATERIAL AND METHODS: Twenty subacute stroke patients in the initial stages (<3 months) participated in the study. Subjects were allocated to 2 groups: the experimental group received HF-rTMS + TOMT and the control group received HF-rTMS. TOMT training was conducted in 10 sessions over 2 weeks for 30 min. rTMS was applied at a 20 Hz frequency over the hand motor area in the cortex of the affected hemisphere for 15 min. Outcomes, including motor-evoked potential (MEP), pinch grip, hand grip, and box and block test, were measured before and after training.
RESULTS: Significant improvements in the MEP and hand function variables were observed in both groups (p<0.05). In particular, hand functions (pinch grip and box and block test) were significantly different between the 2 groups (p<0.05).
CONCLUSIONS: HF-rTMS combined with TOMT had a positive effect on hand function and can be used for the rehabilitation of precise hand movements in acute stroke patients.

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[Abstract] Long-lasting effects of transcranial static magnetic field stimulation on motor cortex excitability

Abstract

Background

Transcranial static magnetic field stimulation (tSMS) was recently added to the family of inhibitory non-invasive brain stimulation techniques. However, the application of tSMS for 10–20 min over the motor cortex (M1) induces only short-lasting effects that revert within few minutes.

Objective

We examined whether increasing the duration of tSMS to 30 min leads to long-lasting changes in cortical excitability, which is critical for translating tSMS toward clinical applications.

Methods

The study comprised 5 experiments in 45 healthy subjects. We assessed the impact of 30-min-tSMS over M1 on corticospinal excitability, as measured by the amplitude of motor evoked potentials (MEPs) and resting motor thresholds (RMTs) to single-pulse transcranial magnetic stimulation (TMS) (experiments 1–2). We then assessed the impact of 30-min-tSMS on intracortical excitability, as measured by short-interval intracortical facilitation (SICF) and short-interval intracortical inhibition (SICI) using paired-pulse TMS protocols (experiments 2–4). We finally assessed the impact of 10-min-tSMS on SICF and SICI.

Results

30-min-tSMS decreased MEP amplitude compared to sham for at least 30 min after the end of the stimulation. This long-lasting effect was associated with increased SICF and reduced SICI. 10-min-tSMS –previously reported to induce a short-lasting decrease in MEP amplitude– produced the opposite changes in intracortical excitability, decreasing SICF while increasing SICI.

Conclusions

These results suggest a dissociation of intracortical changes in the consolidation from short-lasting to long-lasting decrease of corticospinal excitability induced by tSMS. The long-lasting effects of 30-min-tSMS open the way to the translation of this simple, portable and low-cost technique toward clinical trials.

via Long-lasting effects of transcranial static magnetic field stimulation on motor cortex excitability – Brain Stimulation: Basic, Translational, and Clinical Research in Neuromodulation

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[WEB SITE] How Doctors Are Using Brain Imaging to Treat Depression

Typically, depression is diagnosed based on what a patient describes about their emotional and mental state. People who suffer from depression often state that they’re sad more often than not and that things they used to enjoy are no longer enjoyable.

The biggest hurdle in diagnosing depression is overcoming the stigma and embarrassment of possibly having a mental health disorder. It’s hard to talk about such raw, emotional, and personal details. Another issue is the fact that depression manifests itself in different ways. Some patients stop eating, others gain weight and suffer from anxiety. There’s no one-size-fits-all when it comes to depression symptoms.

While there aren’t many biological indicators that can be used to diagnose someone with depression, brain imaging has proven to be useful in diagnosing and helping to shape a treatment plan.

What Does Brain Imaging Show?

A recent study that was published in Nature Medicine discuss biological markers that can be used to distinguish different types of depression. To get a better look at the brain, functional magnetic resonance imaging was used to measure the connection strength between the brain and neural circuits. From these images researchers were able to pinpoint four types of depression.

While further research is needed to confirm initial findings, the potential of using biological indicators paves the way for clearer diagnoses and more personalized and effective therapies that treat the brain.

Based on the research, it was observed that certain patients experienced higher levels of fatigue while others discussed a lack of pleasure. In the future there is hope that certain treatment types can be matched to a type of depression. For example, those who report a lack of pleasure may benefit from a treatment known as transcranial magnetic stimulation (TMS). Because TMS uses a magnet to create small electric currents in the brain, the under-functioning reasons can be restored through TMS therapy.

