Feasibility of Real-Time Ultrasound for Pudendal Nerve Block in Patients With Chronic Perineal Pain

By Rofaeel, Ayman Peng, Philip; Louis, Ihab; Chan, Vincent

Background and Objectives: Compared with conventional fluoroscopic-guided pudendal nerve block, ultrasonography has potential advantages for visualizing anatomical landmarks such as the internal pudendal artery and nerve, the sacrospinous and sacrotuberous ligaments, and local anesthetic spread. We examined the clinical utility of performing pudendal nerve block under real- time ultrasound guidance. Methods: Seventeen patients were studied. With the patient lying prone, a 2 to 5 MHz curved array ultrasound probe was placed at the level of the ischial spine to capture the transverse view of the ischial spine, the sacrospinous and sacrotuberous ligaments (SSL and STL), the internal pudendal artery (confirmed with color Doppler), and the pudendal nerve. A 22-gauge needle was advanced under real-time ultrasound guidance to reach the pudendal nerve in the plane between the STL and SSL. Following confirmation of spread of dextrose 5% solution in the interligamentous plane, a mixture of 5 mL 0.25% bupivacaine with 1:200,000 epinephrine and 40 mg Depo-Medrol (Pharmacia & Upjohn, Kalamazoo, MI) was injected. Assessment included the ease of identification of anatomical structures and local anesthetic spread with ultrasound, and the degree of sensory block in the perineum.

Results: The ischial spine, SSL, STL, internal pudendal artery, and pudendal nerve were easily identifiable with ultrasound in the majority of patients. Local anesthetic spread was seen as a hypoechoic collection around the nerve and expanding between the STL and SSL. All patients developed perineal sensory block following the procedure.

Conclusions: Pudendal nerve block at the ischial spine level can be reliably performed under real-time ultrasound guidance. Reg Anesth Pain Med 2008;33:139-145.

Key Words: Pudendal neuralgia, Pudendal nerve, Ultrasound.

Pudendal neuralgia commonly presents as chronic debilitating pain in the perineal area, classically exacerbated by sitting and partially relieved by standing or lying down.1 It is mainly caused by pudendal nerve entrapment, which can occur along its path, either in between the sacrotuberous and sacrospinous ligaments (Fig I),2 or through Akock’s canal.3 Pudendal nerve block is crucial to the diagnosis and treatment of pudendal neuralgia.

Knowledge of pudendal nerve anatomy is essential for understanding the technique of pudendal nerve block. The pudendal nerve is formed from the anterior rami of the second, third, and fourth sacral nerves (S2, S3, and S4) emerging from the anterior sacral foramina. Accompanied by the internal pudendal artery, the pudendal nerve exits the pelvis through the greater sciatic notch, traveling posterior to the sacrospinous ligament, and abutting the attachment of the latter to the ischial spine. At this level, the nerve is situated between the sacrospinous and sacrotuberous ligaments (interligamentous plane), medial to the pudendal artery.1- 4-7 The nerve then swings anteriorly to enter the pelvis through the lesser sciatic notch and Alcock’s canal,8,9 which is the fascia tunnel formed by the duplication of internal obturator muscles, under the plane of the levator ani muscle, on the lateral wall of the ischiorectal fossa.10- 11 The pudendal nerve subsequently gives off 3 terminal branches: the dorsal nerve of the penis (clitoris), the inferior rectal nerve, and the perineal nerve, providing the sensory branches to the skin of penis (clitoris), perianal area, and the posterior surface of scrotum or labia majora. It also innervates the external anal sphincter (inferior rectal nerve) and deep muscles of the urogenital triangle (perineal nerve).4,7,12

Fig 1. Illustrative anatomy of the pudendal nerve (PN) at the level of ischial spine. The gluteus maximus muscle was split, and part of the sacrotuberous ligament (STL) at the ischial spine level was removed, to show the passage of pudendal nerve in the interligamentous plane. SSL, sacrospinous ligament.

