Epidural Space

The spinal epidural space is the space within the vertebral canal between the spinal dura mater and the surrounding walls of the canal. It extends along the spinal canal and contains fat, loose connective tissue, blood vessels, and components of the spinal nerve roots as they pass toward the intervertebral foramina.

Unlike the cranial cavity, where the periosteal layer of dura is closely adherent to the inner surface of the skull, the vertebral canal contains a true epidural space. The spinal dura consists primarily of the meningeal layer and is separated from the periosteum lining the vertebral canal by the contents of the epidural space.^[1][2]

The epidural space is clinically important because it provides access for epidural anesthesia and analgesia. It is also a potential site for hemorrhage, infection, tumor extension, and accumulation of other material capable of compressing the spinal cord or spinal nerve roots.

Location and Extent

The epidural space lies inside the vertebral canal but outside the spinal dura mater. It extends longitudinally from the upper end of the vertebral canal to the sacral region.

Superiorly, the spinal dura becomes continuous with the cranial dura at the foramen magnum. The epidural space does not continue into the cranial cavity as an equivalent normally patent space because cranial dura is closely associated with the internal surface of the skull.

Inferiorly, the dural sac extends to approximately the level of the S2 vertebra. The epidural space surrounds the dural sac throughout this course. Below the termination of the dural sac, the filum terminale externum continues toward the coccyx and blends with the periosteum.^[1][3]

Laterally, the epidural space communicates with tissues outside the vertebral canal through the intervertebral foramina. Spinal nerve roots and associated vessels pass through these regions as the roots unite to form spinal nerves.

Boundaries

The boundaries of the epidural space are formed externally by the structures making up the vertebral canal and internally by the spinal dura mater. The exact relationships vary according to vertebral level.

The posterior epidural space is generally more accessible than the anterior portion and contains a substantial amount of epidural fat. This posterior region is the usual route through which a needle enters the epidural space during interlaminar epidural procedures.

Contents

The epidural space is not empty. Its contents form a flexible layer between the dural sac and the rigid boundaries of the vertebral canal.

• Epidural fat: Adipose tissue that surrounds and cushions the dural sac and occupies much of the posterior and lateral epidural space.
• Loose connective tissue: Connective tissue contributes to the organization of the space and its relationship with the dura and vertebral canal.
• Internal vertebral venous plexus: A network of thin-walled, valveless veins running longitudinally within the vertebral canal.
• Arteries and small vascular branches: Segmental vessels supply structures within and around the vertebral canal.
• Spinal nerve roots: Root sleeves and nerve roots traverse the lateral epidural region as they approach the intervertebral foramina.

The relative amount of fat and the arrangement of vessels vary with vertebral level. These differences influence the distribution of injected solutions and the appearance of the epidural space on cross-sectional imaging.

Epidural Fat

Epidural fat is a characteristic component of the spinal epidural space. It is especially prominent posteriorly and laterally and provides a deformable layer between the dural sac and the surrounding vertebral structures.

The amount and distribution of epidural fat vary between spinal regions and between individuals. Because fat produces a characteristic signal on MRI, it also provides useful contrast for identifying the dural sac, nerve roots, ligaments, and pathological processes occupying the epidural space.

Excessive accumulation of epidural adipose tissue can narrow the available space within the vertebral canal. This condition, known as spinal epidural lipomatosis, can compress neural structures when sufficiently severe.

Internal Vertebral Venous Plexus

The epidural space contains the internal vertebral venous plexus, part of the extensive vertebral venous system. These veins run longitudinally within the vertebral canal and communicate with veins outside the canal through the intervertebral foramina.

The internal vertebral venous plexus is commonly divided into anterior and posterior components. The anterior internal vertebral veins are prominent along the posterior surfaces of the vertebral bodies, while posterior channels lie closer to the vertebral arches.

A notable feature of the vertebral venous system is the absence of valves. Blood can therefore flow in different directions depending on pressure gradients. The internal plexus communicates with external vertebral veins and with segmental veins at successive spinal levels.^[1][2]

These communications provide alternative pathways for venous return but also create anatomical routes through which infection or malignant cells can spread between the trunk, pelvis, vertebral column, and cranial region.

Relationship to the Spinal Meninges

Understanding the epidural space requires distinguishing it from the spaces associated with the spinal meninges. From superficial to deep, the relevant structures are the epidural space, dura mater, arachnoid mater, subarachnoid space, and pia mater.

This distinction is especially important during neuraxial procedures. Medication delivered into the epidural space remains outside the dura, whereas spinal anesthesia involves entry into the subarachnoid space and direct administration into cerebrospinal fluid.

