Percutaneous sacral neurostimulation of penile erection during radical prostatectomy: surgical technique

Abstract

Intraoperative neurophysiological monitoring (IONM) has been introduced as a potentially useful adjunct technology when performing pelvic surgeries such as radical prostatectomy for improving postoperative functional outcomes. It involves surveying the functional integrity of critical intrapelvic nerve structures in conjunction with registering neuronally mediated functional effects in the pelvic region during surgery, such that intraoperative adjustments can be made with the intent to minimize iatrogenic nerve injury. Such critical nerve structures include the periprostatic cavernous nerves, the source of neurochemical mediators of penile erection and urinary continence. To implement this technology, a neuronal stimulus is needed for activating physiological responses in the pelvic region. As such, we describe an innovative technical procedure for applying neurostimulation at the sacral spinal cord, bearing in mind the significance of this spinal level in neuroregulating pelvic functions, including penile tumescence and urethral sphincteric activity. Our procedure involves percutaneous insertion of 26-gauge, 50-mm monopolar insulated needle electrodes into the low back, guided by external physical landmarks, that contact the laminae at the fourth and fifth lumbar vertebral levels, from which an electrical current is delivered, thereby driving sacral spinal cord neural impulses to pelvic organs and structures. We performed the procedure in a pilot investigation of patients undergoing open radical prostatectomy just prior to surgical dissection of the prostate, assessing tumescence responses using a proprietary Sentinel^® functional near-infrared spectrophotometry sensor cuff placed on the penis. In five patients with mild or no erectile dysfunction preoperatively, we recorded higher than baseline variance in oxygenated and deoxygenated hemoglobin concentrations after neurostimulation (P<0.05), consistent with the onset of penile tumescence. In two patients with moderate to severe erectile dysfunction, neurostimulation did not elicit significant spectrophotometric changes or penile tumescence. There were no adverse events related to this procedure. Our findings suggest that sacral neurostimulation using a percutaneous needle electrode insertion technique is a feasible and safe procedure that may be used intraoperatively during radical prostatectomy to induce penile erection. Its potential role for assessing the functional integrity of neuroregulated erectile function in an IONM platform for radical prostatectomy surgeries is proposed, although further investigation is needed to demonstrate its utility for this purpose.

Highlight box.

Surgical highlights

• The technique described of sacral neurostimulation applies a systematic, minimally invasive procedure of percutaneously inserting two needle electrodes that contact the vertebral laminae at lumbar vertebral levels four and five in a standardized configuration so that electrical current is delivered to pelvic organs and structures.

• The technique has been shown preliminarily to be expediently and effectively applied in an intraoperative neurophysiological monitoring platform during radical prostatectomy.

What is conventional and what is novel/modified?

• Conventional neurostimulation used in intraoperative neuromonitoring for pelvic surgeries has utilized intrapelvic neurostimulatory techniques, which have been shown to be deficient in generating ample neuronal signaling in the pelvic region.

• Sacral neurostimulation, which targets the premier spinal center regulating pelvic functions including micturition, defecation and sexual function, provides an advantageous approach for generating robust and all-inclusive neuronal signaling in the pelvic region.

What is the implication, and what should change now?

• Sacral neurostimulation using a percutaneous needle electrode technique may serve as an adjunct of such neurodiagnostic modalities as intrapelvic neural mapping and neurophysiological monitoring when performing pelvic surgeries, thereby offering to improve the execution and outcomes of these surgeries presently relative to the past.

Introduction

Intraoperative neurophysiological monitoring (IONM) has recently gained interest as a promising adjunct technology when performing pelvic surgeries such as radical prostatectomy aiming to improve postoperative sexual and urinary function outcomes (1). The modality comprises several key components, including the creation of a neuronal stimulus for activating neuronally mediated physiological responses in the pelvic region that then can be registered (2-4). By this intrapelvic functional nervous system registration, the surgeon may then proceed in real time to execute the surgery maximally preserving neuroregulated pelvic functions. As done for various disciplinary surgical settings applying IONM, electrical current delivery to neural tissue constitutes the requisite neuronal stimulus (5,6). Thus far for pelvic surgery, neuromonitoring protocols have commonly implemented electrical neurostimulation techniques that are locally applied within the pelvic region (7-11). However, deficiencies of local intrapelvic neurostimulation, including imprecise neuronal stimulation and recording, have limited their practical use (12,13).

