Study of positioning of fetal conus medullaris by detecting atlanto-axial intervertebral space with three-dimensional ultrasound

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Objective To explore the imaging rate and diagnostic rate of positioning the fetal conus medullaris by three-dimensional ultrasound method to detect atlantoaxial intervertebral space, comparing it with the traditional two-dimensional and three-dimensional ultrasound methods. Methods Consecutively 318 singleton fetuses received routine ultrasound screening during the second trimester were enrolled from November 2017 to December 2018 in Shenzhen Luohu People′s Hospital and Shenzhen People′s Hospital. These fetuses included 276 normal cases and 42 abnormal cases. The abnormal group contained 11 cases tethered cords fetuses(tethered cords group) and 31 cases non-tethered fetuses(non-tethered group). A new ultrasound method named detecting atlanto-axial intervertebral space with three-dimensional ultrasound and traditional two-dimensional and three-dimensional ultrasound methods were used to acquire and store the images. The positions of the fetal conus medullaris were analyzed blindly and recorded by three experienced physicians using three different methods with off-line software. Results ①The χ2 test comparing multiple sample rates was used to compare the imaging acquisition success rate of fetal conus medullaris by three ultrasound methods. The test level was adjusted to be α′=0.05/4=0.0125, the results showed that there were no statistically significant differences between the three methods in the normal group (χ2=7.39, P=0.025) and the abnormal group (χ2=5.32, P=0.070). ②The χ2 test comparing multiple sample rates was used to compare the diagnostic accuracy of fetal conus medullaris position in normal group by three methods, it showed there was no significant difference in the correct rate of conus medullaris position in the normal group (χ2=2.52, P=0.284). ③The χ2 test comparing multiple sample rates was used to compare the diagnostic accuracy of the fetal conus medullaris in tethered cord group and non-tethered group using 3 methods, the difference was not statistically significant in tethered cord group (χ2=1.22, P=0.543), while the difference was statistically significant in non-tethered group(χ2=9.69, P=0.008). Conclusions The method of detecting atlanto-axial intervertebral space with three-dimensional ultrasound has a high imaging rate and diagnostic accuracy in positioning the fetal conus medullaris. Positioning of fetal conus medullaris by detecting atlanto-axial intervertebral space with three-dimensional ultrasound is better than traditional two-dimensional and three-dimensional ultrasound in the abnormal non-tethered fetuses, which can provide more valuable information for prenatal diagnosis consultation and prenatal and postnatal care. Key words: Ultrasonography; Atlanto-axial intervertebral space; Fetal conus medullaris; Tethered cord