Response of external urethral sphincter to high frequency biphasic electrical stimulation of pudendal nerve

Spatiotemporal parameters for energy efficient kilohertz-frequency nerve block with low onset response

BACKGROUND

Electrical nerve conduction block has great potential for treatment of disease through reversible and local inactivation of somatic and autonomic nerves. However, the relatively high energy requirements and the presence of undesired excitation at the onset of the kilohertz-frequency (KHF) signals used for block pose obstacles to effective translation. Frequency, electrode geometry, and waveform shape are known to influence block threshold and onset response, but available data provide a limited understanding of how to select these parameters to optimize nerve block.

METHODS

We evaluated KHF nerve block in rat tibial nerve across frequencies (5-60 kHz), electrode geometries (monopolar, bipolar, and tripolar), and waveform shapes. We present a novel Fourier-based method for constructing composite signals that systematically sample the KHF waveform design space.

RESULTS

The lowest frequencies capable of blocking (5-16 kHz) were not the most energy-efficient among the tested frequencies. Further, bipolar cuffs required the largest current and power to block, monopolar cuffs required the lowest current, and both tripolar and monopolar cuffs required the lowest power. Tripolar cuffs produced the smallest onset response across frequencies. Composite signals comprised of a first harmonic sinusoid at fundamental frequency (f0) superposed on a second harmonic sinusoid at 2f0 could block at lower threshold and lower onset response compared to the constituent sinusoids alone. This effect was strongly dependent on the phase of the second harmonic and on the relative amplitudes of the first and second harmonics. This effect was also dependent on electrode geometry: monopolar and tripolar cuffs showed clear composite signal effects in most experiments; bipolar cuffs showed no clear effects in most experiments.

CONCLUSIONS

Our data provide novel information about block threshold and onset response at the boundary of frequencies that can block. Our results also show an interaction between spatial (cuff geometry) and temporal (frequency and waveform shape) parameters. Finally, while previous studies suggested that temporal parameters could reduce onset response only in exchange for increased block threshold (or vice versa), our results show that waveform shape influences KHF response in ways that can be exploited to reduce both energy and onset responses.

Low pressure voiding induced by stimulation and 1 kHz post-stimulation block of the pudendal nerves in cats

The goal of this study is to induce low-pressure voiding by stimulation and bilateral 1 kHz post-stimulation block of the pudendal nerves. In anesthetized cats, wire hook electrodes were placed on the left and/or right pudendal nerves. Stimulus pulses (30 Hz, 0.2 ms) were applied to one pudendal nerve to induce a reflex bladder contraction and to produce contractions of the external urethral sphincter (EUS). High frequency (1 kHz) biphasic stimulation was applied to block axonal conduction in both pudendal nerves and block EUS activity. In 4 cats, a catheter was inserted into the distal urethra to perfuse and measure the back pressure caused by the EUS contraction. In another 5 cats, a catheter was inserted into the bladder dome and the urethra was left open to allow voiding. The 1 kHz stimulation (30-60 s, 0.5-5 mA) delivered via a wire hook electrode completely blocked pudendal nerve conduction for ≥2 min after terminating the stimulation, i.e., a post-stimulation block. The block gradually disappeared in 6-18 min. The block duration increased with increasing amplitude or duration of the 1 kHz stimulation. Without the 1 kHz block, 30 Hz stimulation alone induced high-pressure (90 cmH₂O) voiding. When combined with the 1 kHz block, the 30 Hz stimulation induced low-pressure (≤50 cmH₂O) voiding with a high voiding efficiency (80%). In summary, a minimally invasive surgical approach might be developed to restore voiding function after spinal cord injury by stimulation and block of the pudendal nerves using lead electrodes.

A general framework for automatic closed-loop control of bladder voiding induced by intraspinal microstimulation in rats

Individuals with spinal cord injury or neurological disorders have problems in voiding function due to the dyssynergic contraction of the urethral sphincter. Here, we introduce a closed-loop control of intraspinal microstimulation (ISMS) for efficient bladder voiding. The strategy is based on asynchronous two-electrode ISMS with combined pulse-amplitude and pulse-frequency modulation without requiring rhizotomy, neurotomy, or high-frequency blocking. Intermittent stimulation is alternately applied to the two electrodes that are implanted in the S2 lateral ventral horn and S1 dorsal gray commissure, to excite the bladder motoneurons and to inhibit the urethral sphincter motoneurons. Asynchronous stimulation would lead to reduce the net electric field and to maximize the selective stimulation. The proposed closed-loop system attains a highly voiding efficiency of 77.2-100%, with an average of 91.28 ± 8.4%. This work represents a promising approach to the development of a natural and robust motor neuroprosthesis device for restoring bladder functions.

Quantitative comparisons of block thresholds and onset responses for charge-balanced kilohertz frequency waveforms

OBJECTIVE

There is growing interest in delivering kilohertz frequency (KHF) electrical signals to block conduction in peripheral nerves for treatment of various diseases. Previous studies used different KHF waveforms to achieve block, and it remains unclear how waveform affects nerve block parameters.

