The thecal sac, often called the dural sac, is a protective sheath within the spine that encases the delicate neural structures of the central nervous system. This membranous tube is a direct extension of the protective layers surrounding the brain, acting as a flexible barrier inside the bony spinal column. Its primary purpose is to safeguard the spinal cord and the nerves that branch off it, ensuring a stable environment necessary for proper neurological function.
Location and Structure of the Thecal Sac
The thecal sac is a sleeve formed by the dura mater, the tough, outermost layer of the three meninges that cover the brain and spinal cord. This sheath begins at the base of the skull, adhering to the bone at the foramen magnum, and extends downward through the vertebral canal. It runs inside the bony ring of the vertebrae, separated from the bone by the epidural space, which contains fat and blood vessels. The sac continues past the end of the spinal cord (L1-L2), enclosing the bundle of spinal nerve roots known as the cauda equina, before terminating at the second sacral vertebra (S2).
The Role of Cerebrospinal Fluid
The main content within the thecal sac is cerebrospinal fluid (CSF), a clear, plasma-like liquid. The thecal sac ensures this fluid environment is maintained for the health of the spinal cord and nerve roots. This fluid acts as a shock absorber, cushioning the neural tissues from sudden movements or trauma. CSF delivers essential nutrients, such as glucose and electrolytes, to the spinal cord and surrounding tissues. Additionally, the fluid helps remove metabolic waste products and provides buoyancy, supporting the spinal cord and nerve roots.
Clinical Relevance: Conditions That Affect the Thecal Sac
The thecal sac is frequently mentioned in medical reports because its compression or deformation is a common cause of back and leg pain. Conditions that narrow the spinal canal can exert pressure on this sac, a phenomenon often referred to as compression or impingement. This narrowing, known as spinal stenosis, is often caused by age-related wear and tear, where ligaments thicken or bone spurs develop.
A common finding in imaging reports is a herniated disc, where the soft inner material of a spinal disc bulges out. This protruding disc material can press directly against the thecal sac, potentially causing pain or neurological symptoms. When this pressure causes the sac to flatten, it is described as thecal sac effacement or indentation. Severe compression can lead to nerve root impingement, resulting in pain that radiates down the leg, known as sciatica, or cause weakness and numbness.
The thecal sac can also be involved in issues such as a cerebrospinal fluid leak. This occurs if the sac is punctured or torn, perhaps as a complication of a medical procedure like a lumbar puncture or spinal surgery, or due to trauma. A CSF leak reduces the fluid volume and pressure, which can result in symptoms like severe headaches and dizziness.