Transcutaneous auricular vagus nerve stimulation in healthy individuals, stroke, and Parkinson’s disease: a narrative review of safety, parameters, and efficacy1.0

1. Introduction: What is taVNS?

Transcutaneous auricular vagus nerve stimulation (taVNS) is a noninvasive technique that stimulates a branch of the vagus nerve in the ear. Signals travel to the brainstem's nucleus tractus solitarius (NTS) and then influence brain regions that control attention, emotions, movement, and autonomic functions like heart rate and digestion. This makes taVNS a potential tool for both healthy individuals and patients with neurological disorders

2. Purpose and Methods

This review looks at taVNS in three groups:

1. Healthy people

2. Stroke patients

3. Parkinson’s disease (PD) patients

We focus on three main aspects:

• Safety

• Stimulation parameters (frequency, intensity, pulse width, on-off cycles)

• Efficacy

A literature search covered PubMed, Google Scholar, Web of Science, Cochrane Library, and Scopus, including 154 studies: 139 on healthy people, 9 on stroke, and 6 on PD.

3. Safety

taVNS is generally safe. Most participants experienced no serious side effects. Mild effects included warmth, tingling, or numbness in the ear, but these were not dangerous. Some studies did not report side effects in detail.

4. Stimulation Parameters

Parameters varied across studies and populations:

• Frequency: mostly 25 Hz

• Pulse width: 200–300 μs

• Intensity: usually below the pain threshold

• On-off pattern: often 30 sec “on” / 30 sec “off”; some PD studies used different cycles or continuous stimulation

• Duration: mostly ≤60 minutes, though healthy participants sometimes received longer or task-specific stimulation

Parameter effects include:

• 2 mA best for pupillary dilation

• 500 μs and 10 Hz most effective for heart rate regulation

• 250 μs, 100 Hz, 3 mA best for pain suppression

• 100 Hz more effective than 25 Hz for cerebellar inhibition

5. Neuromodulatory Effects

taVNS influences three main areas:

1. Brainstem and cortical activity: fMRI and EEG studies show activation of NTS, locus coeruleus, prefrontal cortex, and limbic regions. Attention, motor learning, and emotion regulation are improved.

2. Autonomic nervous system: heart and digestive functions are modulated.

3. Cognition and motor function: taVNS enhances motor execution, attention, and in some studies memory tasks.

Results vary due to differences in stimulation parameters and electrode placement (cymba concha vs tragus).

6. Application in Healthy Individuals

In healthy people, taVNS modulates alpha and beta brainwaves, improving attention, reaction time, and task performance. Combining taVNS with cognitive training or tDCS can further enhance working memory and cognitive performance.

7. Application in Stroke Patients

taVNS may improve motor function and autonomic regulation after stroke. By adjusting cortical excitability and autonomic activity, it can support neurorehabilitation and enhance recovery training effects.

8. Application in Parkinson’s Disease

In PD, taVNS can improve motor control and some cognitive functions, especially when combined with rehabilitation exercises. Variations in stimulation parameters may explain differences in efficacy across studies.

9. Conclusion and Future Directions

taVNS is a safe, noninvasive method that modulates brain function, cognition, motor performance, and autonomic activity in both healthy individuals and patients with neurological disorders. Key challenges include standardizing stimulation parameters and accounting for individual differences. Optimizing these parameters may increase clinical benefits, particularly in neurorehabilitation settings.