Ultrasound-guided caudal epidural block is utilized in a variety of clinical scenarios as a diagnostic, prognostic, and therapeutic maneuver as well as to provide surgical anesthesia for pelvic and lower extremity surgeries. As a diagnostic tool, ultrasound-guided caudal epidural block allows accurate placement of the needle tip within the epidural space when performing differential neural blockade on an anatomic basis in the evaluation of back, groin, pelvic, bladder, perineal, genital, rectal, anal, and lower extremity pain. As a prognostic tool, ultrasound-guided caudal epidural block can be utilized as a prognostic indicator of the degree of motor and sensory impairment that the patient may experience if lower lumbar and/or sacral nerve roots are going to be destroyed in an effort to palliate intractable pain in patients too sick to undergo neurosurgical destructive procedures. In the acute pain setting, ultrasound-guided caudal epidural block with local anesthetics and/or opioids may be used to palliate acute pain emergencies while waiting for pharmacologic, surgical, and/or antiblastic methods to become effective. This technique has great clinical utility in both children and adults when managing acute postoperative and posttrauma pain. Sympathetically mediated pain syndromes including the pain of acute herpes zoster of the lumbosacral dermatomes and the pain of urethral calculi can also be effectively managed with epidurally administered local anesthetics, steroids, and/or opioids. Additionally, this technique is of value in patients suffering from acute vascular insufficiency of the lower extremities secondary to vasospastic and vasoocclusive disease, including frostbite and ergotamine toxicity (Fig. 110.1). There is increasing evidence that the prophylactic or preemptive use of caudal epidural nerve blocks in patients scheduled to undergo lower extremity amputations for ischemia will result in a decreased incidence of phantom limb pain. The administration of local anesthetic and/or steroids via the ultrasound-guided caudal approach to the epidural space is useful in the treatment of a variety of chronic benign pain syndromes, including lumbar radiculopathy, low back syndrome, spinal stenosis, postlaminectomy syndrome, phantom limb pain, vertebral compression fractures, diabetic polyneuropathy, chemotherapy-related peripheral neuropathy, postherpetic neuralgia, reflex sympathetic dystrophy, orchalgia, proctalgia, and pelvic pain syndromes.

Pain of malignant origin involving the groin, back, pelvis, perineum, rectum, and lower extremities as well as spinal metastatic disease (especially from breast and prostate primary cancers) is also amenable to treatment with caudally administered local anesthetics, steroids, and/or opioids.

The five sacral vertebrae are fused together to form the triangular-shaped sacrum (Fig. 110.2). 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. The posterior sacral foramina are smaller than their anterior counterparts. 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. 110.2). These bony projections are called the sacral cornua and represent important clinical landmarks when performing ultrasound-guided caudal epidural nerve block. The U-shaped sacral hiatus is covered posteriorly by the sacrococcygeal ligament, which is also an important clinical landmark when performing ultrasound-guided caudal epidural nerve block (Fig. 110.3). 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 caudal epidural nerve block on a given patient.

Ultrasound-guided caudal epidural block can be carried out by placing the patient in the prone position with the patient’s abdomen resting on a thin pillow (Fig. 110.4). To relax the gluteal muscles, the patient is asked to turn his or her heels

outward (Fig. 110.5). A total of 12 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 and sacral hiatus is then prepped with antiseptic solution, and the sacral hiatus and cornua are palpated using a rocking motion (Fig. 110.6). A high-frequency linear ultrasound transducer is then placed over the lower sacrum in the transverse plane and slowly moved caudally until the sacral cornua are visualized (Figs. 110.7 and 110.8). The classic ultrasound appearance of the sacral cornua and their acoustic shadow is reminiscent of two nuns walking down the street (Fig. 110.9). Lying between the two nuns is the sacral hiatus, which provides access to the epidural space (see Fig. 110.8). Between the necks of the two is the sacrococcygeal ligament, which appears as a hyperechoic band-like structure (Fig. 110.10). Lying just beneath the sacrococcygeal ligament is the hypoechoic caudal
canal (see Fig. 110.10). The floor of the caudal canal will appear as a bright hyperechoic line (see Fig. 110.10).