The Next Steps

Though several studies have been conducted to compare depressed brains to those who don’t have the condition, it will take some time before brain imaging becomes a fool-proof way of diagnosing depression. Doctors and researchers will need to find common ground and patterns between the various types of depression so there is one unified method of determining if a patient has depression and the type.

In the future, it’s hoped that brain imaging can not only be used to diagnose depression but also to:

  • Determine treatment options
  • Determine the success rate of treatment
  • Understand other mental health disorders
  • Diagnose other conditions that may impact depression symptoms

While there is still a way to go in using brain imaging to diagnose and treat depression, the future is bright in this health arena.

Treatment Options

There are several forms of brain treatment that can be used to treat depression. The top two options include electroconvulsive therapy (ECT) and transcranial magnetic stimulation (TMS).

Electroconvulsive Therapy (ECT)

The use of ECT dates back hundreds of years. In fact, ECT is the most commonly used brain treatment for those who suffer from depression. When undergoing ECT treatment, an electric current is formed in the brain that creates a spurt of energy. This causes the patient to have a seizure. Though seizures can be quite scary to experience and even scarier to watch, patients are given anesthesia and a muscle relaxant to avoid the convulsions that are often seen in someone who is having a seizure.

The biggest drawback to ECT is memory loss. Patients often have a hard time remembering past memories so doctors encourage people to create new memories to get that functionality in the brain back up and running.

Transcranial Magnetic Stimulation (TMS)

While electroconvulsive therapy (ECT) is often the go-to procedure for those with severe, long-term, or treatment resistant depression, TMS has proven to be an effective brain treatment for depression. As we better understand how depression impacts regions of the brain, especially the prefrontal cortex, doctors will be able to pinpoint which treatment of combination thereof will produce the best results for a patient.

TMS is beneficial in that it is safe, non-invasive, has minimal side effects, and is designed to target and restore those abnormal connections in the brain. Unlike ECT and other forms of brain treatment options, TMS typically produces minimal to no side effects. Some patients have complained of headache and scalp discomfort but nothing as serious as the memory loss that is often found in those who undergo ECT.

Conclusion

As it stands physical symptoms are the best indicators of whether or not someone has depression. But, with the continued research of using brain imaging to diagnose and determine treatment brings new hope and ideas into the mental health realm.

via How Doctors Are Using Brain Imaging to Treat Depression

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[Abstract] Decreased short-interval intracortical inhibition correlates with better pinch strength in patients with stroke and good motor recovery

Abstract

Background

Deeper short-interval intracortical inhibition (SICI), a marker of GABAA activity, correlates with better motor performance in patients with moderate to severe hand impairments in the chronic phase after stroke.

Objectives

We evaluated the correlation between SICI in the affected hemisphere and pinch force of the paretic hand in well-recovered patients. We also investigated the correlation between SICI and pinch force in controls.

Methods

Twenty-two subjects were included in the study. SICI was measured with a paired-pulse paradigm. The correlation between lateral pinch strength and SICI was assessed with Spearman’s rho.

Results

There was a significant correlation (rho = 0.69, p = 0.014) between SICI and pinch strength in patients, but not in controls. SICI was significantly deeper in patients with greater hand weakness.

Conclusions

These preliminary findings suggest that decreased GABAA activity in M1AH correlates with better hand motor performance in well-recovered subjects with stroke in the chronic phase.

via Decreased short-interval intracortical inhibition correlates with better pinch strength in patients with stroke and good motor recovery – Brain Stimulation: Basic, Translational, and Clinical Research in Neuromodulation

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[PINTEREST Board] transcranial Direct Current Stimulation (tDCS), Transcranial Magnetic Stimulation (TMS)

 

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[WEB SITE] Insights from TMS into recovery after stroke

