Ultrasound-guided pudendal nerve block is useful in the management of the pelvic pain subserved by the pudendal nerve. This technique serves as an excellent adjunct to lumbar epidural block in obstetrics and for local or general anesthesia or as a stand-alone surgical anesthetic when performing surgery on the labia or scrotum, for example, Bartholin cyst surgery (Fig. 132.1). Ultrasound-guided pudendal nerve block with local anesthetic may be used to palliate acute pain emergencies, including postoperative pain, pain secondary to traumatic injuries of the pudenda, and cancer pain, while waiting for pharmacologic, surgical, and antiblastic methods to become effective.

Ultrasound-guided pudendal nerve block can also be used as a diagnostic tool when performing differential neural blockade on an anatomic basis in the evaluation of pelvic pain as well as in a prognostic manner to determine the degree of neurologic impairment the patient will suffer when destruction of the pudendal nerve is being considered or when there is a possibility that the nerve may be sacrificed during surgeries in the anatomic region of the pudendal nerve at the level of the hip. This technique may also be useful in those patients suffering symptoms from compromise of the pudendal nerve. Ultrasound-guided pudendal nerve block may also be used to palliate the pain and dysesthesias associated with stretch injuries to the pudendal nerve that can occur after “straddle injuries” or forceps delivery. Pudendal nerve block with local anesthetic and steroid is also useful in the palliation of pain of malignant origin arising from tumors invading the labia or scrotum or the pudendal nerve itself. The technique may also be useful in palliation of persistent rectal, vulvar, or vaginal itching that has not responded to topical therapy. Destruction of the pudendal nerve is occasionally indicated for the palliation of persistent pelvic or rectal pain after blunt or open trauma to the pelvis or persistent pain mediated by the pudendal nerve after obstetric deliveries or transvaginal surgery or in the palliation of pain of malignant origin.

Electrodiagnostic testing should be considered in all patients who suffer from pudendal nerve dysfunction to provide both neuroanatomic and neurophysiologic information regarding nerve function. Magnetic resonance imaging and ultrasound imaging of the lumbar plexus and the pelvis anywhere along the course of the pudendal nerve are also useful in determining the cause of pudendal nerve compromise.

The pudendal nerve is comprised of fibers from the S2, S3, and S4 nerves (Fig. 132.2). The nerve passes inferiorly between the piriformis and coccygeal muscles. The pudendal nerve leaves the pelvis accompanying the pudendal artery and vein via the greater sciatic foramen. It then passes around the medial portion of the ischial spine to re-enter the pelvis via the lesser sciatic foramen. At this level, the nerve lies between the sacrospinous and sacrotuberous ligaments (Fig. 132.3). The pudendal nerve is amenable to blockade at this point via the transvaginal or ultrasound-guided approach described below. The nerve then divides into three terminal branches: (1) the inferior rectal nerve, which provides innervation to the anal sphincter and perianal region; (2) the perineal nerve, which supplies the posterior two-thirds of the scrotum or labia majora and muscles of the urogenital triangle; and (3) the dorsal nerve of the penis or clitoris, which supplies sensory innervation to the dorsum of the penis or clitoris (Fig. 132.4).

The benefits, risks, and alternative treatments are explained to the patient and informed consent is obtained. The patient is then placed in the prone position (Fig. 132.5). At the midposterior gluteal region, a low-frequency curvilinear ultrasound transducer is placed over the ilium in the posterior midgluteal region, and an ultrasound survey scan is taken (Figs. 132.6 and 132.7). The medial margin of the ilium will appear as a hyperechoic line that is widest at the level of the ischial spine (Fig. 132.8). The ultrasound probe is slowly moved in a caudad direction along the extent of the medial margin of the ilium until the ischial spine comes into view appearing as a straight hyperechoic line (Figs. 132.9 and 132.10). Just above the ischial spine lies the sacrospinous ligament, and sacrotuberous ligaments lies just above the straight hyperechoic line of
the ilium (Fig. 132.11). The pudendal nerve lies between these two ligaments at this level. The sacrospinous ligament can be seen as a slightly less hyperechoic line that is confluent with and just above the ischial spine (see Fig. 132.11). The sacrotuberous ligament is seen just above and parallel to the sacrospinous ligament lying deep within the gluteus maximus muscle (see Fig. 132.11). Color Doppler is then utilized to identify the internal pudendal artery, which lies medial to or just above the pudendal nerve (Fig. 132.12). It should be noted that the inferior gluteal artery, which lies lateral to the tip of the ischial spine and in proximity to the sciatic nerve, can be easily mistaken for the internal pudendal artery if the ischial spine is not first identified (Fig. 132.13). The pudendal nerve should be identifiable just medial to or above the inferior pudendal artery (Fig. 132.14). After the internal pudendal artery and adjacent pudendal nerve are identified, the skin overlying the area
beneath the ultrasound transducer is prepped with antiseptic solution. A sterile syringe containing 10.0 mL of 0.25% preservative-free bupivacaine is attached to a 3½-inch, 22-gauge needle using strict aseptic technique. If the painful condition being treated is thought to have an inflammatory component, 40 to 80 mg of depot steroid is added to the local anesthetic. The needle is placed through the skin ˜1 cm above the medial edge of the transducer and is then advanced using an in-plane approach with the needle trajectory adjusted under realtime ultrasound guidance so that the needle tip rests within proximity, but outside the substance of the pudendal nerve (Fig. 132.14). When the tip of needle is thought to be in satisfactory position, after careful aspiration, a small amount of local anesthetic and steroid is injected under real-time ultrasound guidance to confirm that the needle tip is lying between the sacrospinous and sacrotuberous ligaments and not within the substance of the nerve (Fig. 132.15). After proper needle tip placement is confirmed, the remainder of the contents of the syringe are slowly injected. There should be minimal resistance to injection, and the clinician should stop injecting immediately if the patient experiences any increase in pain during the injection procedure.