The dural sac is a protective tube of membrane that surrounds your spinal cord and spinal nerves, running through the center of your spinal column. Also called the thecal sac, it is filled with cerebrospinal fluid, the clear liquid that cushions and nourishes your central nervous system. If you’re reading about it for the first time, it’s likely because the term showed up on an MRI or imaging report.
What the Dural Sac Is Made Of
Your brain and spinal cord are wrapped in three layers of tissue called the meninges. From outermost to innermost, these are the dura mater, arachnoid mater, and pia mater. The dural sac specifically refers to the sleeve formed by the dura mater and the arachnoid mater together. The dura mater (Latin for “hard mother”) is the tough, thick outer layer. The arachnoid mater sits just beneath it, forming a more delicate middle layer. Between the arachnoid and the innermost pia mater is a fluid-filled space where cerebrospinal fluid circulates.
Inside the dural sac, you’ll find the spinal cord itself (in the upper spine), the bundle of spinal nerve roots that branch off from it, and the cerebrospinal fluid that acts as a shock absorber. This fluid prevents your spinal cord from bumping against bone during movement. The meninges also anchor the spinal cord in place and provide a support network for blood vessels and nerves that supply the central nervous system.
Where It Starts and Ends
The dural sac begins at the base of the skull, where it is continuous with the membranes covering the brain. It extends down through the spinal column but doesn’t reach the very bottom. In most adults, the dural sac ends somewhere between the L5-S1 disc (the lowest lumbar disc) and the upper portion of the S3 vertebra in the sacrum. This is lower than where the spinal cord itself ends. The spinal cord typically stops around the L1 or L2 vertebra, but the dural sac continues below that point, enclosing the dangling bundle of nerve roots known as the cauda equina.
Knowing exactly where the dural sac ends matters in clinical settings, particularly when doctors perform procedures like caudal anesthesia (an injection near the tailbone) and need to avoid puncturing it.
Normal Size and Measurements
On MRI scans, doctors measure the dural sac to check whether it’s being squeezed or abnormally stretched. Two measurements come up most often: the sagittal diameter (front-to-back width) and the cross-sectional area (a slice viewed from above).
In healthy adults, the sagittal diameter of the dural sac averages about 13.3 mm at the L4 vertebra and 9.9 mm at S1, naturally tapering as it descends. Cross-sectional area in people without spinal problems averages roughly 152 mm² at the L4-L5 level. These numbers serve as reference points. When the cross-sectional area drops below 100 mm², it’s considered relative stenosis (narrowing). Below 75 mm² is classified as absolute stenosis, meaning the dural sac is significantly compressed.
Why It Appears on Your MRI Report
The dural sac is one of the key structures radiologists evaluate on spinal MRI. If your report mentions “dural sac indentation” or “effacement of the thecal sac,” it means something is pressing into or flattening the sac. Common causes include bulging or herniated discs, bone spurs, and thickened ligaments. These are hallmarks of spinal stenosis, a condition where the spinal canal narrows and crowds the dural sac and the nerves inside it.
An important nuance: indentation of the dural sac on imaging doesn’t automatically mean the nerves inside are being harmed. Research on cervical spine conditions has shown that multiple dural sac indentations are often visible on MRI, but it’s rare for all of them to be causing symptoms. Not every indentation of the dural sac translates to compression of the spinal cord itself. Studies suggest the spinal cord generally needs to be compressed by at least 30% of its front-to-back diameter before tissue-level damage begins. In other words, a mild indentation noted on your MRI may be an incidental finding rather than the source of pain or numbness.
Dural Sac Compression and Spinal Stenosis
When the dural sac is significantly narrowed, it can squeeze the nerves inside it, leading to symptoms like leg pain, numbness, tingling, and difficulty walking. This is lumbar spinal canal stenosis, one of the most common reasons the dural sac gets mentioned in clinical conversations. People with this condition often notice that their legs feel heavy or weak after walking a certain distance, a pattern called neurogenic claudication, which improves when they sit down or lean forward.
Doctors use the dural sac cross-sectional area as a more sensitive indicator of stenosis severity than the overall spinal canal measurement, because the dural sac is what actually contains the nerves. In cases where the cross-sectional area drops below 50 mm², surgical decompression may be recommended even if symptoms at that spinal level are mild, because the degree of compression raises the risk of worsening problems over time. The goal of decompression surgery is to remove the bone, disc, or ligament tissue that is crowding the dural sac, giving the nerves more room.
Dural Ectasia: When the Sac Expands
The opposite problem, where the dural sac balloons outward rather than being compressed, is called dural ectasia. This expansion of the sac is most closely associated with Marfan syndrome, a genetic connective tissue disorder. In Marfan syndrome, the tissue that makes up the dural sac is weaker than normal and gradually stretches under the pressure of cerebrospinal fluid.
The typical symptom pattern includes low back pain, headache, leg pain (especially in the upper legs), weakness and numbness above and below the knee, and sometimes pain in the genital or rectal area. These symptoms occur because the expanding sac can erode surrounding bone and stretch or irritate nearby nerve roots. Dural ectasia is one of the most common orthopedic features of Marfan syndrome and is sometimes the finding that leads to the diagnosis.
Thecal Sac vs. Dural Sac
If your report says “thecal sac” instead of “dural sac,” don’t worry about a distinction. The two terms are interchangeable. “Thecal sac” comes from the Greek word for sheath, while “dural sac” references the dura mater. Both refer to the same structure: the membrane sleeve containing cerebrospinal fluid and spinal nerves. Radiologists and surgeons use both terms freely, and you may see either one depending on who wrote your report.