Posted in Clinical Review Article on 11th Dec 2017

Transcranial Magnetic Stimulation (TMS) is a non-invasive technique whereby an electro-magnetic coil held over the scalp is used to induce a brief electrical current in the cortex of the underlying brain. TMS may be used in a number of ways to gain in vivo insights into brain physiology and has provided a window into the cascade of physiological changes that occur following stroke. When stimulating the primary motor cortex in a healthy subject stimuli of an intensity above the motor threshold will give rise to a detectable evoked potential in a peripheral muscle. As the stimulus intensity is gradually increased the evoked potentials increase in amplitude up to a maximum, giving rise to a measurable recruitment curve. The motor threshold and the recruitment curve both provide measures of the excitability of the corticospinal tract. This incorporates the excitability of axons within the motor cortex, synaptic inputs onto pyramidal cells and the spinal alpha motor neuron pool. In paired pulse TMS a sub-threshold pulse is delivered a few milliseconds before a second suprathreshold pulse, both through the same coil. The first pulse conditions the response to the second, resulting in inhibition or facilitation depending upon the inter-stimulus interval and reflecting the activity in intracortical regulatory circuits. If two separate TMS coils are used then one may use a similar test-conditioning approach to explore inter-regional interactions, such as interhemispheric inhibition between the two primary motor cortices (Figure 1). Alternatively pulses may be delivered during a motor task, and the resulting effect on behaviour used to infer the stimulated region’s role in task performance, for example during a simple reaction time or movement selection task. See Reis et al1 for a summary of these techniques and their physiological basis.

Figure 1. TMS measures of motor cortical physiology. a) Eliciting a Motor Evoked Potential (MEP) from the primary motor cortex, b) Measures of corticospinal tract excitability, c) Using paired pulse stimulation to measure Intracortical inhibition, d) Using two TMS coils to measure Interhemispheric Inhibition.

Strokes that cause hemiparesis usually disrupt the corticospinal tract, and when testing the stroke hemisphere this may manifest as increased motor thresholds, flattened recruitment curves, or alternatively as the inability to elicit an evoked potential at all. An absent evoked potential in the acute post-stroke period is a poor prognostic indicator for motor recovery2,3 and at that stage these markers of corticospinal excitability, when obtainable, correlate strongly with clinical measures of motor performance.4 If these corticospinal excitability measures improve then they usually do so over the early weeks post-stroke, presumably reflecting spontaneous biological recovery around the infarct, and the physiological change is reflected in clinical improvement in this group. It has been a fairly consistent finding that the correlation between corticospinal excitability and clinical status declines with time, which may reflect reduced reliance on the original corticospinal projection once the network generating the motor output has reorganised. Some studies have shown hyper-excitability of the corticospinal tract from the non-stroke hemisphere during the acute phase in more severely affected patients, which is interpreted by some as a form of diaschisis.

Paired pulse measures have fairly reliably shown reduced inhibitory activity in the intracortical circuits after stroke. Such apparent disinhibition is hard to interpret in the stroke hemisphere, as these measures depend upon an unconditioned evoked potential of reasonable amplitude which may not be available or may require high stimulus intensities. However no such technical issue affects the intact hemisphere, and the absence or reduction of inhibition when tested in the contralesional primary motor cortex suggests a reduction in the tonic level of GABA-ergic inhibitory activity in intracortical circuits that extends far beyond the site of the stroke. In one study such intracortical disinhibition of the non-stroke hemisphere was seen in patients with cortical but not subcortical infarction,5 suggesting that this phenomenon may relate to interruption of the transcallosal projection between the motor cortices. Few longitudinal studies of intra-cortical excitability have been performed but on the basis of the data available it seems that in the acute period disinhibition is seen regardless of clinical status, but that by three months it has resolved in those patients with a better clinical outcome, such that a clinical–physiological correlation emerges at around that time.6 A recent meta-analysis of TMS studies has shown no overall abnormality of excitability in the intact hemisphere:7however if disinhibition were seen only in more severely affected patients then one may see clinical correlation without a group effect.

It is recognised that in healthy humans there is tonic inhibition of each hemisphere by its opposite, a situation that is likely to be important in the generation of unimanual versus bimanual movements. When this inter-hemispheric interaction is measured at rest with two TMS coils using a paired pulse conditioning approach there is robust inhibition. When tested in healthy subjects during a reaction time paradigm the baseline inhibition disappears or reverses as the onset of movement approaches. Murase and colleagues8found that such switching-off of interhemispheric inhibition was impaired in stroke patients, and that the extent of residual interhemispheric inhibition was greater in more severely affected patients. This result has been interpreted as suggesting that after stroke there is an imbalance of such interhemispheric interactions, with pathological inhibition of the recovering stroke hemisphere by the non-stroke hemisphere.