Conventional fluoroscopic-guided technique for pudendal nerve block has gained popularity in clinical practice because of the ease of landmark recognition.13,14 The needle is placed adjacent to the tip of the ischial spine. However, the pudendal nerve is situated in the plane between the sacrospinous and sacrotuberous ligaments (interligamentous plane), which cannot be visualized by fluoroscopy. An approach to the interligamentous plane using computerized tomography (CT) scan has been described, which is also the only approach described that allows blocking the pudendal nerve in Alcock’s canal.5 However, CT scan is not readily accessible by physicians, other than radiologists. Furthermore, this technique is performed without real-time visual control and carries the risks of unintended puncture of adjacent vessels, and exposure to radiation.

Kovacs et al.15 first described an ultrasoundguided technique for localization of the pudendal nerve in volunteers. However, this study did not involve an actual ultrasound-guided needle placement for nerve block, and subsequently no information regarding clinical outcome was available.

We propose that the use of ultrasonographic guidance for pudendal nerve block at the level of the ischial spine is feasible. Ultrasonography may allow direct visualization of the anatomical landmarks in close relationship with the pudendal nerve, such as the ischial spine, internal pudendal artery, and the sacrospinous and sacrotuberous ligaments. Furthermore, the spread of the injected solution can possibly be detected with real-time ultrasound.16 Ultrasonography also eliminates the risk of radiation associated with fluoroscopy. Therefore, ultrasonography may improve the precision and safety of the technique and minimize complications. This study was designed to examine the feasibility of performing pudendal nerve block under real-time ultrasound guidance.

Methods

The study was approved by our local Institutional Review Board and written informed consent was obtained from all patients. The procedure was performed in the operating room, with patients lying in the prone position.

This prospective case series consists of 17 pudendal nerve blocks performed under real-time ultrasound guidance as the primary imaging technique. Patients with a clinical diagnosis of pudendal nerve entrapment who presented mainly with perineal pain were studied. A low frequency 2 to 5 MHz curved array ultrasound probe, and a Philips HDI 5000 unit (Bothell, WA) with color flow Doppler and image-capturing capabilities, were used.

After skin preparation with povidone-iodine, and sterile probe preparation within a transparent plastic sheath, scanning was performed in transverse planes to visualize the ischium forming the lateral border of the sciatic notch. By moving the ultrasound probe in a cephalad-caudal direction, the ischium appeared as a progressively lengthening hyperechoic line and was widest at the ischial spine level. The ischial spine was further confirmed with visualization of the pudendal artery, and the sacrospinous and sacrotuberous ligaments (Figs 2 and 3). The sacrospinous ligament appeared as a hyperechoic line in continuity with the ischial spine, with lower echogenicity than bone. Similarly, the sacrotuberous ligament was seen on ultrasound image as a light hyperechoic line deep to the gluteus maximus muscle and appeared parallel and superior to the sacrospinous ligament. At this level, color Doppler was used to localize the internal pudendal artery pulsations in close proximity to the ischial spine (Fig 3). Localization of the pudendal nerve was targeted in the plane between these 2 ligaments. Under ultrasound guidance, a 20-gauge, 12 cm insulated peripheral nerve-stimulating needle (Uniplex; Pajunk, Gelsingen, Germany) was inserted from the medial aspect of the probe and advanced in line with the ultrasound probe to the medial aspect of the internal pudendal artery (Fig 4). Once the needle passed through the sacrotuberous ligament, a ‘click’ was usually felt and a small volume (1-2 mL) of dextrose 5% solution was injected. The solution appeared as a hypoechoic collection, to identify the plane between the sacrotuberous and the sacrospinous ligaments, and to accentuate the pudendal nerve appearance (Fig 5).

Fig 2. Ultrasound identification of the ischial spine, sacrospinous ligament (SSL), and sacrotuberous ligament (STL).

