Since brain activity is due to the electrical activity of neurons, extrinsic electrical stimulation as a means of modulating this activity has been long studied as a technique which could help to change brain activity patterns either temporarily or permanently. Some of the methods used to this end include transcranial magnetic stimulation, transcranial direct current stimulation (tDCS), transcranial random noise stimulation, and transcranial pulsed current stimulation.
Among these, tDCS is being studied extensively because of its apparent ease of use, the simplicity with which long-term changes are induced after one or a few neurons, and the relative freedom from adverse effects.
tDCS is noninvasive and can reduce or increase neuronal excitability in brain regions through which the current passes. It has a bidirectional mechanism of modulation of cortical activity. Anodal current leads to stimulation while cathodal current reduces the excitability of the neurons by inducing hyperpolarization and depolarization of neuronal membranes. The firing frequency is also altered. The interesting finding is that tDCS has a long-lasting effect, for up to 120 minutes after application of current. This points to long-term effects on neuroplasticity, which are of great research and therapeutic interest. The duration of such effects is correlated with the strength and duration of the current passed. Thus, its potential in treating stroke, epilepsy, chronic pain, Alzheimer’s disease, major depression and addictions.
tDCS is generally felt to be a safe technique and reported side effects have been few and minor. However, most of them have been derived from the results of single-session tDCS performed in healthy volunteers and almost half of all studies do not give the incidence or type of adverse effects.
The most common reported side effects include itching, tingling, headache, burning sensation, and discomfort. These are similar in adults and children. The physical adverse effects are restricted to the site of stimulation. One study in children showed that irritability and acute mood changes were also chief adverse effects in this age group.
Safety of use in children should be especially focused on as the skull thickness, cerebrospinal fluid volume, neuroplasticity and volume of white and gray matter are all different between adults and children. This means that doses which are used in adults may need to be modified in children, based on ongoing research. For instance, one study showed that 2 mA had a greater concentration over the brain surface in children as compared to adults. It is crucial that tDCS does not cause any brain lesions or final fall in intelligence or memory. In non-urgent situations it is probably better to defer the use of this technique until maturity.
Another area of concern is the possible occurrence of skin lesions under the electrode placement sites following stimulation, for instance, lesions due to increased skin temperature may be observed. The thickness of the skin, the current intensity, the greater impedance of the skin, the duration of current flow, and other technical factors, may all correlate with the risk of such lesions. The change in skin impedance with the flow of current, and this change is related to current intensity, flow duration and stimulation density. Thus, a dynamic adjustment of the stimulation voltage should be achieved to ensure that a constant current flows across the skin and the skull to the brain.
Other adverse effects which may be observed include moderate tiredness, headache, insomnia and nausea, most of which are temporary, and well tolerated.
In order to achieve the maximum safety with tDCS, the following safety norms are advised:
- The patient should be confirmed to have no metallic implants in the skin or brain
- The patient should not have any skin disease
- The skin should be cleaned lightly using a swab prior to electrode placement, rather than scrubbing it clean, which may abrade it and lead to lesions
- The skin impedance should be measured to make sure it falls within the limits prescribed by the tDCS device manufacturer
- The electrode sponges are soaked in saline to reduce skin impedance and provide a firm area of skin contact
- The device should supply current at a constant strength
- The density of current is suggested by some researchers to lie between 0.029 and 0.142 mA/cm2, and the charge density at the stimulating electrode to be not more than 40 μC/cm2
- Sessions should not be spaced too closely to allow the brain and superficial tissues to recover from any undesirable effects
- All the manufacturer’s precautions should be closely adhered to
- The therapist or researcher should be thoroughly informed as to the technique and adverse effects and the procedure should be fully supervised
It is vital to note that the effects of tDCS in healthy children have not been fully reported as well. It is also necessary that the long-term effects, as well as rare and unexpected adverse effects, be reported meticulously during all studies.
Reviewed by Chloe Barnett, BSc