Abnormalities Associated With the Cavum Septi Pellucidi on Fetal MRI: What Radiologists Need to Know

Screening the cavum septi pellucidi (CSP), more commonly referred to as the "cavum septum pellucidum," is a required component of the fetal anatomic survey during second-trimester ultrasound (US). The inability to identify the normal appearance of this structure warrants further evaluation because septal insufficiency is associated with multiple brain malformations. In this article, we discuss embryology, normal anatomy, and prenatal evaluation of the CSP as well as the differential diagnosis of associated abnormalities.The CSP is an essential part of the CNS evaluation on second-trimester US; if its normal appearance cannot be confirmed by 20 weeks' gestational age, further evaluation is warranted. Fetal MRI, in either the second or third trimester, has become an important tool in further characterization of the associated abnormalities. However, when fetal MRI is not possible, postnatal MRI can also be used and will help to differentiate primary from secondary absence and will aid in providing prognostic information and therapeutic options for patients.