Infratentorial Midline Cyst

Gregory L. Katzman, MD, MBA

DIFFERENTIAL DIAGNOSIS

Common

Mega Cisterna Magna
Arachnoid Cyst

Less Common

Neurocysticercosis
Dandy-Walker Continuum
Obstructive Hydrocephalus (“Trapped” or “Encysted” 4th Ventricle)
Pilocytic Astrocytoma
Hemangioblastoma
Epidermoid Cyst
Dermoid Cyst
Enlarged Perivascular Spaces

Rare but Important

Congenital Vermian Hypoplasia
Ganglioglioma
Pleomorphic Xanthoastrocytoma
Neurenteric Cyst

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Is mass intra- or extra-axial?
If extra-axial, cistern or 4th ventricle?
- CSF cistern (mega cisterna magna, Dandy-Walker continuum, arachnoid cyst, epidermoid cyst)
- 4th ventricle (encysted ventricle, neurocysticercosis, dermoid or epidermoid cyst, cystic neoplasm)
If intra-axial, pons, vermis, or cerebellum?
- Cerebellum (enlarged perivascular spaces, cystic neoplasm)
- Vermis (cystic neoplasm, vermian hypoplasia)
- Pons (cystic neoplasm > > enlarged perivascular spaces)

Helpful Clues for Common Diagnoses

Mega Cisterna Magna
- Enlarged posterior fossa CSF space
- Normal vermis completely covers 4th ventricle (rules out Dandy-Walker malformation/variant)
- May show striking scalloping of skull (due to CSF pulsations)
Arachnoid Cyst
- Sharply demarcated extra-axial cyst
- Follows CSF attenuation/signal
- Suppresses on FLAIR, no DWI restriction
- Size varies from few millimeters to giant
- Often asymptomatic, found incidentally

Helpful Clues for Less Common Diagnoses

Neurocysticercosis
- Best clue: Cyst with “dot” inside
  - ± Discrete eccentric scolex
  - Cyst slightly hyperintense to CSF
- Cisterns > 4th ventricle
Dandy-Walker Continuum
- DWC: Broad spectrum of cystic posterior fossa malformations
- DW malformation: Large posterior fossa + large CSF cyst, normal 4th ventricle absent, lambdoid-torcular inversion
- DW variant: Failure of “closure” of 4th ventricle, vermian hypoplasia
- Includes persistent Blake pouch cyst, mega cisterna magna
- 2/3 have associated CNS &/or extracranial anomalies
Obstructive Hydrocephalus (“Trapped” or “Encysted” 4th Ventricle)
- Due to obstructing lesions of 4th ventricle; all foramina must be involved (Magendie, Luschka, aqueduct)
- May be from hemorrhage, infectious, inflammatory, or neoplastic causes
- Ventricle enlarged but maintains basic shape
- CSF intensity/attenuation
Pilocytic Astrocytoma
- Cystic cerebellar mass with enhancing mural nodule
- Cerebellum > vermis, 4th ventricle
- Child > adult
Hemangioblastoma
- Best diagnostic clue: Adult with intra-axial posterior fossa mass with cyst, enhancing mural nodule abutting pia
- Size varies from tiny to several centimeters
- 1-2% of primary intracranial tumors, 7-10% of posterior fossa tumors
- May be associated with von Hippel-Lindau syndrome
Epidermoid Cyst
- Congenital inclusion cyst
- Lobulated, irregular, insinuating CSF-like mass with “fronds”
- Cerebellopontine angle cistern > 4th ventricle
- FLAIR usually doesn’t completely null; restricts on DWI
Dermoid Cyst
- Congenital inclusion cyst
- Looks like fat
  - Use fat-suppression sequence to confirm
  - ± Rupture (fat droplets in cisterns, sulci, ventricles)
  - May cause chemical meningitis, extensive enhancement
Enlarged Perivascular Spaces
- Pial-lined interstitial fluid-filled structures that accompany penetrating arteries but do not communicate directly with subarachnoid space
- Cluster of variably sized intra-axial cysts
- Off-midline (dentate nuclei) > midline (vermis, pons)
- Follow CSF
  - Suppress completely on FLAIR
  - No restriction on DWI
  - No enhancement
- “Leave me alone” lesion that should not be mistaken for serious disease

Helpful Clues for Rare Diagnoses

Congenital Vermian Hypoplasia
- Prototype = Joubert syndrome
- Inherited hypoplasia or aplasia of vermis characterized by transient episodic hyperpnea, oculomotor abnormalities, ataxia, variable mental retardation
- “Molar tooth” brainstem; “bat wing” or “umbrella” shaped 4th ventricle; vermian remnant variable size
- Midline anomalies common (holoprosencephaly, frontonasal dysplasia, facial clefting)
Ganglioglioma
- Best diagnostic clue: Partially cystic, enhancing, cortically based mass in child/young adult
- Cortical dysplasia commonly associated
- Excellent prognosis if surgical resection complete
- Malignant degeneration rare, approximately 5-10% (glial component)
Pleomorphic Xanthoastrocytoma
- Supratentorial cortical mass with adjacent enhancing dural tail
- Cyst and enhancing mural nodule typical
- 98% supratentorial, rarely found in cerebellum
- Despite circumscribed appearance, tumor often infiltrates
Neurenteric Cyst
- Benign malformative endodermal CNS cyst
- Round/lobulated nonenhancing mass
- Anterior to pontomedullary junction, slightly off-midline
- Slightly/moderately hyperintense compared to CSF