Anomalies of Spinal Cord Length and Filum Thickness

18 Anomalies of Spinal Cord Length and Filum Thickness

Shokei Yamada, Javed Siddiqi, and Shoko M. Yamada

Adults with tethered cord syndrome (TCS) are divided into two groups: group 1 adult TCS patients with spinal dysraphism and group 2 patients without dysraphism who develop signs and symptoms in adulthood.1–3 A significant number of group 2 patients failed to show an elongated spinal cord and thickened filum terminale that Hoffman et al described in pediatric patients.⁴ This chapter reports the statistics derived from the studies on the location of the caudal end of the spinal cord and the thickness of the filum terminale in group 2 adult TCS patients (N = 104). Only 35.6% of them were found to have both elongated cord and thickened filum. The data indirectly support that the essential factor for development of TCS is the inelastic filum.

Based on the neurological signs and symptoms localized in the lumbosacral cord and consistent imaging and operative findings, Hoffman et al adopted the term tethered spinal cord.⁴ They attributed the motor and sensory deficits and incontinence to an increased tension in the elongated spinal cord anchored by a thickened filum terminale. As neurosurgeons’ experience continued to increase, however, the concept of tethering-induced disorder broadened to the recognition of an inelastic filum as an essential mechanical factor for development of TCS.^5,6 Earlier in 1993, Warder and Oakes reported that 18% of TCS patients, including both children and adults, had the caudal end of the spinal cord at L1–2 intervertebral space or higher.⁷ The authors present the retrospective analysis of the data on the caudal end of the spinal cord and filum thickness obtained from group 2 TCS in adult and late teenage patients as previously discussed.⁸

Materials and Method

One hundred and four patients within the group 2 adult and late teenage TCS, from 17 to 81 years of age, 39 males and 65 females, presented with typical signs and symptoms of TCS (Chapter 15). All the patients lacked spinal dysraphism on imaging studies as well as at operation, except for five patients with bony spina bifida occulta of the S3 through coccygeal vertebrae shown by plain x-ray films. During surgery, the level of the caudal end of the spinal cord was determined by the exit of the lowest coccygeal nerve root, and its location was expressed in relation to the lumbar and sacral vertebral bodies. The filum thickness was measured by the lateral diameter (usually slightly greater than the anteroposterior diameter).

Results

The locations of the caudal end of the conus and the diameter of the filum terminale are listed in Table 18.1. Only 35.6% of the group 2 TCS patients showed cord elongation and a thickened filum. The caudal end of the spinal cord was found at the L2–3 intervertebral space or above in 37 patients (35.6%), and below the L2–3 level in 67 patients (64.4%). The diameter of the filum was less than 2 mm (assumed to be in the normal range) in 60 patients (57.7%) and 2 mm or greater in 44 patients (42.3%).

Table 18.1 Locations of the Caudal End of the Conus and the Diameter of the Filum Terminale (from total 104 adult cases with TCS)

1. Normal range of caudal end and filum thickness	30 cases (28.8%)
2. Normal range of caudal end and abnormally thick filum	7 cases (6.7%)
3. Low-lying caudal end and normal range filum thickness	30 cases (28.8%)
4. Low-lying caudal end and abnormally thick filum	37 cases (35.6%)

Adding 1. and 2., 37 patients (35.5%) had the caudal end at L2–3 or above, and adding 3. and 4. the diameter of the filum was less than 2 mm in 60 patients (57.7%).

Of 67 patients with the caudal end below the L2–3, 41 (39.4% of total) had the caudal end opposite to the L3 vertebra, leaving only 26 (25%) with the caudal end below the L3 vertebra. None of the cases with the caudal end at the L3 level was detected by imaging studies as having an elongated spinal cord (Chapter 3).

Discussion

Our data clearly demonstrate the variability of the cord length and filum thickness in adults and late teenage patients with TCS, which correspond to the previous report.⁸ Warder and Oakes first described no elongation of the spinal cord in adult and pediatric TCS patients.⁷ There are also an increasing number of pediatric TCS patients who failed to show these two features (Knierim DS and Won DJ, personal communication, 2007). Other authors diagnosed TCS in small children with a chief complaint of incontinence but without these two anatomical features.^9–11 These facts explain how difficult it is to diagnose TCS by relying on only two features, an elongated cord and a thickened filum. Despite these recent reliable findings, it is still widely believed that an elongated spinal cord and thick filum are the two fundamental features to establish the diagnosis of TCS. Further, this logic extends to such an assumption as the patients without two features are excluded from TCS or tethered spinal cord, even if they present with typical signs and symptoms of TCS (Chapter 15). The data reported by Yundt et al are noticeable; the normal range of filum thickness was measured as 1.1 to 1.2 mm,¹² whereas the filum thickness in TCS patients ranged from 1.0 to 2.0 mm.^13–15

We emphasize that the clinical findings¹⁶ are the primary tools for the diagnosis of TCS (Chapter 15), and are assisted by the magnetic resonance imaging (MRI) findings, such as the two features mentioned earlier, fat signal in the filum and syringomyelia.¹⁷ The most consistent MRI finding is the posterior displacement of the conus and filum. This feature is confirmed by intrathecal endoscopy before widely opening the dura and arachnoid membrane, or preoperative percutaneous endoscopy.⁶ The further step to prove the lack of elasticity in the filum is the stretch test after exposing the filum. This is the logical approach that utilizes the experimental work on redox changes in cytochrome a,a₃¹⁸ proportionate to cord elongation induced by traction,^18–20 detailed in Chapter 3.

Although fat tissue in the filum has been emphasized for the diagnosis of TCS,¹⁷ fat itself is soft and elastic, and TCS may not develop until fibrous tissue is increased to sufficiently reduce filum elasticity. MRI studies with currently available resolution do not identify the caudal end of the spinal cord, particularly when it is located at the L3 vertebral level. This difficulty can be explained by the inability to locate the exit of the lowest coccygeal nerve root (100 µm in diameter),² which is too small to be identified as a landmark of the caudal end of the spinal cord. Commonly, the caudal end at the L3 vertebra is not detected in the group of low-lying cord before surgery.1–3 It is advisable to consider the conus localization by MRI studies (see Chapter 5, Fig. 5.3). In some cases, fat tissue that extends from the filum into the conus makes the definition of the conus-filum junction impossible by imaging studies. Only at operation, the junction can be determined by the exit of the lowest coccygeal root with microscopic observation (see Chapter 15). The elongation of the spinal cord usually occurs in lumbosacral segments,¹³ and the conus diameter is often greater than the normally located conus (S2–4 segments). The relatively large bulk of the conus in these patients may resist traction forces as much as the lumbar cord segments do, and explain widely spread lumbosacral cord dysfunction in TCS.

Conclusion

The length of the spinal cord and the thickness of the filum terminale are of relative importance for the diagnosis of the TCS. Neurological and musculoskeletal abnormalities that indicate or strongly suggest a stretch-induced lumbosacral functional lesion make imaging studies useful. The symptomatic protocol presented in Chapter 15 will assist in the diagnosis for TCS in adults as well as in children.