The epidural space is a narrow, fat-filled gap that runs the length of your spinal canal, sitting between the protective membrane around your spinal cord (called the dura mater) and the bones and ligaments of your vertebral column. It ranges from less than half a millimeter wide in some areas to about 7.5 mm at its widest. This space matters in medicine because it provides a route for delivering pain-relieving and anti-inflammatory medications directly to spinal nerve roots, which is why you hear about it most often in the context of epidural anesthesia during childbirth or steroid injections for back pain.
Where the Epidural Space Sits
The space starts at the base of the skull, where two layers of the dura mater fuse together at an opening called the foramen magnum. From there it extends all the way down to the bottom of the sacrum, the triangular bone at the base of your spine. Think of it as a sleeve that wraps around the dural sac (the fluid-filled tube protecting your spinal cord and nerves) with a thin cushion of tissue separating the sac from bone.
The boundaries are defined by the structures surrounding it on all sides. In front, the space is bordered by the back surfaces of the vertebral bodies, the discs between them, and a ligament called the posterior longitudinal ligament. Behind, it’s bordered by the laminae (flat plates of bone on each vertebra), the capsules of the facet joints, and an important elastic ligament called the ligamentum flavum. On either side, the bony pedicles of each vertebra and the openings where spinal nerves exit (intervertebral foramina) form the lateral walls.
What’s Inside
The epidural space is not empty. It contains three main components: fat, veins, and loose connective tissue. The fat is unencapsulated, meaning it grows freely through the space rather than being contained in a defined pocket. This epidural fat acts as a cushion and shock absorber for the dural sac and nerve roots during spinal movement.
Threaded through that fat is an extensive network of veins known as the spinal epidural venous plexus. This network includes veins running along both the front and back of the space, connected by smaller veins that drain blood from the vertebral bodies themselves. These veins have no valves, which means blood can flow in either direction depending on pressure changes in the chest and abdomen. The venous plexus serves as an alternative drainage route for blood from the spine and pelvis.
The space also contains spinal nerve roots passing through on their way out of the spinal canal. The anterior (front) portion houses the ventral nerve roots that carry motor signals, while the posterior (back) portion contains the dorsal nerve roots responsible for sensation. This is precisely why medications injected into the space can block pain signals so effectively.
How Wide It Is
The epidural space is not uniform. Its width varies significantly depending on the spinal level, and these measurements matter for anyone performing procedures in the area. At the junction of the cervical and thoracic spine (C7-T1), the space narrows to just 0.4 mm. In the upper thoracic region, the posterior space reaches its maximum width of about 7.5 mm. In the lumbar spine, where epidurals are most commonly placed, the widest point is about 5 mm at the L2 vertebra. Throughout most of the spinal canal, the space measures roughly 4 to 6 mm from the bony borders to the dura.
These dimensions explain why lumbar epidurals are the most common approach. The space is wide enough in the lower back to safely accommodate a needle and catheter, while the cervical region is so tight that the margin for error shrinks considerably.
Pressure Inside the Space
The epidural space maintains a pressure of approximately 9 mmHg when you’re lying on your back. This pressure fluctuates in a wave-like pattern, influenced by your breathing, arterial and venous blood flow in and around the brain and spinal cord, and even changes in abdominal pressure. When you bear down, cough, or strain, abdominal pressure rises, blood gets pushed into the epidural veins, and the pressure in the space temporarily increases.
This pressure behavior is clinically useful. When a clinician places an epidural needle, they rely on a technique called “loss of resistance.” As the needle passes through the dense ligamentum flavum (the tough elastic ligament at the back of the space), there’s notable resistance. The moment the needle tip crosses into the epidural space, that resistance drops. The ligamentum flavum forms the anatomic basis for this technique, essentially acting as the doorway into the space.
How It Appears on Imaging
On both CT and MRI scans, the normal epidural space shows up as a thin layer of fat surrounding the dural sac. On CT, it appears with the same density as fat elsewhere in the body. On MRI, it has the characteristic bright signal of fat on certain sequences. The venous plexus within the space is usually hard to see on standard imaging because flowing blood tends to produce a dark signal void, though dilated veins sometimes become visible in people with prominent epidural fat.
Imaging becomes important when something goes wrong in the space. Blood collections (hematomas), infections (abscesses), or excessive fat buildup (epidural lipomatosis) all show up as abnormal areas that compress the dural sac or nerve roots. Calcification of the surrounding ligaments, particularly the ligamentum flavum, is easier to spot on CT, while soft tissue abnormalities are better characterized on MRI.
Medical Uses of the Space
The epidural space is one of the most commonly accessed areas in pain medicine. Epidural anesthesia, used during labor, surgeries, and post-operative pain management, works by injecting anesthetic medications into the space where they bathe the spinal nerve roots and block both sensory and motor signals. During placement, clinicians often inject 5 to 10 mL of saline first to gently expand the space and reduce the risk of injuring a blood vessel.
Epidural steroid injections are another major application, used since the 1950s to treat pain from herniated discs, spinal stenosis, degenerative disc disease, and a range of other conditions that irritate or compress spinal nerve roots. In a herniated disc, the soft center of the disc pushes through its outer layer and pinches an adjacent nerve root, causing inflammation. Steroids injected into the epidural space reduce that inflammation at the source. A large systematic review of 70 studies found good evidence that lumbar epidural steroid injections work for disc herniations, fair evidence for spinal stenosis, and poor evidence for failed back surgery syndrome.
The list of conditions treated through epidural injections is broad: bone spurs pressing on nerve roots, thickened ligaments narrowing the spinal canal, compression fractures causing radiating pain, post-surgical pain syndromes, and even nerve pain following shingles.
Risks of Epidural Procedures
The most serious rare complication of epidural catheter placement is an epidural hematoma, a collection of blood within the space that can compress the spinal cord or nerve roots. A large review of 43,200 epidural catheterizations found that about 1 in 430 patients developed symptoms concerning enough to warrant imaging, but only 6 confirmed hematomas were identified across all those cases. That works out to roughly 1 in 7,200 procedures. While the absolute risk is small, it’s the reason clinicians screen carefully for bleeding disorders and blood-thinning medications before performing epidural procedures.