APPROACH

We quantified the effects of waveform on KHF block of the rat tibial nerve in vivo and in computational models. We compared block thresholds and onset responses across current-controlled sinusoids and charge-balanced rectangular waveforms with different asymmetries and duty cycles.

MAIN RESULTS

Sine waves had higher block thresholds than square waves, but used less power at block threshold. Block threshold had an inverse relationship with duty cycle of rectangular waveforms irrespective of waveform asymmetry. Computational model results were consistent with relationships measured in vivo, although the models underestimated the effect of duty cycle on increasing thresholds. The axonal membrane substantially filtered waveforms, the filter transfer function was strikingly similar across waveforms, and filtering resulted in post-filtered rms block thresholds that were approximately constant across waveforms in silico and in vivo. Onset response was not consistently affected by waveform shape, but onset response was smaller at amplitudes well above block threshold. Therefore, waveforms with lower block thresholds (e.g. sine waves or square waves) could be more readily increased to higher amplitudes relative to block threshold to reduce onset response. We also observed a reduction in onset responses across consecutive trials after initial application of supra-block threshold amplitudes.

SIGNIFICANCE

Waveform had substantial effects on block thresholds, and the amplitude relative to block threshold had substantial effects on onset response. These data inform choice of waveform in subsequent studies and clinical applications, enhance effective use of block in therapeutic applications, and facilitate the design of parameters that achieve block with minimal onset responses.

Counted cycles method to measure the block inception time of kiloHertz frequency mammalian motor nerve block

BACKGROUND

Kilohertz frequency alternating currents (KHFAC) produce rapid nerve conduction block of mammalian peripheral nerves and have potential clinical applications in reducing nerve hyperactivity. However, there are no experimental measurements of the block inception time (BIT) for the complete block of mammalian motor axons, i.e. the time from the start of delivery of the KHFAC to the axons reaching a fully blocked state.

NEW METHOD

A "counted cycles" method (CCM) was designed to exploit characteristics of the onset response, which is typical of KHFAC block, to measure the BIT with a millisecond time resolution. Randomized and repeated experiments were conducted in an in-vivo rodent model, using trains of KHFAC over a range of complete cycle counts at three frequencies (10, 20, and 40 kHz).

RESULTS

Complete motor nerve conduction block was obtained in the rat sciatic nerve (N = 4) with an average BIT range of 5 ms-10 ms. The fastest BIT measured was 2.5 ms-5 ms. There was no statistical difference between the block inception times for the three frequencies tested.

COMPARISON WITH EXISTING METHODS

There are no comparable methods to measure the KHFAC BIT.

CONCLUSION

The KHFAC BIT is faster than previously estimated. KHFAC motor nerve block is established in milliseconds. These results may assist in the design of methods to eliminate the onset response produced by KHFAC nerve block.

Measurement of block thresholds in kiloHertz frequency alternating current peripheral nerve block

BACKGROUND

Kilohertz frequency alternating currents (KHFAC) produce rapid nerve conduction block of mammalian peripheral nerve and have potential clinical applications in reducing peripheral nerve hyperactivity. The experimental investigation of KHFAC nerve block requires a robust output measure and this has proven to be the block threshold (BT), the lowest current or voltage at which the axons of interest are completely blocked. All significant literature in KHFAC nerve block, both simulations and experimental, were reviewed to determine the block threshold method that was used. The two common methods used are the High-Low method experimentally and the Binary search method for simulations.

NEW METHOD

Four methods to measure the block threshold (High-Low, High-Low-High, Binary and Random) at three frequencies (10, 20 and 30 kHz) were compared through randomized repeated experiments in the in-vivo rodent sciatic nerve-gastrocnemius model.

RESULTS

The literature review showed that more than 50% of publications did not measure the block threshold. The experimental results showed no statistical difference in the BT value between the four methods.

COMPARISON WITH EXISTING METHOD(S)

However, there were differences in the number of significant onset responses, depending on the method. The run time for the BT determination was the shortest for the High-Low method.

CONCLUSIONS

It is recommended that all research in electrical nerve block, including KHFAC, should include measurement of the BT. The High-Low method is recommended for most experimental situations but the Binary method could also be a viable option, especially where onset responses are minimal.

Effect of high-frequency alternating current transcutaneous stimulation over muscle strength: a controlled pilot study

Background

High-frequency alternating currents of greater than 1 kHz applied on peripheral nerves has been used in animal studies to produce a motor nerve block. It has been evidenced that frequencies higher than 5 kHz are necessary to produce a complete peripheral nerve block in primates, whose nerve thickness is more similar to humans. The aim of the study was to determine the effect on muscle strength after the application of a high-frequency stimulation at 5 and 10 kHz compared to sham stimulation in healthy volunteers.

Findings

Transcutaneous stimulation at 5 kHz, 10 kHz and sham stimulation were applied to eleven healthy volunteers over the ulnar and median nerves for 20 min. Maximal handgrip strength was measured before, during, immediately after the intervention, and 10 min after the end of intervention. The 10 kHz stimulation showed a lower handgrip strength during the intervention (28.1 N, SEM 3.9) when compared to 5 kHz (31.1 N, SEM 3.6; p < 0.001) and to sham stimulation (33.7 N, SEM 3.9; p < 0.001). Furthermore, only stimulation at 10 kHz decreased handgrip strength when compared to baseline.

