34 – Corpus Callosum and Septum Pellucidum Anomalies

Anomalies of the corpus callosum (ACC) and septum pellucidum are disorders of prosencephalic midline development, which are often associated with other brain abnormalities or may be part of a syndrome. Diagnosis of ACC is usually impossible before 18 to 20 weeks' gestation because the formation of the corpus callosum is incomplete. ACC can be complete or partial. The main indirect US signs of complete ACC include colpocephaly, absent CSP, and increased separation of the hemispheres. The definitive diagnosis relies on the demonstration of the absence of the corpus callosum on midsagittal and coronal views of the fetal brain. Partial ACC could be difficult to identify as in 1/3 of the cases there are no indirect signs. Fetal magnetic resonance imaging (MRI) is more sensitive than US in the recognition of possible associated cerebral anomalies. The overall prognosis of ACC is controversial and may range from asymptomatic with normal intellectual capacities to severe mental retardation. Several studies have reported a worse prognosis in the presence of additional anomalies. Cases of isolated ACC seem to have a better prognosis. Absence of the septum pellucidum can be diagnosed by US in the second trimester of pregnancy. Some associated brain abnormalities are not detectable before the third trimester, and in most cases it is impossible to determine the presence of SOD before birth. MRI can visualize the size of the optic nerves and chiasma (in late pregnancy) to search for optic nerve or chiasma hypoplasia (SOD) and to show possible associated cortical abnormalities (SOD-plus). Clinical severity of ASP varies, and only 30% of patients manifest the complete clinical triad. Children with isolated ASP have a favorable neurological outcome, but 18% of fetuses diagnosed with ASP are at risk for SOD.