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Transcranial Direct Current Stimulation (tDCS) utilizes scalp electrodes to provide a continuous flow of low-intensity current to the brain without causing any damage to the brain tissue. As a result, currents circulate through the cortex’s outer layer. Low-level electrical stimulation does not trigger an action potential directly in brain neurons, but rather changes a neuron’s receptivity to other exogenous stimuli, making it more or less excitable. A stimulus-inducing anodal current raises excitability, while a resting cathodal current lowers excitability. A single stimulus has a lasting effect of between 30 and 120 minutes. 

Unipolar depression, followed by substance use disorders and schizophrenia, is the most widely studied psychiatric disease to date, according to a recent study exploring the therapeutic properties of tDCS. There is limited evidence to support tDCS use in the management of disorders of anxiety including post-traumatic stress disorder. Some case reports are available to date in the fields of anorexia nervosa, generalized anxiety disorder, and treatment-resistant obsessive-compulsive disorder, among other conditions.

Major Depressive Disorder (MDD) has a lifetime prevalence of between 6 percent and 12 percent and a yearly prevalence of between 3 percent and 11 percent throughout the world. After one year of antidepressant treatment, about 80% of individuals experience a recurrence of symptoms of depression, with up to 33% of patients failing to achieve complete remission after two or three pharmaceutical trials. The heterogeneity and complexity of MDD – with differences in its response to treatment, course, symptoms, and etiology – necessitates additional study that targets to improve our understanding of the fundamental neurobiology, with the ultimate objective of identifying brain areas and biochemical circuits associated with this disease.

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Some encouraging evidence suggests that tDCS is effective in the treatment of depression. A consensus of European specialists graded tDCS as a possibly beneficial technology in the treatment of depression in the most recent CANMAT (Canadian Network for Mood and Anxiety Treatments) edition. The pioneer pilot trials of tDCS in depression were conducted more than a decade ago and yielded encouraging outcomes; though, the samples were limited.

Recently, two randomized-controlled trials (N = 245 participants and N = 120 respectively) investigated the role of tDCS in the context of pharmaceutical therapy. According to the findings of the first trial, tDCS in combination with sertraline has additional efficacy than either sertraline or tDCS on their own. Secondly, another experimental study compared tDCS (2 mA prefrontal stimulus treatments for fifteen successive weekdays, trailed by seven weekly sessions) with escitalopram (10mg per day for three weeks, then 20 mg per day afterward) was conducted.

The major results revealed that escitalopram was superior to tDCS, and that tDCS was superior to placebo in terms of effectiveness. This shows tDCS is less effective on average as a monotherapy than escitalopram and should not be regarded as a substitute for antidepressant medication. However, because tDCS was more effective and associated with minimal adverse effects than a placebo, it can be deliberated as a treatment in situations where escitalopram or another pharmacological treatment is not commended, or where patients prefer non-pharmacological treatments over pharmaceutical treatments.

New tDCS procedures should examine how to optimize protocols and procedures in order to further improve patient results. Current research is focused on developing household tDCS, in which the patient (or caregiver) is trained to manage tDCS at home under the remote supervision of clinicians. This type of tDCS treatment has the potential to greatly expand the feasibility and comfort of tDCS treatment. This method necessitates the use of specialized equipment that has been certified for remote use as well as guidelines for tracking progress.

It has been demonstrated that bilateral prefrontal stimulation is useful in the treatment of depression. The anode on the left was put over the dorsolateral prefrontal cortex, and the cathode on the right was positioned in the prefrontal cortex area. The intensity of the current used was 2 milliamps, and the session lasted 30 minutes. As the number of sessions was increased, it was discovered that the efficacy was increasing. Other procedures have been tried, however, they all utilized anodal stimulation above the dorsolateral prefrontal cortex as their main stimulus source.

Anode placement over the left dorsolateral prefrontal cortex (DLPFC) and cathode placement over the right DLPFC, supraorbital, or extra-cephalic is used in the treatment of unipolar depression. The research that led to the development of the prefrontal asymmetry concept of depression, which specifies that right prefrontal function is greater than left in depressed patients, as well as clinical studies with repetitive Transcranial magnetic stimulation (rTMS) using facilitatory excitation over the left DLPFC and inhibitory activity over the right DLPFC, provide the basis for this montage.