Conclusions

These findings suggest high-frequency stimulation has an inhibitory effect over muscle strength. Future studies are required in patients that are characterized by motor hyperactive such as spasticity or tremors.

Clinical trial registration

NCT, NCT03169049. Registered on 30 May 2017

Detrusor-External Sphincter Dyssynergia: Review of Minimally Invasive and Endoscopic Management

Detrusor-external sphincter dyssynergia (DSD) is a debilitating problem in patients with spinal cord injury. DSD carries a high risk of complications, and even life expectancy can be affected. Management of this condition includes the use of antimuscarinic agents in combination with intermittent catheterization, indwelling urethral catheterization, suprapubic catheterization, and a variety of surgical options, depending on patient and physician preference. This paper will review the current literature and data on minimally invasive and endoscopic management of DSD.

Electrical stimulation for the treatment of lower urinary tract dysfunction after spinal cord injury

Electrical stimulation for bladder control is an alternative to traditional methods of treating neurogenic lower urinary tract dysfunction (NLUTD) resulting from spinal cord injury (SCI). In this review, we systematically discuss the neurophysiology of bladder dysfunction following SCI and the applications of electrical stimulation for bladder control following SCI, spanning from historic clinical approaches to recent pre-clinical studies that offer promising new strategies that may improve the feasibility and success of electrical stimulation therapy in patients with SCI. Electrical stimulation provides a unique opportunity to control bladder function by exploiting neural control mechanisms. Our understanding of the applications and limitations of electrical stimulation for bladder control has improved due to many pre-clinical studies performed in animals and translational clinical studies. Techniques that have emerged as possible opportunities to control bladder function include pudendal nerve stimulation and novel methods of stimulation, such as high frequency nerve block. Further development of novel applications of electrical stimulation will drive progress towards effective therapy for SCI. The optimal solution for restoration of bladder control may encompass a combination of efficient, targeted electrical stimulation, possibly at multiple locations, and pharmacological treatment to enhance symptom control.

Combined KHFAC + DC nerve block without onset or reduced nerve conductivity after block

OBJECTIVE

Kilohertz frequency alternating current (KHFAC) waveforms have been shown to provide peripheral nerve conductivity block in many acute and chronic animal models. KHFAC nerve block could be used to address multiple disorders caused by neural over-activity, including blocking pain and spasticity. However, one drawback of KHFAC block is a transient activation of nerve fibers during the initiation of the nerve block, called the onset response. The objective of this study is to evaluate the feasibility of using charge balanced direct current (CBDC) waveforms to temporarily block motor nerve conductivity distally to the KHFAC electrodes to mitigate the block onset-response.

APPROACH

A total of eight animals were used in this study. A set of four animals were used to assess feasibility and reproducibility of a combined KHFAC + CBDC block. A following randomized study, conducted on a second set of four animals, compared the onset response resulting from KHFAC alone and combined KHFAC + CBDC waveforms. To quantify the onset, peak forces and the force-time integral were measured during KHFAC block initiation. Nerve conductivity was monitored throughout the study by comparing muscle twitch forces evoked by supra-maximal stimulation proximal and distal to the block electrodes. Each animal of the randomized study received at least 300 s (range: 318-1563 s) of cumulative dc to investigate the impact of combined KHFAC + CBDC on nerve viability.

MAIN RESULTS

The peak onset force was reduced significantly from 20.73 N (range: 18.6-26.5 N) with KHFAC alone to 0.45 N (range: 0.2-0.7 N) with the combined CBDC and KHFAC block waveform (p < 0.001). The area under the force curve was reduced from 6.8 Ns (range: 3.5-21.9 Ns) to 0.54 Ns (range: 0.18-0.86 Ns) (p < 0.01). No change in nerve conductivity was observed after application of the combined KHFAC + CBDC block relative to KHFAC waveforms.

SIGNIFICANCE

The distal application of CBDC can significantly reduce or even completely prevent the KHFAC onset response without a change in nerve conductivity.

Surgical management of functional bladder outlet obstruction in adults with neurogenic bladder dysfunction

BACKGROUND

The most common type of functional bladder outlet obstruction in patients with neurogenic bladder is detrusor-sphincter dyssynergia (DSD). The lack of co-ordination between the bladder and the external urethral sphincter muscle (EUS) in DSD can result in poor bladder emptying and high bladder pressures, which may eventually lead to progressive renal damage.

OBJECTIVES

To assess the effectiveness of different surgical therapies for the treatment of functional bladder outlet obstruction (i.e. DSD) in adults with neurogenic bladder dysfunction.

SEARCH METHODS

We searched the Cochrane Incontinence Group Specialised Register, which contains trials identified from the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, MEDLINE In-Process, and handsearching of journals and conference proceedings (searched 20 February 2014), and the reference lists of relevant articles.

