Ultrasound-assisted neuraxial blocks

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.

Technique

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.