Graded electrical stimulation, at 1 Hz up to 1.5 mA, was applied as an attempt to elicit paresthesia in the perineal and genital areas. After the needle was in the final position, fluoroscopy was performed to examine the distance from the needle tip to the ischial spine, and the agreement between the ultrasound and fluoroscopy was noted. A disagreement was defined as a discrepancy of 1 cm or more from the ischial spine in fluoroscopy. The need for repositioning the needle tip after ultrasound localization, based on fluoroscopy, was recorded.

Fig 3. Color Doppler identification of the internal pudendal artery (IPA). IS, ischial spine; SSL, sacrospinous ligament; STL, sacrotuberous ligament.

Fig 4. Ultrasound image of the pudendal nerve (PN) before injection of dextrose 5% solution. IS, ischial spine; SSL, sacrospinous ligament; STL, sacrotuberous ligament.

Satisfactory positioning of the needle tip was determined by its location in the interligamentous plane medial to the pudendal artery and satisfactory spread of dextrose 5% solution in the interligamentous plane. Subsequently, a mixture of local anesthetic (5 mL 0.25% bupivacaine with 1:200,000 epinephrine) and steroid (40 mg Depo-Medrol; Pharmacia & Upjohn, Kalamazoo, MI) was injected, and the adequacy of local anesthetic spread around the nerve during injection was reassessed with ultrasound. Fig 5. Ultrasound image of the pudendal nerve (PN) after injection of dextrose 5% solution. Accentuation of the nerve image is noted (as a hyperechoic structure) surrounded by hypoechoic collection. IS, ischial spine; SSL, sacrospinous ligament; STL, sacrotuberous ligament.

Table 1. Visibility of Anatomic Structures

The procedures were performed by 2 investigators, who graded the ease of visualization of anatomical structures (ischial spine, sacrospinous and sacrotuberous ligaments, internal pudendal artery, pudendal nerve) and local anesthetic spread around the pudendal nerve as: good (structure was easily detectable on ultrasound), fair (structure was detectable on ultrasound after a few attempts), or poor (structure was not detectable on ultrasound). Successful blockade of the pudendal nerve, defined as the study’s primary outcome, was determined by loss of temperature and pinprick sensation in the perineum 15 minutes after injection, as well as the ability to sit with minimal or no pain. Side effects were also monitored, including bruising, temporary foot drop, numbness along the sciatic nerve distribution, temporary loss of control of bowel and/or bladder, infection, and nerve injury. All patients were asked to chart their pain scores while lying supine versus sitting on a verbal pain rating scale (from 0, no pain, to 10, worst pain) before (baseline) and hourly after the procedure for the first day then 4 times daily for a period of 1 week. Daily median pain scores were compared with baseline scores using the Student t test.

Results

Seventeen patients (6 males and 11 females, 58 +- 8 years old, 59 +- 8 kg) successfully completed the study. Grading of visibility of all anatomical landmarks is displayed in Table 1. The ischial spine was easily identified in all patients, representing the most prominent and reliable anatomical landmark in close proximity to the pudendal nerve. The depth of the ischial spine was 5.2 +- 1.1 cm. The internal pudendal artery was also easily identified medial to the ischial spine in all but 1 patient. Although the artery could not be localized with the color Doppler in one particular patient, transmitted arterial pulsations were detectable in the surrounding tissues with real-time ultrasound.

We were unable to visualize the pudendal nerve in only 2 patients (12%). When visualized, it lies medial to both the pudendal artery and the ischial spine. We did not find electrical stimulation helpful as only 5 patients developed paresthesia in the perineal area. However, the inferior gluteal artery (in close proximity with the sciatic nerve) was mistaken as the pudendal artery in 1 patient and nerve stimulation elicited plantar flexion of the ankle. Subsequent repositioning of the probe revealed the pudendal artery, which was more medial to the inferior gluteal artery.

Spread of the injected solution was visualized in 88% of patients as a hypoechoic shadow surrounding the nerve and expanding within the plane bounded by the sacrospinous and sacrotuberous ligaments, which in turn provided confirmation for accurate needle placement in close proximity to the targeted nerve. There was an agreement between ultrasonographic and fluoroscopic localization of needle tip position in relation to the ischial spine in 14 (82.4%) patients. In those patients with disagreement between ultrasound and fluoroscopic images, the needle was noticed to be medial to the ideal position determined by fluoroscopic-guided technique. However, injection of dextrose 5% solution showed good spread in the interligamentous plane and the needle position was accepted.

