The thecal sac, also known as the dural sac, is a protective membrane within the spinal column. This sheath surrounds the delicate spinal cord and the bundle of nerves that extend from it. It plays a role in safeguarding these crucial components of the central nervous system. Maintaining its integrity is important for proper nerve function.
Understanding Its Structure and Location
The thecal sac is a tube-like structure formed by the dura mater, the outermost and toughest of the three layers of meninges that cover the brain and spinal cord. This robust, fibrous membrane creates a barrier that encases the spinal cord and the nerve roots. The sac extends from the base of the skull, specifically adhering to the bone at the foramen magnum, and continues downwards.
This protective sheath reaches the level of the second sacral vertebra (S2) in the lower back, where it tapers to cover the filum terminale. Along most of the spinal canal, the thecal sac is separated from the inner bony surface by the epidural space. Projections from the sac, known as dural root sheaths, follow the spinal nerves as they exit the vertebral canal.
Its Essential Function and Contents
The primary role of the thecal sac is to provide a protective environment for the spinal cord and the spinal nerve roots. It acts as a barrier, cushioning these delicate structures from external pressures and mechanical forces. This protective function is important for maintaining the uninterrupted transmission of signals between the brain and the rest of the body.
Within the thecal sac, the cerebrospinal fluid (CSF) circulates, providing buoyancy and nourishment to the spinal cord and nerve roots. This fluid also helps to maintain a stable pressure and fluid balance around the spinal cord. The sac encloses the spinal cord itself, which ends around the first to second lumbar vertebrae (L1-L2), and the cauda equina, a bundle of nerve roots resembling a horse’s tail that extends below the end of the spinal cord.
How Thecal Sac Integrity Can Be Compromised
The integrity of the thecal sac can be affected by various mechanical issues, potentially compromising its protective function. Compression is a common problem, often resulting from conditions such as disc herniations, where a disc bulges out and presses against the sac. Spinal stenosis, a narrowing of the spinal canal, can also exert pressure on the thecal sac, as can tumors growing within or around the spine.
When the thecal sac is compressed or “effaced,” it can lead to symptoms like back or neck pain, numbness, or weakness in the limbs. Severe compression can impact the spinal cord and nerve roots, potentially causing more serious neurological issues. Additionally, trauma or surgical procedures can sometimes lead to tears in the thecal sac, which may result in cerebrospinal fluid leaks.