Interventional neurophysiology of the sacral nervous system

Intraoperative Monitoring of the External Urethral Sphincter Reflex: A Novel Adjunct to Bulbocavernosus Reflex Neuromonitoring for Protecting the Sacral Neural Pathways Responsible for Urination, Defecation and Sexual Function

PURPOSE

Intraoperative bulbocavernosus reflex neuromonitoring has been utilized to protect bowel, bladder, and sexual function, providing a continuous functional assessment of the somatic sacral nervous system during surgeries where it is at risk. Bulbocavernosus reflex data may also provide additional functional insight, including an evaluation for spinal shock, distinguishing upper versus lower motor neuron injury (conus vs. cauda syndromes) and prognosis for postoperative bowel and bladder function. Continuous intraoperative bulbocavernosus reflex monitoring has been utilized to provide the surgeon with an ongoing functional assessment of the anatomical elements involved in the S2-S4 mediated reflex arc including the conus, cauda equina and pudendal nerves. Intraoperative bulbocavernosus reflex monitoring typically includes the electrical activation of the dorsal nerves of the genitals to initiate the afferent component of the reflex, followed by recording the resulting muscle response using needle electromyography recordings from the external anal sphincter.

METHODS

Herein we describe a complementary and novel technique that includes recording electromyography responses from the external urethral sphincter to monitor the external urethral sphincter reflex. Specialized foley catheters embedded with recording electrodes have recently become commercially available that provide the ability to perform intraoperative external urethral sphincter muscle recordings.

RESULTS

We describe technical details and the potential utility of incorporating external urethral sphincter reflex recordings into existing sacral neuromonitoring paradigms to provide redundant yet complementary data streams.

CONCLUSIONS

We present two illustrative neurosurgical oncology cases to demonstrate the utility of the external urethral sphincter reflex technique in the setting of the necessary surgical sacrifice of sacral nerve roots.

The diagnostic accuracy of neuromonitoring for detecting postoperative bowel and bladder dysfunction in spinal oncology surgery: a case series

PURPOSE

Postoperative bowel and bladder dysfunction (BBD) poses a significant risk following surgery of the sacral spinal segments and sacral nerve roots, particularly in neuro-oncology cases. The need for more reliable neuromonitoring techniques to enhance the safety of spine surgery is evident.

METHODS

We conducted a case series comprising 60 procedures involving 56 patients, spanning from September 2022 to January 2024. We assessed the diagnostic accuracy of sacral reflexes (bulbocavernosus and external urethral sphincter reflexes) and compared them with transcranial motor evoked potentials (TCMEP) incorporating anal sphincter (AS) and external urethral sphincter (EUS) recordings, as well as spontaneous electromyography (s-EMG) with AS and EUS recordings.

RESULTS

Sacral reflexes demonstrated a specificity of 100% in predicting postoperative BBD, with a sensitivity of 73.33%. While sensitivity slightly decreased to 64.71% at the 1-month follow-up, it remained consistently high overall. TCMEP with AS/EUS recordings did not identify any instances of postoperative BBD, whereas s-EMG with AS/EUS recordings showed a sensitivity of 14.29% and a specificity of 97.14%.

CONCLUSION

Sacral reflex monitoring emerges as a robust adjunct to routine neuromonitoring, offering surgeons valuable predictive insights to potentially mitigate the occurrence of postoperative BBD.

The science behind programming algorithms for sacral neuromodulation

AIM

Sacral neuromodulation (SNM) is a widely adopted treatment for overactive bladder, non-obstructive urinary retention and faecal incontinence. In the majority, it provides sustained clinical benefit. However, it is recognized that, even for these patients, stimulation parameters (such as amplitude, electrode configuration, frequency and pulse width) may vary at both initial device programming and at reprogramming, the latter often being required to optimize effectiveness. Although some recommendations exist for SNM programming, the scientific data to support them are understood by few clinicians.