All patients developed diminished cold and pinprick sensation in the perineum immediately following the procedure. Before the pudendal nerve block, no patient had baseline numbness to pinprick but 18% had diminished sensation to cold. One patient developed transient manifestations of sciatic nerve block. No subsequent complications were reported in the remaining patients. The overall pain scores were significantly reduced over the first week after the procedure compared with baseline (P < .05; Fig 6).

Discussion

This study demonstrates the feasibility of reliably determining the sonoanatomy of the area adjacent to the pudendal nerve and to perform real-time needle advancement for nerve block under ultrasound guidance.

Fig 6. Verbal analog pain scores (VAPS) shown for lying versus sitting positions, before (baseline) and after pudendal nerve block (days 1-7). *Significant difference compared with baseline.

Robert et al. suggested that the pudendal nerve is located in the interligamentous plane between the sacrospinous and sacrotuberous ligaments at the ischial spine level.1 This forms the target for needle placement in our proposed ultrasound-guided technique, and is also the target in the CT scan-guided technique.5 To visualize the sacrospinous ligament, one needs to identify the ischial spine, as it appears as a hyperechoic line medial to the ischial spine in transverse scan. Our experience with localization of the ischial spine was successful in all studied patients, and compares favorably with the ultrasound techniques previously described by Kovacs et al.,15 and Chan et al.17 So far, the ischial spine has been reported as the best site for fluoroscopy,13,14 as well as for ultrasound- guided techniques, for pudendal nerve localization.6,15 The sacrotuberous ligament appears as a light hyperechoic line deep to the gluteus maximus muscle. It may not be easily visualized in patients with high body mass index. We had poor visualization of the sacrotuberous ligament in 2 patients. However, with real-time ultrasound guidance, one can feel a “click” when the needle is advanced through the ligament; its shadow is also accentuated following injection with dextrose 5% solution or local anesthetics.

In the vicinity of the ischial spine, we were able to easily detect the internal pudendal artery with color Doppler ultrasonography, and the artery was located medial to the ischial spine. This finding closely resembles previous studies,6,17 in which the internal pudendal artery was consistently localized within a 10 mm distance medial to the tip of the ischial spine in most cases. Both the ischial spine and the internal pudendal artery generate easily identifiable bony and vascular signals on real-time ultrasound images and represent reliable cues lateral to the pudendal nerve location. If the arterial pulsation was found ‘lateral’ to the ischial spine, it likely was the inferior gluteal artery (in close proximity to sciatic nerve) and that level was likely cephalad to the true ischial spine. Failure to recognize this would result in muscle contraction from sciatic nerve stimulation.

Because the pudendal nerve is principally found medial to the pudendal artery (76%-100%) at the level of ischial spine, the needle tip was positioned just medial to the artery when we could not locate the nerve.6,15 The final position of the needle was thus medial to the ischial spine and internal pudendal artery. Therefore, it is not a surprise to find that the final needle position using the ultrasoundguided technique is more medial to that of a fluoroscopically-guided position. In our series, there were 3 patients where the needle position was not in agreement with that of the fluoroscopic-guided position because all appeared in a more medial position. We accepted the ultrasound-guided position and all patients eventually developed sensory block.

