Anatomy of the Epidural Space

The epidural space surrounds the dural sac and is bounded by the posterior longitudinal ligament anteriorly, the ligamenta flava and the periosteum of the laminae posteriorly, and the pedicles of the spinal column and the intervertebral foramina containing their neural elements laterally. The space communicates freely with the paravertebral space through the intervertebral foramina. Superiorly, the space is anatomically closed at the foramen magnum where the spinal dura attaches with the endosteal dura of the cranium. Functionally, however, local anesthetics can diffuse intracranially during excessively high epidural block. Caudally, the epidural space ends at the sacral hiatus which is closed by the sacrococcygeal ligament. The epidural space contains loose areolar connective tissue, semiliquid fat, lymphatics, arteries, an extensive plexus of veins, and the spinal nerve roots as they exit the dural sac and pass through the intervertebral foramina.

Investigators have defined the anatomy of the epidural space using anatomical dissection, epidural injections of resins,¹ MRI,^2, CT epidurography,³ epiduroscopy in cadavers and patients,^4,5,6 and most recently by cryomicrotome sectioning in cadavers frozen soon after death.^7,8 Now considered the gold standard of anatomic investigation due to the minimal amount of artifact associated with the technique, cryomicrotome sectioning has resulted in findings that differ from previous studies.

The lumbar epidural space in adults is segmented and discontinuous.
Areas of epidural fat under the ligamentum flavum extend under the laminae but are separated by areas where the posterior dura contacts, but does not adhere to, the periosteum of the lamina. This segmentation may impede the passage of an epidural catheter and promote coiling and misplacement. Contact with the pedicles also divides the posterior epidural space from the lateral epidural space. The anteroposterior dimension of the posterior space is greatest in the lumbar region and averages 5.0 - 6.0 mm in adult males (Figures 1,2,3,4,5,6).

The posterior epidural space becomes more continuous in the thoracic region.
In the thoracic region the anteroposterior dimension of the posterior epidural space decreases but the space becomes more continuous. A thin layer of epidural fat extends between the lamina and the dura (Figure 7). Epidural catheters placed thoracically may pass easier because areas where the dura meets bone are fewer. In more cephalad cervicothoracic regions, the epidural fat disappears and the dura directly contacts lamina (Figure 8). The shallow space provides little room for excessive needle advancement.

A homogenous semifluid fat pad free of vessels or fibrous septation occupies the posterior epidural space.
A delicate smooth capsule surrounds the fat and attaches it to the dorsal midline through a connective tissue pedicle. Typically, the capsule glides freely against the surface of the lamina and ligamentum flavum, but occasionally sends attachments to the spinal roots and dura. The fat pad may absorb local anesthetics when an epidural needle and catheter are introduced directly into it using a midline approach through the pedicle. When the epidural space is entered off the midline, however, local anesthetics likely spread in the tissue planes around the fat pad and dissect the capsule away from the boney and ligamentous walls of the spinal canal. These differences may account for some of the variability in response observed with epidural anesthetics.

The lateral epidural space comminicates freely with the paravertebral space through the interverterbal foramina
The intervening pedicle in contact with the dura separates the lateral epidural space from the posterior epidural compartment. Spinal roots, septated fat, and vessels fill the space. The space typically communicates freely with the paravertebral space through the intervertebral foramina. The open intervertebral foramina transmits intrabdominal pressure directly to the epidural space. Degenerative joint disease and aging can narrow the intervertebral foramina and prevent the spread of local anesthetic out the foramina, resulting in greater longitudinal spread of local anesthetics in the epidural space.

A rich venous plexus almost entirely fills the anterior epidural space
The anterior dura adheres tightly to the posterior longitudinal ligament, which stretches across the intervertebral discs to form the anterior epidural space between the posterior longitudinal ligament and the periosteum of the vertebral body. The dura and posterior longitudinal ligament blend with the annular ligament, dividing the anterior epidural space into vertical compartments at each vertebral level. In areas immediately next to the intervertebral discs, dense connective tissue extensions extend superiorly and inferiorly, further dividing the anterior epidural space into lateral halves. In lumbar but not midvertebral levels, a membranous extension of the posterior longitudinal ligament joins with the neural elements laterally and isolates the anterior epidural space from the posterior and lateral epidural space.

