High frequency electrical conduction block of the pudendal nerve

Abstract

A reversible electrical block of the pudendal nerves may provide a valuable method for restoration of urinary voiding in individuals with bladder-sphincter dyssynergia. This study quantified the stimulus parameters and effectiveness of high frequency (HFAC) sinusoidal waveforms on the pudendal nerves to produce block of the external urethral sphincter (EUS). A proximal electrode on the pudendal nerve after its exit from the sciatic notch was used to apply low frequency stimuli to evoke EUS contractions. HFAC at frequencies from 1 to 30 kHz with amplitudes from 1 to 10 V were applied through a conforming tripolar nerve cuff electrode implanted distally. Sphincter responses were recorded with a catheter mounted micro-transducer. A fast onset and reversible motor block was obtained over this range of frequencies. The HFAC block showed three phases: a high onset response, often a period of repetitive firing and usually a steady state of complete or partial block. A complete EUS block was obtained in all animals. The block thresholds showed a linear relationship with frequency. HFAC pudendal nerve stimulation effectively produced a quickly reversible block of evoked urethral sphincter contractions. The HFAC pudendal block could be a valuable tool in the rehabilitation of bladder-sphincter dyssynergia.

Figure 1

Schematic of experimental setup with two nerve cuff electrodes on the pudendal nerve and instrumentation for pressure recording from bladder and EUS. Inset shows details of the nerve cuff electrode design used for the HFAC block.

Figure 2

Electrodes on pudendal nerve (bottom) and responses (top) to applied proximal and distal stimulation with the HFAC sinusoidal block. Horizontal arrowhead points to the peak onset response. Vertical arrowhead points to the minimum pressure during block. Lower dotted line: proximal stimulus at 1 Hz applied with cuff electrode PSE on left. Solid line: HFAC applied with cuff electrode BE at center. Upper dotted lines: distal stimulus at 0.5 Hz with hook electrode DSE. The evoked EUS twitches from stimulus at the DSE, during HFAC input, ruled out fatigue of the EUS.

Figure 3

Typical EUS pressure responses to HFAC are shown in the traces on the left, with histograms of block ratios on the right. In each case, 1 = complete (100%) block, 0 = no block. (a) Block ratio without proximal stimulation; (1 – P_b/P_a) 1 – (minimum EUS pressure during block/pre block baseline pressure), n = 70, mean = 1.055 ± 0.410. (b) Block ratio with proximal 1 Hz stimulation; P_d/P_c = ratio (average pre block response–minimum response during block)/average pre block response; n = 131, mean = 0.803 ± 0.517. (c) Block pre block response; with proximal 20 Hz stimulation; P_f/P_e = (average pre block response–minimum response during block)/average pre block response; n = 33, mean = 1.009 ± 0.142.

Figure 4

Threshold voltage and HFAC frequency, with a linear regression fit line ( p < 0.0001) and bivariate normal ellipse ( p 0.950). The individual trials for each of the four test animals are= shown by different markers. The data show an increase in the voltage required to achieve complete HFAC block, with increasing frequency.