Cavum septum pellucidum

Last revised by Yaïr Glick on 30 Apr 2023

Cavum septum pellucidum (CSP) is a normal variant CSF space between the leaflets of the septum pellucidum.

While the term "cavum septum pellucidum" is generally accepted, it is grammatically incorrect. Since it denotes a space (cavum meaning cave) of the septum pellucidum, the second part (septum pellucidum) should be in the genitive noun case, which would be inflected as cavum septi pellucidi

It has been posited that rendering it as "cavo septum pellucidum", would also be correct, which means "septum pellucidum with a cave". However this form has never been used.

Historically it has also been called the fifth ventricle, but this use is now advised against as the cavum does not usually have any direct communication with the ventricular system.

A cavum septum pellucidum is present in the normal fetus, but over 85% of them fuse by 3-6 months of age meaning that a cavum septum pellucidum persists in ~15% of the adult population. 

The cavum septum pellucidum commonly occurs with, and is often confused with, the cavum vergae, which is situated posterior to the anterior columns of the fornix. During development, these spaces obliterate posteroanteriorly - the cavum vergae followed by the cavum septum pellucidum - and it is not uncommon that both occur together as one contiguous space (see cases 3 and 4), aptly termed "cavum septum pellucidum (or septi pellucidi) et vergae".

Cavum septum pellucidum is a Latin term deriving from 'cavum', meaning 'space', 'septum', meaning 'fence', and therefore a dividing structure, and 'pellucidus' meaning 'transparent'.

It has been loosely associated with 3-5:

However, an absent cavum septum pellucidum in antenatal imaging is a concerning feature and is associated with significant CNS anomalies 6.

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