Regional Anatomy

The dimensions and contents of the epidural space are not uniform throughout the vertebral canal. Its shape changes with the anatomy of the cervical, thoracic, lumbar, and sacral regions.

Cervical Region

The cervical epidural space is relatively narrow, particularly compared with the lumbar region. The spinal cord occupies a substantial proportion of the cervical vertebral canal, leaving less distance between the dura and surrounding canal walls.

Thoracic Region

In the thoracic region, the epidural space surrounds the dural sac within a vertebral canal whose dimensions and orientation differ from those of the cervical and lumbar regions. The overlapping thoracic spinous processes and narrower interlaminar spaces also influence posterior procedural access.

Lumbar Region

The lumbar epidural space is particularly important clinically. It contains abundant epidural fat and the vertebral venous plexus and surrounds the dural sac and descending nerve roots.

Because the adult spinal cord usually terminates near L1-L2, lower lumbar approaches occur below the spinal cord itself. The dural sac, however, continues to approximately S2 and contains the cauda equina within the cerebrospinal fluid of the subarachnoid space.

Sacral Region

The epidural space continues into the sacral canal around the lower portion of the dural sac. Below the termination of the dural sac at approximately S2, the sacral canal contains epidural tissues and the structures continuing toward the lower sacral and coccygeal regions.

The sacral hiatus provides an inferior route into the epidural space and can be used for caudal epidural anesthesia.

Epidural Anesthesia and Analgesia

The epidural space provides a route for administering local anesthetics, analgesics, and other medications near spinal nerve roots without intentionally entering the subarachnoid space. Drugs introduced into the space can spread across multiple spinal levels and act on neural structures as they traverse the vertebral canal.

In a common lumbar midline approach, the needle passes sequentially through:

1. Skin.
2. Subcutaneous tissue.
3. Supraspinous ligament.
4. Interspinous ligament.
5. Ligamentum flavum.
6. Epidural space.

The ligamentum flavum is an important landmark because the epidural space lies immediately deep to it in a posterior midline approach. Clinically, entry into the space is commonly identified using a loss-of-resistance technique.^[2][3]

An epidural catheter can be advanced through the needle and left within the space, allowing repeated or continuous administration of medication. Epidural techniques are used in several settings, including obstetric analgesia and perioperative pain control.

Epidural Space and Spinal Anesthesia

Epidural and spinal anesthesia use different anatomical compartments. In epidural anesthesia, the needle tip remains outside the dura. In spinal anesthesia, the needle passes through the dura and arachnoid to enter the subarachnoid space.

This distinction affects drug distribution and technique. Medication placed directly into cerebrospinal fluid acts differently from medication introduced into epidural tissues and diffusing toward spinal nerve roots.

Clinical Significance

The epidural space is clinically important not only as a route for anesthesia but also because its location within the rigid vertebral canal means that expanding lesions can compress nearby neural structures.

Spinal Epidural Hematoma

A spinal epidural hematoma is an accumulation of blood within the epidural space. As the collection enlarges, it can displace the dural sac and compress the spinal cord or cauda equina, depending on its level.

The neurological consequences depend on the location, size, and rate of expansion of the hematoma. The epidural venous plexus and other vessels within the space provide potential sources of bleeding.

Spinal Epidural Abscess

A spinal epidural abscess is an infectious collection within the epidural space. Because the space extends longitudinally along the vertebral canal, infection may spread across more than one vertebral level.

An expanding collection can compress the spinal cord or nerve roots. The epidural fat and venous connections are important anatomical features when considering the location and potential spread of infection.

Tumor and Metastatic Disease

The epidural space can be involved by tumors arising from or extending through vertebral structures. Metastatic disease involving a vertebral body may extend posteriorly into the epidural space and reduce the space available for the spinal cord or nerve roots.

The valveless vertebral venous plexuses also provide communication between vertebral veins and venous systems of the thorax, abdomen, and pelvis. These connections are anatomically relevant to the spread of metastatic disease to the vertebral column.^[1]

Imaging Anatomy

MRI is particularly useful for evaluating the epidural space because epidural fat provides natural contrast around the dural sac. Displacement or replacement of normal epidural fat can help identify masses, hemorrhage, infection, disc-related pathology, and other processes extending into the vertebral canal.

The spinal epidural space forms an important interface between the vertebral canal and the spinal meninges. Its fat, venous plexuses, segmental vessels, and relationship with the spinal nerve roots give it both anatomical and clinical importance, while its accessibility makes it a major route for neuraxial anesthesia and analgesia.