The cortical bite sign: a mid-trimester sonographic marker of focal cortical dysplasia in fetuses with complete agenesis of the corpus callosum

To describe a new mid-trimester US sign, the “cortical bite sign” (CBS), a specific sonographic feature of cortical focal dysplasia, which is consistently seen at brain US in fetuses with complete agenesis of the corpus callosum (ACC). Retrospective study from 2018 to 2021, including patients referred to five referral centres in the second trimester (19+0–22+0 weeks of gestation) with suspected ACC. All cases with the diagnosis of complete ACC were submitted to an axial US assessment of the fetal brain on the transventricular plane. On this plane, we investigated the mesial profile of both cerebral hemispheres at the level of the frontoparietal cortex and looked for an abnormal invagination of the cortical surface along the widened interhemispheric fissure, which was referred to as the CBS. All fetuses were submitted to dedicated antenatal MRI to reassess the US findings. Cases with additional CNS anomalies which did not involve the cortex were excluded. The final diagnosis was confirmed at postnatal brain MRI or at postmortem examination, for cases undergoing TOP. The primary outcome was to evaluate the presence and the laterality of the CBS in fetuses with complete ACC.at antenatal US and MRI. In the study period, 65 cases of ACC were selected for the study purposes. 50 (76.9%) cases underwent TOP, and 15 (23.1%) pregnancies resulted in a livebirth. Sonographically, the CBS was found in 13/65 (20.0%) cases, and targeted MRI detected 16/65 (24.6%). Moreover, the CBS was confined to only one of the cerebellar hemispheres in all cases. Interestingly, there was a predominant majority, albeit non-significant, of male fetuses (75.0% vs. 25.0%, P = 0.12) in the group of complete ACC with CBS. Finally, 15/16 (93.8%) cases with ACC and CBS underwent TOP. The CBS is a specific marker of focal cortical dysplasia which seems to characterise a large group of fetuses with complete ACC. The etiology and prognostic role of the CBS remains to be investigated.