Non-invasive transcranial brain modulation for neurological disorders treatment: A narrative review

Noninvasive brain stimulation/modulation is a rapidly emerging technique that has been implemented in different clinical applications. The commonly noninvasive techniques used in neurological manipulations include photobiomodulation (PBM), transcranial electrical stimulation (TES), transcranial magnetic stimulation (TMS), and ultrasound stimulation (USS). These techniques have the ability to excite, inhibit, or modulate neuronal activity in targeted brain areas to obtain the required therapeutic effects. However, each technique owns its unique mechanism of action that relies on specific parameters suitable for treating certain neurological disorders. Neurological disorders such as epilepsy, Parkinson's disease (PD), Alzheimer's disease (AD), and depression have been treated by one or more of these noninvasive techniques. The therapeutic outcomes of these techniques for neurological diseases are promising, yet with limitations. In the present review, the mechanisms of action of these different brain stimulation/modulation modalities were explored and a synopsis of their applications in the treatment of certain neurological disorders was provided. Moreover, methodological issues, limitations, and open questions were presented. Furthermore, some future directions were suggested.