Prepuncture ultrasound scanning is helpful to determine the midline, the depth from the skin, the desired level, and rotation of the spine.
There are limited outcome data on the real-time guidance with ultrasound for neuraxial blocks.
The available evidence suggests that the use of ultrasound may improve the success rate from the first attempt, reduce the number of attempts, and improve patient comfort.
The use of ultrasound for epidural access is a technically advanced procedure. It requires adequate experience with ultrasound scanning and ultrasound guidance of the needle at deep levels with a less-than-optimal angle of incidence.
Thorough understanding of the neuraxial anatomy and conceptual visualization of the different echogenic structures are necessary for ultrasound scanning of the epidural space.
Relevant sonoanatomy of the spine
There are a couple of challenges for ultrasound imaging of the spine and the neuraxial structures. The depth of the spine makes it less than optimal for the ultrasound beams to produce higher-resolution images. Also, the osseous structure of the laminae and articular processes conceal the underlying neuraxial structures of interest to perform the blocks. Accordingly, it is important to have a visual appreciation of the spine outline when scanning for neuraxial blocks.
In general, scanning the vertebrae demonstrates three hyperechoic levels: the spinous process, the lamina, and the articular facet joint with the transverse process.
The spinous processes reflect the most superficial hyperechoic shadow closest to the skin. Careful scanning, cephalad and caudad to the spinous process, reveals an acoustic window representing the interspinous space occupied with the less echogenic interspinous ligaments. Careful examination of the deeper layer of the ligamentous structure shows the ligamentum flavum as a slightly more hyperechoic layer separated from another hyperechoic layer—the posterior dura—by the epidural space. The spinal canal underneath represents the next anechoic layer deeper to the posterior dura. On the deeper side of the spinal canal (anterior), the anterior dura with the postlongitudinal ligament form a hyperechoic structure called the anterior complex .
With the aforementioned echogenic characteristics of the neuraxial structures in mind, scanning the different levels of the spine leads to different views. These views also depend on the scanning plane and orientation of the ultrasound beam. These are the main planes for scanning the spine:
Median sagittal plane : A longitudinal scan along the midline where the beam of the ultrasound is parallel to the long axis of the spine on top of the spinous processes (unless the spine is scoliotic).
Paramedian sagittal plane : A longitudinal scan parallel to the long axis of the spine but off the midline. The beams are usually on top of the transverse processes or the laminae, with the articular joints (facets) between the adjacent spines.
Paramedian sagittal oblique plane: Another longitudinal plane that is similar to the paramedian sagittal plane with the probe tilted medially to direct the beams toward midline. The ultrasound beams usually travel across the laminae with the intervertebral foraminae in between. Access to the ligaments, dura, and spinal canal are usually accessed with the beam within the interlaminar windows.
Transverse axial view : A transverse view where the beam of the ultrasound is perpendicular to the long axis of the spine. With this orientation, the ultrasound probe can be on top of the vertebra where the beams cross the spinous process, lamina, and transverse process. If the beam is steered cephalad or caudad by sliding the probe or tilting it, an interspinous (acoustic) window is obtained. The ligaments, epidural space, and spinal canal can be visualized through this window.
Except for the caudal block, the neuraxial scanning requires a low-frequency (2–5 MHz) curvilinear probe to see the depth of the target structures. In addition to the ability to scan deeper structures, the curved probe provides a divergent beam, giving a wider field of vision and helping to scan the different anatomic structures in a single view compared with the limited field of vision produced by the linear probe. The disadvantage of the curved probe is a lack of spatial resolution at deeper levels, making viewing the needle a challenge when performing the block. Scanning the spine for procedures can be performed in sitting, lateral, and prone positions depending on the level of the procedure to be performed.
Caudal epidural block
Because the sacral hiatus is a relatively superficial structure, a high-frequency linear probe (6–13 MHz) is usually used for caudal scanning. The block is performed in the prone position with a pillow under the pelvis. The ultrasound probe is placed in a transverse orientation (axial scan) to scan the sacral cornua as two hyperechoic, reversed U-shaped structures. The sacrococcygeal ligament connecting both cornua, forming the superficial boundary of the sacral hiatus, appears as a hyperechoic band. The anterior boundary of the sacral canal is formed by the posterior surface of the sacrum, which appears as another hyperechoic linear structure anterior (deep) to the sacrococcygeal ligament. The sacral hiatus appears as a hypoechoic space between these two described hyperechoic lines. With this view, the needle can be introduced in the middle of the probe, perpendicular to the ultrasound beams, out of the plane approach, and targeting the sacral hiatus ( Fig. 39.1 ).