Nerve Fiber Activation During Peripheral Nerve Field Stimulation: Importance of Electrode Orientation and Estimation of Area of Paresthesia

Introduction and Aim: Low back pain is one of the indications for using peripheral nerve field stimulation (PNFS). However, the effect of PNFS varies between patients; several stimulation parameters have not been investigated in depth, such as orientation of the nerve fiber in relation to the electrode. While placing the electrode parallel to the nerve fiber may give lower activation thresholds, anodal blocking may occur when the propagating action potential passes an anode. Methods: A finite element model was used to simulate the extracellular potential during PNFS. This was combined with an active cable model of Aa and Ad nerve fibers. Itwas investigated howthe angle between the nerve fiber and electrode affected the nerve activation and whether anodal blocking could occur. Finally, the area of paresthesia was estimated and compared with any concomitant Ad fiber activation. Results: The lowest threshold was found when nerve and electrode were in parallel, and that anodal blocking did not appear to occur during PNFS. The activation of Aa fibers was within therapeutic range (< 10V) of PNFS; however, within this range, Ad fiber activation also may occur. The combined area of activated Aa fibers (paresthesia) was at least two times larger than Ad fibers for similar stimulation intensities. Conclusion: No evidence of anodal blocking was observed in this PNFS model. The thresholds were lowest when the nerves and electrodes were parallel; thus, it may be relevant to investigate the overall position of the target nerve fibers prior to electrode placement.