Ultrasound-Guided Caudal, Lumbar, and Epidural Injection

Part 1: Caudal Epidural Steroid Injection: A Primer on the Combined Use of Ultrasound and Fluoroscopy for Pain Management

Imanuel R. Lerman

Joseph C. Hung

Dmitri Souzdalnitski

The use of ultrasonography in pain medicine procedures allows the pain physician to eliminate or significantly reduce exposure to the ionizing radiation associated with fluoroscopy and computed tomography (CT)-guided procedures.¹^, ²^, ³^, ⁴^and ⁵ Ultrasound guidance in interventional pain procedures provides a unique advantage over the use of fluoroscopy alone by visualizing the advancing needle tip in real time in addition to the surrounding nerves, vasculature, bones, tendons, and musculature. This ability to visualize soft-tissue structures and especially the underlying vasculature in real time is touted as one of the major advantages of ultrasound guidance. Recent studies have shown that inadvertent vascular transgression can be avoided when ultrasound guidance is used in interventional pain procedures.⁶^, ⁷^, ⁸^, ⁹^and ¹⁰ In contrast, fluoroscopic guidance does detect intravascular transgression but only after the needle has entered the vessel.

However, there are clear limitations to the use of ultrasound as a sole imaging modality. This is especially true when performing interventional procedures near bony structures, as bony artifacts can obscure the visualization of the advancing needle tip and shaft. Bony artifacts or obscuration of the needle is termed bone shielding. Besides bone shielding, there can be significant decrement in the axial resolution of an ultrasound image when attempting deeper ultrasound-guided interventions. Accordingly, obtaining a clear ultrasound image in deeper tissues of an overweight or obese patients can be difficult. Without proper visualization, accuracy of needle placement also suffers and can result in inaccurate needle placement.¹¹^, ¹²^and ¹³ The use of fluoroscopic guidance may be more beneficial in this situation, as x-rays are able to penetrate bone and deeper tissues without a significant loss of image resolution, targeted structures, or the needle tip.

Sonofluoroscopic-guided caudal epidural steroid injection

There are established advantages when ultrasound guidance is used for caudal epidural steroid injection, including decreased exposure to ionizing radiation, visualization of the advancing
needle tip in real time, and delineation of soft-tissue anatomy. Clear visualization of the underlying anatomy of the sacral hiatus can be especially helpful, as this portion of the sacrum can have significant anatomical variation.¹⁴^, ¹⁵^, ¹⁶^, ¹⁷^and ¹⁸

The disadvantages of using ultrasound as the sole image guidance modality in the caudal epidural steroid injection is largely associated with inadequate visualization of the needle tip as it advances under bony structures, including the sacral corneae and apex of the sacral hiatus. At this point in the procedure, fluoroscopic guidance is necessary for clear visualization of the advancing needle tip. Likewise, it is necessary to confirm that the needle is extravascular and extradural by the injection of nonionic radiographic contrast under live fluoroscopy or digital subtraction angiography. Of note, intravascular uptake has been shown to occur at rates ranging from 9% to 40% even with the use of fluoroscopic guidance.¹⁹^and ²⁰ Moreover, needle aspiration has been shown to be insensitive, as well as nonspecific, for detecting intravascular needle placement when controlled with fluoroscopic digital subtraction angiography imaging.¹³^,²⁰^,²¹

Background: Caudal epidural steroid injection (via the sacral hiatus) offers an alternative and safe entry point for the epidural space. The caudal epidural steroid injection is considered to be of particular use in patients who have undergone prior lumbar surgery or instrumentation, as the prior surgery will result in a high likelihood of scarring at the site, which can obliterate the potential epidural space.

When the landmark palpation technique is used for caudal epidural access, failure rates range from 14% to 56%.²²^, ²³^, ²⁴^and ²⁵ This wide range in success rates has been attributed to sacral anatomical variation as well as differences in body habitus.¹³^, ¹⁴^, ¹⁵^, ¹⁶^, ¹⁷^and ¹⁸ Although ultrasound-guided caudal epidural steroid injection is a relatively new technique (first described by Klocke²⁶ in 2003), recent studies have confirmed that this approach is accurate, with success rates ranging from 96% to 100% when confirmed with contrast-enhanced fluoroscopic imaging.¹³^,²⁷

Anatomy: The sacral hiatus is defined as a natural defect secondary to incomplete fusion of the posterior midline section of the S4 and S5 vertebrae. This incomplete fusion results in a posterior opening between sacrum and coccyx, a defect that has been termed the sacral hiatus.

The sacral hiatus is bounded posterior superficially by the sacrococcygeal ligament, named for its location spanning between the tip of the sacrum to the coccyx. The sacrococcygeal ligament is covered by subcuTaneous fat and skin. The rostral-to-caudad length of the sacrococcygeal membrane has been measured in multiple studies and averages between 22 and 26 mm.¹⁴^, ¹⁵^, ¹⁶^and ¹⁷ The floor of the sacral hiatus comprises the posterior aspect of the sacrum, which can also be referred to as the dorsal surface of the anterior sacral plate. The medial cephalad boundary of the sacral hiatus, termed the sacral hiatus apex, is formed by the medial sacral crest at the caudalmost tip of the posterior sacral plate. The lateral boundaries of the sacral hiatus are formed by two sacral corneae. The axial disTance between the two sacral cornua is approximately 10 to 17 mm.¹⁷^,¹⁸ It should be noted that large anatomical studies have consistently demonstrated the absence of the sacral hiatus or one or both sacral cornua in 3% to 10% and 50% of patients examined, respectively.¹⁵^,¹⁷^,¹⁸^,²⁸ Anatomic structures found within the sacral hiatus include the caudal epidural canal and epidural venous plexuses. The anteroposterior diameter of the sacral hiatus just caudal to the apex ranges from 4.11 to 4.66 mm.¹⁵^, ¹⁶^and ¹⁷ Of note, the maximum anteroposterior diameter of the sacral hiatus has been shown to be consistently in the upper third of the sacral hiatus.¹⁶

The epidural space is contiguous from above the dens to the level of the sacral hiatus. The sacral epidural space proximal to the sacral hiatus contains nerve roots, spinal vessels, and the filum terminale. The dural sac usually terminates between levels S1 and S3.¹⁶^,¹⁷^,²² The disTance from the apex of the sacral hiatus to the caudal edge of the S2 vertebral level has been estimated at approximately 30 to 60 mm.¹⁶^,

Only gold members can continue reading. Log In or Register to continue