Interpretation of the physiological changes observed after stroke remains a matter of debate. TMS as a technique operates at the level of the whole system, drawing inferences from the effect of manipulations on the overall corticospinal output, but the pathological changes observed may be the result of dysfunction at one or more of several levels. These may include the effects of cytotoxic changes on local neurochemistry, altered inhibitory vs excitatory synaptic activity, or diaschisis due to disruption of inter-regional tracts. The TMS finding of wide- spread intracortical disinhibition is in keeping with MR Spectroscopy studies that show reduced cortical GABA content during this period after stroke.9 A rapid reduction in GABA is also seen in healthy humans as a response to motor training or to experimental deafferentation of one arm by ischaemic nerve block, which likewise causes intracortical disinhibition as assessed by TMS. In those contexts it is felt that such disinhibition creates a more favourable environment for synaptic plasticity to occur, and it is tempting to conclude that the same is occurring in the post-stroke period as a way of driving reorganisation of the motor output. It is also conceivable that disinhibition allows cortical regions that are disconnected from their usual corticospinal output projection to access instead the horizontal cortico-cortical connections that are prevalent in cortical layers 2 and 3, thereby reaching an alternative output projection. Such a phenomenon would allow for the shifts in cortical motor output maps that are well documented after stroke.10 In one longitudinal study the correlation between disinhibition and clinical status was strong at three months but then became weaker in the chronic phase.6 We interpreted this as suggesting that ongoing disinhibition becomes less important for motor function as the reorganised motor network becomes better established with time. Direct evidence for such a process is lacking however, and some would argue that disinhibition is an epiphenomenon rather than an adaptive response to injury. This alternative conclusion would be supported by the observation that reduced intracortical inhibition as measured by paired pulse TMS may be observed in other pathological states, including dystonia and Attention Deficit Hyperactivity Disorder.

The concept of an imbalance between the two hemispheres and of excessive interhemispheric inhibition of the stroke hemisphere has gained a lot of traction and has provided the rationale for therapeutic approaches that aim to redress it. Non-invasive brain stimulation, either by repetitive TMS or transcranial direct current stimulation (tDCS), can induce measurable changes in cortical excitability that outlast the period of stimulation. Depending on the stimulation paradigm used one can induce either increases or reductions lasting minutes or in some cases hours. The most common approaches are either to apply excitatory stimulation to the stroke hemisphere or alternatively inhibitory stimulation to the non-stroke hemisphere (Figure 2), the aim being to enhance the response to conventional therapy by stimulating either before or during treatment. The concept of an overactive non-stroke hemisphere resonates with the widely reproduced functional imaging finding of increased movement-related brain activation on that side in more severely affected patients, which may normalise as clinical recovery progresses.6

 

Figure 2. The interhemispheric rivalry model, whereby the intact hemisphere exerts a pathological degree of interhemispheric inhibition on the stroke hemisphere and hinders motor function in the paretic limb. This has given rise to the therapeutic strategies of either increasing excitability in the stroke hemisphere or reducing it in the intact hemisphere.

It is by no means clear however that this contralesional activity is the same phenomenon as that which generates pathological inhibition of the recovering hemisphere, or that it is necessarily maladaptive. There is evidence that at least some regions on the non-stroke side may support movement of the paretic side, such as the contralesional dorsal premotor cortex which displays increased functional connectivity to the primary motor cortex of the stroke in more affected patients and appears to support hand movement.11,12Furthermore a recent study suggested that reducing intact hemisphere excitability may in fact be detrimental to upper limb function in more impaired patients.3