It is conceivable that alternate locations along the neuroaxis from the brain and spinal cord to peripheral nerves could be applied as neurostimulatory sites for IONM during pelvic surgeries. One may propose applying the sacral spinal cord for IONM (14-16). The rationale in targeting this site in neurostimulatory protocols for IONM platforms in pelvic surgery has recently been presented (17). Principally, based on its neuroregulatory influence, the sacral spinal cord is contended to be exploitable in a specialized manner for intraoperative diagnostic testing in the pelvic region. This application differs from that of sacral neuromodulation, an approach for treating such chronic pelvic disorders as prostatitis/chronic pelvic pain syndrome, overactive bladder, non-obstructing urinary retention and neurogenic lower urinary tract dysfunction (18,19).

A putative advantage of using sacral neurostimulation for IONM of pelvic surgeries, relative to other modes of nervous system stimulation, resides with the fact that the sacral spinal cord region and its neural projections serve as the primary spinal source for neuroregulated functions in the pelvis (14-17). At the lumbosacral vertebral level, various technical approaches for neurostimulation may be devised and implemented, consistent with the employment of assorted electrode montages and stimulator devices in managing spinal cord injuries (20). In this report, we present sacral neurostimulation via percutaneous needle electrodes as a surgical technique to generate intrapelvic neuronal signaling and induce neuroregulated pelvic function responses, adapted from a derivation of percutaneous spinal neurostimulation as described by Owen and colleagues (21). We present this article in accordance with the SUPER reporting checklist (available at https://tau.amegroups.com/article/view/10.21037/tau-2026-1-0167/rc).

Preoperative preparations and requirements

Because patients undergoing major pelvic surgery such as radical prostatectomy bear the surgical risk of iatrogenic intrapelvic nerve injury, they are potentially eligible candidates for sacral neurostimulation, under the scheme of an IONM platform. Standard preoperative workup and medical clearance for the intended primary surgical procedure should be performed. Prior neurosurgical procedures of the low back area such as laminectomy or discectomy of lumbar vertebrae represent relative contraindications in applying this technique, because of the potential procedural risk of spinal cord injury.

The technique is incorporated into an IONM platform that is performed in an operating room theater on a patient under general anesthesia. Intact neuromuscular transmission throughout all testing and monitoring phases is necessary for successful IONM applied to pelvic surgeries. This requirement accords with the recording of myogenic functional responses of pelvic structures, concomitantly obtaining intrapelvic neurogenic measurements, as a methodology that differs from that applied in recording solely neurogenic signals, which involves full neuromuscular blockade (22,23). If neuromuscular blockade is required for airway management or patient positioning, it must be limited to induction only and allowed to resolve fully or be pharmacologically reversed prior to the initiation of IONM. Adequate recovery of neuromuscular function must be confirmed before commencing neurostimulation or functional recording. The level of neuromuscular blockade is assessed by recording the compound muscle action potential from a target muscle in an extremity (24). A complete neuromuscular blockade is assessed as a train of four stimuli. Coordination between the anesthesia and IONM teams is necessary to maintain anesthetic conditions that support reliable stimulation-based responses throughout the surgical procedure.

