Definition and Overview
Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique that transmits a weak direct current of 1 to 2 mA through the skull to the brain cortex through electrodes attached to the scalp. It is being actively studied in neuroscience and clinical fields because it can change the excitability of neurons in the brain cortex without surgical procedures or anesthesia.
tDCS has been studied since the early 20th century, but clinical research in its modern form began with the work of Nitsche and Paulus in 2000. Since then, clinical evidence has been accumulating in various areas such as depression, chronic pain, stroke rehabilitation, and cognitive function improvement.
Principle and mechanism
electrical principles
A direct current of about 1 to 2 mA flows between two electrodes (anode, cathode) attached to the scalp. The electric current passes through the scalp, skull, and cerebrospinal fluid to reach the cerebral cortex. In reality, the current density reaching the brain is greatly attenuated as it passes through the skull.
regulation of neuronal excitability
- Anodal stimulation: Increases spontaneous firing rate by shifting the resting membrane potential of cortical neurons below the electrode toward depolarization. Cortical excitability increases.
- Cathodal stimulation: Conversely, it causes hyperpolarization and reduces neuron firing rate. Cortical excitability is suppressed.
synaptic plasticity
The excitability changes of tDCS last for a certain period of time (tens of minutes to several hours) not only during stimulation but also after the end of stimulation. This is due to plastic changes similar to synaptic long-term potentiation (LTP) and long-term depression (LTD), which are associated with changes in NMDA receptors, calcium-dependent mechanisms, and brain neurotrophic factor (BDNF) expression.
Clinical indications
International clinical guidelines (2017), the level of evidence is confirmed in the following diseases:
Established indications
- Fibromyalgia: Bipolar stimulation of the primary motor cortex shows a significant effect in reducing pain.
- Depression: Anodal stimulation of the left dorsolateral prefrontal cortex (DLPFC) is effective in improving depressive symptoms. In a meta-analysis, tDCS showed a significant reduction in depressive symptoms compared to placebo, and its effectiveness was also reported in drug-resistant depression.
- Rehabilitation of motor function after stroke: Stimulation of the motor cortex of the damaged hemisphere promotes motor recovery.
- Aphasia after stroke: Stimulation of the language center shows an adjuvant effect on language recovery.
Indications under study
- Chronic Pain and Neuropathic Pain
- Control of autonomic function
- Enhancing cognitive function and adjuvant treatment for Alzheimer's disease
- Obsessive-compulsive disorder, PTSD
- chronic fatigue syndrome
- Cognition and Gait Function in Parkinson's Disease
Effects on the autonomic nervous system
tDCS stimulation of the left prefrontal cortex (DLPFC) or insula cortex can affect the central autonomic network (CAN), which is involved in regulating the autonomic nervous system. Studies have reported changes in heart rate variability (HRV) and blood pressure response after tDCS, and research on the mechanism for improving autonomic function is in progress.
In particular, the insula is a key cortical structure in autonomic nervous regulation, and the hypothesis that tDCS stimulation of this area may be involved in regulating sympathetic-parasympathetic balance is being studied.
Treatment method
Instrument and electrode installation
Standard tDCS devices supply a stable direct current of 1 to 2 mA. The electrode is a 35cm² square or circular sponge pad, and is sufficiently moistened with saline solution before the procedure to reduce contact resistance with the skin.
target area
Set the positions of the anode and cathode according to the purpose of treatment. For example, when treating depression, the anode is placed in the left DLPFC (F3 location, based on 10-20 EEG system), and the cathode is placed in the right superior superior parietal process.
Procedure time and frequency
One treatment takes 20 to 30 minutes, and the treatment process consists of 5 to 20 sessions (daily or every other day) depending on the disease and purpose. It can be performed as an outpatient procedure, and other treatments or cognitive training can be combined during the procedure.
safety
According to the 2016 international safety guidelines, tDCS performed within the recommended range (current density of 0.029 mA/cm² or less, total charge of 7.2 C or less) is safe without serious side effects.
Minor side effects include skin erythema at the electrode site, tingling, and headaches. Difficulty concentrating and drowsiness may also appear temporarily.
Absolute contraindications: intracranial metal implants (DBS electrodes, aneurysm clips), intracranial electrical stimulation devices.
Relative precautions: epilepsy, pregnancy, scalp skin lesions, pacemakers.
Comparison of tDCS and TMS
| Item | tDCS | TMS | |------|------|-----| | Stimulation method | weak direct current | strong magnetic field induced current | | Mechanism of effect | Excitability regulation | Neuron direct firing | | Device size | Compact, portable | Large stationary | | Treatment time | 20~30 minutes | 30~40 minutes | | Noise/Inconvenience | Almost none | Hitting sound, scalp irritation | | Insurance Coverage | LIMITED | Partially applied | | Evidence of indication | Growing | Accumulating more evidence |