Sonographic Evaluation of Fetal Conus Medullaris and Filum Terminale

Sonographic evaluation of the fetal conus medullaris (CM) level is not reproducible. The objectives of this study were to determine the normal position of the fetal CM during pregnancy as well as the normal intradural filum terminale (FT) length and to evaluate their use in detecting tethered cord.

This is a prospective evaluation of normal singleton pregnancies examined by sonography from 17 weeks of gestation to term. Each sonographer had to identify the top of the first sacral vertebra (S1) to measure the distance between it and the conus extremity (CM-S1 distance). The intradural FT distance was measured with 5- to 8-MHz probes.

194 consecutive pregnant women were included. The CM and intradural FT were demonstrated clearly in 164 (84%) cases. The mean CM-S1 distance was 20.6 mm (range 0.5-42). The mean intradural FT distance was 27.9 mm (range 6.6-49.3). Linear regression analysis showed a significant association between both those distances and gestational age (p < 0.05). In cases of tethered cord, the mean CM-S1 distance and the mean intradural FT distance were both below the 5th percentile.

Prenatal evaluation of the CM and the intradural FT is feasible and reproducible and seems useful in detecting tethered cord.