An epidural delivers medication into the epidural space, a narrow gap inside your spinal column that sits just outside the protective membrane (called the dura mater) surrounding your spinal cord. The needle enters your lower back, passes through several layers of tissue and ligament, and stops in this fat-filled space where nerves branch off from the spinal cord. From there, medication bathes those nerve roots and blocks pain signals before they reach your brain.
The Epidural Space in Your Spine
Your spinal cord runs through a bony canal formed by stacked vertebrae. Within that canal, the cord is wrapped in a tough, fluid-filled sac called the dura mater. The epidural space is the gap between the outside of that sac and the walls of the spinal canal itself. It runs nearly the full length of your spine, from the base of your skull down to your tailbone.
This space isn’t hollow. It’s filled with fat, connective tissue, blood vessels, and the nerve roots that branch off the spinal cord and exit through gaps between each vertebra. Those branching nerve roots are the target. When an anesthetic is injected into this space, it soaks into the nerve roots and temporarily blocks them from carrying pain signals. The spinal cord itself, sealed inside its protective sac, is not directly contacted.
What the Needle Passes Through
During placement, you’ll sit upright or lie curled on your side, rounding your back to open the spaces between vertebrae. The needle travels through a specific sequence of tissue layers before reaching its target:
- Skin and subcutaneous fat, the soft tissue of your lower back
- Supraspinous ligament, a tough band connecting the tips of your vertebrae
- Interspinous ligament, a deeper ligament between adjacent vertebrae
- Ligamentum flavum, a dense, elastic ligament that forms the back wall of the epidural space
The ligamentum flavum is the final barrier. Once the needle passes through it, it enters the epidural space. The clinician knows the needle is in the right place because of a technique called “loss of resistance.” As the needle advances through the tough ligaments, there’s significant resistance against a syringe attached to it. The moment the tip enters the epidural space, that resistance drops suddenly and saline or air flows easily from the syringe. This distinct change in feel is the primary way clinicians confirm they’ve reached the correct depth.
A thin, flexible catheter (a soft plastic tube) is then threaded through the needle into the epidural space. The needle is removed, and the catheter stays taped to your back so medication can be delivered continuously or in repeated doses. The needle used for this, called a Tuohy needle, has a curved tip with a side-facing opening that guides the catheter in the right direction rather than pushing it straight ahead into the dura.
Where Along the Spine It’s Placed
The vertebral level where the epidural goes depends entirely on what kind of pain needs to be controlled. For labor and delivery, the needle is inserted in the lumbar region, typically between the L1 and L5 vertebrae in your lower back. This is also below the point where the spinal cord ends in adults (around the L1-L2 level), which adds a margin of safety.
For surgeries on the chest or upper abdomen, the epidural is placed higher, in the thoracic region (T10 through L1). Thoracic epidurals are more technically demanding. Studies have found that first-attempt success rates for thoracic placement using traditional landmarks are around 35%, compared to nearly 69% when ultrasound guidance is used. Both approaches ultimately succeed over 90% of the time, but ultrasound tends to require fewer needle redirections.
How the Medication Works Once Inside
After injection, the anesthetic spreads through the fat and tissue of the epidural space and penetrates the nerve roots where they exit the spinal cord’s protective sac. The medication doesn’t need to reach the spinal cord itself. Instead, it blocks the nerve roots at the point where they’re most accessible, surrounded by epidural fat rather than sealed inside the dura.
This is different from a spinal block, which goes one layer deeper. A spinal injection pierces through the dura and delivers medication directly into the fluid surrounding the spinal cord. That produces faster, more complete numbness but without the option for continuous dosing through a catheter. An epidural is slower to take effect (typically 10 to 20 minutes) but can be topped up for hours, which is why it’s preferred for labor.
How Clinicians Confirm Correct Placement
Because the epidural space is only a few millimeters wide in some areas, confirming the catheter is in the right place matters. Beyond the initial loss-of-resistance technique, several checks help verify placement. A simple one involves holding up the saline-filled catheter and watching for a “meniscal drop,” a visible fall in the fluid level that occurs because of the slight negative pressure inside the epidural space. If the catheter were sitting in subcutaneous fat instead, the fluid wouldn’t drop.
Clinicians also check that they can’t draw back blood or spinal fluid through the catheter. Blood would suggest the tip is in an epidural vein, and clear fluid would mean it has punctured through the dura into the spinal fluid space. A small test dose of medication is often given while monitoring your heart rate and blood pressure for signs that the catheter has ended up in a blood vessel or too deep.
What Can Go Wrong With Placement
The most common concern is accidental dural puncture, where the needle goes slightly too far and pierces the dura mater. A large retrospective study of over 26,000 patients found this complication occurred in about 0.36% of labor epidurals. When it happens, spinal fluid can leak through the puncture site, which sometimes causes a distinctive positional headache (worse when upright, better when lying flat) in the days afterward. This headache is treatable, and the puncture typically heals on its own.
Incomplete pain relief is another possibility. Because the epidural space contains connective tissue and compartments that vary from person to person, medication doesn’t always spread evenly. This can result in a “patchy” block where some areas remain sensitive. Repositioning the catheter or adjusting the dose usually helps, though occasionally the epidural needs to be replaced entirely.
The space itself also changes shape depending on where along the spine you look. In the lumbar region, the epidural space is widest at the midline, roughly 5 to 6 millimeters deep. In the thoracic spine, it narrows considerably. These anatomical differences are part of why placement technique and level selection matter so much for effective pain control.