Posterior complex: clue for suspicion of partial agenesis of corpus callosum at fetal brain screening

Developmental anomalies of the corpus callosum (CC), even if isolated, have been linked to multiple neurodevelopmental disorders, and there is growing interest in assessing this brain structure in the fetus1. Complete agenesis of the CC is relatively easy to diagnose on prenatal ultrasound, based on indirect signs observed on basic axial examination of the fetal brain and significant distortion of some components of the anterior complex, and is confirmed by the absence of CC visualization in the midsagittal plane2. Nonetheless, despite significant effort in recent years to improve its detection, partial agenesis of the CC (pACC) remains a challenging condition to diagnose prenatally. There are multiple reasons for this. Firstly, the morphological definitions of CC abnormalities, including pACC, are highly heterogeneous1. Secondly, quantitative assessment of the CC in the midsagittal plane is problematic due to the significant heterogeneity in charts for CC length, which raises the question as to whether routine measurement of the CC should be performed during fetal neurosonography1, 2. Perhaps the biggest challenge concerning fetal pACC is the diagnosis itself. The first suspicion of pACC is usually based on a 'short CC' or lack of CC splenium observed during the direct visualization of this structure in the midsagittal plane. However, assessment in the sagittal plane, although recommended strongly by many authors, is not part of all basic ultrasound guidelines for fetal brain screening. Moreover, it has been suggested recently that a 'short CC' is a non-entity in the postnatal classification of CC abnormalities, and that this misnomer should be avoided in the definition of CC pathology in the fetus3. The same authors suggest that the term 'pACC' should be used only when one or more of the four components of the CC (rostrum, genu, corpus or splenium) is absent3. Homogenizing the definition of CC abnormalities and improving the methodology used in CC biometric studies is of the utmost importance in the prenatal diagnosis of pACC. Until then, our efforts should be focused on finding clues that raise suspicion of pACC at mid-trimester ultrasound screening. Since it was first reported by our group4 and later extrapolated to large populations of second- and-third trimester fetuses5, the posterior complex has proved to be a valuable tool in the assessment of the posterior portion of the midline in an axial plane. Slicing cranially from the transventricular plane, slightly superior to the quadrigeminal cistern, provides direct visualization of the posterior complex, which includes the CC splenium crossing the midline, the posterior callosal sulcus, the interhemispheric fissure, the parieto-occipital fissures, and the lateral ventricles and their choroid plexuses, with their normal oblique position relative to the midline4. Figure 1 shows two cases in which abnormality of the posterior complex was an important clue for the suspicion of pACC. Upon confirmation of the diagnosis in the midsagittal plane, it was observed that part of the splenium was absent in both cases and the rostrum was missing in one. Optimization of the anatomical targets to be screened could enhance prenatal ultrasound diagnosis, and incorporation of the posterior complex into basic ultrasound examination of the fetal brain has the potential to improve the recognition and understanding of pACC. Data available on request due to privacy/ethical restrictions.