SELECTION CRITERIA

Randomised controlled trials (RCTs) or quasi-RCTs comparing a surgical treatment of DSD in adults suffering from neurogenic bladder dysfunction, with no treatment, placebo, non-surgical treatment, or other surgical treatment, alone or in combination.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed trial quality and extracted data.

MAIN RESULTS

We included five trials (total of 199 participants, average age of 40 years). The neurological diseases causing DSD were traumatic spinal cord injury (SCI), multiple sclerosis (MS), or congenital malformations.One trial compared placement of sphincteric stent prosthesis with sphincterotomy. For urodynamic measurements, results for postvoid residual urine volume (PVR) and cystometric bladder capacity were inconclusive and consistent with benefit of either sphincteric stent prosthesis or sphincterotomy at three, six, 12, and 24 months. Results for maximum detrusor pressure (Pdet.max) were also inconclusive at three, six, and 12 months; however, after two years, the Pdet.max after sphincterotomy was lower than after stent placement (mean difference (MD) -30 cmH2O, 95% confidence interval (CI) 8.99 to 51.01).Four trials considered botulinum A toxin (BTX-A) injection in the EUS, either alone or in combination with other treatments. The comparators included oral baclofen, oral alpha blocker, lidocaine, and placebo. The BTX-A trials all differed in protocols, and therefore we did not undertake meta-analysis. A single 100 units transperineal BTX-A injection (Botox®) in patients with MS resulted in higher voided urine volumes (MD 69 mL, 95% CI 11.87 to 126.13), lower pre-micturition detrusor pressure (MD -10 cmH2O, 95% CI -17.62 to -2.38), and lower Pdet.max (MD -14 cmH2O, 95% CI -25.32 to -2.68) after 30 days, compared to placebo injection. Results for PVR using catheterisation, basal detrusor pressure, maximal bladder capacity, maximal urinary flow, bladder compliance at functional bladder capacity, maximal urethral pressure, and closure urethral pressure at 30 days were inconclusive and consistent with benefit of either BTX-A injection or placebo injections. In participants with SCI, treatment with 200 units of Chinese manufactured BTX-A injected at eight different sites resulted in better bladder compliance (MD 7.5 mL/cmH2O, 95% CI -10.74 to -4.26) than participants who received the same injections with the addition of oral baclofen. Results for maximum uroflow rate, maximal cystometric capacity, and volume per voiding were inconclusive and consistent with benefit of either BTX-A injection or BTX-A injection with the addition of oral baclofen. However, the poor quality of reporting in this trial caused us to question the relevance of bladder compliance as an adequate outcome measure.In participants with DSD due to traumatic SCI, MS, or congenital malformation, the results for PVRs after one day were inconclusive and consistent with benefit of either a single 100 units transperineal BTX-A (Botox®) injection or lidocaine injection. However, after seven and 30 days of BTX-A injection, PVRs were lower (MD -163 and -158 mL, 95% CI -308.65 to -17.35 and 95% CI -277.57 to -39.03, respectively) compared to participants who received lidocaine injections. Results at one month for Pdet.max on voiding, EUS activity in electromyography, and maximal urethral pressure were inconclusive and consistent with benefit of either BTX-A or lidocaine injections.Finally, one small trial consisting of five men with SCI compared weekly BTX-A injections with normal saline as placebo. The placebo had no effect on DSD in the two participants allocated to the placebo treatment. Their urodynamic parameters were unchanged from baseline values until subsequent injections with BTX-A once a week for three weeks. These subsequent injections resulted in similar responses to those of the three participants who were allocated to the BTX-A treatment. Unfortunately, the report presented no data on placebo treatment.Only the trial that compared sphincterotomy with stent placement reported outcome measures renal function and urologic complications related to DSD. Results for renal function at 12 and 24 months, and urologic complications related to DSD at three, six, 12, and 24 months were inconclusive and consistent with benefit of either sphincteric stent prosthesis or sphincterotomy.Adverse effects reported were haematuria due to the cystoscopic injection and muscle weakness, of which the latter may be related to the BTX-A dose used.All trials had some methodological shortcomings, so insufficient information was available to permit judgement of risk of bias. At least half of the trials had an unclear risk of selection bias and reporting bias. One trial had a high risk of attrition bias, and another trial had a high risk of reporting bias.

AUTHORS' CONCLUSIONS

Results from small studies with a high risk of bias have identified evidence of limited quality that intraurethral BTX-A injections improve some urodynamic measures after 30 days in the treatment of functional bladder outlet obstruction in adults with neurogenic bladder dysfunction. The necessity of reinjection of BTX-A is a significant drawback; a sphincterotomy might therefore be a more effective treatment option for lowering bladder pressure in the long-term.However, because of the limited availability of eligible trials, this review was unable to provide robust evidence in favour of any of the surgical treatment options. More RCTs are needed, measuring improvement on quality of life, and on other types of surgical treatment options for DSD since these are lacking. Future RCTs assessing the effectiveness of BTX-A injections also need to address the uncertainty about the optimal dose and mode of injection for this specific type of urological condition.