METHODS

This is a narrative review of the literature covering some of the science behind stimulating a mixed peripheral nerve and available preclinical data in the field of SNM. It covers electrode configuration, amplitude, frequency, pulse width and cycling considerations. The review is targeted at clinicians with an interest in the field and does not seek to provide exhaustive detail on basic neuroscience.

RESULTS AND CONCLUSIONS

Knowledge of the science of neuromodulation provides some guiding principles for programming but these are broad. These principles are not refuted by preclinical data but specific parameters in clinical use are not strongly supported by animal data, even after the limitations of small and large animal models are considered. The review presents a shortlist of programming principles on a theoretical basis but acknowledges that current practice is as much derived from evolved experience as science.

Pelvic Floor Muscle Electromyography as a Guiding Tool During Lead Placement and (Re)Programming in Sacral Neuromodulation Patients: Validity, Reliability, and Feasibility of the Technique

PURPOSE

To assess the validity, reliability, and feasibility of electromyography (EMG) as a tool to measure pelvic floor muscle (PFM) contractions during placement and (re)programming of the tined lead electrodes in sacral neuromodulation (SNM) patients.

MATERIALS AND METHODS

Single tertiary center, prospective study conducted between 2017 and 2019 consisting of three protocols including a total of 75 patients with overactive bladder (wet/dry) or nonobstructive urinary retention. PFM EMG was recorded using the multiple array probe (MAPLe), placed intravaginally. All stimulations (monophasic pulsed square wave, 210 μsec, 14 Hz) were performed using Medtronic's standard SNM stimulation equipment. During lead implantation, all four lead electrodes were stimulated with fixed increasing stimulation intensities (1-2-3-5-7-10 V). During lead electrode (re)programming, five bipolar lead electrode configurations were stimulated twice up to when an electrical PFM motor response (EPFMR), sensory response, and pain response were noted (i.e., the threshold), respectively. Additionally, amplitude and latency of the EPFMRs were determined. Validity, reliability, and feasibility were statistically analyzed using the intraclass correlation coefficient, weighted Cohen's kappa and linear regression, respectively.

RESULTS

Validity: EPFMRs were strongly associated with visually detected PFM motor responses (κ = 0.90). Reliability: EPFMR amplitude (ICC = 0.99) and latency (ICC = 0.93) showed excellent repeatability. Feasibility: linear regression (EPFMR threshold = 0.18 mA + 0.76 * sensory response threshold) showed an increase in the sensory response threshold is associated with a smaller increase in EPFMR threshold, with the EPFMR occurring before or on the sensory response threshold in 83.8% of all stimulations.

CONCLUSIONS

Measuring PFM contractions with EMG during placement and (re)programming of lead electrodes in SNM patients is valid, reliable, and feasible. Therefore, the use of PFM EMG motor responses could be considered as a tool to assist in these procedures.

Neural pathway of bellows response during SNM treatment revisited: Conclusive evidence for direct efferent motor response

BACKGROUND

In sacral neuromodulation (SNM) patients, it is thought the bellows response elicited upon sacral spinal nerve stimulation is reflex-mediated. Therefore the mechanism of action of SNM is considered to be at the spinal or supraspinal level. These ideas need to be challenged.

OBJECTIVE

To identify the neural pathway of the bellows response upon sacral spinal nerve stimulation.

DESIGN, SETTING, AND PARTICIPANTS

Single tertiary center, prospective study (December 2017-June 2019) including 29 patients with overactive bladder refractory to first-line treatment.

INTERVENTION

Recording of the pelvic floor muscle response (PFMR) using a camcorder and electromyography (EMG) (intravaginal probe and concentric needles) upon increasing stimulation during lead or implantable pulse generator placement.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

The lowest stimulation intensity needed to elicit a visual PFMR and electrical PFMR was determined. Electrical PFMRs were subdivided according to their latency.

