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Caudal epidural injection is the administering of medications into the epidural space via the sacral hiatus route.1,2 It is an injection technique for the treatment of low back pain caused by disease entities such as lumbosacral nerve root compression.2 Successful caudal epidural injection relies on accurate placement of a needle into the epidural space (sacral canal) through the sacral hiatus. Even with experienced physicians, the failure rate of inserting the needle accurately into the caudal epidural space via the sacral hiatus can be up to 25% when using the blind method.3 Chen et al.2 have demonstrated that musculoskeletal ultrasound can be used as an adjuvant tool in placing the needles accurately into the caudal epidural space.
Ultrasound-Guided Caudal Epidural Injection—How It Is Approached
With the patient placed in a prone position, the ultrasound transducer is first placed transversely at the midline to obtain the transverse sonographic view of the sacral hiatus. It is under this transverse view that the cornua of the sacrum, the sacrococcygeal ligament, and the sacral hiatus opening can be observed (Fig. 1).2 Then, the transducer is rotated 90 degrees to rest between the two cornua and to obtain the longitudinal sonographic view of the sacral hiatus. It is under the longitudinal view that the caudal epidural needle is guided by ultrasound and inserted into the caudal epidural space.2 The advancement of the needle between the two cornua to the sacral hiatus and then into the caudal epidural space can be observed through continuous and real-time sonographic imaging.
When the needle pierces through the sacrococcygeal ligament and enters the sacral hiatus, the portion of the needle inside the caudal epidural space can no longer be observed under sonography (Fig. 2). This is because ultrasound waves cannot penetrate through the posterior sacral bony surface situated on the top of the needle. Ultrasound-guided injection technique has been shown to have 100% accuracy in correct needle placement.2 The sacral canal is filled with fluid, fat, and loose areolar connective tissue. These structures allow the spread of the injected medical solution in a rostral direction. Complications after caudal injection often result from misplacement of the needle into the intravascular, intraosseous, and even intrathecal locations, possibly leading to detrimental injection technique failure, systemic toxicity, or accidental spinal anesthesia. As a result, it is crucial that a negative pressure should be applied to the syringe first to ensure that no blood is aspirated before further injection can be continued.4
Ultrasound can provide sonographic images that reveal anatomic variations of the sacral hiatus, such as a closed or small sacral hiatus.5 It has been shown that ultrasound-guided caudal epidural injection cannot be performed in patients with sonographic images indicating a closed sacral hiatus (no opening to insert the needle into the sacral canal). Sacral hiatus with diameters ranging from 1.2 to 1.6 mm (significantly smaller as compared with the average diameter) may indicate a higher failure rate in performing caudal epidural injections.3
Ultrasound-guided caudal injection technique has been shown to have 100% accuracy in correct needle placement into the sacral canal for subsequent epidural injection. This video thoroughly explains the anatomic structures of the sacral hiatus when viewed under musculoskeletal ultrasound and how the injection needle is guided into the sacral canal. Before injecting, it is recommended that a negative pressure should be applied to the syringe first to ensure that no blood is aspirated. This is to prevent possible detrimental intravascular, intraosseous, and even intrathecal injections.
1. Chen CP, Lew HL, Tsai WC, et al.: Ultrasound-guided injection techniques for the low back and hip joint. Am J Phys Med Rehabil
2011; 90: 860–7
2. Chen CP, Tang SF, Hsu TC, et al.: Ultrasound guidance in caudal epidural needle placement. Anesthesiology
2004; 101: 181–4
3. Chen CP, Wong AM, Hsu CC, et al.: Ultrasound as a screening tool for proceeding with caudal epidural injections. Arch Phys Med Rehabil
2010; 91: 358–63
4. Park JH, Koo BN, Kim JY, et al.: Determination of the optimal angle for needle insertion during caudal block in children using ultrasound imaging. Anaesthesia
2006; 61: 946–9
5. Sekiguchi M, Yabuki S, Satoh K, et al.: An anatomic study of the sacral hiatus: A basis for successful caudal epidural block. Clin J Pain
2004; 20: 51–4