As the role of contralesional brain regions in movement appears to differ depending upon factors such as clinical severity, extent of corticospinal tract disruption and possibly stroke location it would seem that reducing excitability on that side equally in all stroke patients may represent rather a blunt therapeutic approach. This is especially true of tDCS, whose effects incorporate most of the stimulated hemisphere. However, positive studies may be found in the literature for both repetitive TMS and tDCS when applied to either side of the brain (sometimes both), and although a comprehensive review of the outcomes is beyond the scope of this article the most promising approach appears to be inhibition of the non-stroke hemisphere by tDCS.13 Non-invasive brain stimulation has not as yet entered routine clinical practice however, and there are a number of reasons why this may be, such as the large number of stimulation protocols available and uncertainty regarding the optimal time to apply stimulation. However, for the reasons discussed above it is important to consider the heterogeneity of the clinical syndrome when designing further trials. Opinions differ as to whether progress will be made using a ‘one size fits all’ design, the hope being that larger sample sizes will take care of heterogeneity, or alternatively whether targeting stimulation according to clinical and physiological factors would have a greater chance of success. This is likely to be worth getting right, as a large negative study would present an obstacle to further investigation in this field. It is likely to be the case that applying brain stimulation to the right patients could significantly enhance the outcome of post-stroke rehabilitation, with a clinically meaningful reduction in resulting impairment and disability.

References

  1. Reis J, Swayne OB, Vandermeeren Y, Camus M, Dimyan MA, Harris-Love M, Perez MA, Ragert P, Rothwell JC, Cohen LG. Contribution of transcranial magnetic stimulation to the understanding of cortical mechanisms involved in motor control. J Physiol 2008; 586(2):325-51.
  2. Stinear CM, Barber PA, Smale PR, Coxon JP, Fleming MK, Byblow WD. Functional Potential in chronic stroke patients depends on corticospinal tract integrity. Brain 2007;130:170-180.
  3. Bradnam LV, Stinear CM, Barber PA, Byblow WD. Contralesional hemisphere control of the proximal paretic upper limb following stroke. Cereb Cortex. 2012 Nov;22(11):2662-71.
  4. Traversa R, Cicinelli P, Pasqualetti P, Filippi M, Rossini PM. Follow-up of interhemispheric differences of motor evoked potentials from the ‘affected’ and ‘unaffected’ hemispheres in human stroke. Brain Research 1998; 803:1-8.
  5. Manganotti P, Patuzzo S, Cortese F, Palermo A, Smania N, Fiaschi A. Motor disinhibition in affected and unaffected hemisphere in the early period of recovery after stroke. Clin Neurophysiol 2002; 113:936-943.
  6. Swayne OB, Rothwell JC, Ward NS, Greenwood RJ. Stages of Motor Output Reorganization after Hemispheric Stroke Suggested by Longitudinal Studies of Cortical Physiology. Cereb Cortex 2008; 18:1909- 1922.
  7. McDonnell MN, Stinear CM. TMS measures of motor cortex function after stroke: A meta-analysis. Brain Stimul. 2017 Jul – Aug;10(4):721-734.
  8. Murase N, Duque J, Mazzocchio R, Cohen LG. Influence of interhemispheric interactions on motor function in chronic stroke. Ann Neurol 2004; 55:400-409.
  9. Blicher JU, Near J, Næss-Schmidt E, Stagg CJ, Johansen-Berg H, Nielsen JF, Østergaard L, Ho YC. GABA levels are decreased after stroke and GABA changes during rehabilitation correlate with motor improvement. Neurorehabil Neural Repair 2015 Mar-Apr;29(3):278-86.
  10. Delvaux V, Alagona G, Gerard P, De Pasqua V, Pennisi G, de Noordhout AM. Post-stroke reorganization of hand motor area: a 1-year prospective follow-up with focal transcranial magnetic stimulation. Clin Neurophysiol 2003; 114:1217-1225
  11. Bestmann S, Swayne O, Blankenburg F, Ruff CC, Teo J, Weiskopf N, Driver J, Rothwell JC, Ward NS. The role of contralesional dorsal premotor cortex after stroke as studied with concurrent TMS-fMRI. J Neurosci 2010; 30(36):11926-37.
  12. Johansen-Berg H, Rushworth MF, Bogdanovic MD, Kischka U, Wimalaratna S, Matthews PM. The role of ipsilateral premotor cortex in hand movement after stroke. Proc Natl Acad Sci U S A 2002; 99:14518- 14523.
  13. Kang N, Summers JJ, Cauraugh JH. Transcranial direct current stimulation facilitates motor learning post- stroke: a systematic review and meta-analysis. J Neurol Neurosurg Psychiatry. 2016 Apr;87(4):345-55.

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