Vagus nerve stimulation in the treatment of nervous system disease: a review article

The vagus nerve (VN), the longest cranial nerve and an essential part of the parasympathetic system, connects the central nervous system to respiratory, cardiovascular, immune, gastrointestinal, and endocrine systems and is involved in the maintenance of homeostasis by controlling these systems. Vagus nerve stimulation (VNS) is related to any method that would stimulate the vagal nerve via electrical stimulation. VNS is a Food and Drug Administration (FDA)-approved treatment for medication-resistant depression, drug-resistant epilepsy, and migraine. However, VNS has also been studied for various other conditions, such as Alzheimer's disease and tinnitus, by targeting the VN in the neck and ear. Currently, there are two methods for VNS: a) invasive-VNS (iVNS), which requires surgical implantation of a pulse generator under the anterior chest wall, that is linked through a wire to an electrode cuff that wraps around a cervical vagus nerve, b) non-invasive transcutaneous VNS which is separated into cervical transcutaneous vagal nerve stimulation (ctVNS) and auricular transcutaneous vagal nerve stimulation (atVNS). The non-invasive transcutaneous VNS techniques are well tolerated and have no significant side effects, making them effective in clinical research for brain diseases. Because with these newer methods, the electrical stimulation is carried out through the skin. The results of this study were collected using the advanced search in Scientific Information Database (SID), Google Scholar, PubMed, and Scopus between 2011 to 2021. Out of 671 articles surveyed, we used 53 articles in the study after the evaluation. Medical Subject Headings (MeSH) and Keyword Searching was carried out through the MeSH database. VNS has been shown to alter neural activity in multiple areas of the brain related to the regulation of the affective states. However, the precise mechanism of VNS action on the clinical consequences is still unknown. This study aimed to review the therapeutic effect of both methods of VNS in neuropsychiatric and neurological disorders such as depression, migraine, seizure, tinnitus and Alzheimer's disease and discuss several hypotheses on the mechanism of VNS, as a new approach, in the treatment of such disorders. It considers that a brain-mapping approach is needed to discover the therapeutic mechanisms of VNS in brain diseases.