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OC03.07: Prenatal diagnosis of malformations of cortical development associated to midbrain‐hindbrain anomalies

Objectives: The description of the banana sign (Nicolaides et al, 1986) caused by the caudal displacement of the brainstem with consecutive obliteration of the cisterna magna has improved the diagnosis of open spina bifida (OFB) at the 20 weeks scan significantly. There is evidence, that the changes in the fetal posterior fossa can be visualised already at the first trimester scan (Chaoui et al, 2011; Lachmann et al, 2011; Scheier et al, 2011; Egle et al, 2011). The aim of this study was to investigate a practicable approach to integrate the screening for OSB in the routine first trimester screening. Methods: This is a prospective multicentre study on women undergoing first trimester ultrasound scans between April 2011 and september 2015. Measurements of the brain stem (BS) diameter and the brain stem to occipital bone distance (BSOB) were measured as described by Lachmann et al, 2011. The cisterna magna (CM) was visualised in the tilted axial view according to the method of Egle et al, 2011. Scans were performed by certified trainees and supervised by consultants. Results: There were 4949 fetuses included in the study. Pregnancy outcome was available in 4673 cases. At the first trimester scan 5 fetuses with OSB were diagnosed. The sensitivity and specificity of the BS/BSOB ratio > 1 was 100%. The sensitivity and specificity of the cisterna magna width < 5th centile was 100% and 89%, respectively. In 2.8% the BS/BSOB ratio was between 0.9 – 1.0 and in these cases 87% hat a normal CM and 13% a CM ≤ 5th centile. Later in pregnancy two fetuses were diagnosed with spina bifida occulta and both showed normal values of CM, BS and BSOB as well as BS/BSOB ratio. Conclusions: Screening for spina bifida is feasible in routine first trimester scan. The BS/BSOB ratio shows an excellent sensitivity and specificity. In cases with near normal values of the BS/BSOB ratio the CM width might be helpful for further assessment.