Thecal sac compression is a frequent finding on spinal imaging reports, such as Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scans. It describes a structural change where the protective tube surrounding the spinal cord and its nerves is physically squeezed or flattened. When a radiology report mentions indentation, effacement, or encroachment, it signifies a reduction in the space available for the sensitive neurological structures within the bony spinal column.
Anatomy of the Thecal Sac
The thecal sac, also called the dural sac, is a membranous sheath that runs the length of the spine within the vertebral canal. It is composed of the dura mater, the outermost layer of meninges covering the central nervous system. This sac functions as a protective enclosure, shielding the delicate structures of the spine from friction and injury.
Inside this sheath, the spinal cord and its nerve roots are bathed in cerebrospinal fluid (CSF). The CSF acts as a hydraulic cushion, providing buoyancy and shock absorption for the neural tissue. The thecal sac extends from the base of the skull down to the second sacral vertebra (S2), where it tapers off.
In the lower back (lumbar spine), the spinal cord ends around the first or second lumbar vertebra. Below this point, the thecal sac contains a bundle of individual nerve roots known as the cauda equina, Latin for “horse’s tail.” These roots are responsible for motor and sensory function in the lower limbs and pelvic organs.
What Compression Means
Compression occurs when external structures encroach upon the spinal canal, physically reducing the available space. This narrowing exerts pressure on the sac, the cerebrospinal fluid, and the nerve roots housed inside. The result is a flattening or deformation of the sac, which is visible on cross-sectional imaging.
Radiologists classify the degree of compression into grades: mild, moderate, or severe. This grading is based on the obliteration of the cerebrospinal fluid (CSF) space surrounding the nerve roots. Mild compression shows partial effacement of the CSF, but nerve roots remain distinct.
Moderate compression occurs when the CSF space is further reduced, causing nerve roots to begin clumping together. In severe compression, the CSF space is almost completely obliterated, and the nerve roots aggregate into a single bundle. The radiological severity of compression does not always align with the patient’s symptoms.
Common Conditions That Cause Compression
The physical narrowing that leads to thecal sac compression is most frequently a result of degenerative changes associated with aging and wear on the spine. One of the most common causes is a herniated disc, where the soft, gel-like center of an intervertebral disc pushes through a tear in the exterior layer. The displaced disc material protrudes directly into the spinal canal, pressing against the nearby thecal sac.
Spinal stenosis is another primary contributor, referring to the abnormal narrowing of the spinal canal itself. This narrowing can be caused by the formation of bone spurs, known as osteophytes, which develop as the body attempts to stabilize joints affected by arthritis. Furthermore, the ligaments that support the spine, particularly the ligamentum flavum, can thicken and harden over time, encroaching on the space and squeezing the thecal sac from behind.
Another structural issue is spondylolisthesis, a condition where one vertebra slips forward over the one below it. This misalignment reduces the diameter of the central canal, directly compressing the sac and its contents. Less common causes include tumors or trauma, which can cause immediate compression from fracture fragments or hematomas.
Recognizing the Symptoms and Severity
Symptoms depend on the degree of pressure and which specific nerve roots are affected. The most common neurological consequence is radiculopathy, characterized by pain, numbness, tingling, or weakness that radiates away from the compressed area. In the lower back, this is often felt as sciatica, with symptoms traveling down the buttock and leg along the path of the sciatic nerve.
Compression interferes with the function of the nerves, leading to sensory changes like a “pins and needles” feeling or muscle weakness in the arms or legs. If the compression is significant, it can lead to Cauda Equina Syndrome (CES), a severe and time-sensitive condition requiring immediate medical intervention.
The distinguishing features of CES include sudden and severe low back pain combined with specific neurological deficits. Patients may experience saddle anesthesia, which is numbness or altered sensation in the areas that would touch a saddle, including the buttocks, perineum, and inner thighs. Additionally, CES involves new-onset dysfunction of the bladder or bowels, such as difficulty urinating (retention) or loss of control (incontinence). Recognizing these “red flag” symptoms is necessary for prompt diagnosis and urgent surgical decompression to prevent potential permanent nerve damage.