Pudendal nerve stimulation and block by a wireless-controlled implantable stimulator in cats

OBJECTIVE

The study aims to determine the functionality of a wireless-controlled implantable stimulator designed for stimulation and block of the pudendal nerve.

MATERIALS AND METHODS

In five cats under α-chloralose anesthesia, the stimulator was implanted underneath the skin on the left side in the lower back along the sacral spine. Two tripolar cuff electrodes were implanted bilaterally on the pudendal nerves in addition to one bipolar cuff electrode that was implanted on the left side central to the tripolar cuff electrode. The stimulator provided high-frequency (5-20 kHz) biphasic stimulation waveforms to the two tripolar electrodes and low-frequency (1-100 Hz) rectangular pulses to the bipolar electrode. Bladder and urethral pressures were measured to determine the effects of pudendal nerve stimulation (PNS) or block.

RESULTS

The maximal (70-100 cmH2O) urethral pressure generated by 20-Hz PNS applied via the bipolar electrode was completely eliminated by the pudendal nerve block induced by the high-frequency stimulation (6-15 kHz, 6-10 V) applied via the two tripolar electrodes. In a partially filled bladder, 20-30 Hz PNS (2-8 V, 0.2 ms) but not 5 Hz stimulation applied via the bipolar electrode elicited a large sustained bladder contraction (45.9 ± 13.4 to 52.0 ± 22 cmH2O). During cystometry, the 5 Hz PNS significantly (p < 0.05) increased bladder capacity to 176.5 ± 27.1% of control capacity.

CONCLUSIONS

The wireless-controlled implantable stimulator successfully generated the required waveforms for stimulation and block of pudendal nerve, which will be useful for restoring bladder functions after spinal cord injury.

Post stimulus effects of high frequency biphasic electrical current on a fibre's conductibility in isolated frog nerves

OBJECTIVE

High frequency biphasic (HFB) electrical currents are widely used in nerve blocking studies. Their safety margins largely remain unknown and need to be investigated.

APPROACH

This study, exploring the post stimulus effects of HFB electrical currents on a nerve's conductibility, was performed on bullfrog sciatic nerves. Both compound action potentials (CAPs) and differential CAPs (DCAPs, i.e. control CAPs subtracted by CAPs following HFB currents) were obtained, and N1 and N2 components, which were the first and second upward components of DCAPs, were used for analyses of the effects introduced by HFB electrical stimulation.

MAIN RESULTS

First, HFB currents of 10 kHz at a completely blocking threshold were applied for 5 s. The maximum amplitudes and conducting velocities of the CAPs were significantly (P < 0.02) decreased within the observed period (60 s) following HFB currents. The DCAPs displayed clear N1 and N2 components, demonstrating respectively the losses of the fibres' normal conductibility and the appearances of new delayed conductions. Decreases of N1 amplitudes along time, regarded as the recovery of the nerve's conductibility, exhibited two distinct phases: a fast one lasting several seconds and a slow one lasting longer than 5 min. Further tests showed a linear relationship between the HFB stimulation durations and recovering periods of N1 amplitudes. Supra-threshold blocking did not cause higher N1 amplitudes.

SIGNIFICANCE

This study indicates that HFB electrical currents lead to long lasting post stimulus reduction of a nerve's conductibility, which might relate to potential nerve injuries. A possible mechanism, focusing on changes in intracellular and periaxonal ionic concentrations, was proposed to underlie the reduction of the nerve's conductibility and potential nerve injuries. Greater caution and stimulation protocols with greater safety margins should be explored when utilizing HFB electrical current to block nerve conductions.

Effects of high-frequency alternating current on axonal conduction through the vagus nerve

High-frequency alternating current (HFAC) is known to disrupt axonal conduction in peripheral nerves, and HFAC has much potential as a therapeutic approach for a number of pathological conditions. Many previous studies have utilized motor output as a bioassay of effects of HFAC on conduction through medium- to large-diameter motor axons. However, little is known about the effectiveness of HFAC on smaller, more slowly conducting nerve fibres. The present study tested whether HFAC influences axonal conduction through sub-diaphragmatic levels of the rat vagus nerve, which consists almost entirely of small calibre axons. Using an isolated nerve preparation, we tested the effects of HFAC on electrically evoked compound action potentials (CAPs). We found that delivery of charge-balanced HFAC at 5000 Hz for 1 min was effective in producing reversible blockade of axonal conduction. Both Aδ and C components of the vagus CAP were attenuated, and the degree of blockade as well as time to recovery was proportional to the amount of HFAC current delivered. The Aδ waves were more sensitive than C waves to HFAC blockade, but they required more time to recover.