With acknowledgement that our approach for IONM in the pelvis is investigational, this technique was developed under a clinical research protocol. All procedures performed in this study were in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the Institutional Review Board of the Johns Hopkins University School of Medicine (No. 00300979). Patient consent for publication of this article and accompanying images was waived owing to the use of anonymized data. Penile near-infrared spectrophotometry was used to measure penile blood volume effects, ascertained as recordings of blood oxygenation (oxygenated and deoxygenated hemoglobin) concentrations per patient (25,26). Measurements were determined by converting raw signals (20-second segments before and after neurostimulation) to hemoglobin concentrations using the modified Beer-Lambert law (27) and modeling the effect of penile tumescence on the variance of the hemoglobin concentration using a linear mixed-effects model (28). The abridged, 5-item version of the International Index of Erectile Function (IIEF-5) questionnaire was used as the outcome metric for assessing the presence and severity of erectile dysfunction (29). Statistical analyses were conducted using the MNE-Python software package for descriptive characteristics, with statistical significance set at P<0.05 (27).

Step-by-step description

After general anesthesia has been induced, the patient is placed laterally on the operating room table standardly and then repositioned in a fetal position (tight curl) (Figure 1). This position maximally exposes the procedural area. Using a standard surgical prep set, the skin is sterilized using an antiseptic solution. The prep area consists of the low back, specifying the expanse between the iliac crests bilaterally and above the crease between the patient’s buttocks. Sterile drapes are placed to delimit a rectangle framing the procedural area.

Figure 1.

Patient positioning and sacral spinal orientation. Illustration depicting a patient placed laterally on the operating room table in the fetal position (tight curl), which displays the low back area for performing the procedure. The precise sites for inserting the anode and cathode needle electrodes at the fourth and fifth lumbar vertebral levels, respectively, are shown. This work credits David A. Rini, MFS, CMI, FAMI, Art as Applied to Medicine, Johns Hopkins School of Medicine.

External physical body landmarks provide a bearing for percutaneous electrode placement. Readily identifiable surface landmarks of the low back area include the posterior superior iliac crests, the posterior superior iliac spines, the greater sciatic notches, and the tip of the coccyx at the midline (30). The fourth lumbar vertebra (L4) vertebra is located at the approximate level of the iliac crest (30). Sacral nerve root stimulation at the L4–L5 vertebral level is targeted because it corresponds with the second through fourth sacral spinal cord (S2–S4) segments, the origin of spinal parasympathetic neural input to pelvic organs including the penis (Figure 2). Consistent with many neuroanatomical studies showing that segments of the spinal cord generally do not correspond to the respective vertebral level, the sacral spinal cord segments classically localize to the axilla of the first lumbar (L1) spinal nerve roots and adjacent vertebral bodies (31,32).

Figure 2.

Anatomy of the fourth and fifth lumbar vertebrae. Illustration in a posterior view depicting the lumbar vertebrae at the lower end of the vertebral column, within which the sacral spinal nerves and nerve roots emanate and supply pelvic organs and structures. Reprinted from G. Zuidema, Johns Hopkins Atlas of Human Functional Anatomy, Johns Hopkins University Press, ©1977. Ant. long. lig., anterior longitudinal ligament; Ant. primary division, anterior primary division; Inf. facet of L-5, inferior facet of L5; L-4 vert., L4 vertebra; L4 n., L4 nerve; Post. long. lig., posterior longitudinal ligament; Post. primary division, posterior primary division; Trans. process, transverse process.

Needle electrode insertion is performed at either side of midline at the fourth and fifth lumbar vertebrae (L4–L5) level. The technique uses monopolar long-beveled needle electrodes that are insulated except at the tip (50 mm length, 26 gauge or alternate 75 mm length, 26 gauge, for obese patients; SPES Medica, Genoa, Italy). Two needle electrodes are placed, one anode and one cathode. The anode is placed cephalad and the cathode is placed caudad at a contralateral location relative to the midline spinous process (Figure 1). An optional additional anode can be placed contralateral to the initial anode placement, creating a montage of stimulating electrodes for evaluating two location combinations. The separation distance between anode and cathode is approximately 2.5 cm, which corresponds with the average length (27.7±3.1 mm) of sacral spinal cord segments (31). Accordingly, the electrodes are spaced in such a way to contact one vertebra lamina apart from the next.

