Corpus: Pellucid septum

The pellucid septum is a vertical, membranous partition in the brain that runs sagittally between the two anterior horns of the lateral ventricles.

The pellucid septum is a 1.5 to 3 mm wide, crescent-shaped septum that extends between the corpus callosum and the fornix (crura fornicis). It extends in a sagittal direction from the genu of the corpus callosum to the splenium of the corpus callosum, where it tapers to a point. It is made up of two thin layers (laminae of the pellucid septum) through which connecting veins run to the choroid plexus.

The septum forms the medial boundary of the anterior horns of the lateral ventricles, which are convex towards the midline, and of parts of the central part of the lateral ventricles. It also represents the medial hemispheric wall. The lateral ventricles detach rostrally and occipitally from the septum and diverge laterally. The two interventricular foramina are located at the lower edge of the pellucid septum.

The pellucid septum only occurs in more highly developed mammals. Development begins in the 10th to 12th week of pregnancy. Due to the growth in size of the corpus callosum, the pellucid septum is stretched out like a sail and is fully developed by the 17th week of pregnancy. The temporary cavity between the two layers is called the cavity of corpus callosum. It can persist into adulthood as a cavity of septum pellucidi alone or together with a cavum vergae.

The laminae consist of glial cells and neurons on the medial side. On the lateral surface, the pellucid septum is lined with ependymal cells.

5.1. Cavity of septum pellucidum

During embryogenesis, an approximately 10 mm wide, fluid-filled cavity forms between the layers of the pellucid septum. If this cavity does not regress during pediatric development, it remains into adulthood and is referred to as the cavity of septum pellucidum. It is located at the rostral end of the septum. The average size is 6 x 7.5 mm with a width of 2 to 5 mm. The prevalence is about 2.2 %.

5.2. Cavum vergae

The cavum vergae is another normal variant located at the caudal end between the layers of the pellucid septum. It almost exclusively occurs together with a cavity of septum pellucidum. The prevalence of an isolated cavum vergae is around 0.4 %.

5.3. Cavum septi pellucidi et vergae

The simultaneous occurrence of a cavity of septum pellucidum and cavum vergae represents a continuous cavity between the layers of the pellucid septum and occurs with a prevalence of approximately 3 %. The average size is 9 x 25 mm with a width of 6 mm.

A rare communication variant between cavity of septum pellucidum and cavum vergae is called the aqueductus septi, a very narrow connection occurring when the fornix legs are close together.

The exact function of the pellucid septum is not fully understood (2023). The septum is connected to the hippocampus and hypothalamus by bundles of nerve fibers and is part of the limbic system. The fiber bundles running through it form an interface between the autonomic hypothalamic system and the hippocampus, amygdala, habenula, and brain stem (reticular formation). The pellucid septum likely plays a role in processes such as consciousness, sleep, and emotions. Other functions may include foraging, sexuality, and fight-or-flight reactions.

Lesions of the pellucid septum can lead to neuropsychiatric abnormalities, such as learning disorders and mental retardation, due to the important neuronal connections of the limbic system passing through it.

7.1. Cysts

Neuroepithelial cysts of the pellucid septum are round, encapsulated cavities in the septal wall filled with cerebrospinal fluid.

Small cysts (< 5 mm) that communicate with the ventricular system are usually asymptomatic. Cysts near the interventricular foramina can become symptomatic as they may promote hydrocephalus development or cause compression of neighboring nerve tissue, resulting in neurological deficits.

Surgical treatment of symptomatic cysts involves drainage or creating a communicating connection to the ventricular system.

7.2. Agenesis

The absence of the septum pellucidum is associated with cerebral malformations, such as:

• Holoprosencephaly
• Septo-optic dysplasia
• Dysgenesis of the corpus callosum
• Apert syndrome
• Hypoplasia of the brain
• Heterotopia of the grey matter
• Schizencephaly
• Miller-Dieker syndrome
• Congenital hydrocephalus
• Arnold-Chiari malformation (type II)

Acquired causes of pellucid septum destruction include:

• Prolonged hydrocephalus
• Craniocerebral trauma
• Meningitis
• Porencephaly

7.3. Tumors

Neoplasms of the pellucid septum are very rare. Described tumors include oligodendrogliomas and ectopic pinealomas originating directly from the septum. Infiltration by tumors of the neighboring brain tissue (e.g., oligodendrogliomas, subependymomas, glioblastomas) is more common. Tumor diseases of the pellucid septum typically present on CT scans as irregular lines and thickening of more than 3 mm.

7.4. Other pathologies

Vascular malformations (venous angiomas), calcifications, and neurocysticercosis in the pellucid septum have been described.

• Sarwar M: The septum pellucidum: normal and abnormal Am J Neuroradiol 1989 Sep-Oct;10(5):989-1005. PMID: 2505543 PMCID: PMC8335275