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DOI: 10.1055/s-0041-1739766
3D sonographic evaluation of the position of the fetal conus medullaris at 1st trimester
Introduction The prenatal diagnosis of closed spinal dysraphism (CSD) is challenging because of the absence of a visible defect at the level of the lumbar spinal cord and indirect cranial signs of spinal dysraphism. The presence of a low-lying conus medullaris (CM) may represent an indirect sign of CSD. We aimed to assess the position of the CM at first trimester 3D ultrasound in a cohort of structurally normal fetuses.
Methods This was a multicenter prospective study involving a consecutive series of structurally normal fetuses between 11–13 weeks of gestation (CRL between 45–84 mm). All fetuses were submitted to 3D transvaginal ultrasound using a sagittal view of the spine as the starting plane of acquisition. At offline analysis, the position of the CM was evaluated by two independent operators with a quantitative and a qualitative method: 1) the distance between the most caudal part of the conus medullaris to the distal end of the coccyx (CMCd) was measured; 2) a line perpendicular to the fetal spine joining the tip of the conus medullaris to the anterior abdominal wall was traced to determine the level of this line in relation to the umbilical cord insertion (Conus to-abdomen-line or CAL). Interobserver agreement for the CCMd was evaluated. Linear regression analysis was used to determine the association between CMCd and CRL. The absence of congenital anomalies was confirmed in all cases after birth.


Results Over the period of 4 months, 143 fetuses were recruited. In 130 fetuses (90.9%), the visualization of the CM was feasible. The mean value of the CMCd was 1.09±0.16 cm. The 95% limits of agreement for the interobserver variability in measurement of the CMCd were −0.24 and 0.26 cm. The interobserver variability based on the ICC for the CCMd was good (ICC=0.81). We found a positive linear relationship between the CCMd and the CRL. In all these fetuses, the CAL encountered the abdominal wall at or above the level of the cord insertion.
Conclusion The evaluation of the position of the CM is feasible in the first trimester using 3D ultrasound with a good interobserver agreement. In normal cases, the level of the CM was never found to be below the fetal umbilical cord insertion, whereas the CMCd was noted to increase with advancing gestational age, confirming the “ascension” of the CM during fetal life. We envisage that our data may contribute to elucidate the natural history of skin-covered spinal dysraphism and to improve its sonographic detection during fetal life.
Publication History
Article published online:
26 November 2021
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