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|Year : 2019 | Volume
| Issue : 2 | Page : 424-426
Non-invasive brain stimulation in stroke: Do the electrical currents have the potential to enhance neuroplasticity?
Kamal N Arya
Pandit Deendayal Upadhyaya National Institute for Persons with Physical Disabilities, New Delhi, India
|Date of Web Publication||13-May-2019|
Dr. Kamal N Arya
Pandit Deendayal Upadhyaya National Institute for Persons with Physical Disabilities, New Delhi
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Arya KN. Non-invasive brain stimulation in stroke: Do the electrical currents have the potential to enhance neuroplasticity?. Neurol India 2019;67:424-6
Stroke rehabilitation has reached the stage of directly altering the structure and function of the brain. The objective of most of the neuromodulation methods is to induce or enhance the lost or impaired functions of the lesioned cortex. Various non-invasive brain stimulation (NIBS) techniques (both electrical and non-electrical) have been investigated in post-stroke subjects, for instance, motor imagery, action observation, mirror therapy, and transcranial stimulation.
The brain hemispheres communicate with each other to coordinate the motor control via the corpus callosum. The synchronization is crucial for motor learning. Post-stroke, the ipsilesional cortical activity reduces, whereas the activity of the contralesional brain increases, thus inducing an interhemispheric competition. In addition to this, intra-hemispheric changes can be observed in the form of alterations in the representation of cortical maps, such as shifting of the hand area to the shoulder area. During the acute phase, the stimulation of the contralesional motor cortex may enhance the motor recovery; though the response gradually reduces after a year. In the chronic phase, facilitation of the ipsilesional brain may augment recovery.
Neuroplasticity is the ability of the brain to reorganize its structure and function in response to external or internal stimulus. The cortical changes may be favourable in response to the environment or therapy and may be unfavourable in response to the presence of a lesion or poor experiences. The neuroplasticity changes could occur in various forms such as synaptic plasticity (change in the strength of the synaptic connection), synaptogenesis, neurogenesis, and protein synthesis. Further, there may also be augmentation of the synaptic plasticity, known as metaplasticity. The pre-stroke state of synaptic activity would be an important factor for inducing plasticity after stroke. Electroencephalography, magnetoencephalography, functional magnetic resonance imaging (MRI), magnetic resonance spectroscopy, positron emission tomography are some of the commonly used measures to discern neuroplastic changes.
There are ranges of transcranial stimulation methods being investigated that enhance or alter cortical activity, for instance, transcranial electrical stimulation (TES) which includes transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation; transcranial magnetic stimulation (TMS); transcranial pulsed current stimulation; transcranial random noise stimulation; and transcranial ultrasound stimulation. Out of all these approaches, tDCS and TMS have been largely investigated for post-stroke motor issues.
In this issue of Neurology India, Solomons and Shanmugasundaram, by their review, highlighted the array of investigations related to TES, specifically the tDCS among stroke subjects. The authors have elaborated the standard protocol as well as the adjunctive therapies, and have focused on the motor recovery related to the tDCS studies.
In tDCS, a low voltage, weak current of 1-2 mv (delivered by a 9-volt battery) is applied to stimulate the cortex through the scalp electrodes (placed over the desired brain areas). The procedure creates an electrical field between the anode and cathode placed on the brain. The technique has been considered to influence the cortical activity by either depolarizing (anode) or hyperpolarizing (cathode) the resting membrane potential of the neurons. Accordingly, the anode is placed over the lesioned brain (to increase the neuronal activity), whereas the cathode is placed over the non-affected brain (to retard the neuronal activity). The altered cortical activity retained even after the withdrawal of the stimulation, supports the criteria of neuroplasticity and is believed to enhance motor recovery. Neurotransmitters, such as GABA (gamma-aminobutyric acid), may be considered as neuroplastic biomarkers in response to TES. The tDCS application reduces the GABA level; the alteration has been evident during the augmentation of motor learning. Animal studies have demonstrated a reduction in the infarct volume in acute stroke by the performance of cathodal tDCS.
The tDCS regimen has been analysed with or without other rehabilitative training separately for the upper and lower limbs. The upper limb tDCS training may be combined with peripheral stimulation methods, other non-invasive brain stimulation (NIBS) activities such as mirror therapy, and advanced techniques such as virtual reality and robotic training. In the lower limb, tDCS may be coupled with body weight support, treadmill training and neuromuscular electrical stimulation. However, explicit indications for the adjunctive therapies are still being explored.
tDCS is a portable device that allows the incorporation of other therapies also. A favourable motor recovery has been observed in response to tDCS when combined with other rehabilitative therapies, such as constraint-induced movement therapy. Thus, electrical stimulation may also facilitate the response of motor therapies, complementing cortical plasticity. TES is considered to be safe with a few, very short-lasting adverse effects such as scalp irritation and a mild tingling sensation. Though simple to use, any type of TES should preferably be applied by a trained professional. The inclusion criteria of the subjects as well as the adjunctive therapies should be critically decided. The regime should be followed by standard motor and functional rehabilitation so as to translate the facilitated neuronal activity into motor learning and functional performance. The evidence of neuroplastic biomarkers and associated motor recovery should be ascertained in clinical and research practice. Although safety, ethical, and regulatory guidelines are available globally, the researchers and clinicians should ensure that their respective national directives are being followed, if they exist. The formulation of the guidelines at appropriate levels should be given due importance for the countries lacking the regulations. Most importantly, there should be a consensus regarding the dosimetry of the intervention.
The TES method is considered to be effective enough to enhance post-stroke recovery. However, the technique has limited clinical applications due to different reasons. The post-stroke subjects respond variably to the electrical stimulation. The type of stroke, post-stroke duration, area of involvement, type of electrodes placed, site of electrode placement, therapy-dosage, and associated motor regimes followed, are some of the variables other than the usual stroke-recovery related factors. Evidence suggests that the motor recovery is favourably associated with the charge density, current density, and pad size of the tDCS arrangement. It has also been indicated that the magnitude of motor recovery is large among the chronic subjects. In contrast, TES may also induce imbalance between the functional brain networks. Thus, apart from the targeted brain area and the desired motor outcome, the related neural networks should be cautiously considered. Additionally, the integrity of the cortico-spinal tract is one of the key variables that determines the effectiveness of any NIBS technique.
In TMS, an electric current induced by a pulsed magnetic field is applied via a magnetic coil to stimulate (excite or inhibit) the cortex. The technique modulates excitatory and inhibitory neurotransmitters, thus inducing neuroplasticity. The TMS technique is utilized for interventions as well for establishing the diagnosis. Repetitive TMS has demonstrated an improvement in the hand dexterity among stroke subjects. In contrast to tDCS, TMS is more effective in the acute stage of stroke. However, a high quality evidence for the regimen is still warranted. Mild headache and anxiety are some of the reported adverse events of TMS.
The generalizability of review of Solomons and Shanmugasundaram should be judged in view of the available evidence on TES. In the Cochrane review cited by the present study, a systematic analysis of 12 trials (analysing secondary outcome) indicated no favourable upper extremity function in response to tDCS intervention. Moderate-to-low quality evidence for spasticity reduction has also been reported. Further, the tDCS paradigm has exhibited moderate evidence for facilitating gait speed among post-stroke individuals.
The role of TES intervention has also been investigated for language, psycho-behaviour, as well as perceptual and cognitive deficits in stroke. Since these impairments are usually associated with motor paresis, an integrated application of the technique requires further research. In view of variability in the treatment response, an in-depth investigation for the selection of the patient-type and the variant of electrical stimulation that should be selected in order to augment neuroplasticity, is also warranted.
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