Application of a computational model of vagus nerve stimulation

Abstract

Objectives -The most widely used and studied neurostimulation procedure for medically refractory epilepsy is vagus nerve stimulation (VNS) Therapy. The goal of this study was to develop a computational model for improved understanding of the anatomy and neurophysiology of the vagus nerve as it pertains to the principles of electrical stimulation, aiming to provide clinicians with a systematic and rational understanding of VNS Therapy. Materials and methods -Computational modeling allows the study of electrical stimulation of peripheral nerves. We used finite element electric field models of the vagus nerve with VNS Therapy electrodes to calculate the voltage field for several output currents and studied the effects of two programmable parameters (output current and pulse width) on optimal fiber activation. Results -The mathematical models correlated well with strength-duration curves constructed from actual patient data. In addition, digital constructs of chronic versus acute implant models demonstrated that at a given pulse width and current combination, presence of a 110-lm fibrotic tissue can decrease fiber activation by 50%. Based on our findings, a range of output current settings between 0.75 and 1.75 mA with pulse width settings of 250 or 500 ls may result in optimal stimulation. Conclusions -The modeling illustrates how to achieve full or nearly full activation of the myelinated fibers of the vagus nerve through output current and pulse width settings. This knowledge will enable clinicians to apply these principles for optimal vagus nerve activation and proceed to adjust duty cycle and frequency to achieve effectiveness.

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“…Furthermore, we are still in need for early clues to refine stimulation parameters without having to wait for changes in body weight. It can be speculated that brain-imaging methods together with computational model of VNS (Helmers et al, 2012) might be of significant help towards this clinical requirement.…”

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“…Consequently, higher stimulation may be required for the activation of the nerve fibers present in the area not covered by the electrode. In contrast, fibers located near the perineurium of a fascicle are exposed to a stronger electric field (Helmers et al, 2012). In addition, the large variation in epineurial connective tissue might influence the effectiveness of VNS (Helmers et al, 2012).…”

“…In contrast, fibers located near the perineurium of a fascicle are exposed to a stronger electric field (Helmers et al, 2012). In addition, the large variation in epineurial connective tissue might influence the effectiveness of VNS (Helmers et al, 2012). Verlinden et al (Verlinden et al, 2016) have shown that the right cervical VN has a 1.5 times larger effective surface area than the left nerve and that there is a broad spreading within the individual nerves.…”

“…The battery lasts between 7 and 10 years depending on the frequency of stimulation and intensity of the current. Battery life is correlated with charge, the lower pulse widths (250 and 130 μs) and their respective threshold currents conserve battery life more effectively than the longer pulse widths (500, 750, or 1000 μs) (Helmers et al, 2012). Battery demand is also affected by duty cycle and signal frequency (increases in both of these parameters will negatively affect battery longevity) (Helmers et al, 2012).…”

“…Achieving full activation of the VN requires selecting the right combination of VNS therapy parameter settings. Based on a computational model, Helmers et al (Helmers et al, 2012) have shown that a range of output current settings between 0.75 and 1.75 mA with pulse width settings of 250 or 500 μs may result in optimal stimulation. The spiral electrode wrapped around the cervical left VN does not fully encircle the VN, but wraps approximately 270 degrees around it.…”