A rich venous plexus surrounded by minimal amounts of fat almost entirely fills the anterior epidural space. In the thoracolumbar region (T10 - L2) the basivertebral vein originates from this venous plexus and extends into the vertebral bodies. As the size of the dural sac relative to the epidural space decreases at the L4-L5 level, the posterior longitudinal ligament falls away from the anterior dura, and fat fills the anterior epidural space. The increasing amounts of epidural fat anteriorly may contribute to the long latency of epidural anesthesia typically observed in the L5 and S1 nerve roots.

The dura mater blends with the connective tissue of the epineurium in the dural cuff region
The pia and arachnoid membranes continue with the spinal nerve roots as they leave the spinal cord and exit through the intervertebral foramina, where they blend with the perineurium of the spinal nerves. The dura mater also extends over the nerve roots laterally, but becomes much thinner and blends with the connective tissue of the epineurium. Spinal arteries, veins, and lymphatics pierce the dura in this region as they pass to the spinal cord through the subarachnoid space.

A midsagittal gap between the two halves of the ligamentum flavum is common in the thoracic and cervical regions.
The lateral halves of the ligamenta flava meet variably in the midline at an angle less than 90 degrees and form a steeply arched roof over the lumbar posterior epidural space. A midsagittal gap between the ligamenta flava in the midline is common in the thoracic and cervical regions, where it occurs in half of the segments (Figure 9). The midsagittal gap may contribute to a variable loss of resistance when the midline approach is used to enter the epidural space, although advancing a needle through the interspinous ligament probably produces a loss of resistance despite the absence of the ligamentum flavum.

Cryomicrotome sectioning has found no evidence of a midline dorsal connective tissue band or any septation of the posterior epidural space.
Several studies support the existence of dorsomedian ligamentous strands that extend from the ventral side of the vertebral arch and draw the dura posteriorly in a dorsomedian dural fold, the plica mediana dorasalis (Figure 10). Latex casts of the epidural space exhibit filling defects in the dorsal midline, which occasionally are associated with asymmetric filling of the lateral epidural compartments.¹ Under epiduroscopy, the plica appears to fix the dura mater closely to the ligamentum flavum and cause a tenting of the dura and a narrowing of the epidural space in the midline.^4,5,6 CT epidurography also demonstrates a connective tissue band that divides the posterior epidural space in the midline, as well as a transverse connective tissue plane that further divides the posterior epidural space into ventral and dorsal compartments.³ The plica as described functionally divides the posterior epidural space into lateral compartments and narrows the space in the midline. Investigators have proposed that this segmentation of the epidural space may occasionally impede epidural catheter placement, or cause maldistribution of local anesthetics and unilateral or patchy anesthesia.

In contrast to previous studies, cryomicrotome sectioning has found no evidence of the plica or any septation of the posterior epidural space.^7,8 The appearance of the connective tissue band may be due to the presence of the epidural fat pad that has fallen away from the lateral ligamentous and periosteal lining of the spinal canal, yet remains tightly connected by its pedicle to the apex of the epidural space. Distortion by the injection of latex, radiocontrast, or air into the epidural space may compress the fat pad and exaggerate the appearance of a thin connective tissue band and tenting of the dura into a dorsomedian dural fold.

Epidural veins
The epidural venous plexus is a valveless system that communicates with the basivertebral vein, the intracranial sigmoid, occipital, and basilar venous sinuses, and the azygous system. Drugs, air, or other material injected into the epidural space can potentially reach the heart or brain directly through this route. Abdominal and thoracic veins connect with the venous plexus through the intervertebral foramina, and transmit intraabdominal and intrathoracic pressure to the epidural space. Inferiorly, the venous plexus connects with the iliac veins through the sacral venous plexus.

Chronically increased intraabdominal pressure or obstruction of the inferior vena cava (as in late trimester pregnancy or in the presence of a large intraabdominal tumor) can distend the epidural venous plexus, with important implications for epidural anesthesia. This increases the risk of intravascular cannulation with an epidural catheter. It effectively decreases epidural space volume, allowing local anesthetics to distribute more widely with resulting greater degrees of block. Exposure to greater vascular surface area also potentially increases the risk for local anesthetic toxicity due to absorption from the epidural space.

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