High-frequency stimulation selectively blocks different types of fibers in frog sciatic nerve

Conduction block using high-frequency alternating current (HFAC) stimulation has been shown to reversibly block conduction through various nerves. However, unlike simulations and experiments on myelinated fibers, prior experimental work in our lab on the sea-slug, Aplysia, found a nonmonotonic relationship between frequency and blocking thresholds in the unmyelinated fibers. To resolve this discrepancy, we investigated the effect of HFAC waveforms on the compound action potential of the sciatic nerve of frogs. Maximal stimulation of the nerve produces a compound action potential consisting of the A-fiber and C-fiber components corresponding to the myelinated and unmyelinated fibers' response. In our study, HFAC waveforms were found to induce reversible block in the A-fibers and C-fibers for frequencies in the range of 5-50 kHz and for amplitudes from 0.1-1 mA. Although the A-fibers demonstrated the monotonically increasing threshold behavior observed in published literature, the C-fibers displayed a nonmonotonic relationship, analogous to that observed in the unmyelinated fibers of Aplysia. This differential blocking behavior observed in myelinated and unmyelinated fibers during application of HFAC waveforms has diverse implications for the fields of selective stimulation and pain management.

Mechanism of conduction block in amphibian myelinated axon induced by biphasic electrical current at ultra-high frequency

The mechanism of axonal conduction block induced by ultra-high frequency (≥ 20 kHz) biphasic electrical current was investigated using a lumped circuit model of the amphibian myelinated axon based on Frankenhaeuser-Huxley (FH) equations. The ultra-high frequency stimulation produces constant activation of both sodium and potassium channels at the axonal node under the block electrode causing the axonal conduction block. This blocking mechanism is different from the mechanism when the stimulation frequency is between 4 kHz and 10 kHz, where only the potassium channel is constantly activated. The minimal stimulation intensity required to induce a conduction block increases as the stimulation frequency increases. The results from this simulation study are useful to guide future animal experiments to reveal the different mechanisms underlying nerve conduction block induced by high-frequency biphasic electrical current.

The role of slow potassium current in nerve conduction block induced by high-frequency biphasic electrical current

The role of slow potassium current in nerve conduction block induced by high-frequency biphasic electrical current was analyzed using a lumped circuit model of a myelinated axon based on the Schwarz-Reid-Bostock model. The results indicate that nerve conduction block at stimulation frequencies above 4 kHz is due to constant activation of both fast and slow potassium channels, but the block at stimulation frequencies below 4 kHz could be due to either anodal or cathodal dc block depending on the time of the action potential arriving at the block electrode. When stimulation frequency was above 4 kHz, the slow potassium current was about 3.5 to 6.5 times greater than the fast potassium current at blocking threshold, indicating that the slow potassium current played a more dominant role than the fast potassium current. The blocking location moved from the node under the blocking electrode to a nearby node as the stimulation intensity increased. This simulation study reveals that in mammalian myelinated axons, the slow potassium current probably plays a critical role in the nerve conduction block induced by high-frequency biphasic electrical current.

Modulation of axonal excitability by high-frequency biphasic electrical current

The modulation of axonal excitability by high-frequency biphasic (HFB) electrical current was analyzed using a lumped-circuit model of the myelinated axon based on Schwarz-Reid-Bostock (SRB) equations. The results show that axonal excitability could be either increased or decreased by HFB current depending on the current intensity. The increase of axonal excitability is due to the high level of sodium channel activation, whereas the activation of both fast and slow potassium channels plays an important role in decreasing axonal excitability. As the HFB current intensity increases, the location determining the axonal excitability changes from the nodes under the electrode within the "main lobe" region of the activating function to the nodes away from the electrode in the "side lobe" region of the activating function. This simulation study also shows that the modulation of axonal excitability by HFB electrical current could be potentially useful to selectively activate the small nerve fibers in a compound nerve trunk without activating the large fibers. Understanding how HFB electrical current modulates the axonal excitability will further elucidate the possible mechanisms underlying the nerve conduction block induced by HFB electrical current.

Transcutaneously coupled, high-frequency electrical stimulation of the pudendal nerve blocks external urethral sphincter contractions

BACKGROUND

Detrusor-sphincter dyssynergia is a condition in which reflexive contractions of the external urethral sphincter occur during bladder contractions, preventing the expulsion of urine. High-frequency stimulation (kHz range) has been shown to elicit a fast-acting and reversible block of action potential propagation in peripheral nerves, which may be a useful technique in the management of this condition.

OBJECTIVE

The aim of these experiments was to see if a newly developed stimulus delivery system, capable of transmitting current transcutaneously to remote peripheral nerves using a passive implanted conductor, was an effective way to transmit high-frequency waveforms to the pudendal nerve to block ongoing sphincter contractions.

METHODS

High-frequency waveforms were delivered through the skin to the pudendal nerve using a passive implanted conductor in 6 adult cats anesthetized with isoflurane. Five of the experiments were acute, terminal procedures, and the remaining cat was implanted with a permanent electrode system allowing evaluation for 6 months. Typical stimulation parameters were in the range of 1 to 10 kHz and 1 to 10 mA.

RESULTS

Complete blocking of external urethral sphincter contractions was achieved in 5 of the 6 animals. High-frequency stimulation was also tested in the chronically implanted animal without anesthesia, and the stimulation was tolerated with minimal aversive reactions.

