Neuraxial Block Anatomy

Neuraxial blocks—spinal, epidural, and caudal—are the most widely used regional blocks. The main reasons for their popularity are that neuraxial blocks have well-defined end points and the anesthesiologist can produce the blocks reliably with a single injection. The first step in being able to use neuraxial blocks effectively is to gain an understanding of neuraxial anatomy.

To understand the neuraxial anatomy it is necessary to develop a concept of the relationship between surface and bony anatomy pertinent to the neuraxial structures (Fig. 39-1). Beginning cephalad, the spinous process of the seventh cervical vertebra, the vertebral prominence, is the most prominent midline structure at the base of the neck. A line drawn between the lower borders of the scapula crosses the vertebral axis at approximately the spinous process of T7. The lower extent of the spinal cord, the conus medullaris, ends in the adult at approximately L1. (In the infant the conus medullaris may extend to L3.) The line between the iliac crests (Tuffier’s or the intercrestal line) most often crosses through the spinous process of L4. A line drawn between the posterior superior iliac spines identifies the level of the second sacral vertebra and the caudal extent of the dural sac containing cerebrospinal fluid (CSF).

Figure 39-1. Neuraxial anatomy: surface relationships.

The 33 vertebrae from C1 to the tip of the coccyx have a number of common features as well as differences that should be highlighted. Each vertebra contains a spinous process joined to the lamina, from which a transverse process extends laterally into both lamina and pedicle. The pedicle joins this posterior assembly to the vertebral body, which relates to the neighboring vertebral bodies through both superior and inferior facet joints (Fig. 39-2). Figure 39-3 outlines the general relationship of these structures at levels that correspond to common sites for cervical, thoracic, and lumbar punctures of the neuraxis.

Figure 39-2. Neuraxial anatomy: lumbar vertebra.

Figure 39-3. Neuraxial anatomy: vertebral column relationships.

The lateral, oblique, and posterior views highlight two features of the bony anatomy that need emphasis. First, in the cervical and lumbar vertebrae, the spinous process assumes a more horizontal orientation than in the mid-thoracic region. The caudal angulation of the spinous process in the mid-thoracic region highlights why needle puncture of the neuraxial structures in this area requires a more cephalad needle angulation. Conversely, in both the cervical and lumbar regions, it is possible to use a more direct (perpendicular) needle angle to reach the neuraxial structures. The second feature is the angulation of the lamina immediately lateral to the spinous process in the three regions. As illustrated by the black line in the lateral view of the vertebral bodies, from cephalad to caudad the vertebral laminae become more vertical. Both of these features are important for understanding the technique of “walking” needles off the lamina into the desired neuraxial locations.

In addition to the bony relationships of the vertebral bodies, there are important ligamentous relationships. As illustrated in Figure 39-4, defining the posterior limit of the epidural space is the ligamentum flavum, or “yellow ligament.” This ligament extends from the foramen magnum to the sacral hiatus. Although classically portrayed as a single ligament, it is really composed of two ligamenta flava, the right and the left, which join in the midline. The ligamentum flavum is not uniform from skull to sacrum or even within an intervertebral space. Within an individual intervertebral space, the ligamentum flavum is thicker caudally than cephalad and thicker in the midline than on its lateral borders. Immediately posterior to the ligamentum flavum are either the lamina and spinous processes of the vertebral body, or the interspinous ligament. Extending from the external occipital protuberance to the coccyx posterior to these structures is the supraspinous ligament, which joins the vertebral spinous processes.

Figure 39-4. Neuraxial anatomy: lumbar vertebral ligaments.

(From Zarzur E. Anatomic studies of the human lumbar ligamentum flavum. Anesth Analg. 1984;63:499-502, with permission.)

