Non electrical and non pharmacological ways of vagus nerve stimulation: Overview, pathways and clinical implications

Vagal nerve stimulation is gaining recognition as an important therapeutic approach especially for non-communicable diseases such as cardiovascular disease (CVD), Depression, Fibromyalgia, Arthritis, Type 2 Diabetes, Alzheimer, and certain types of cancer. In turn, decreased vagal tone its implicated in the prognosis of a wide range of diseases. Afferent vagal stimulation seems to have the key to reset certain pathophysiological states of the body leading to health improvement by reshaping neural networks. The most studied therapeutic option of vagal nerve stimulation is electrical either by transcutaneous or subcutaneous stimulation, although both are costly and unpleasant to some degree. Over twenty other options of non electrical and non pharmacological vagal nerve stimulation exist, yet most clinicians and researchers are not aware of these techniques. There are no systematic reviews available, neither on ideas about dose and response relationships or effect sizes of the different options nor on potential therapeutic utility e.g. whether a certain technique may allow low cost implication in the clinical or home setting. We attempt to provide an overview of natural non electrical and non pharmacological vagal nerve stimulation with previously described interventions, identifying stimulated pathways and reporting potential effects. Results are divided in four distinct categories to describe different groups of plausible biological pathways for vagal stimulation including vagal reflexes and sustained stimulation to the nucleus tractus solitarius (NTS). Activities may include breathing techniques, facial immersion in cold water face, praying, muscle stretching, certain food and beverage intake among others. This review offers a basic understanding of useful tools that stimulate vagal pathways and that are promising therapeutic adjuvants in a wide range of diseases.