For the most part, the same time and position variables are used in all of the stimulation protocols used to treat serious depression (bifrontal montage for twenty-minute). Some investigations went into greater detail about the many parameters that may be employed. For example, one study discovered that a combination of tDCS stimulation for 30 minutes with sertraline resulted in better symptom relief than combining with treatment for 20 minutes alone. Another study by Martin et al. discovered that extra-cephalic electrodeposition was associated with a stronger antidepressant effect in a group of individuals who did not respond effectively to bifrontal stimulation. They also discovered that having weekly or fortnightly maintenance sessions resulted in remission rates of 80 percent after three months and 50 percent after six months, after starting the program.

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Clinical research has shown that tDCS effectively reduces the clinical symptoms of severe depression. One of the first studies to use 1-mA anodal stimulation for twenty minutes a day over the left DLPFC for five days by Fregni et al. showed significant improvement in symptoms. Different stimulation procedures and patient subpopulations were examined in several randomized clinical trials with variable outcomes. Researchers used meta-analyses of randomized clinical studies to show that active tDCS had a greater response and remission rates and improved depression scores in comparison to sham tDCS. One meta-analysis, on the other hand, found no difference between sham and active stimulation groups in terms of response or remission rates. It’s clear that evaluating different stimulation methods and small sample sizes is an issue because the results are so inconsistent.

The results of the efficacy of Transcranial direct current stimulation (tDCS) in treating MDD are varied due to the variety of patient samples, the intensity of symptoms of depression, and the small sample sizes. Moreover, the combination of cognitive tasks and stimulation is also being investigated as a viable strategy for increasing the effectiveness of tDCS for the management of Depression.

Schizophrenia affects 0.5-1.5% of the general population. The disorder’s clinical symptoms can be broken down into three categories: positive symptoms, negative symptoms, and cognitive deficits. The typical positive symptoms include delusions, hallucinations, disturbed thinking, and uncontrollable movements. It can cause negative symptoms like depression, such as depression, and lethargy. Chronic, often-refractory symptoms are difficult to treat with traditional antipsychotic drugs because of their poor efficacy. 

To treat the various aspects of the disease, tDCS montage is used on patients with schizophrenia. Anodal Transcranial direct current stimulation (tDCS) over the left dorsomedial prefrontal cortex (DLPFC) and cathodal stimulation over the left temporal lobe (2 milliamps, twenty minutes, five days) significantly reduced auditory hallucinations in individuals with schizophrenia when compared to sham. It was shown that blocking the prefrontal cortex, which has been found to be hypoactive in schizophrenia, was effective in treating negative symptoms while activating the temporoparietal cortex, which is linked to positive symptoms. Further research may be able to repeat this finding. 

Patients with schizophrenia are expected to show abnormal patterns of P50 suppression linked with a diminished capability to figure out unnecessary or redundant stimuli, so the mechanisms behind improvements in positive symptoms may be related to a tDCS-induced rise in sensory gating as analyzed by the P50 event-related potential. In one study, smoking was reported to decrease the efficacy of tDCS in the treatment of hallucinations.

Three primary conclusions have emerged from studies on the impact of Transcranial direct current stimulation (tDCS):

  1. Fronto-temporal tDCS montage for auditory verbal hallucinations (the anode is put over the left dorsolateral prefrontal cortex combined with the cathode over the left temporoparietal junction).
  2. Frontal montage of negative signs and symptoms (the anode placed over the left dorsolateral prefrontal cortex combined with the cathode put on the right supraorbital region, right dorsolateral prefrontal cortex, or extra-cranially)
  3. The use of various tDCS montages to enhance cognitive skills.

There have been encouraging results obtained for all three of these outcomes. tDCS has been shown to reduce symptoms by 30% while simultaneously improving a wide range of cognitive abilities (eg, facial emotion recognition, self-monitoring, facial emotion recognition). Open-label studies and case reports with small samples have formed up the majority of the research to date. In order to prove the efficacy of Transcranial direct current stimulation (tDCS) in schizophrenia, larger randomized controlled studies are required.

