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Cavum Septi Pellucidi (CSP) :: Brain imaging

Definition  & Terminology

  • Cystic CSF cavity of septum pellucidum (SP)
  • Occurs ± cavum vergae (CV)

Diagnostic checklist 

  1. Elongated finger-shaped CSF collection between lateral ventricles
  2. Cavum septi pellucidi (CSP): Between frontal horns of lateral ventricles
  3. Cavum vergae: Posterior extension between fornices
  4. Size : varies from slit like to several mm, occasionally > 1 cm
  5. Septum Pellucidium : invariably cystic in fetus
  6. Width of fetal cavum Septum Pellucidium increases between 19-27 weeks
  7. Plateaus at 28 weeks
  8. Gradually closes in rostral direction between 28 weeks and term
  9. CSP present in 100% of premature, 85% of term infants
  10. CSP seen in up to 15-20% of adults


  • Asymmetric lateral ventricles
  • Cavum velum interpositum
  • Ependymal cyst
  • Absent SP


• CSP forms if fetal SP fails to obliterate
• CSP is not "5th ventricle"
• CV is not "6th ventricle"


• Usually asymptomatic, incidental
• Headache (relationship to cyst unclear)
• CSP frequent among athletes with history of repeated traumatic brain injury, such as boxers


• CV almost never occurs without CSP

Imaging Gallery

Coronal graphic with axial insert shows classic cavum septi pellucidi (CSP) with cavum vergae (CV) . Note the finger-like CSF collection between the lateral ventricles. 

Axial T2WI MR shows cavum septi pellucidi as a CSF collection between the leaves of the septum pellucidum (blue opened arrows). Although seen incidentally, some studies have report that CSP is frequent among athletes with a history of repeated traumatic brain injury (TBI), such as boxers, and American professional football players.

Coronal T1WI MR shows a classic large CSP between the frontal horns (blue arrows)  bowing the leaves of the septum pellucidum laterally (curves blue arrows).

Axial FLAIR MR shows a large CSP with CV as a large CSF collection between the leaves of the septum pellucidum (blue arrows) continuing directly posteriorly with the CSF collection, splaying the fornices laterally (curved blue arrows).

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