Four Steps in Diagnosing Complete Agenesis of the Corpus Callosum in Prenatal Life

Among congenital brain anomalies, complete agenesis of the corpus callosum (cACC) including cases of callosal hypoplasia has a prevalence of 1.8 per 10 000 in the general population. It is also one of the most challenging brain anomalies to detect during the mid-trimester ultrasound scan. Standard axial planes do not provide enough information to make the definitive diagnosis of cACC.

From our library of images and ultrasound reports, we reviewed our most recent cases of complete agenesis of the corpus callosum in the fetus at the mid-trimester scan. In our analysis we included only cases that were confirmed postnatally or by autopsy. Exams were performed between January 2010 and June 2012. All of the patients were scanned transabdominally by means of 2 D and static 3 D. From the 2 D and 3 D images we identified 4 anatomical views that consistently gave us enough information to identify cACC: axial biparietal transthalamic view (AX1); axial biparietal falx view (AX2); coronal transthalamic view (COR); mid-sagittal view (SAG).

From our library 30 cases were selected with confirmed cACC postnatally or in autopsy findings. The mean gestational age at the time of referral to our center was 20.7 weeks (range 19 - 23 weeks). In all analyzed cases sufficient 2 D images were found and in 93.3 % of them informative 3 D volumes were also available for off-line review. We identified the following patterns of cACC at the mid-trimester scan: A- normal size of 3 rd ventricle + normal size of the lateral ventricles or mild ventriculomegaly; B1- dilated 3 rd ventricle + normal size of the lateral ventricles; B2- dilated 3 rd ventricle + mild or moderate ventriculomegaly; C- dilated 3 rd ventricle + severe ventriculomegaly; D- gross dilatation of 3 rd ventricle with the appearance of interhemispheric cyst. The AX1 view revealed: absence of the cavum septum pellucidi in all cases; dilatation of the third ventricle in 86.6 % of cases; separation of frontal horns in 83.3 % of cases; ventriculomegaly in 73.3 % of cases, including 13.6 % with severe forms. The AX2 view showed separation of the interhemispheric fissure (IHF) in 90 % of cases and upward displacement of the 3 rd ventricle in 80 % of cases. The COR view confirmed separation of the interhemispheric fissure in 90 % of cases including gross separation in 7.4 % of cases; absence of CC fibers at this level and descent of the falx towards the roof of the 3 rd ventricle in all cases, and upward displacement of the 3 rd ventricle in 80 % of cases. The SAG view revealed the absence of the CSP-CC complex in all cases and dilatation of the 3 rd ventricle in 86.6 % of cases.

1. We suggest a stepwise ultrasound diagnostic approach for cACC and variations of this condition. 2. We suggest a classification of characteristic patterns found in fetuses with proven cACC based on findings presented in axial views.