Transcutaneous vagus nerve stimulation (TENS) involves the stimulation of the left and/or
right auricular branch of the vagus nerve in the area of the cymba concha with
low-frequency electrical impulses. The auricular branch of the vagus nerve runs
superficially, which makes it a favorable target for non-invasive stimulation techniques
to modulate vagal activity. It gained popularity due to minimal side effects and low
cost.
This method is a new, cost-effective alternative to invasive cervical vagus nerve
stimulation (iVNs), which is an FDA-approved treatment of depression resistant to the
treatment, epilepsy and other pathologies.
The use of TENS has shown similar positive results as iVNs, for example, in reducing
symptoms in patients with depression and changing the early visual processing of negative
emotional stimuli in adolescent depression. Similarly, positive effects of TENS have also
been found in chronic pain and epilepsy. These similarities in effects can be explained
by the similarity of brain network activation achieved by iVNs and TENS.
The lack of similarity between behavioral studies and numerous theories of physiological
processes in TENS make it necessary to determine a reliable biomarker of successful
activation of the vagus nerve using TENS. Although many potential biomarkers have been
proposed, most studies have focused on HRV.
Despite the physiological justification of HRV as a biomarker for TENS, the data on the
effects of TENS on HRV are ambiguous. In some studies, a significant decrease in the
ratio of spectral characteristics (LF/HF) in active TENS compared to fictitious
stimulation (sham) was found, indicating an increase in the parasympathetic component of
HRV.
However, other studies have not revealed an increase in HRV. Large methodological
differences between studies, such as different stimulation devices, sides and places of
stimulation, experimental schemes, reported HRV parameters and stimulation protocols,
reduce comparability between studies.
One of the most striking examples is the use of various control conditions. While in most
studies active TENS are compared with imitation of the earlobe as an independent
variable, as recommended, in some studies active stimulation of the tragus was compared
with a control state without stimulation or with a fictitious state without stimulation
when the electrode is placed on the ear, but no electric current is applied.
The development of international agreed consensus guidelines on TENS research reporting
should address these issues. Although TENS represents a potential treatment option for
many disorders and it is an interesting tool for experimental research, it needs to be
studied in an objective and reliable way before its true place as a
neuro-immunomodulatory intervention can be determined.
The investigators plan to conduct a study on the assessment of TENS on the dynamics of
HRV parameters with reporting according to the international consensus document
https://www.frontiersin.org/articles/10.3389/fnhum.2020.568051/full#B235