OUTCOME

the association between visual and electrical PFMRs. Statistical analyses were performed using the weighted kappa coefficient.

RESULTS

Three different electrical PFMRs could be identified by surface and needle EMG, corresponding with a direct efferent motor response (R1), oligosynaptic (R2), and polysynaptic (R3) afferent reflex response. Only the R1 electrical PFMR was perfectly associated with the visual PFMR (κ = 0.900).

CONCLUSIONS

The visual PFMRs upon sacral spinal nerve stimulation are direct efferent motor responses. A reopening of the discussion on the mechanism of action of SNM is possibly justified.

Intraoperative neurophysiological monitoring of the bulbocavernosus reflex during surgery for conus spinal lipoma: what are the warning criteria?

OBJECTIVE

Despite the surge in the intraoperative use of the bulbocavernosus reflex (BCR) during lumbosacral surgeries, there are as yet no widely accepted BCR warning criteria for use with intraoperative neurophysiological monitoring (IONM). The author's aim was to find clinically acceptable warning criteria for use in IONM of the BCR.

METHODS

Records of IONM of the BCR in 164 operations in 163 patients (median age 5 months) with a conus spinal lipoma who underwent surgery between August 2002 and May 2016 were retrospectively analyzed. The outcomes of IONM of the BCR were grouped by the residual amplitude at the end of surgery: group 1, ≥ 50%; group 2, 25%-50% (including the lower bound, but not the upper); and group 3, < 25%. Cases in which the BCR was lost were separately assessed as a subgroup of group 3. The postoperative urinary complication rate was used to verify the warning criteria zones.

RESULTS

The BCR could be monitored in 149 surgeries (90.9%). There were 118 surgeries (79.2%) in group 1, 18 (12.1%) in group 2, and 13 (8.7%) in group 3. Two surgeries (11.1%) in group 2 and 6 (46.2%) in group 3 resulted in urinary complications. In the group 3 subgroup (lost BCR), all 5 surgeries resulted in urinary complications. The cutoff value of the BCR amplitude reduction was placed between groups 1 and 2 (zone 1: cutoff 50%), groups 2 and 3 (zone 2: cutoff 25%), and group 3 and its subgroup (zone 3: cutoff zero, present or lost). In zone 1, the positive predictive value (PPV) was 25.8% and the negative predictive value (NPV) was 100%. In zone 2, the PPV was 53.8% and the NPV 98.5%. In zone 3, the PPV was 100% and the NPV 97.9%. The PPV was highest in zone 3. The NPV was highest in zone 1, but its PPV was low (25.8%).

CONCLUSIONS

The "lost or remained" criterion of BCR amplitude (zone 3: cutoff zero) can be used as a predictor of postoperative urinary function. As a warning criterion, the cutoff value of the BCR amplitude reduction at 75% (zone 2) may be used. This preliminary clinical report on the warning criteria for the BCR may contribute to improving the safety of surgery for conus spinal lipoma.

Normalization of substance P levels in rectal mucosa of patients with faecal incontinence treated successfully by sacral nerve stimulation

Background

Sacral nerve stimulation (SNS) may improve faecal incontinence by modulating rectal sensation. This study measured changes in the peripheral expression of various neural epitopes in response to SNS.

Methods

Rectal mucosal biopsies were taken from 12 patients before and after temporary SNS, and from ten responders at 90 days after permanent stimulation. Sections were immunostained for substance P, transient receptor potential vanilloid (TRPV) 1, vasoactive intestinal peptide (VIP) and calcitonin gene-related peptide (CGRP). Levels were compared with those in nine continent controls.

