Ultrasound-guided sacral nerve block is useful in the evaluation and management of radicular and perineal pain thought to be subserved by the sacral nerve. This technique serves as an excellent adjunct to caudal and epidural nerve block for surgical anesthesia. Ultrasound-guided sacral nerve block with local anesthetic may be used to palliate acute pain emergencies, including postoperative pain, pain secondary to traumatic injuries of the sacrum, and cancer pain, while waiting for pharmacologic, surgical, and antiblastic methods to become effective.

Ultrasound-guided sacral nerve block can also be used as a diagnostic tool when performing differential neural blockade on an anatomic basis in the evaluation of radicular or perineal pain as well as in a prognostic manner to determine the degree of neurologic impairment the patient will suffer when destruction of the sacral nerve or nerves is being considered or when there is a possibility that the nerve may be sacrificed during surgeries in the anatomic region of the sacral nerves. Ultrasoundguided sacral nerve block with local anesthetic and steroid is occasionally used in the treatment of sacral root or perineal pain when the pain is believed to be secondary to inflammation of the sacral nerve or when entrapment of the sacral nerve is suspected. Ultrasound-guided sacral nerve block with local anesthetic and steroid is also indicated in the palliation of pain associated with diabetic neuropathy and is useful in the treatment of bladder dysfunction after injury to the cauda equina. Destruction of the sacral nerves via this approach is occasionally used in the palliation of persistent perineal pain secondary to invasive tumor or bladder dysfunction that is mediated by the sacral nerves and has not responded to more conservative measures. The technique can also be utilized to identify the posterior sacral foramina for introduction of stimulating electrodes for bladder dysfunction.

Because of the potential for hematogenous spread via Batson plexus, local infection and sepsis represent absolute contraindications to ultrasound-guided sacral nerve block. Pilonidal cyst and congenital abnormalities of the dural sac and its contents also represent relative contraindications to this technique.

Plain radiographs of the sacrum and bony pelvis are indicated in all patients who present with sacral and perineal pain to identify occult bony pathology (Fig. 128.1). Based on the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, sedimentation rate, and antinuclear antibody testing. Electrodiagnostic testing should be considered in all patients who suffer from sacral nerve dysfunction to provide both neuroanatomic and neurophysiologic information regarding nerve function. Magnetic resonance imaging and/or computerized tomographic scanning of the lumbar spine and the pelvis are also useful in determining the cause of sacral nerve compromise.

The five sacral vertebrae are fused together to form the triangular-shaped sacrum (Fig. 128.2). The dorsally convex sacrum inserts in a wedge-like manner between the two iliac bones with superior articulations with the fifth lumbar vertebra and caudad articulations with the coccyx. On the anterior concave surface, there are four pairs of unsealed anterior sacral foramina that allow passage of the anterior rami of the upper four sacral nerves (Fig. 128.3). The posterior sacral foramina are smaller than their anterior counterparts (Fig. 128.4). Leakage of drugs injected into the sacral canal is effectively prevented by the sacrospinal and multifidus muscles. The vestigial bony remnants that are the result of the incomplete fusion of the inferior articular processes of the lower half of the S4 and all of the S5 vertebrae project downward on each side of the sacral hiatus (see Fig. 128.2). These bony projections are called the sacral cornua and represent important clinical landmarks when performing ultrasoundguided sacral nerve block. The U-shaped sacral hiatus is covered posteriorly by the sacrococcygeal ligament, which is also an important clinical landmark when performing ultrasound sacral nerve block. Penetration of the sacrococcygeal ligament provides direct access to the epidural space of the sacral canal. Although there are gender- and race-determined differences in the shape of the sacrum, they are of little importance relative to the ultimate ability to successfully perform sacral nerve block on a given patient.

The benefits, risks, and alternative treatments are explained to the patient, and informed consent is obtained. Ultrasoundguided sacral nerve block can be carried out by placing the patient in the prone position with the patient’s abdomen resting on a thin pillow (Fig. 128.5). To relax the gluteal muscles, the patient is asked to turn his or her heels outward (Fig. 128.6). A total of 16 mL of local anesthetic suitable for epidural administration is drawn up in a 20-mL sterile syringe. 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 skin overlying the sacrum is then prepped with antiseptic solution, and the medial sacral crest is palpated using a rocking motion (Fig. 128.7). A lowfrequency curvilinear ultrasound transducer is then placed in the transverse plane over the superior medial sacral crest, and an ultrasound survey scan is taken (Fig. 128.8). The dorsal median sacral crest will be seen as a hyperechoic line that curves downward toward the sacral foramina in a shape reminiscent of baseball player Rollie Fingers’ famous mustache (Figs. 128.9 and 128.10). After the median sacral crest is identified, the

transversely placed ultrasound transducer is slowly moved caudally and laterally toward the affected side until the first dorsal sacral foramen is visualized (Fig. 128.11). The foramen will appear as a cone-shaped indentation of the dorsal surface of the sacrum. The ligament covering the foramina may be visible (see Fig. 128.11). Color Doppler may assist in identification of the sacral foramen by allowing identification of the foraminal branch of the lateral sacral artery, which exits the inferolateral aspect of each sacral foramina (Fig. 128.12).