The Vagus Nerve Somatosensory-evoked Potential in Neural Disorders: Systematic Review and Illustrative Vignettes

Abstract

Objective. To review the scientific publications reporting vagal nerve somatosensory-evoked potential (VSEP) findings from individuals with brain disorders, and present novel physiological explanations on the VSEP origin. Methods. We did a systematic review on the papers reporting VSEP findings from individuals with brain disorders and their controls. We evaluated papers published from 2003 to date indexed in PubMed, Web of Science, and Scielo databases. We extracted the following information: number of patients and controls, type of neural disorder, age, gender, stimulating/recording and grounding electrodes as well as stimulus side, intensity, duration, frequency, and polarity. Information about physiological parameters, neurobiological variables, and correlation studies was also reviewed. Representative vignettes were included to add support to our conclusions. Results. The VSEP was studied in 297 patients with neural disorders such as Parkinson’s disease (PD), Alzheimer’s disease, vascular dementia, mild cognitive impairment, subjective memory impairment, major depression, and multiple sclerosis. Scalp responses marked as the VSEP showed high variability, low validity, and poor reproducibility. VSEP latencies and amplitudes did not correlate with disease duration, unified PD rating scale score, or heart function in PD patients nor with cerebrospinal fluid β amyloid, phosphor-τ, and cognitive tests from patients with mental disorders. Vignettes demonstrated that the VSEP was volume conduction propagating from muscles surrounding the scalp recording electrodes. Conclusion. The VSEP is not a brain-evoked potential of neural origin but muscle activity induced by electrical stimulation of the tragus region of the ear. This review and illustrative vignettes argue against assessing the parasympathetic system using the so-called VSEP.

Closed-loop transcutaneous auricular vagus nerve stimulation for the improvement of upper extremity motor function in stroke patients: a study protocol

“…Although the ECG features can be utilized to assess vagal tone activation and the EMG features may reflect motor function progress and the sympathetic tone to some extent, these results may not sufficiently unveil the neural mechanisms underlying the rehabilitative effects of EMG triggered closed-loop taVNS on motor deficits. Therefore, additional electrophysiological assessments such as sympathetic skin responses, motor evoked potentials via transcranial magnetic stimulation (TMS), vagus somatosensory evoked potentials (VSEP) using electroencephalography (EEG) recordings ( 36 , 37 ), or TMS-evoked potentials (TEP) should be considered. Designing studies to explore modifications in spinal ascending and descending tracts and circuits would provide valuable insights into the modulation of motor control and corticospinal motor networks following taVNS.…”