Discussion Author(s): A. James Barkovich, MD
Early in embryogenesis, the spinal cord extends to the caudal end of the spinal canal. At that time, each neural segment is at the exact same level as the corresponding segment of the spinal canal. Each nerve root extends directly laterally to its neural foramen. As the embryo matures, the most distal portion of the cord undergoes a process known as retrogressive differentiation. In this process, the most distal fibers degenerate and elongate. At the same time, the vertebral bodies are growing more quickly than the cord. This combination of factors results in a relative ascent of the spinal cord within the spinal canal. The precise level of the conus medullaris within the spinal canal at the time of birth is debated. However, it has been established that the conus is usually positioned at the L1-L2 level by the age of 3 months. The conus is generally considered abnormal if it extends below the bottom of L2.
In some patients, the process of retrogressive differentiation goes awry. In these patients the conus medullaris remains below the bottom of L2. As a consequence, the distal spinal cord is stretched and does not function normally.
Patients with tethered spinal cords can present at any age. All patients tend to suffer from difficulty with locomotion, ranging from muscle stiffness to actual weakness. All patients have abnormal lower extremity reflexes. The patients can also exhibit bladder dysfunction, sensory changes, and orthopedic deformities of the lower extremities (most commonly club foot). Back pain and radiculopathy are also common complaints; in fact, most adults with tethered cords present with radiculopathy.
The diagnosis of a tethered spinal cord is easily made with MR. The major findings are the conus medullaris ending below the bottom of L2 and a thickened filum terminale. The normal filum terminale measures less than or equal to 2 mm in diameter at the L5- S1 level. A word of caution should be issued concerning identification of the level of the conus medullaris. On sagittal images, it is sometimes difficult to differentiate the bottom of the spinal cord from the proximal cauda equina. Therefore, it is advisable to obtain axial, T1-weighted images if there is any question whatsoever about the level of the conus. It should be noted that approximately 25% of patients with tethering of the spinal cord will have a mild dilatation of the central canal of the distal cord. This mild dilatation does not represent frank syringohydromyelia and does not need to be directly treated. Often, this mild dilatation will go away after treatment of the tethered cord.
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