Cistern, Subarachnoid Space Normal Variants

Karen L. Salzman, MD

DIFFERENTIAL DIAGNOSIS

Common

Less Common

Rare but Important

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Helpful Clues for Common Diagnoses

Cavum Septi Pellucidi (CSP)
- Elongated finger-shaped CSF collection between frontal horns of lateral ventricles
- Posterior continuation between fornices often associated (cavum vergae)
Mega Cisterna Magna
- Enlarged cisterna magna communicates freely with 4th ventricle & basal cisterns
- Large posterior fossa
- Normal vermis
- Cistern crossed by falx cerebelli, tiny veins
- Occipital bone may appear scalloped
MR Artifacts, Flow-Related
- CSF flow artifact is common in basal cisterns, ventricles
- Commonly seen on FLAIR MR
- Artifact often extends outside skull
Enlarged Subarachnoid Spaces
- Idiopathic enlargement of subarachnoid spaces (SAS) during first year of life
- Increased head circumference (> 95%)
- Resolves without therapy by 12-24 months

Helpful Clues for Less Common Diagnoses

Cavum Velum Interpositum (CVI)
- Triangular-shaped CSF space between bodies of lateral ventricles, below fornices, above 3rd ventricle
- Often elevates, splays fornices & causes inferior displacement of internal cerebral veins & 3rd ventricle
Enlarged Optic Nerve Sheath
- May occur as normal variant
- Occurs in idiopathic intracranial hypertension (pseudotumor cerebri), NF1

Helpful Clues for Rare Diagnoses

Blake Pouch Cyst
- Failure of regression of Blake pouch cyst causes compression of basal cisterns
- Free communication of 4th ventricle with prominent inferior CSF space
Liliequist Membrane
- Thin arachnoid membrane separates suprasellar, interpeduncular, & prepontine cisterns