CONCLUSIONS

The transcutaneous passive implanted conductor stimulus delivery system is an effective way to stimulate the pudendal nerve at high frequency, leading to sphincter relaxation. This system may provide a simple means to implement this stimulation paradigm in people with detrusor-sphincter dyssynergia.

Relationship between temperature and stimulation frequency in conduction block of amphibian myelinated axon

The temperature-frequency relationship in nerve conduction block induced by high-frequency, biphasic electrical current was investigated by computer simulation using an amphibian myelinated axon model based on Frankenhaeuser-Huxley (FH) equations. For an axon of diameter 10 microm, the minimal blocking frequency was changed from 6 to 3 kHz as the temperature was decreased from 37 degrees C to 15 degrees C. The maximal blocking temperature below which the axon could be blocked was increased from 22 degrees C to 37 degrees C as the stimulation frequency was increased from 4 to 8 kHz. The maximal blocking temperature was not influenced by axon diameter. Simulation analysis also revealed that activation of potassium channels might determine the temperature-frequency relationship. This study indicates that temperature might be one of the factors that cause the frequency discrepancy as reported in previous animal studies.

Influence of temperature on pudendal nerve block induced by high frequency biphasic electrical current

PURPOSE

We determined the influence of temperature on the minimal stimulation frequency required to block pudendal nerve conduction.

MATERIALS AND METHODS

The pudendal nerve block induced by high frequency, biphasic electrical current was investigated at different temperatures using cats under alpha-chloralose anesthesia. Urethral pressure was measured to indicate pudendal nerve activation or block.

RESULTS

As stimulation frequency was increased above a frequency threshold, the urethral pressure response was decreased and the pudendal nerve was blocked. The minimal stimulation frequency required to block the pudendal nerve was decreased from 6 to 4 kHz as the temperature was decreased from 37C to 15C. At a 4 kHz frequency the maximal temperature below which the pudendal nerve could be blocked was 24.5C.

CONCLUSIONS

To block pudendal nerve conduction at body temperature (37C) the stimulation frequency must be greater than 6 kHz. This study provides a practical guide for blocking the pudendal nerves to restore efficient voiding after spinal cord injury.

Voiding reflex in chronic spinal cord injured cats induced by stimulating and blocking pudendal nerves

AIMS

To induce efficient voiding in chronic spinal cord injured (SCI) cats.

METHODS

Voiding reflexes induced by bladder distension or by electrical stimulation and block of pudendal nerves were investigated in chronic SCI cats under alpha-chloralose anesthesia.

RESULTS

The voiding efficiency in chronic SCI cats induced by bladder distension was very poor compared to that in spinal intact cats (7.3 +/- 0.9% vs. 93.6 +/- 2.0%, P < 0.05). In chronic SCI cats continuous stimulation of the pudendal nerve on one side at 20 Hz induced large amplitude bladder contractions, but failed to induce voiding. However, continuous pudendal nerve stimulation (20 Hz) combined with high-frequency (10 kHz) distal blockade of the ipsilateral pudendal nerve elicited efficient (73.2 +/- 10.7%) voiding. Blocking the pudendal nerves bilaterally produced voiding efficiency (82.5 +/- 4.8%) comparable to the efficiency during voidings induced by bladder distension in spinal intact cats, indicating that the external urethral sphincter (EUS) contraction was caused not only by direct activation of the pudendal efferent fibers, but also by spinal reflex activation of the EUS through the contralateral pudendal nerve. The maximal bladder pressure and average flow rate induced by stimulation and bilateral pudendal nerve block in chronic SCI cats were also comparable to those in spinal intact cats.

CONCLUSIONS

This study shows that after the spinal cord is chronically isolated from the pontine micturition center, bladder distension evokes a transient, inefficient voiding reflex, whereas stimulation of somatic afferent fibers evokes a strong, long duration, spinal bladder reflex that elicits efficient voiding when combined with blockade of somatic efferent fibers in the pudendal nerves.

Influence of frequency and temperature on the mechanisms of nerve conduction block induced by high-frequency biphasic electrical current

The influences of stimulation frequency and temperature on mechanisms of nerve conduction block induced by high-frequency biphasic electrical current were investigated using a lumped circuit model of the myelinated axon based on Schwarz and Eikhof (SE) equations. The simulation analysis showed that a temperature-frequency relationship was determined by the axonal membrane dynamics (i.e. how fast the ion channels can open or close.). At a certain temperature, the axonal conduction block always occurred when the period of biphasic stimulation was smaller than the action potential duration (APD). When the temperature decreased from 37 to 15 degrees C, the membrane dynamics slowed down resulting in an APD increase from 0.4 to 2.4 ms accompanied by a decrease in the minimal blocking frequency from 4 to 0.5 kHz. The simulation results also indicated that as the stimulation frequency increased the mechanism of conduction block changed from a cathodal/anodal block to a block dependent upon continuous activation of potassium channels. Understanding the interaction between the minimal blocking frequency and temperature could promote a better understanding of the mechanisms of high frequency induced axonal conduction block and the clinical application of this method for blocking nerve conduction.

