Introduction: The retrogressive differentiation of the fetal spinal cord results in the relative ascension of the conus medullaris in relation to the bones of the spine. This can be hindered by an abnormal filum terminale or another abnormal structure that tethers the cord caudally, resulting in tethered cord syndrome. However, the rate and timeframe of conus ascent are not well understood.

Methods: We reviewed radiographic images of 219 infants born with concerns for low-lying conus medullaris. All patients were subdivided into 3 groups, based on the gestational age at which the first ultrasound was obtained: Group A (=42 weeks of gestation) Group B (43-50 weeks), and Group C (=51 weeks). The level of the conus was assigned a numerical value (e.g. L1 = 1, L2 = 2, etc.) In patients where the conus was located opposite to the disk space at the junction of two vertebral bodies, the numerical value was averaged.

Results: All 219 patients had an initial ultrasound of the lumbosacral spine. Of all patients, 25 (11%) had a second ultrasound, 132 (60%) had an MRI following the initial ultrasound, and 62 (28%) had both a second ultrasound and an MRI. In Group A, the level of the conus differed by 0.5 vertebral segments between first and second ultrasounds (2.9±0.7 vs. 2.4±0.5). This ascent was statistically significant (p < 0.001). Between the first ultrasound and MRI the conus rose 0.2 vertebral segments (p = 0.044). There was no significant difference in conus level between serial imaging studies in Groups B and C.

Conclusions: Our study demonstrates that the conus medullaris almost certainly ascends within 50 weeks of gestational age. No ascension was noted past this period. Therefore, when patients are being evaluated for spinal cord tethering, there is likely no benefit in obtaining new imaging after 50 weeks of gestation.

Patient Care: These findings should reduce unnecessary imaging studies past the period of expected conus ascent, reducing cost and inconvenience to patients.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) Recognize the timeframe in which the conus medullaris normally ascends 2) Adapt serial imaging practices to the general timeframe of conus ascent

References: 1. Yamada, S., D.J. Won, and S.M. Yamada, Pathophysiology of tethered cord syndrome: correlation with symptomatology. Neurosurg Focus, 2004. 16(2): p. E6. 2. Hertzler, D.A., 2nd, et al., Tethered cord syndrome: a review of the literature from embryology to adult presentation. Neurosurg Focus, 2010. 29(1): p. E1.