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Brown: Atlas of Regional Anesthesia, 3rd ed., Copyright © 2006 Saunders, An Imprint of Elsevier
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Chapter 47 – Neuraxial Anatomy

Neuraxial blocks—spinal, epidural, caudal—are the most widely used regional blocks. The main reasons for their popularity are that the blocks have well defined endpoints, 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 develop an understanding of the neuraxial anatomy.

As illustrated in Figure 47-1 , to understand the neuraxial anatomy it is necessary to develop a concept of the relation between the surface and bony anatomy pertinent to neuraxial structures. Beginning cephalad, the spine 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 T7 spine. The lower extent of the spinal cord, the conus medullaris, ends in the adult at approximately L1. (In infants the conus medullaris may extend to L3.) The line between the iliac crests, Tuffier’s or intercrestal line, most often crosses through the L4 spine. A line drawn between the posterosuperior iliac spines identifies the level of the second sacral vertebra and the distal extent of the dural sac containing cerebrospinal fluid (CSF).

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Figure 47-1  Neuraxial anatomy: surface relations.


The 33 vertebrae from C1 to the tip of the coccyx have a number of features in common, as well as differences, which 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. 47-2 ). Figure 47-3 outlines the general relations of these structures at levels that correspond to common sites for cervical, thoracic, and lumbar punctures of the neuraxis.

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Figure 47-2  Neuraxial anatomy: lumbar vertebrae.


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Figure 47-3  Neuraxial anatomy: vertebral column relations.


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 does the spinous process 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 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 that requires emphasis 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 (see Fig. 47-3 ), from cephalad to caudad the vertebral laminae become more vertical in orientation. Both these features are important for understanding the concept of “walking” needles off the lamina into the desired neuraxial locations.

In addition to the bony relations of the vertebral bodies, there are important ligamentous relations. As illustrated in Figure 47-4 , 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 the spinous processes of vertebral bodies 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 spines.

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Figure 47-4  Neuraxial anatomy: lumbar vertebral ligaments.  (From Zarzur E: Anatomic studies of the human lumbar ligamentum flavum. Anesth Analg 63:499–502, 1984, with permission.)



Most neuraxial blocks are performed in the lumbar region. Figures 47-5 , 47–6 , and 47–7 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, they 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. Between the pia and the arachnoid 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 a 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. 47-6 ). 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 utilized 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.

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Figure 47-5  Neuraxial anatomy: posterior lumbar details.


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Figure 47-6  Neuraxial anatomy: lateral lumbar details.


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Figure 47-7  Neuraxial anatomy: cross-sectional 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 the 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 follows 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 the unilateral epidural block must be considered in light of other cryomicrotome 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 fusion of the five sacral vertebrae, whereas the sacral hiatus results from 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 that is a functional counterpart to the ligamentum flavum ( Fig. 47-8 ). 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. 47-8 ). In the sacral canal, the nerves of the cauda equina continue their routes until they exit via their respective vertebral foramina. Once again, the dural sac continues to the level of S2, or the line joining the posterosuperior spines.

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Figure 47-8  Neuraxial anatomy: sacrum.


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