Electrodes are inserted perpendicular to the skin, angled in a slightly caudal direction to avoid entering the spinal canal (Figure 3). Electrodes are advanced for a depth until contact is made with cancellous bone (lamina of the vertebra spine). Once the electrodes are inserted, they along with a strain loop are taped in place, and a small foam doughnut is placed around the exposed hubs. The leads are connected to an extension cable, which is then plugged into the electrical stimulator device that comprises a nerve monitor system (Inomed Inc., Westchester, IL, USA). The setup and execution time for the procedure is regularly less than 10 minutes.

Figure 3.

Needle electrode placement for sacral neurostimulation. Illustrations in lateral (A), lateral oblique (B) and superior (C) views depicting the alignment of a needle electrode inserted percutaneously with its tip abutting the lamina of the fourth lumbar vertebra. This work credits David A. Rini, MFS, CMI, FAMI, Art as Applied to Medicine, Johns Hopkins School of Medicine.

Test stimulation is done to confirm electrode placement and ascertain the neuronal signaling reliability of the stimulating electrodes. To establish optimal stimulatory parameters for the erection response, tetanic stimulation is done initially consisting of a train of 20-30 shocks per second for one to two seconds at provisional parameters that produces a response threshold. This manner of neurostimulation optimization differs from that used for measuring continence, whereby a single shock at a fixed mAmp intensity level sufficiently establishes stimulatory parameters. Stimulation parameters commonly consist of a stimulation rate (frequency) of 150–1,500 Hz, current amplitude of 10–80 mAmp, and pulse duration of 0.2–0.3 ms (33,34).

The response indicating successful electrode placement is penile tumescence. This outcome is established by a variety of determinants. A crude determination is the visible observation of penile tumescence. Strain gauges and near infrared spectrophotometry also may serve as erection monitoring instruments (29). Complementary techniques that may be used to confirm sufficient sacral neurostimulation include free-running and triggered electromyography (EMG, measures of electrical activity in muscles) and motor unit number estimation (MUNE, measures of muscle strength/weakness) testing of the external urethral and anal sphincters (35).

If an adequate neurostimulated response is not confirmed, electrode adjustments can be made. The electrode position can be changed with minimal movements without withdrawing it beyond the level of the skin. As an additional consideration, another needle electrode (usually the anode) can be placed at a slightly higher vertebral level, or both anode and cathode electrodes can be newly placed ipsilaterally at slightly different levels. Additional strategies are to reverse the delivered polarity or switch to alternate neurostimulation methods.

We applied our surgical technique in a pilot investigation of patients undergoing open radical prostatectomy prior to prostatic surgical dissection. Patient enrollment was defined by their eligibility for radical prostatectomy. Erection responses to sacral neurostimulation ability were assessed in seven patients, whose preoperative erection ability was defined according to IIEF-5 questionnaire scores: five patients having mild or no erectile dysfunction (functional erections), as indicated by scores equal to or greater than 17 (median score 23); and two patients having moderate to severe erectile dysfunction (erection inability), as indicated by scores equal to or less than 11 (scores 2 and 10). Needle electrodes were methodically placed after general anesthesia and prior to surgical positioning and standard prep and drape procedures. Procedure time was uniformly less than 10 minutes. After surgical preparation and urethral catheterization, a proprietary Sentinel^® functional near-infrared spectrophotometry sensor cuff was placed with sensors situated uniformly at the lateral proximal extent of the penis.

We recorded higher than baseline variances in oxygenated and deoxygenated hemoglobin concentrations before and after neurostimulation: 0.30±0.88 vs. 7.43±12.90 moles/L, z=2.0, P=0.047 and 0.04±0.13 vs. 1.15±1.99 moles/L, z=2.0, P=0.046, respectively, consistent with the onset of observed penile tumescence in five patients with preoperatively functional erections. We did not obtain measurably distinct spectrophotometric variances from baseline or observe penile tumescence changes after neurostimulation in two patients with erection inability preoperatively. We did not observe any adverse events in performing this technique. After completing sacral neurostimulation with erectile function testing, radical prostatectomy surgeries were performed routinely unimpeded by the neuromonitoring equipment. Needle electrodes and the spectrophotometric cuff were simply removed from the patient’s back and penis, respectively, at the conclusion of the prostate surgery.

