| Discussion: |
The septum pellucidum normally has a cavity of 1 to 2mm in width. When this cavity is larger, it is called cavum septi pellucidi or fifth ventricle. The term fifth ventricle is a misnomer because unlike ventricular cavities it does not contain cerebrospinal fluid nor is it lined by ependyma. The incidence of cavum septi pellucidi was 20.3% in 1032 brains studied.
The cavurn septi pellucidi varies from 2 to 95% depending to author; it is more prevalent in the young (infants) than older adults. The extreme differences in the reported frequency is highly suspect.
The cavum septum pellucidum is present at birth but in more than 80% of individuals it is obliterated by the age of 3 to 6 months. It is up to 1cm in width and the walls are parallel. It is an enclosed space and is not part of the ventricular system or connected with the subarachnoid space in man. A cyst of the cavum septum pellucidum may occur which is distinguished by the lateral bowing of the walls of the septum pellucidum which are more than 1 cm apart. These may be symptomatic. The significance of a persistent cavum septi pellucidi in the adult population is uncertain but may be associated with abnormalities of the limbic system.