The limited direct visibility of the pudendal nerve by ultrasound is in agreement with previous literature,6,15 and can be accounted for by several factors. The average diameter of the pudendal nerve at the level of the ischial spine is approximately 4 mm to 6 mm.4,6,9 Nerves of this size are generally difficult to detect with ultrasound at a depth of 5.2 +- 1.1 cm. At the level of the ischial spine, 30% to 40% of pudendal nerves are 2-trunked or 3- trunked.4,7,9 This reduces the chance of direct depiction of the nerve with ultrasound and may also account for the poor response to the nerve stimulator. Lastly, the pudendal nerve itself may be embedded in dense connective or fatty tissue,6 which may limit its delineation. Nevertheless, it has been reported that ultrasound identification of nerves may be facilitated with injection of quiescent solution, which can serve as reverse contrast, outlining the borders of the anesthetized nerve.18 Although visualization of the pudendal nerve was not possible in all cases, accurate placement of medication in the interligamentous plane should allow medication to spread around the nerve and that was why all the patients in our series developed sensory block following injections. In 2 patients, we were unable to visualize the spread of dextrose 5% solution in real-time. In both cases, the needle was placed on the medial side of pudendal artery just about the SSL and sensory blockade was achieved in both patients. Following our initial experience with these 17 patients, we had performed more than 30 similar procedures and we were able to visualize the spread of solution in the interligamentous plane in all patients.

Based on our experience with this case series, the technique we recommend is to locate the ischium at the ischial spine level first. At this level, the internal pudendal artery is just medial to the ischial spine. Adjustment of the ultrasound probe position allows the visualization of the sacrospinous ligament medial to the ischial spine. Deep to the gluteus maximus muscle, the sacrotuberous ligament appears as a light hyperechoic line. If the pudendal nerve cannot be visualized, the needle can be placed just medial to the internal pudendal artery. As the needle advances deep to the gluteus maximus muscle, a ‘click’ can always be felt. Injection of a small amount of dextrose 5% solution or normal saline will show the spread of the solution in the interligamentous plane, and the correct positioning of the needle. There are a few limitations with this case series, which examined the feasibility of performing pudendal nerve block under real-time ultrasound guidance and was not intended to compare the improvement in outcome with that of a fluoroscopicguided technique. The assessment of the visualization of the landmarks was performed by authors not blind to the procedure. This might result in bias of assessment. However, all procedures were successfully performed without further fluoroscopic guidance. Nerve stimulation was set as an a priori criterion for successful block, similar to other peripheral nerve block techniques. However, our experience showed that it was not a reliable confirmation test, possibly because of the size of the nerve at the interligamentous plane. The confirmation of the pudendal nerve therefore relies on the visualization of the nerve and surrounding structures, and on the spread of the solution in the interligamentous plane.

In summary, real-time ultrasound technology can be used to provide high-quality images of the anatomical landmarks in the vicinity of the ischial spine, guide needle placement to target the pudendal nerve, and reliably monitor the distribution of the injected therapeutic solution. While our data demonstrate favorable clinical outcome, future studies are required to further elaborate the evidence for the clinical utility of ultrasound-guided pudendal nerve block compared with other techniques.

References

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15. Kovacs P, Gruber H, Piegger J, Bodner G. New, simple, ultrasound-guided infiltration of the pudendal nerve: ultrasonographic technique. Dis Colon Rectum 2001;44:1381-1385.

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17. Chan VW, Nova H, Abbas S, McCartney CJ, Perlas A, Xu DQ. Ultrasound examination and localization of the sciatic nerve: a volunteer study. Anesthesiology 2006,104:309-314.

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Ayman Rofaeel, M.D., Philip Peng, M.B.B.S., F.R.C.P.C, Ihab Louis, M.D., and Vincent Chan, M.D., F.R.C.P.C.

From the Department of Anesthesia, Toronto Western Hospital, University of Toronto, Toronto, Canada.

Accepted for publication October 1, 2007.

Source of funding: Institutional. Equipment Support: Philips Medical System (Bothell, WA). This study was presented in part at the 2006 ASA Annual Meeting, October 17, 2006, Chicago, IL (Abstract 1579).

Reprint requests: Philip Peng, M.B.B.S., F.R.C.P.C, Department of Anesthesia, McL 2-405, Toronto Western Hospital, 399 Bathurst Street, Toronto, Ontario M5T 2S8, Canada. E-mail: philip.peng@uhn.on.ca

(c) 2008 by the American Society of Regional Anesthesia and Pain Medicine.

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doi:10.1016/j.rapm.2007.10.004

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