Tethered Spinal Cord Syndrome: Pathophysiology and Radiologic Diagnosis
The diagnosis of Tethered Cord Syndrome (TCS) relies primarily on clinical and radiologic criteria. Similar to a variety of other spinal pathologies, TCS may manifest as diffuse or localized lumbar pain, lower extremity motor or sensory deficits, muscular atrophy, and urinary incontinence. TCS has been broadly sub-categorized as either primary/childhood or acquired/adult-onset. Recent research efforts have focused on at unraveling the underlying pathological mechanisms that differentiate these two forms of TCS. While positive-contrast myelography and computerized tomography (CT) with intrathecal contrast have previously been used to diagnose intrinsic spinal cord pathology, magnetic resonance (MR) imaging is now considered the imaging modality of choice due its ability to portray anatomic details in high resolution. While a thickened filum terminale and a low-lying conus medullaris have classically characterized TCS on MR imaging, observational studies have revealed that a significant population of TCS patients may have atypical radiographic findings. In this paper, we provide an updated review on the pathophysiology behind TCS and its characterization on MR imaging.
A comprehensive literature search was performed using the PubMed and Google Scholar database for all journal articles published prior to and including July, 2010. Key words used in this search were tethered cord syndrome, MR imaging, pathophysiology, filum terminale, conus medullaris, and diagnosis. All terms were searched alone or in combination. The articles selected were based upon the quality of the scientific investigations and included clinical case studies, retrospective analysis analyses and experimental studies using animal models. All papers meeting these criteria and relevant to either the pathophysiology or the radiologic diagnosis of TCS were included in this report.
The purpose of this review is to report the experimental and clinical data that were aimed towards uncovering the pathophysiology of TCS. Similarly, the diagnostic findings of TCS on MR imaging will also be reviewed using post-mortem anatomic studies, patient-based observations and retrospective analyses. The results reported will include a brief discussion of caudal spinal cord development followed by an etiological description of TCS in the adult and pediatric populations. Experimental studies describing the pathologic basis for, precipitation of, and the events leading to neuronal damage in TCS will also be discussed. Finally, we report the various MR imaging findings of TCS and their clinical significance.
Embryology of the Caudal Spinal Cord
Development of the caudal spinal cord is initiated as the neural tube elongates. The caudal portion of the neural tube, also termed the “caudal neurpore,” ultimately forms the CM, the ventriculus terminalis and the FT. Until the eighth week of gestation, the spinal cord terminates at the thecal sac. The FT then develops through canalization and retrograde differentiation of the caudal neural segments. Extending from the tip of the intradural CM to its extradural position at the first coccygeal segment, the FT elongates proportionally with the spinal cord. As development progresses, the differential growth rate of the vertebral bodies and the spinal cord ultimately positions the CM at the level of L4 by the seventeenth week of gestation.1 Both the vertebral bodies and spinal cord will continue to grow, with the CM reaching the level of L2 at approximately two months of age in 98% of the population.2-6
Tethered Cord Syndrome
Tethered Cord Syndrome (TCS) is a condition in which the filum terminale (FT), the anchoring segment of the cauda equina, is prevented from hanging freely within the spinal canal. Originally described in the pediatric population, it was hypothesized that a tight FT may result in prolonged spinal cord traction leading to caudal displacement of the conus medullaris (CM).7 Neurologic deficits, especially incontinence, were thought to be attributed to the effects of the tethered FT because these patients often improved clinically after surgical un-tethering. In 1981, Yamada et al described children with tethering of the FT who were responsive to surgical un-tethering as having “primary” tethered cord syndrome.8-10 This population of patients includes those whose neurologic deficits were related to spinal cord traction, as well as those patients with a tethered FT due to a congenital malformation—such as myelomeningocele, lipomyelomeningocele, diastematomyelia, or spinal lipomas.2,5,11 In both patient populations, the diagnosis of TCS pre-operatively depends primarily on MR imaging. A low-lying CM or a thickened FT is often used as sensitive indicators of TCS.8-10,12,13
Currently, MR imaging remains the standard of care in the diagnosis and characterization of TCS in both the adult and pediatric populations. MR imaging is extremely useful in visualizing the shape and location of the CM and FT in high resolution anatomic detail. The majority of patients in the adult population with clinically-suspected TCS will exhibit suggestive radiographic findings of TCS, such as the low-lying CM or thickened FT. However, a small subset of adult patients with TCS will have a normally positioned CM with the presence or absence of a thickened FT. Hence, additional radiographic findings, such as craniocaudal movement of the CM, have been studied as a means to diagnosis TCS definitively in these situations.12,14-17
Pediatric Versus Adult TCS Onset
Children with documented TCS often present clinically with neurologic deficits beginning in childhood and so contributes to early diagnosis. Due to the relative differential growth rates of the vertebral bodies and spinal cord during childhood, the FT elongates to allow ascent of the CM.2,4,14,18,19 Hence, lesions that prevent the FT from elongating or tether the FT at an inappropriate spinal level will cause a significant amount of traction to be placed on the CM. Embryologic defects that disrupt neurulation of the caudal spinal cord can inevitably anchor the FT prematurely.2,4,5,20-22 As mentioned earlier, TCS is common in children who present with myelomeningocele or meningocele—with both caused by a defect in the closure of the caudal neuropore resulting in spinal dysraphism. Exposure of paraxial mesoderm, secondary to defects within the neural ectoderm, allows infiltration of fat leading to spinal lipomas or even lipomyelomeningoceles. Diastematomyelia refers to total separation of the spinal canal due to an osseous, fibrous or fibrocartilaginous septum. Both legs of the spinal cord reunite caudally, tapering to form a shortened, pathologic FT. Although genetic abnormalities associated with TCS have been described, the molecular details involving its pathogenesis remain unclear.1,10,15,22,23