Tethered Cord Syndrome and Occult Spinal Dysraphism
Daryl E. Warder, M.D., Ph.D. Department of Surgery, Division of Neurosurgery, The University of Texas Medical Branch, Galveston, Texas
Abstract and Introduction
Abstract
Tethered cord syndrome is a progressive form of neurological deterioration that results from spinal cord tethering by various dysraphic spinal abnormalities. The syndrome, treatments, outcomes, and current controversies are reviewed.
Introduction
Progressive neurological deterioration localized to the lower spinal cord resulting from traction on the conus medullaris has been termed TCS and has been described in children[23,24] and adults.[57,61] This syndrome of neurological deterioration was described and treated in 1891 by Jones.[29] It was again described in 1918, [5] and the recommendation for surgical intervention was made. It was rediscovered in the mid-20th century.[19,27,44,45] In its classic form, TCS involves traction on a low-lying conus medullaris;[17,57] however, TCS has also been described in the setting of a conus medullaris in the "normal" position.[69,70] Features associated with the syndrome include the dysraphic spinal elements, cutaneous stigmata of OSD, vertebral anomalies, orthopedic abnormalities (scoliosis and extremity abnormalities), neurological deterioration at the level of the lower spinal cord including bowel and bladder dysfunction, and anorectal malformations.[12,71]
Spinal dysraphism is a term that refers to all forms of developmental abnormalities occurring in the midline of the back - from the skin externally to the vertebral bodies internally.[26] Although the true incidence of spinal dysraphism is unknown, authors of some studies have estimated an incidence of 0.05 to 0.25 per 1000 births.[33,67] These abnormalities usually involve the lumbosacral spine, although lesions in the cervical and thoracic region do occur. Spinal dysraphism, which may result in a tethered spinal cord, exists in an open form, spina bifida aperta, and in a closed form, spina bifida occulta. The most common expression of spina bifida aperta is the myelomeningocele. Common forms of spina bifida occulta include the lipomyelomeningocele and diastematomyelia.
The same term, spina bifida occulta, when it refers to a benign bone cleft in the L-5 or S-1 spinous process that occurs in approximately 17% of the total population and 30% of normal individuals aged 1 to 10 years,[4,40] does not imply an association with a tethered spinal cord or TCS.
Tethered Cord Syndrome and Occult Spinal Dysraphism
Human intrauterine development is divided into the embryonic and fetal periods.[16,43] The embryonic period includes the first 50 to 62 days postconception,[16] and has itself been divided into 23 stages.[68] Each stage encompasses approximately 2 to 3 days. The fetal period includes the subsequent 7 months of gestation.[16] Stages 8 through 14 (Days 18-32) are of particular importance with respect to the development of the central nervous system. During these stages the three major steps of the development of the central nervous system occur. These steps include neurulation, canalization of the tail bud, and regression. Neurulation, the process by which the neural tube is formed, occurs during stages 8 through 20 (Days 18-48). During this process, the flat neural plate folds on itself into a neural tube which is covered by a continuous layer of cutaneous ectoderm.[16] The closure of the neural tube begins in the upper cervical region and then extends caudally and cephalically. During stage 12, the caudal portion of the neural tube closes at the level L-1 or L-2.[16] Neurulation gives rise to the spinal cord only down to the lumbar spine region. The spinal cord caudal to this is formed by the process of canalization. Errors during neurulation may lead to various congenital malformations such as myelomeningocele, meningocele, lipomyelomeningocele, SCMs, the dermal sinus, and intraspinal tumors such as dermoids and epidermoids. With the completion of neurulation, the neural tube is totally covered by cutaneous ectoderm.[16]
The terminal filum and cauda equina are formed from the caudal portion of the neural tube by regression. The ventriculus terminalis marks the level of the future conus medullaris and is a dilation of the central canal that can be identified at stages 18 through 20 (Days 43-48) at which time it lies at the coccygeal level. The tip of the vertebral coccygeal segments contains an epidermal cell rest, the coccygeal medullary vestige. The terminal filum is formed when the caudal neural tube regresses between the ventriculus terminalis and the coccygeal medullary vestige and is first present at stage 23 (Day 52). During the fetal period, the vertebral canal grows faster than the neural tube, resulting in the "ascent" of the spinal cord. At the time of birth, the conus medullaris has reached the L2-3 space in the majority of individuals,[3] and it has reached the adult level by age 3 months.
Associated Clinical Features of Spinal Dysraphism
Cutaneous Stigmata
The cutaneous changes associated with OSD and TCS include the midline lumbosacral cutaneous hemangiomas (Figs. 1A and 2B), lumbosacral hypertrichosis (Figure 1B), the lumbosacral dermal sinus (Figure 1A), the midline lumbosacral subcutaneous lipoma (Figs. 1A and 2), and the lumbosacral skin appendage (Figure 2A).[27] The cutaneous stigmata of OSD are present in approximately 50%70 of patients who present with TCS.
Figure 1 (click image to zoom) . Photographs showing cutaneous stigmata of OSD. A: One of three dermal sinuses (arrow). Also present are a subcuta-neous lipoma and cutaneous hemangioma. B: A hairy patch of the lumbosacral spine is displayed.
