Elsevier

The Lancet Neurology

Volume 5, Issue 8, August 2006, Pages 708-712
The Lancet Neurology

Rapid Review
Non-invasive brain stimulation: a new strategy to improve neurorehabilitation after stroke?

https://doi.org/10.1016/S1474-4422(06)70525-7Get rights and content

Summary

Background

Motor impairment resulting from chronic stroke can have extensive physical, psychological, financial, and social implications despite available neurorehabilitative treatments. Recent studies in animals showed that direct epidural stimulation of the primary motor cortex surrounding a small infarct in the lesioned hemisphere (M1lesioned hemisphere) elicits improvements in motor function.

Recent developments

In human beings, proof of principle studies from different laboratories showed that non-invasive transcranial magnetic stimulation and direct current stimulation that upregulate excitability within M1lesioned hemisphere or downregulate excitability in the intact hemisphere (M1intact hemisphere) results in improvement in motor function in patients with stroke. Possible mechanisms mediating these effects can include the correction of abnormally persistent interhemispheric inhibitory drive from M1intact hemisphere to M1lesioned hemisphere in the process of generation of voluntary movements by the paretic hand, a disorder correlated with the magnitude of impairment. In this paper we review these mechanistically oriented interventional approaches.

What next?

These findings suggest that transcranial magnetic stimulation and transcranial direct current stimulation could develop into useful adjuvant strategies in neurorehabilitation but have to be further assessed in multicentre clinical trials.

Introduction

Recovery of motor function after stroke is typically incomplete.1 6 months after the episode, two-thirds of stroke survivors are unable to take part in activities of daily living with their paretic hand to the extent they were before2 and only a few are able to carry out professional work.3 It remains a desirable goal to develop strategies to improve the beneficial effects of neurorehabilitative treatments.

Neuroimaging studies showed increased activity of M1intact hemisphere with movements of the paretic hand in patients with motor impairment.4, 5 The role of activity in the intact hemisphere on motor control, presently under investigation, varies depending on lesion site, time from stroke, and magnitude of impairment.6, 7 Patients with stroke experience changes in motor cortical excitability8, 9, 10, 11 and an abnormally high interhemispheric inhibition from M1intact hemisphere to M1lesioned hemisphere with movements of the paretic hand that is more prominent in cases with more substantial motor impairment.1 These findings, consistent with interhemispheric competition models of sensory and motor processing,12, 13, 14, 15 raised the hypothesis that purposeful modulation of excitability in motor regions of the intact and affected hemisphere may contribute to improvements in motor function.16

Recent studies in animals showed that motor recovery after focal lesions in the hand motor representation can improve with direct epidural cortical stimulation.17, 18, 19 The feasibility and safety of this invasive approach in patients is under investigation;20 an alternative approach is the use of non-invasive cortical stimulation.

Non-invasive brain stimulation is a powerful method to modulate human brain function.21, 22, 23 Transcranial magnetic stimulation is a painless procedure that modulates cortical excitability and has contributed to the understanding of mechanisms underlying cognitive processes.21, 24 The procedure involves a short strong electrical current that is delivered through an insulated coil of wire placed over the scalp (magnetic coil). The induced electrical currents modulate neuronal excitability at the stimulated sites. Depending on stimulation parameters, transcranial magnetic stimulation can upregulate or downregulate excitability to different extents25 in the neural structures under the stimulating coil.26 Transcranial direct-current stimulation is a procedure used to polarise brain regions through the non-invasive application of weak direct currents.23, 27, 28 The procedure elicits focal reversible shifts in cortical excitability depending on the polarity, strength, and duration of stimulation.28 Both techniques can purposefully modulate brain function, are painless and non-invasive, and can be used in double-blind, experimental designs27, 29, 30, 31, 32, 33, 34—although the mechanisms underlying these features may differ (panel22, 23, 27, 35, 36, 37, 38).

Interhemispheric competition models16 suggest possible strategies to influence function in the paretic hand (figure): upregulation of excitability in M1lesioned hemisphere and downregulation of excitability in M1intact hemisphere. In this paper we describe recent studies that report the effects of both transcranial magnetic stimulation and transcranial direct current stimulation on cortical excitability and motor function in the paretic hand after chronic stroke. Additional options under investigation include modulation of excitability in non-primary motor regions like the dorsal39, 40 and ventral41 premotor cortices or the supplementary motor area.

Section snippets

Upregulation of excitability in the affected hemisphere

Two non-invasive strategies have been used to increase excitability in the affected hemisphere: anodal transcranial direct current stimulation and rapid-rate transcranial magnetic stimulation (panel24, 26, 28, 35). Anodal transcranial direct current stimulation delivered to M1lesioned hemisphere was studied in patients with chronic stroke in sham-controlled double-blind crossover experimental designs.29, 30, 31 These studies showed improvements in performance of motor tasks that mimic

Where next?

Performance improvements reported so far with transcranial magnetic stimulation and transcranial direct current stimulation have been moderate in magnitude (10–30%) and, depending on the study, relatively transient. Implementation of repetitive stimulation and synchronous application with established rehabilitative treatments may lead to more prominent or longer lasting performance improvements. Areas for future investigation are the optimisation of parameters and sham controls, the role of

Search strategy and selection criteria

References for this review were identified by searches of MEDLINE between 1969 and April 2006 and references from relevant articles. The search terms used were “brain stimulation”, “stroke”, “recovery”, “TMS”, “tDCS”, “plasticity”, “imaging”, “interhemispheric inhibition”, “intracortical inhibition”, “interhemispheric competition”. Abstracts and reports from meetings were also included. The final reference list was generated based on originality and relevance to the topics covered in the

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