The thecal sac, also known as the dural sac, is a protective membrane within the spinal column. This sheath safeguards the delicate structures of the central nervous system that extend into the spine. It provides an enclosed environment for the spinal cord and its associated nerves.
Understanding the Thecal Sac
The thecal sac is a membranous tube formed by two of the three meningeal layers: the dura mater and the arachnoid mater. The dura mater is the outermost and toughest layer. The arachnoid mater lies beneath the dura and is a thinner, web-like membrane. Together, these layers create a continuous sheath that surrounds the spinal cord and nerve roots.
Precise Location in the Spine
The thecal sac is situated within the vertebral canal, the hollow space within the vertebral column. It begins superiorly at the foramen magnum, a large opening at the base of the skull where the brainstem transitions into the spinal cord. From this point, the sac extends downwards.
The sac continues inferiorly, reaching and tapering at the second sacral vertebral level (S2) in adults. It is surrounded by bone and ligaments of the vertebral column, providing additional external protection. The thecal sac encases the lower spinal cord and the bundle of nerve roots known as the cauda equina.
Role and Contents of the Thecal Sac
The thecal sac acts as a fluid-filled cushion, absorbing shocks and protecting the spinal cord and nerve roots from physical trauma. It contains cerebrospinal fluid (CSF), a clear fluid that surrounds the brain and spinal cord.
The spinal cord is housed within this fluid, including its lower tapering end called the conus medullaris. Below the conus medullaris, the thecal sac encloses the cauda equina, a collection of spinal nerve roots that resemble a horse’s tail. The CSF continuously circulates within the sac, providing buoyancy to the spinal cord and nerve roots, which helps reduce their effective weight. Additionally, CSF delivers nutrients to these neural tissues and removes waste products, maintaining a healthy environment for nerve function.
Common Conditions Affecting the Thecal Sac
Medical conditions can impact the thecal sac, leading to various symptoms. One common issue is thecal sac compression. This can be caused by a herniated disc, where the soft inner material of an intervertebral disc pushes outward, or by spinal stenosis, a narrowing of the spinal canal. Tumors or osteophytes (bone spurs) can also exert pressure on the thecal sac. Compression may result in pain, numbness, or weakness in the limbs, depending on which nerve roots are affected.
Cerebrospinal fluid (CSF) leaks are another concern, often resulting from trauma, surgery, or occurring spontaneously. A breach in the thecal sac allows CSF to escape, leading to symptoms such as headaches that worsen when upright and improve when lying down. Thecal sac cysts, such as Tarlov cysts, are fluid-filled sacs that form on nerve roots, often in the sacral region. While some Tarlov cysts are asymptomatic, larger ones can cause localized pain, sciatica, or even bowel and bladder dysfunction due to compression of the contained nerve roots.