Prenatal Ultrasound Evaluation of the Position of Conus Medullaris for the Diagnosis of Tethered Cord Syndrome.

To determine the conus distance between the end of the conus medullaris and the distal end of the last vertebral body in healthy fetuses with various gestational ages using ultrasonography for its diagnostic value in tethered cord syndrome (TCS). This retrospective study included 540 healthy and 8 autopsy-confirmed TCS fetuses. Ultrasonographic measurement of the conus distance was performed when the fetus was in a prone position within the spine in the near field at 14 to 41 weeks of gestational age. Linear correlation analysis was performed to analyze the relationship between the conus distance and the gestational age, biparietal diameter, femur length, head circumference, and abdominal circumference. The normal results were compared with 8 cases of postnatally confirmed TCS. In 526 (95.9%) of 548 fetuses, the conus distance was successfully measured. The 95% limits of agreement in measurement of conus distance were -2.2 to 2.6 mm for the intraobserver variability and -3.7 to 3.1 mm for the interobserver variability. Significant correlations between the conus distance and the gestational age, biparietal diameter, femur length, head circumference, and abdominal circumference were observed. The most marked association was found to be between conus distance and femur length. The conus distance was significantly less in TCS fetuses than in healthy fetuses. Ultrasonographic measurement of conus distance is an easy and reliable method to evaluate the position of the conus medullaris and, therefore, can be helpful in the prenatal diagnosis of TCS.