tCS (transcranial Current Stimulation of the brain) has been used in the past 60 years to treat different indications. In the last five years a growing base of clinical and behavioral studies indicate that there is a significant potential for the treatment of: pain, post stroke rehabilitation, depression and cognitive enhancement. The most explored variant of tCS is tDCS (where the current is maintained constant during the stimulation session). Following is a short review of the effects of tDCS in these indications.
tDCS and Pain Relief
The typical current strategy for pain relief is pharmacological, with excellent results in some applications but modest or even no improvement in others. Most pharmacology treatments for pain could have side effects. Thus pain treatment is not a fully resolved mater. tDCS has been proposed as an alternative or complementary help for pain, mainly for its non invasiveness and excellent tolerability. Several applications of tDCS in pain related pathologies have been explored, including: chronic central pain, neuropathic pain, fibromyalgia, postoperative pain, complex regional pain, migraine headache, post-stroke pain and multiple sclerosis pain.
For example in the chronic central pain of multiple sclerosis -poorly responsive to current conventional medications- a randomized, double blind, sham controlled study showed anodal tDCS is able to reduce pain-scale scores and that this effect outlasts the period of stimulation. A recent review suggested the option of tDCS for multiple sclerosis patients no responders when treated with other therapies. Moreover, the available evidence suggests that tDCS applied to the motor cortex may have short-term effects on chronic pain, and preliminary recent observations support the use of tDCS for the treatment of pain in selected patient populations -i.e. in a common illnesses as migraine-.
tDCS in Stroke
After a stroke event there is a reorganization of functionality of remaining non-directly affected brain tissue that is mediated by neuroplasticity. These intrinsic neural changes could be favorable for the recovery of the patients, but in certain cases may interfere with the rehabilitation. Indeed, recovery of motor function after stroke may depend on a balance of inhibitory relation in the neural network involving the affected and the unaffected motor cortices. tDCS have the potential to interact with these complex spontaneous processes taking profit of its ability of producing selective neuroplastic changes in specific areas of the brain, either enhancing its activity (anodal stimulation) or decreasing its activity (cathodal stimulation).
Data from several studies in healthy subjects and stroke patients revealed tDCS increase corticomotor excitability with a dependence on current density and duration time of application. Although the fine underlying mechanisms are not completely understood, stroke studies show a trend in favor of motor function rehabilitation following tDCS suggesting that this technique could develop into useful adjuvant strategies in stroke neurorehabilitation. A potential role for tDCS has been proposed for improvement of post-stroke related memory deficits and post-stroke dysphagia. Also, preliminary findings suggest that non-invasive brain stimulation techniques may be useful for facilitating post-stroke aphasia recovery particularly for speech production.
tDCS for Depression
The established treatments for depression could produce side effects in the case of antidepressant drugs, and are very time consuming of high skilled professionals with the consequent economical costs in the case of psychotherapy. Moreover, a significant amount of patients with depression symptoms are no responders or not fully responders to the habitual treatments.
A third alternative way that may complement or substitute in the future traditional therapeutics of depression overcoming the limitations is highly desirable. tDCS could address this unmet need due to its limited side effects and low cost. Some recent preliminary evidence support earlier positive reports of the efficacy and safety of tDCS in the treatment of depression, and the first meta-analysis encountered an effect favoring active versus sham group.
However, despite preliminary promising results, further research is needed.
Cognitive enhancement has been defined as interventions in humans that aim to improve mental functioning beyond what is necessary to sustain or restore good health. tDCS has the capability of modified the short and long term functionality of nervous tissue. As described previously the effects could be either activating or reducing both locally and at the network level the nervous functionality. This opened an exceptional opportunity to influence selectively functions of the brain supported by areas that are accessible to the stimulation with tDCS or related with these areas, and provide the basis to its application to cognitive enhancement in healthy people and in patients.
Recent abundant findings support improvement of a variety of cognitive skills in normal subjects when adequate tDCS stimulation is applied fitting the specific cerebral areas related with the task. Facilitation of insight, enhancement of planning activity, improvement or impairment of declarative memory, enhancement of language performance, more cautious driving behavior, and a large list of other enhanced cognitive effects have been currently reported.
This is a scientific area of an enormous potential, however due to novelty these findings have not been replicated yet, more work is necessary to fully explore the strengths and limitations.