Purpose or Case Report: Evaluate temporal change in position of the tip of the conus medullaris in infants with borderline or low position of the conus on initial spine ultrasound (US) performed at less than 3 months of age.
Methods & Materials: This is an IRB-approved study. From the radiology information system, we retrieved all spine US studies from 2004-2018 in children younger than 3 months of age. Based on the radiology report, we included only studies with conus level from L2-L3 to L3-L4 who had follow-up spine US or MRI within one year. All patients with neurological abnormalities related to the lower cord, including lower extremity and bladder dysfunction, or known spinal dysraphism were excluded.

Study indication, age, gender, and time interval between US and follow-up studies were documented. We noted whether the level of the tip of the conus on the initial spine US and follow-up US or MRI study was at the intervertebral level, or at the upper third, middle third, or lower third of a vertebral body level. If this detailed information was not available on the radiology report, it was determined by reviewing the study.
Results: The study group included 72 children (34 females), with age range 0-79 days (average 19 days) at initial spine US. The most common indication for spine US was sacral dimple (34/72, 45.8%). 15 infants had US follow-up and 57 had MRI follow-up. The average follow-up time interval was 118 days.
On the initial US, the tip of the conus was in a borderline position (L2-L3) in 19/72 (25.7%) children and at the upper L3 to L3-L4 level in 53/72 (74.3%). On follow-up, the tip of the conus was normal (at or above L2) in 43/72 (59.7%) children, borderline in position (L2-L3) in 28/72 (37.8%), and at the level of upper L3 in one child.
On average, there was ascent of the tip of the conus by 0.73 vertebral body level over 3.9 months time.
Conclusions: In asymptomatic infants less than 3 months of age, isolated finding of borderline or low conus medullaris (L2-L3 to L3-L4) on spine US may not be a reliable indicator of tethered cord as most will ascend to a normal level in later infancy. Therefore, follow-up imaging of the spine should be considered in selected patients.