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Normally, the spine of the unborn fetus grows faster than the spinal cord, so that by the time of birth, the bulk of the spinal cord only extends to the vicinity of the top of the pelvis. The space below the cord is occupied by spinal fluid and nerve roots - which is why a lumbar puncture or "spinal tap" is safely performed below the top of the pelvis).
Rarely in early fetal development the spinal cord does not properly thin out between the cord and the bony sacrum. It remains "tethered" by a thick ropelike cord rather than the normal threadlike "filum terminale." As the spinal column grows, the abnormally attached spinal cord is stretched. This stretching, especially with bending of the spine, can cause compression of spinal blood vessels and thus neurologic symptoms.
Tethered cord can be manifest at birth, or may not cause symptoms until later in life. Affected infants may have abnormal leg growth or foot deformities. Older children may present with bladder function abnormalities, progressive scoliosis, or diffuse pain in the legs.
Tethered cord is especially associated with myelomeningocoele, skin tags over the sacral area, fatty tumors (lipomas) or hemangiomas over the lower spine, and spina bifida. When a tethered cord is suspected, it is diagnosed by MRI examination. Treatment is surgical, by cutting the thickened filum terminale.