The spinal epidural space extends from the foramen magnum to the sacral hiatus. The caudal epidural space can be accessed through the sacrococcygeal ligament that covers the sacral hiatus. Caudal blocks provide anesthesia for genitourinary and anorectal surgical procedures. This procedure is normally performed by placing a needle or catheter through the sacrococcygeal ligament for injection of local anesthetic drugs. Unlike subarachnoid blocks, caudal blocks are relatively easy to perform in prone position. The caudal approach is usually the most superficial access to the epidural space.

The sacral hiatus is the caudal termination of the sacral canal. The volume of the epidural space within the sacral canal is highly variable, with estimates ranging from 10 to 26 mL in adults (Table 56-1).¹ In this study of 37 adults (23 female, 14 male), the sacrococcygeal membrane was significantly thicker in females than in males (mean values, 3.6 and 2.5 mm, respectively).¹ The sacral canal volume was significantly smaller in females than in males (mean values 13.2 and 16.5 mL, respectively).¹ Because the sacral canal volume varies, the dose required to achieve a given level of caudal epidural block varies from individual to individual.

Table 56-1 Magnetic Resonance Imaging Estimates of Caudal Space Anatomy in Adults

Adapted from Crighton IM, Barry BP, Hobbs GJ. A study of the anatomy of the caudal space using magnetic resonance imaging. Br J Anaesth 1997;78:391–5.

In adults, the dural sac of the subarachnoid space ends between the S1 and S2 sacral segments.¹ In this study, the distance between the dural sac and sacrococcygeal ligament ranged from 34 to 80 mm. The S5 and coccygeal nerves normally exit the sacral canal through the sacral hiatus. The sacral and coccygeal cornua (horns) articulate to form an archlike structure. Table 56-2 summarizes the estimates of the level of the caudal termination of the dural sac in adults gathered from several studies.

Variations in sacral anatomy are relatively common. A number of conditions can make caudal block difficult, including narrowing or complete absence of the sacral hiatus.^9,10 These conditions occur in 3% to 6% of anatomic specimens. The sacral cornua are prominent (>3 mm of bony prominence on each side) in only 21% of adult sacrums,⁹ and therefore assessment by palpation is problematic.

The sacral epidural space is highly vascular. Inadvertent intravenous injection is relatively common during caudal block, occurring in about 5% to 10% of these procedures.¹¹

Landmarks for caudal block are traditionally assessed by palpation. The posterosuperior iliac spines (the superolateral sacral crests of the sacrum) form an equilateral triangle with the sacral hiatus.¹⁰ Although this approximation is accurate, the clinical assessment of landmark position can be difficult. Traditional techniques also rely on tactile sense of needle entry into the caudal space. However, the sacrococcygeal ligament is soft in children and may therefore not be easy to detect manually by needle advancement.¹²

Ultrasound can be used to guide caudal blocks in pediatric and adult patients.^13,14 Sonography can determine the location and size of the sacral hiatus for needle tip placement. In addition, ultrasound can be used to image the distribution after caudal epidural injection. However, the bone of the sacrum prevents ultrasound imaging of most of the sacral canal. One concern is that acoustic shadowing from the overlying bone can prevent detection of intravascular injection during caudal blocks, particularly in adults. Ultrasound imaging may be of particular utility in guiding caudal injections in patients with spinal dysraphism.¹⁵

Suggested Technique

Caudal block with ultrasound is optimally performed in prone position with sterile transducer cover and skin preparation. The wide variety of transducer selections for caudal block depend on patient size.¹¹ In thin adult patients, a 5- to 10-MHz small-footprint “hockey-stick” transducer is appropriate. Average-sized adults image well with a standard linear transducer for the procedure. For obese patients, a lower frequency curvilinear array is optimum.

The sacral hiatus is easiest to image in transverse view over the midline.¹⁴ The sacral cornua appear as two reverse U-shaped hyperechoic structures. The sacrococcygeal ligament and base of the sacrum appear as two parallel bandlike structures between the cornua. The sacral hiatus lies between these two hyperechoic bands.

A 21-gauge, 5- to 7-cm echogenic needle can be used for in-plane caudal block through the sacral hiatus. Two common approaches are the longitudinal in-plane approach and transverse out-of-plane approach.

There is a characteristic tent and recoil when the sacrococcygeal ligament is punctured by the block needle. This is more likely to be detected with ultrasound imaging than by tactile sense. As the needle punctures the sacrococcygeal ligament, the needle tip disappears owing to acoustic shadowing from the overlying bone.

Ultrasound imaging can be used to assess the injection distribution for caudal block.¹³ Some investigators have favored a transverse view to assess the test injection distribution.¹⁶ With the probe in transverse position, it is possible to observe bilateral spread. Anterior displacement of the posterior dura occurs in more than 90% of caudal epidural injections.¹⁵ Turbulence of the injection, as manifested by a mosaic pattern on color Doppler, also indicates successful injection. Longitudinal paramedian view can be advantageous for assessing the level of injection in adults with the sacrum in the field as a reference point.

