Comparison of Effectiveness In-ear and Behind-ear Vagus Nerve Stimulation

Overview

The use of vagus nerve stimulation (VSS), one of the examples of neuromodulation therapies, continues to increase in the medical field. Basically, invasive (iVSS) and transcutaneous VSS (tVSS) can be performed. tVSS is more preferred because of its advantages such as easy application, not requiring surgical procedure and having fewer side effects. VSS has long been an approved treatment modality in the treatment of epilepsy and refractory depression. tVSS can be administered from the auricular-auricular or posterior ear region. The vagus nerve (VS) belongs to the parasympathetic branch of the autonomic nervous system and has an effect on vital functions. There are also studies in the literature showing that VS stimulation reduces inflammation. In addition to this effect, the vagus nerve may also show analgesic activity through pain-related pathways. Due to its anti-inflammatory and analgesic effects, VSS has become a new treatment method in diseases in which various inflammatory processes are shown in the etiology such as inflammatory bowel diseases, musculoskeletal system diseases and central nervous system diseases.

There are no generally accepted values for taVNS (transcutaneous auricular vagus nerve stimulation) application parameters (amplitude, frequency, intensity, duration, side); therefore, there are heterogeneous applications in studies. In a meta-analysis on the safety profile of taVSS, local skin irritation (18. 2%), headache (3.6%), nasopharyngitis (1.7%), dizziness (1.4%) and nausea (1.1%) were reported and taVSS treatment was reported as an effective and safe treatment with these side effects, most of which were transient and did not require additional intervention. During taVNS, autonomic activities increase in favour of the parasympathetic system and this results in an increase in heart rate variability (HRV). Although an increase in HRV has been demonstrated by different studies, the factors that can predict the change in HRV are not clear. In a study by Geng et al. using taVSS, it was observed that HRV indicators representing vagal nerve activity increased significantly when stimulation was performed in the morning, but evening stimulation did not yield similar results. In addition, they found that improvements in measures of autonomic balance were more pronounced in the presence of lower vagal activity; they also found that by increasing the duration of stimulation, the effect of taVNS on HRV was not regulated by duration. In other words, HRV changes had the best effect only at the beginning of stimulation.