Intraoperative Laryngeal Muscle and Heart Rate Responses to Implanted Vagus Nerve Stimulation

Area of Science:

• Neurology
• Neuroscience
• Biomedical Engineering

Background:

• Implanted cervical vagus nerve stimulation (VNS) treats epilepsy, depression, stroke, and rheumatoid arthritis.
• Understanding neural element recruitment by VNS is crucial for elucidating therapeutic mechanisms.
• Clinical trials for VNS have shown variable response rates, necessitating further investigation into stimulation parameters and targeted pathways.

Purpose of the Study:

• To quantify VNS-evoked laryngeal muscle activation (electromyogram [EMG]) and heart rate (HR) changes.
• To compare VNS thresholds for laryngeal EMG and HR changes.
• To validate noninvasive laryngeal EMG measurements in participants undergoing VNS implantation.

Main Methods:

• Recruited adult participants with treatment-resistant epilepsy undergoing VNS implant or generator replacement.
• Delivered VNS across varying pulse widths (50-1000 μs) and amplitudes (0.05-28 mA).
• Recorded laryngeal EMG and HR responses during VNS delivery.

Main Results:

• Median stimulation amplitudes for 50% maximal laryngeal EMG response were lower at longer pulse widths (0.34-1.32 mA).
• Median stimulation amplitudes for 10% HR decrease were significantly higher (3.53-13.39 mA), approximately 6-63 times greater than EMG thresholds.
• Clinician-selected stimulation amplitudes in chronic implants were above EMG thresholds but below HR-affecting thresholds.

Conclusions:

• Bradycardia thresholds were approximately two times higher than clinician-selected VNS amplitudes.
• This suggests VNS may target larger diameter fibers for epilepsy compared to those influencing heart rate.
• Therapeutic effects in epilepsy might be achieved with less fiber activation than required to induce bradycardia.