Intracranial Cysts: Midline

Anne Kennedy, MD

DIFFERENTIAL DIAGNOSIS

Common

Cavum Vergae
Agenesis of the Corpus Callosum
Dandy-Walker Continuum: Classic
Dandy-Walker Continuum: Variant
Alobar Holoprosencephaly
Semilobar/Lobar Holoprosencephaly
Arachnoid Cyst

Less Common

Glioependymal Cyst
Cystic Teratoma
Syntelencephaly

Rare but Important

Vein of Galen Malformation
Arteriovenous Fistula

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Is it a vascular structure?
Could it be a thrombosed arteriovenous fistula (AVF)?
Where is it?
- Supratentorial
  - Are there 2 separate cerebral hemispheres?
  - Is the corpus callosum (CC) intact?
  - Are there any solid components?
  - Is it simple or multiloculated?
- Infratentorial
  - Is the cerebellar vermis normal?
  - Is the cyst in continuity with the 4th ventricle?
  - Is the torcular (confluence of sinuses) elevated?

Helpful Clues for Common Diagnoses

Cavum Vergae
- Anatomic variant
- Cavum vergae is the posterior extension of the cavum septi pellucidi (CSP)
Agenesis of the Corpus Callosum
- Absence of CC and high-riding 3rd ventricle create prominent midline cerebrospinal fluid (CSF) space
- Associated with midline cysts (usually glioependymal)
- Absent CSP
- Parallel lateral ventricles
- Colpocephaly (tear-drop shaped ventricles)
- “Steer horn” or “trident” configuration of frontal horns on coronal view
- Abnormal branch pattern of anterior cerebral artery
- Stenogyria: Gyri in radial “sunray” distribution in sagittal plane
Dandy-Walker Continuum: Classic
- Vermis severely hypoplastic or absent
- Cystic dilatation of 4th ventricle
- Large posterior fossa with big CSF cyst
- 4th ventricle appears “open” and contiguous with cyst
- Elevation of torcular
- Ventriculomegaly may be present
Dandy-Walker Continuum: Variant
- Inferior vermis absent/dysplastic
- Torcular position normal
- Posterior fossa not enlarged
- “Keyhole” appearance of 4th ventricle
Alobar Holoprosencephaly
- Monoventricle
- Absent CSP
- Absent falx
- Often abnormal facies
  - Hypotelorism, cyclopia, facial cleft
  - Proboscis, absent nose, ethmocephaly, cebocephaly
- Associated with aneuploidy, particularly trisomy 13
Semilobar/Lobar Holoprosencephaly
- Monoventricle anteriorly
- Absent CSP
- Separation into two lobes posteriorly
- May be associated with abnormal facies
Arachnoid Cyst
- Extra-axial, avascular, simple
- Majority over convexities but may be midline
  - 1/3 in posterior fossa in the fetus

Helpful Clues for Less Common Diagnoses

Glioependymal Cyst
- Avascular, may be multiloculated
- More commonly midline and associated with agenesis of corpus callosum
Cystic Teratoma
- Part cystic, part solid mass
- Gross distortion of cerebral architecture
- Hydrocephalus
- Polyhydramnios
Syntelencephaly
- Separate ventricle anteriorly and posteriorly
- Parietal gyral continuity
- Fusion of ventricles in prefrontal area
- Associated with 13q-deletion
  - Syndactyly
  - Hypoplastic thumbs

Helpful Clues for Rare Diagnoses

Vein of Galen Malformation
- Elongated tubular midline vascular structure
  - Aneurysmal dilatation of median prosencephalic vein of Markowski
- Located in cistern of velum interpositum and quadrigeminal plate cistern
- Look for enlarged neck vessels
- Look for cardiomegaly/hydrops from high output state
- May cause hydrocephalus
- May cause ischemic encephalomalacia
Arteriovenous Fistula
- Tubular structures in midline
- Look for flow on Doppler
- Thrombosed AVF looks hypoechoic, low level internal echoes, occasional echogenic clot within hypoechoic area
- Associated with intracranial hemorrhage
  - Echogenic clot in ventricle
  - Echogenic ependyma
- Associated with ischemic encephalomalacia
  - Loss of grey white matter differentiation
  - Ventriculomegaly
  - Porencephalic cyst
  - Microcephaly

Other Essential Information

Prognosis varies with etiology
- Cavum vergae: No clinical significance
- Agenesis of the corpus callosum
  - Prognosis depends on any associated syndromes/brain malformations
- Dandy-Walker continuum
  - Outcome worse for classic malformation
  - Cognitive outcome depends upon associated syndromes/other brain malformations
- Holoprosencephaly spectrum
  - Alobar: Many die in utero, rare survival beyond neonatal period reported
  - Semilobar/lobar outcome varies with type and severity of malformation
  - Developmental delay, seizure disorder, hypothalamic pituitary malfunction and visual impairment all reported
- Arachnoid or glioependymal cyst
  - Prognosis depends on underlying brain malformation
  - Shunt placement required for obstructive hydrocephalus
- Cystic teratoma
  - Dismal prognosis; 97% mortality if diagnosed before 30 weeks
- Vascular malformations
  - Poor outcome if associated intracranial hemorrhage or ischemic encephalomalacia