Due to the DLPFC’s involvement in the processing of imminent danger, tDCS applied to the DLPFC may be a viable therapy option for anxiety disorders. However, empirical data is still scant. Heeren et al. demonstrated that a single treatment of anodal tDCS over the left DLPFC significantly reduced individuals’ attentional bias for social threat in a probe discrimination task in contrast with sham stimulation. The authors conclude that while tDCS may be an intriguing approach for elucidating the fundamental mechanisms of social anxiety disorders, making direct conclusions about tDCS-based therapies would be premature.

As exposure-based psychotherapy is the standard technique for treating anxiety disorders, it may also be valuable to study noninvasive brain stimulation techniques such as tDCS for their capacity to enhance or augment extinction learning, a critical phase in therapies based on exposure. The ventromedial prefrontal cortex was selected as a target in this study because it is involved in extinction learning and consequent memory retention. In a recent study with twenty-eight veterans suffering from post-traumatic stress disorder, the authors discovered that tDCS across the ventromedial prefrontal cortex had additional benefits during the consolidation of fear extinction than during threat/fear elimination learning itself. However, because fear extinction was not evaluated in the setting of individual traumatic experiences, but rather in a standardized experimental model, findings on the use of tDCS in the management of post-traumatic stress disorder would also be inappropriate.

To conclude, Transcranial direct current stimulation (tDCS) is a promising therapy for anxiety disorders, but outcomes are still experimental. The optimal dose regimen, therapy targets, and mechanism of action remain unknown. The combination of Transcranial direct current stimulation with cognitive-behavioral approaches appears to be an especially strong fit for the treatment of anxiety disorders.

Individuals suffering from bipolar depression were given left prefrontal anodal stimulation as a supplementary treatment to their pharmaceutical therapy in an experimental study by Sampaio-Junior et al. Those who received active stimulation had much fewer symptoms than those who received sham tDCS treatment. Patients with bipolar depression who received tDCS for one week saw an improvement in their depression symptoms, according to a meta-analysis. TDCS and pharmaceutical treatment were used in one published case study on a patient suffering from a severe episode of manic depression. Anodal Transcranial direct current stimulation (tDCS) was used in conjunction with a pharmaceutical intervention to treat manic symptoms, and the results lasted for 72 hours following stimulation.

In a nutshell, studies have shown that tDCS can help people with depressive symptoms of bipolar disorder. The effectiveness and appropriateness of tDCS as a treatment modality in the patient population must be defined in bigger randomized controlled studies and further research should look into how often people develop hypomania or mania after receiving tDCS.

The DLPFC, an important brain region for the regulation of seeking behavior, has been proven to be clinically beneficial in the management of drug addiction by tDCS over the DLPFC. Prefrontal tDCS was found to reduce relapse risk and increase the quality of life in a randomized controlled experiment including 33 patients with alcohol dependency. The increase in brain activation caused by alcohol might be inhibited by active tDCS in a short trial with sham controls. tDCS has also been shown to reduce food cravings in patients with food addiction and disordered eating habits triggered by visual stimuli. The therapeutic potential of tDCS in addiction will undoubtedly be further illuminated by a clinical trial presently underway with 340 alcoholic patients and follow-up duration of 24 weeks.

As a result of interception in award centers between prefrontal areas, Transcranial direct current stimulation may have therapeutic effects. A comprehensive assessment of tDCS efficacy in addiction appears to be challenging at the time despite these encouraging outcomes, as research with consistent stimulation methods, sufficient sample sizes, and a satisfactory study duration are needed.

Cognitive-behavioral therapy and medication are being utilized to manage this disease, however, tDCS may help rectify the abnormality in the function of the cortico-striato-thalamocortical neuronal circuits. The premotor and motor system’s pathological hyperexcitability is reduced, making it a viable alternative to deep brain stimulation, which is currently employed to treat refractory conditions. There is presently a clinical trial going on involving electrodes being placed on the pre-supplementary motor area and the deltoid that may allow electricity to reach these relevant cortical and subcortical locations in the brain that are associated with obsessive compulsion.