Postoperative considerations and tasks

After the pelvic surgical procedure is completed, the needle electrodes are withdrawn and the needle insertion sites are covered with a sterile surgical dressing. The sites are briefly inspected for hemorrhage, and if observed brief compression may be done. Although serious biological complications such as dural puncture headache and neurological damage are rare occurrences (36,37), patients are assessed postoperatively for any such complications and administered appropriate care with neurologist consultation as needed.

Tips and pearls

Percutaneous sacral neurostimulation is relatively simple to perform, despite its minimal invasiveness. There is no requirement for specialized training to perform this procedure. The participation of a neurophysiology technician or neurosurgeon is not required (33). The technical success of the procedure relies on following basic principles of proper patient positioning, vertebral level identification based on physical body landmarks, and adjustments of the needle electrode insertions as needed. In our experience, the urologic surgeon or surgical physician assistant capably inserted the needle electrodes according to these directions in all cases. The neurophysiologic success of the procedure relies on conducting test stimulation by protocol that confirms and optimizes electrical current delivery.

Discussion

This presentation of an innovative percutaneous needle electrode technique for sacral neurostimulation introduces an uncomplicated and convenient method for creating neuronal signaling in the pelvic region. We applied this procedure as a possible application in a pilot investigation of an IONM platform for radical prostatectomy demonstrating its feasibility and safety. Our findings indicate that this technique effectively elicits a quantifiable penile erection by way of a diagnostic spectrophotometric assessment of significantly increased blood filling the penis after initiating neurostimulation in patients with preoperatively functional erections but it does not produce this effect in patients with erection inability preoperatively. Observations of verifiable erection responses after sacral neurostimulation signify that the neurostimulatory technique is effective.

In this report, we performed our technique at the beginning of the radical prostatectomy surgeries, demonstrating that the technique effectively produces erection responses. In this way, this technique may be suggested to have a potential role for an objective determination of erection integrity prior to proceeding with pelvic surgeries, during which iatrogenic surgical nerve injury in the pelvis may occur and impact postoperative erectile function recovery. In the future, its further use may extend to intrapelvic neural mapping and neurophysiological monitoring, for which repeated performances of neurostimulated erection testing throughout pelvic surgical dissections may be necessary. Such modalities are proposed for use in pelvic surgeries for identifying and protecting nerves in the pelvic surgical field thereby lessening risks of iatrogenic nerve injury in hopes of improving upon historically known unsatisfactory postoperative functional recovery outcomes (17). It is acknowledged that such modalities are presently investigational, and various considerations of their neurostimulatory parameters and protocols, electrophysiological recording procedures, machine learning systems, real-time surgeon feedback and navigational tools, and anesthesia practices require systems integration and optimization before their roles can be affirmed.

Attention to the sacral spinal cord as a neurostimulatory site is rooted in the knowledge that this neural center plays a premier role in the neurophysiology of the pelvic region (38,39). Based on anatomical, histochemical and electrophysiological studies, the sacral spinal cord represents the premier spinal relay station for neural pathways of the pelvis for regulating diverse functions including micturition, defecation and sexual function (38,39). With respect to the autonomic nervous system, it is the origin of preganglionic parasympathetic neurons projecting into the pelvic region (38,39). This aspect is particularly advantageous in applying our technique for eliciting penile erections, because it exploits the autonomic pro-erectile regulatory source for this physiologic response while eluding anti-erectile sympathetic neural pathways descending into the pelvis (40,41). It is acknowledged that this mechanism of neurostimulation may also drive somatic neuronal responses in the pelvic region, thereby offering a means to harness and register somatically regulated responses simultaneously with autonomic functional effects. Understandably, our technique does not preclude applying possibly alternate spinal stimulatory methodologies for generating pelvic responses that are referable to superior locations of the spinal cord.

