Coronal suture: ultrasonographic window of fetal cranial and brain abnormalities

We report on two cases referred to our center in the second trimester of pregnancy due to poor visualization of the anterior part of the brain, appearing to be 'too much in shadow'. Fetal central nervous system (CNS) screening involves a basic examination of the fetal skull and brain anatomy and requires the use of the cephalic axial planes. Evaluation of the axial planes allows visualization of certain cerebral anatomical landmarks1 depending on the relative ultrasound transparency of the fetal skull bones and, mainly, owing to the acoustic windows provided by the open coronal, lambdoid and temporoparietal sutures and sphenoid fontanelle. Considering that the indication for the referral was 'poorly visualized brain', it appears that the use and understanding of the images generated through these acoustic windows have not necessarily been mastered by the practitioners who carry out CNS screening on a daily basis. This realization prompted us to document the following observations. If the coronal, lambdoid and temporoparietal sutures or fontanelle are abnormal, either because of premature closure (Figures 1a,b and 2a) or abnormal widening associated with offset of the bone tables (Figures 1d,e and 2b), changes in the propagation of the acoustic waves can result in a shadow cone. In our first case, we observed a shadow cone roughly in the anterior 1/3 of the brain (Figure 1a,b). The septal cavity and fornix columns were not visible due to premature closure of the coronal suture (coronal craniosynostosis) (Figure 2a). Such shadowing was also recently reported in a case with bilateral coronal craniosynostosis as early as 24 weeks' gestation by Dall'Asta et al. and termed the 'brain shadowing' sign2. In contrast to the case reported by these authors, we additionally found, at the same gestational age, a change in skull shape (brachycephaly) associated with an abnormal fetal profile (Figure 1c). In our second case, a shadow cone was also present but only in roughly the anterior 1/5 of the brain (Figure 1d,e). The septal cavity and the anterior columns of the fornix were visible and the fetal forehead was receding (Figure 1f). In this particular case, the shadow cone appeared to have moved slightly forward but did not correspond to that observed in a fetus with normal skull and brain, in which the shadow was present in a smaller part (1/7) of the brain (Figure 1g,h) and the anterior interhemispheric fissure was more visible. The shadow cone observed in our second case can be explained by the step-like appearance of the frontal and parietal bones located on either side of the coronal suture (Figure 2b), which is abnormally wide. This offset of the bone tables was due to hypoplasia of the frontal lobes, owing to microcephaly, which resulted in the frontal bone not being sufficiently raised to the level of the corresponding parietal bone. The purpose of our report is not to quantify the extent of acoustic shadow in coronal craniosynostosis and microcephaly, but to point out that two different conditions can be identified by the same indirect sign that is explained by two different mechanisms according to acoustic considerations. Thus, some cranial and cerebral anomalies can be detected indirectly by the presence of an abnormally large anterior intracerebral shadow cone. Examination of the fetal cephalic pole (skull and brain), both in screening and diagnosis, cannot be performed without good knowledge and use of the natural acoustic windows represented by the cranial sutures and fontanelles.