Measurement of vermis-crest angle in the evaluation of fetal posterior fossa

OBJECTIVE: Posterior fossa anomalies constitute a heterogeneous group of malformations seen in one in 5000 live births. In the evaluation of the fetal central nervous system, in addition to basal ultrasonography(USG), the use of three and four-dimensional(4D) ultrasonography, which has emerged with technological developments, is of great importance. Especially evaluation of the presence, rotation and volume of vermis in the midsagittal plane, and normal posterior fossa anatomy and the evaluation of the presence of space-occupying formations in the posterior fossa can be performed by three-dimensional(3D) ultrasonography. With the widespread use of three-dimensional ultrasonography with the developing technology, the brainstem-vermis angle (BVA) and the measurement of the brainstem-tentorium angle (BTA) are used to evaluate the rotation of the vermis in the posterior fossa. In recent studies, a new angle called "vermis-crest angle (VCA)" has come to the fore, which can help the evaluation of normal rotation of vermis, evaluation of normal posterior fossa anatomy, and prenatal diagnosis in the presence of posterior fossa anomaly. The aim of this study is; in the evaluation of normal posterior fossa anatomy in pregnant women with no congenital anomaly in fetal anomaly screening, measuring the vermis-crest angle with three-dimensional ultrasonography, determining the distribution of this angle according to the week of gestation, comparing intra-observer and inter-observer repeatability to evaluate its applicability in clinical practice. METHOD: Patients between 18-22 weeks of gestation were included in our study at the Dokuz Eylül University Gynecology and Obstetrics Outpatient Clinic. The patients had visited the clinic between March 2019 and February 2020. A total of 162 pregnant women were included in our study. After the two-dimensional routine fetal anomaly scanning was completed with the C2-9 convex probe of the Voluson E8 ultrasound device, the fetal brain structures were imaged in the transventricular or transcerebellar plane with the three-dimensional RAB6-D convex probe and the 'corpus callosum' option. Transvers cerebellar diameter(TCD), cisterna magna(CM) size and horizontal diameter(HD) of vermis were measured in the axial plane. In the midsagittal plane, superoinferior(SID) and anteroposterior diameter(APD) of vermis and VCA were measured. Vermis volume and vermis area were calculated with the help of measured diameters. All measurements were made by two different observers. RESULTS: The total number of patients included in the study is 162. The average age of the 162 pregnant women evaluated in the study was 28.85 ± 5.31 (18-42) and the mean weeks of gestation of the cases were 20.96 ± 1.18 (18-23). The mean VCA value was calculated as 58.49º ± 16.47. VCA, volume and area values measured by both observers were found to be statistically highly positive in both intraobserver and interobserver comparisons (p <0.001). There was no statistically significant relationship (p> 0.05) between the VCA values and the gestational age. There was a statistically significant positive correlation (p <0.001) between the volume and area values and the gestational age. DISCUSSION: In our study, the mean VCA value was evaluated as 58.49 ° ± 16.47 and it was found that there was no statistically significant correlation (p> 0.05) between angle and age, gestational week, and obstetric history characteristics. Considering the results, it was revealed that this angle is applicable and reproducible. Good interobserver consistency was found with a high intraclass correlation coefficient (ICC = 0.906) and a small 95% confidence interval (95% CI = 0.832-0.947). Further studies with large numbers of cases, including pathological cases, are needed to confirm and reinforce our results. KEYWORDS: The vermis-crest angle(VCA), three-dimensional ultrasonography, posterior fossa anatomy