Tethered cord is not synonymous with malformation of the spinal cord. In caudal regression syndrome, absence of the lower sacral spinal cord segments and according neurological deficits often occur without associated tethering lesions. Tethered cord describes a potentially harmful pathological fixation of the spinal cord, either directly or via inelastic tissue (bone, fat, fibrous or fatty filum terminale). It is typically located in the lumbosacral region and most commonly caused by spinal cord malformations. The negative impact on the spinal cord is believed to result from a stretching effect brought about by everyday activities, especially flexion of the spine and pelvis. Elongation has been demonstrated to be most severe at the point of fixation and extends upward for up to five segments, but not beyond the lowest insertion of the dentate ligament at the thoracolumbar junction. Such traction on the spinal cord causes impairment of capillary blood flow and results in reduced oxygenation of the spinal cord. Experimental studies in animals have shown a
Fig. 18.2 Magnetic resonance imaging findings typical for tethered cord in patients with anorectal malformations. A Thickened filum terminale (see arrow in A2), absence of the lower segments of the sacral bone. B Lipoma of the spinal cord (arrow). C Terminal myelocystocele. Note the terminal ven
Fig. 18.2 Magnetic resonance imaging findings typical for tethered cord in patients with anorectal malformations. A Thickened filum terminale (see arrow in A2), absence of the lower segments of the sacral bone. B Lipoma of the spinal cord (arrow). C Terminal myelocystocele. Note the terminal ven tricle (v) dorsal to the malformed spinal cord, which is pushed out far above the fascial plane by an enlarged subarachnoid space (sa). D Ventral sacral meningocele with large dermoid (d) adjacent to the subarachnoid space close correlation between the severity and duration of traction, and reduction of blood flow, deterioration of interneuron potentials and neurological deficits . These results have been corroborated by intraoperative measurements of spinal cord blood flow  and cytochrome-C redox state . Intraoperative improvement of those parameters was correlated with the neurological outcome.
Spinal cord malformations causing tethered cord are a heterogeneous group of lesions resulting from maldevelopment of mesoderm invagination and pre-neurulation midline formation, errors of primary and secondary neurulation and post-neurulation events. As would be expected, the spectrum of entities associated with ARM consists of the subgroup related to secondary neurulation and subsequent processes (see Fig. 18.2 for typical MRI findings). Most common is a thickened, shortened or fatty filum terminale, followed by lumbosacral lipoma or lipomyelomeningo-cele. Other lesions are terminal myelocystocele and ventral sacral meningocele. Caudal regression syndrome, also labelled caudal suppression syndrome or, better, caudal dysplasia syndrome, occurs without tethering lesion or in combination with the lesions mentioned above. The complexity of these lesions and their impact on neurological function, natural history, difficulty of surgical repair, outcome and related long-term problems varies enormously, similar to the spectrum of ARM.
Pathological filum is the prototype of pure spinal cord tethering. Dysplasia of the spinal cord and cauda is minor or absent, unless caused by associated malformations. The dura is intact, the arachnoid membranes only slightly abnormal if at all. Patients are less likely to have neonatal neurological deficits, deteriorate later than patients with more severe malformations, but continue to be at risk for deterioration throughout their life [25,26].
Spinal cord lipomas and lipomyelomeningoceles occur in a wide spectrum of mild to severe cases in terms of malformation size, degree of spinal cord involvement, extent of dysplasia of the spinal cord and cauda and intensity of spinal cord tethering and/or compression. At birth, about 20% have functional deficits attributable to the malformation, the majority will deteriorate during infancy and childhood [27-30].
Terminal myelocystocele is a malformation that is not rarely associated with cloacal malformations [31,32]. It has all the features of a lipomyelomenin-gocele (i.e. extension of the spinal cord into a menin-gocele, where it is infiltrated by and tethered to fat tissue). In addition, the central canal of the spinal cord is distended into a terminal ventricle, which forms another component of the cele. The spinal cord tends to be both dysplastic and hypertrophic at the level of the malformation, a syrinx may extend upwards into functionally normal spinal cord segments. Patients tend to be more severely affected at birth and throughout life than average spinal cord lipoma patients.
Ventral sacral meningocele together with ARM and dysplasia of the sacral bone (scimitar sacrum) often presents as a component of the Currarino syndrome. A high proportion of cases are hereditary with autosomal dominant inheritance . Tethered cord is a common finding in ventral sacral meningoceles. The spinal cord is attached to the caudal wall of the meningocele, either directly or by way of a pathological filum terminale. The spinal cord can be elongated to an extent that it may be addressed as a mere pathological filum on imaging. Dermoid tumours are commonly associated, located in or adjacent to the caudal wall of the meningocele. Comorbidities other than faecal abnormalities are uncommon. Early progression of tethered cord syndrome is uncommon, but we have seen some cases with deterioration even in adult age.
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