Absent septum pellucidum Absence of septum pellucidum UMLS:C0431371 In contrast to the septum pellucidum, the cavum septum pellucidum (CSP) is a potential cavity between the membranous leaves of the septum pellucidum, separated by at least 1 mm, and is considered a normal anatomical variation. SNOMEDCT_US:253143001 Agenesis of the septum pellucidum Absence of the septum pellucidum Missing septum pellucidum HP:0006969 Absence of the septum pellucidum (meaning translucent wall in Latin - SP), also known as the ventricle of Sylvius. The septum pellucidum is a thin, triangular double membrane separating the frontal horns of the right and left lateral ventricles of the brain. It extends between the anterior portion of the corpus callosum, and the body of the fornix and its width varies from 1.5 to 3.0 mm. HP:0001331 Abnormal cerebral morphology