Bladder inhibition or voiding induced by pudendal nerve stimulation in chronic spinal cord injured cats

AIMS

To investigate pudendal-to-bladder spinal reflexes in chronic spinal cord injured (SCI) cats induced by electrical stimulation of the pudendal nerve.

METHODS

Bladder inhibition or voiding induced by pudendal nerve stimulation at different frequencies (3 or 20 Hz) was studied in three female, chronic SCI cats under alpha-chloralose anesthesia.

RESULTS

Voiding induced by a slow infusion (2-4 ml/min) of saline into the bladder was very inefficient (voiding efficiency=7.3%+/-0.9%). Pudendal nerve stimulation at 3 Hz applied during the slow infusion inhibited reflex bladder activity, and significantly increased bladder capacity to 147.2+/-6.1% of its control capacity. When the 3-Hz stimulation was terminated, voiding rapidly occurred and the voiding efficiency was increased to 25.4+/-6.1%, but residual bladder volume was not reduced. Pudendal nerve stimulation at 20 Hz induced large bladder contractions, but failed to induce voiding during the stimulation due to the direct activation of the motor pathway to the external urethral sphincter. However, intermittent pudendal nerve stimulation at 20 Hz induced post-stimulus voiding with 78.3+/-12.1% voiding efficiency. The voiding pressures (39.3+/-6.2 cmH2O) induced by the intermittent pudendal nerve stimulation were higher than the voiding pressures (23.1+/-1.7 cmH2O) induced by bladder distension. The flow rate during post-stimulus voiding induced by the intermittent pudendal nerve stimulation was significantly higher (0.93+/-0.04 ml/sec) than during voiding induced by bladder distension (0.23+/-0.07 ml/sec).

CONCLUSIONS

This study indicates that a neural prosthetic device based on pudendal nerve stimulation might be developed to restore micturition function for people with SCI.

Simulation of high-frequency sinusoidal electrical block of mammalian myelinated axons

High frequency alternating current (HFAC) sinusoidal waveforms can block conduction in mammalian peripheral nerves. A mammalian axon model was used to simulate the response of nerves to HFAC conduction block. Sinusoidal waveforms from 1 to 40 kHz were delivered to eight simulated axon diameters ranging from 7.3 to 16 microm. Conduction block was obtained between 3 to 40 kHz. The minimum peak to peak current at which block was obtained, defined as the block threshold, increased with increasing frequency. Block threshold varied inversely with axon diameter. Upon initiation, the HFAC waveform produced one or more action potentials. These simulation results closely parallel previous experimental results of high frequency motor block of the rat sciatic and cat pudendal nerve. During HFAC block, the axons showed a dynamic steady state depolarization of multiple nodes, strongly suggesting a depolarization mechanism for HFAC conduction block.

Simulation analysis of conduction block in myelinated axons induced by high-frequency biphasic rectangular pulses

Nerve conduction block induced by high-frequency biphasic rectangular pulses was analyzed using a lumped circuit model of the myelinated axon based on Frankenhaeuser-Huxley (FH) equations. At the temperature of 37 degrees C, axons of different diameters (2-20 microm) can be blocked completely at supra-threshold intensities when the stimulation frequency is above 10 kHz. However, at stimulation frequencies between 6 kHz and 9 kHz, both nerve block and repetitive firing of action potentials can be observed at different stimulation intensities. When the stimulation frequency is below 6 kHz, nerve block does not occur regardless of stimulation intensity. Larger diameter axons have a lower threshold intensity to induce conduction block. When temperature is reduced from 37 degrees C to 20 degrees C, the lowest frequency to completely block large axons (diameters 10-20 microm) decreased from 8 kHz to 4 kHz. This simulation study can guide future animal experiments as well as optimize stimulation waveforms for electrical nerve block in clinical applications.

High frequency electrical conduction block of the pudendal nerve

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.

Pudendal-to-bladder reflex in chronic spinal-cord-injured cats

The effects of pudendal nerve stimulation on reflex bladder activity were investigated in cats with chronic spinal cord injury (6-12 months) under alpha-chloralose anesthesia. Electrical stimulation of the pudendal nerve on one side at different frequencies and intensities induced either inhibitory or excitatory effects on bladder activity. The inhibitory effect peaked at a stimulation frequency of 3 Hz and gradually decreased at lower or higher frequencies. The inhibitory effect could occur at stimulation intensities between 0.3 and 1 V (pulse width 0.1 ms) and increased at intensities up to 10 V. Stimulation of the central end of transected pudendal nerve also inhibited bladder activity, indicating that afferent axons in pudendal nerve are involved. Nerve transections also showed that both hypogastric and pelvic nerves might be involved in the inhibitory pudendal-to-bladder spinal reflex. Pudendal nerve stimulation at 20 Hz and at the same intensities (1-10 V) elicited a bladder excitatory response. Although this excitatory effect could not sustain a long lasting bladder contraction at small bladder volumes, it did induce continuous rhythmic bladder contractions at large bladder volumes. This study indicated the possibility of developing a neuroprosthetic device based on pudendal nerve electrical stimulation to restore micturition function after spinal cord injury.