The epidural space is an anatomical region running the length of the spine, situated within the bony vertebral canal but outside the central nervous system’s immediate coverings. It acts as a protective layer surrounding the spinal cord and its delicate membranes. The space is crucial in human anatomy, serving a structural function and playing a significant role in modern medical procedures. The term “epidural” literally means “upon the dura,” referring to its location relative to the outermost protective sheath of the spinal cord.
The Exact Location Within the Spinal Column
The epidural space is a thin, tubular region that extends vertically from the base of the skull down to the tailbone. Its superior boundary is defined by the foramen magnum, the large opening at the bottom of the skull where the spinal cord begins.
At this point, the inner and outer layers of the dura mater fuse, effectively sealing the space cranially. The space continues downward until it reaches the sacral hiatus, a bony opening near the coccyx, where it is sealed by the sacrococcygeal membrane.
Internally, the space is bordered by the dura mater, which is the tough, fibrous outermost layer of the three meninges that cover the spinal cord. The external and lateral boundaries are formed by the inner lining of the vertebral canal, including the bones of the vertebrae, the posterior longitudinal ligament, and the ligamentum flavum.
The depth of this space is not uniform, often measuring between four and six millimeters, with variations along the different sections of the spine. The epidural space completely encircles the spinal cord’s protective casing, running the entire length of the column.
Components and Purpose of the Epidural Space
The epidural space is not empty but is filled with a specific collection of tissues that serve a protective and structural function. A significant component is loose connective tissue and a considerable amount of fatty tissue, known as epidural fat. This fat acts as a shock absorber and cushion, protecting the dural sac and the nerves within it from trauma or pressure changes that occur with spinal movement.
The space also contains a complex network of vessels, including lymphatic channels and a rich, valveless system of veins called the epidural venous plexus. Spinal nerve roots also pass through the epidural space as they exit the central nervous system, making their way through the intervertebral foramina.
Why This Area is Used for Pain Management
The clinical relevance of the epidural space lies in its direct proximity to the spinal nerve roots. By delivering medication into this area, medical professionals can achieve regional anesthesia or analgesia, which is targeted pain relief. Local anesthetics injected here diffuse across the dura mater and bathe the nerve roots, temporarily blocking the transmission of pain signals to the brain.
This technique allows for pain relief without requiring a patient to lose consciousness, making it a common approach in various settings. Epidurals are frequently used during labor and delivery, and they are also utilized for post-operative pain control following major surgeries. A thin catheter can provide sustained relief for several days.
Understanding the Difference Between Epidural and Spinal Injections
A common point of confusion is the difference between an epidural injection and a spinal injection, which are distinct procedures targeting different anatomical locations. The epidural space is situated outside the dura mater. A spinal injection, or spinal block, involves inserting a much finer needle deeper to bypass the dura mater and enter the subarachnoid space.
The subarachnoid space is the inner layer, which contains the cerebrospinal fluid (CSF) that directly bathes the spinal cord and nerves. Because the medication is injected directly into the fluid surrounding the nerves, a spinal injection typically provides a rapid onset of anesthesia. In contrast, an epidural injection takes longer, often 10 to 20 minutes, for the medication to diffuse through the epidural fat and tissue to reach the target nerves.
The ability to place a catheter in the epidural space allows for continuous medication delivery and prolonged pain management. This feature is not typically available with a single-shot spinal injection.