Results

Baseline levels of percentage area immunoreactivities of substance P (median 0·51 (95 per cent confidence interval 0·31 to 0·73) versus 0·13 (0·07 to 0·27) per cent; P &lt; 0·001) and TRPV1 (0·76 (0·41 to 1·11) versus 0·09 (0·04 to 0·14) per cent; P &lt; 0·001), but not of VIP (1·26 (0·37 to 2·15) versus 1·28 (0·39 to 2·17); P = 0·943), were significantly greater than in controls. Successful SNS resulted in a significant decrease in substance P immunostaining after temporary (0·15 (0·06 to 0·51) per cent; P = 0·051) and permanent (0·17 (0 to 0·46) per cent; P = 0·051) stimulation. Immunoreactivity of TRPV1, VIP, CGRP and neural markers showed no qualitative change.

Conclusion

Patients with faecal incontinence demonstrate normalization of raised rectal mucosal substance P levels following successful SNS.

Neurophysiological testing in anorectal disorders

The neurophysiological techniques currently available to evaluate anorectal disorders include concentric needle electromyography (EMG) of the external anal sphincter, anal nerve terminal motor latency (TML) measurement in response to transrectal electrical stimulation or sacral magnetic stimulation, motor evoked potentials (MEPs) of the anal sphincter to transcranial magnetic cortical stimulation, cortical recording of somatosensory evoked potentials (SEPs) to anal nerve stimulation, quantification of electrical or thermal sensory thresholds (QSTs) within the anal canal, sacral anal reflex (SAR) latency measurement in response to pudendal nerve or perianal stimulation, and perianal recording of sympathetic skin responses (SSRs). In most cases, a comprehensive approach using several tests is helpful for diagnosis: needle EMG signs of sphincter denervation or prolonged TML give evidence for anal motor nerve lesion; SEP/QST or SSR abnormalities can suggest sensory or autonomic neuropathy; and in the absence of peripheral nerve disorder, MEPs, SEPs, SSRs, and SARs can assist in demonstrating and localizing spinal or supraspinal disease. Such techniques are complementary to other methods of investigation, such as pelvic floor imaging and anorectal manometry, to establish the diagnosis and guide therapeutic management of neurogenic anorectal disorders.

The application of an electrophysiological bulbocavernosus reflex test in male dogs

The electrophysiological bulbocavernosus reflex test consists of recording the electromyographic activity of the bulbocavernosus muscle following electrical stimulation of the glans penis. This study has been carried out to show the applicability of the electrophysiologic bulbocavernosus reflex (EBCR) in dogs and to determine its normal latency value. Ten healthy male dogs were used. Responses to stimulation of the bulbus glandis were recorded from the left bulbocavernosus muscle with a concentric needle-recording electrode. In 10 dogs of maximum size 31 kg and maximum age 3 years, EBCR had response latencies between 17.6 and 28.8 ms with the mean value of 24.4 ms. The results of this study show that the electrophysiological bulbocavernosus reflex test could be employed as a routine diagnostic method in small animal clinics for evaluation of the sacral reflex arc. Among several advantages are easy applicability and the objective result, with the latency exceeding 29 ms not being seen in the dogs used in our study.

External anal sphincter responses after S3 spinal root surface electrical stimulation

AIMS

The aim of this study is to present the normative data of direct and reflex motor anal sphincter responses, simultaneously evoked by S3 surface electrical stimulation. By this method, it is possible to test the functional integrity of the nervous pathways activated during sacral neuromodulation (SNM).

METHODS

Twenty healthy subjects were studied. Motor-evoked potentials (MEPs) were recorded by concentric needle electrode from external anal sphincter (EAS). Electrical stimulation was applied by means of a bipolar surface electrode over the S3 right or left sacral foramina.

RESULTS

Direct (R1) and reflex responses (R2 and R3) were found at latencies of 6.98, 25.12, and 50.31 msec, respectively. The two first responses were recorded in all the cases; the last response is steadily recorded in 17 out of 20 subjects.

CONCLUSIONS

Our data can serve as reference values for future study in patients with pelvic floor dysfunction. EAS responses following S3 percutaneous electrical stimulation can represent a useful aid in the selection of candidates to SNM.