There is conflicting evidence as to whether Transcranial direct current stimulation is effective in improving cognitive function in healthy adults. Studies have shown that even slight gains in cognitive function can have a big impact. However, one review was criticized for failing to consider data from multiple-session tDCS trials and within-subject effects. Other studies showed no results at all.

There is no compelling data to indicate a considerable cognitive effect, negative or positive, in healthy individuals with single-session tDCS according to an analysis of the results from hundreds of tDCS tests conducted in 2015. tDCS had little to no effect on any neurophysiologic outcome, according to a follow-up study by the same researchers.

There is insufficient data to support the use of tDCS to treat memory problems associated with Alzheimer’s disease, schizophrenia, or non-neuropathic pain. A few clinical investigations on the use of tDCS to improve memory problems in patients with Parkinson’s disease, Alzheimer’s disease, and healthy participants have yielded inconsistent results. As of 2013, research in schizophrenia discovered that while substantial effect sizes were first observed for symptom improvement, subsequent and larger trials discovered decreased effect sizes. The majority of study has focused on pleasant symptoms such as auditory hallucinations; there is a dearth of research on negative symptoms.

There is no compelling evidence that tDCS can aid in the recovery of upper limb function following stroke. Till 2014, studies indicate that tDCS is ineffective at refining the function of the upper limb following stroke. Some investigations have advocated that tDCS may be beneficial for post-stroke aphasia, a Cochrane review study of 2015 found no benefit from combining conventional therapy and tDCS. Till 2013, research indicates that tDCS might be beneficial in resolving visual problems following stroke.

Because early and prompt treatment of mental disorders has been shown to result in a better outcome and shorter period of illness with the use of tDCS is being investigated to take advantage of this age group’s increased brain plasticity and to gain a better understanding of the comparative contribution of a respective brain region to different mental pathological conditions. For example, it has been reported to be beneficial in Broca’s region for facilitating language learning in autistic children who have a meager vocabulary. However, when dealing with kids, the risk of aberrant neurological development as a result of extrinsic electrical current exposure should always be considered.

Research on the usage of tDCS to manage pain concluded that the investigation yielded poor outcomes and could not be utilized to promote the usage of tDCS to treat disorders of pain. In persistent pain resulting from spinal cord damage, high-quality research has established that tDCS is ineffective. Additionally, tDCS has been explored in the field of addiction. There is judicious (level B) evidence that tDCS may be suitable for treating craving problems and fibromyalgia in addition to major depressive disorder.

tDCS has been utilized in neuroscience studies to investigate the relationship between certain brain areas and particular psychological problems or cognitive tasks. The cerebellum has been a subject of investigation due to its dense population of nerve cells, its placement directly beneath the skull, and its numerous mutual anatomical connections to the associative and motor brain regions. While the majority of this research examines the effect of cerebellar tDCS on cognitive, motor, and emotional functioning in healthy and diseased populations, some also use tDCS over the cerebellum to investigate the cerebellum’s functional connections to other parts of the brain.

While early trials investigating the use of tDCS in a variety of mental and cognitive diseases demonstrated significant promise, much more study is needed to determine the optimal dosage, pattern of electrode placement, and the number of sessions employed. While the adverse effects appear to be minimal, their clinical value has not been shown.

The US Food and Drug Administration (FDA) has not yet approved Transcranial direct current stimulation (tDCS). It means that the FDA in the United States has not assessed or accepted a company’s “marketing” application for approval of tDCS devices as a therapeutic modality. No definitive decision has been made by the FDA on the safety or efficacy of tDCS for any therapeutic specification, such as depression or pain. In the United States, using tDCS for medical purposes is termed “Investigational.” The FDA solely responds to “marketing” petitions from individual companies and does not evaluate clinical trials before making judgments.

Currently, guidelines (as endorsed by the European Chapter of the International Federation of Clinical Neurophysiology) recommend clinical applications in just the following disciplines as having a reasonable chance of efficacy:

  1. Fibromyalgia
  2. Non-drug-resistant Major depressive illness
  3. Addiction to drugs or drug cravings

There is some evidence of efficacy in the treatment of chronic neuropathic pain in the lower leg, but it is believed to be unsuccessful in the treatment of tinnitus and treatment-resistant severe depression.