Percutaneous sacral neurostimulation for eliciting pelvic functional responses intraoperatively during pelvic surgeries is a novel concept. However, the technique for spinal stimulation involving needle electrodes contacting vertebral laminae is not new. Following original descriptions of this derivative technique of percutaneous spinal stimulation (21), it has been used customarily in other locations of the spinal cord than the sacral level for neurostimulation with good success (33). Its origins accord with percutaneous technical advances in spinal surgery made a few decades ago as a simpler and less invasive alternative approach for performing spinal stimulation than spinal neurosurgical techniques (42,43). In principle, the technical approach of neurostimulation via the vertebral lamina’s electrode contact relies on the transmission of electrical energy as a neuronal stimulus to the adjacent neural root with descending output. Its application at the lumbosacral vertebral level then serves to drive sacral spinal cord neural impulses to pelvic organs and structures.

Our procedure is readily performed, guided by basic technical steps as described herein. The technique for needle placement readily adheres to identifying discernible external physical features of the low back. Achieving proper needle insertions is favored by targeting the lamina of the lumbar vertebrae, which are considerably ample targets that can be readily appreciated by the distinct hardness of bone when contacted by the tips of the needles. Simple adjustments of needle electrode placements can be made to ensure the requisite delivery of electrical current. The procedure was preliminarily demonstrated in pilot studies of an IONM platform during radical prostatectomy as a methodology for neurostimulating penile tumescence and urethral sphincteric activity responses (34,35).

Besides the technique of percutaneous needle electrode contact with lumbar vertebral laminae, other neurostimulatory techniques that exploit sacral neural pathways for pelvic functional responses can be postulated and developed as well. Similar to our adaptation of a technique that has been used successfully at locations elsewhere in the spinal cord, alternative potential options that could be adapted for this purpose include percutaneous epidural electrode insertion techniques (44,45) as well as use of transcutaneous surface electrodes and extracorporeally magnetic devices (46,47). Besides transspinal neurostimulatory techniques, peripheral neurostimulatory techniques, which drive a neurally reflexive response at the sacral spinal cord level by afferent neurotransmission (48), may also be investigated. Posterior tibial nerve stimulation (24), for example, may be considered.

It is acknowledged that further investigations using the percutaneous needle electrode sacral neurostimulatory technique are needed for its validation. Although the present investigation demonstrated the outcome of erection induction under the sacral neurostimulation protocol, additional assessment is necessary to evaluate the success of this technique in inducing other functional effects in the pelvic region. Establishment of optimal electrical stimulation parameters will also come with further testing. It can also be presumed that comparative studies involving different modes of sacral neurostimulation will likely determine the preferable neurostimulatory methodology for this purpose. Lastly, we acknowledge limitations of our erection monitoring device, penile spectrophotometry, used in this study; although this device may be acceptable for assessing erection integrity under neurostimulatory influences, it is imprecise for determining the etiology of the erection non-responses to neurostimulation (i.e., neuropathic, vasculopathic, or combined pathologies).

Conclusions

A percutaneous needle electrode technique for sacral neurostimulation is shown to be a readily and safely performed way to generate intrapelvic neuronal signaling. Herein, in a small case series, we have shown that this technique induces penile tumescence, as a marker of its efficacy. With further investigation, it may be shown to have effects in neuromediating other pelvic functional responses. Conceivably, this technique may well be incorporated into pioneering intraoperative neurophysiological testing and monitoring in the pelvic region for radical prostatectomy and possibly other pelvic surgeries as well. Further study in the manner of well-designed, randomized, controlled trials evaluating IONM and other such proposed technologies is required to establish their actual success in improving functional outcomes after pelvic surgeries. Additionally, by way of improving functional outcomes, such advances in this field potentially will lead to reduced societal costs associated with sexual and urinary problems (e.g., work productivity loss, activity impairment, health-related quality of life burden) (49).

Supplementary

The article’s supplementary files as

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Institutional Review Board of the Johns Hopkins School of Medicine (No. 00300979). Patient consent for publication of this article and accompanying images was waived owing to the use of anonymized data.