Midline Variants of the Septum Pellucidum, Corpus Callosum, and Massa Intermedia

Abstract

The cavum septum pellucidum (CSP) is a normal variant fluid potential space of CSF density (on CT) and signal intensity (on MRI). A CSP may simulate a cystic lesion and can occasionally even appear to cause mass effect or can be confused with hydrocephalus. Three general variations of the septum pellucidum exist: the standard cavum septum pellucidum (CSP), the cavum septum pellucidum et vergae (CEV), and the cavum velum interpositum (CVI). To understand the difference between these three variations, a brief description of the anatomy and embryology is necessary. The normal septum pellucidum consists of two leaflets/septations of white matter that are separated in nearly all fetuses in utero. Typically, these septations (i.e., septi pellucidi) fuse in the posterior to anterior direction soon before or after the neonatal period, especially by adulthood. Thus, the CSP exists when the two leaves fail to fuse; it is situated somewhere posterior to the genu of the corpus callosum but anterior to the columns of the fornix.