Most neuraxial blocks are performed in the lumbar region. Figures 39-5, 39-6, and 39-7 illustrate the lumbar anatomy in the posterior, lateral, and horizontal planes, respectively. Surrounding the spinal cord in the bony vertebral column are three membranes. From the immediate overlay of the cord to the periphery, these are the pia mater, the arachnoid mater, and the dura mater. The pia mater is a highly vascular membrane that closely invests the spinal cord. The arachnoid mater is a delicate, nonvascular membrane that is closely attached to the outermost layer, the dura mater. Between the pia mater and the arachnoid mater is the space of interest in spinal anesthesia, the subarachnoid space. In this space are the CSF, spinal nerves, a trabecular network between the two membranes, blood vessels that supply the spinal cord, and the lateral extensions of the pia mater, the dentate ligaments. These dentate ligaments supply lateral support from the spinal cord to the dura mater and may become important conceptually when unilateral or patchy spinal anesthesia results from what appears to be a technically adequate block. The third and outermost membrane in the spinal canal is the longitudinally organized fibroelastic membrane called the dura mater (or theca). This layer is the direct extension of the cranial dura mater and extends as spinal dura mater from the foramen magnum to S2, where the filum terminale (an extension of the pia mater beginning at the conus medullaris) blends with the periosteum on the coccyx (see Fig. 39-6). There is a potential space between the dura mater and the arachnoid, the subdural space, which contains only small amounts of serous fluid to allow the dura and arachnoid to move over each other. This space is not intentionally used by anesthesiologists, although injection into it during spinal anesthesia may explain the occasional failed spinal anesthetic and the rare “total spinal” after epidural anesthesia, when there was no indication of errant injection of the local anesthetic into the CSF.

Figure 39-5. Neuraxial anatomy: posterior lumbar details.

Figure 39-6. Neuraxial anatomy: lateral lumbar details.

Figure 39-7. Neuraxial anatomy: cross-sectional (horizontal) lumbar details.

Surrounding the dura mater, and in its posterior extent immediately anterior to the ligamentum flavum, is another space effectively used by anesthesiologists, the epidural space. The spinal epidural space extends from the foramen magnum to the sacral hiatus and surrounds the dura mater anteriorly, laterally, and, more usefully, posteriorly. Contents of the epidural space include the nerve roots that traverse it from the intervertebral foramina to peripheral locations, as well as fat, areolar tissue, lymphatics, and blood vessels, which include the well-organized venous plexus of Batson.

Advances in epiduroscopy and epidurography provide an anatomic explanation for the occasional unilateral anesthesia that may follow an apparently adequate epidural technique. The almost universal appearance of a dorsomedian connective tissue band in the midline of the epidural space has been noted with these invasive imaging techniques as well as with some anatomic dissection specimens. This explanation for the occurrence of unilateral epidural block must be considered in view of microscopic thin-section anatomic evidence suggesting that the structure called the “dorsomedian connective tissue band” is really a midline posterior fat pedicle.

The anatomy important during caudal anesthesia is an extension of the epidural anatomy, although the frequent variations in sacral anatomy deserve emphasis. The sacrum results from the fusion of the five sacral vertebrae, whereas the sacral hiatus results from the failure of the laminae of S5 and usually part of S4 to fuse in the midline. The sacral hiatus results in a variably shaped and sized, inverted “V”-shaped bony defect, covered by the posterior sacrococcygeal ligament, which is a functional counterpart to the ligamentum flavum (Fig. 39-8). The hiatus may be identified by locating the sacral cornu (remnants of the S5 articular processes). This bony defect allows percutaneous access to the sacral canal, although the frequent anatomic variation of the sacral hiatus can make caudal block confusing. The sacral canal is functionally the distal extent of the epidural space, and from this canal the pelvic sacral foramina open ventrally toward the ischial rectal fossa, whereas the dorsal sacral foramina open in a posterior direction (see Fig. 39-8). In the sacral canal, the nerves of the cauda equina continue until they exit through their respective vertebral foramina. Once again, the dural sac continues to the level of S2, or the line joining the posterior superior iliac spines.

Figure 39-8. Neuraxial anatomy: sacrum.