Clinical Pearls

General Comments

• Traditional approaches to caudal epidural blocks can be challenging in adults in part because the sacral cornua are difficult to palpate.

• Pilonidal cyst is a relative contraindication to caudal block.

Caudal Epidural Anatomy

• The sacral canal is a continuation of the vertebral canal that terminates as the sacral hiatus. The sacral hiatus is formed by nonfusion of the fifth sacral vertebral arch and is covered by the sacrococcygeal membrane. There is wide variation in the anatomy of the sacrum, particularly in pediatrics.^17,18 The caudal canal is narrowest at the sacral hiatus. Sonography can be used to determine whether the sacral hiatus is patent.

Caudal Epidural Ultrasound Anatomy

• The sacrococcygeal ligament can be visible as one or two layers in transverse view. These layers are much less echobright than the underlying bone of the sacral canal.

• The transducer must be tilted to enhance echoes from the sacrococcygeal ligament because it exhibits a high degree of anisotropy. For this same reason it is difficult to visualize the sacrococcygeal ligament along its entire length.

• In longitudinal view, the caudal epidural space appears as hypoechoic and tapering underneath the fibrillar sacrococcygeal ligament.

The Most Important Views

• The “two nuns” view is important because it clearly shows the sacral hiatus in transverse section.

• Perform the caudal block where there is the widest separation gap between the anterior and posterior surfaces of the sacral canal (the largest step-off).

Needle Priming

• Potential strategies to reduce tissue coring include use of a styletted needle, skin nicks, and priming the needle with fluid.

Needle Angle

• A shallow angle of approach is essential to place the needle within the epidural space of the sacral canal.

• The optimal angle for needle insertion, defined by the base of the sacrum and the skin surface, is about 21 degrees in children.¹²

• In adults, the angle of the sacral canal varies with gender. This angle is about 20 degrees in men and 30 to 45 degrees in women.

Needle Advancement

• Because the sacrococcygeal ligament is thick (≈3 mm in adults),¹ the needle should be advanced after visible and tactile sense of tent and recoil so that the entire bevel opening is within the caudal epidural space.^19,20

Verification of Caudal Epidural Needle Tip Placement

• Cardiac oscillations of a saline column waveform also can be used to confirm caudal epidural placement, similar to that described for other parts of the epidural space.²¹

Aspiration

• Aspirate to help rule out intravascular needle tip placement. Aspirate frequently because the caudal epidural space is highly vascular.

• Because of the overlying cornual bone, probe compression does not help identify epidural arteries and veins.

Injection

• Because caudal epidural injections have low resistance, the visible color distribution should fill the entire epidural space within the field of view.

• Caudal epidural injections can appear similar to intravascular injections with ultrasound imaging.

• Tenting of the sacrococcygeal ligament and caudal displacement of the canal are signs of correct caudal epidural injection.

• One dominant color indicates successful caudal epidural steroid injection.¹⁹

Clinical Assessment

• The lateral aspect of the foot is reliably innervated by S1 for cutaneous assessment of the upper extent of sacral caudal block.

• Narrow sacral canal has been associated with failure of caudal epidural injections. Specifically, no patients with sacral canal diameters less than 2 mm (as measured at the apex of the sacral hiatus) had successful blocks in one clinical series.²²

FIGURE 56-1 Longitudinal in-plane approach to caudal epidural block. Sonogram obtained before needle placement. In this sonogram, the sacrococcygeal ligament can be seen overlying the sacral hiatus.

FIGURE 56-2 Transverse view of the sacral cornua (A). These bones appear as inverted U shapes in transverse view with acoustic shadowing. The hyperechoic lines between the cornua are the sacrococcygeal ligament and underlying sacral bone, with the sacral hiatus lying between these two structures. More distally, the caudal epidural space narrows as the transverse processes of the first coccygeal vertebra come into view (B). This indicates the transducer is caudal to the sacral hiatus.

FIGURE 56-3 Longitudinal view of the sacrum demonstrating the presence (A) or absence (B) of the sacral cornua in the imaging plane.

FIGURE 56-4 Longitudinal in-plane approach to caudal epidural block (A). The corresponding transverse view is shown (B).

FIGURE 56-5 Transverse out-of-plane approach to caudal epidural block. The needle tip crosses the plane of imaging as an echogenic dot.

FIGURE 56-6 Pediatric caudal epidural guided by ultrasound. These transverse sonograms show the sacral cornua and caudal epidural space before (A) and after (B) injection of local anesthetic. The injection of local anesthetic distributes throughout the caudal epidural space and tents the sacrococcygeal ligament.