Interhemispheric Fissure Cysts

Anne G. Osborn, MD, FACR

DIFFERENTIAL DIAGNOSIS

Common

Pineal Cyst

Less Common

Callosal Dysgenesis
Neurocysticercosis
Arachnoid Cyst

Rare but Important

Holoprosencephaly (HPE)
Medial Atrial Diverticulum
Atretic Cephalocele
Dermoid Cyst
Epidermoid Cyst
Tumor-Associated Cysts
Aicardi Syndrome

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Anatomic sublocation key
- Pineal/quadrigeminal region
  - Is it pineal cyst or cystic-appearing pineal tumor (e.g., pineocytoma)
  - CSF-like: Arachnoid cyst, epidermoid cyst, medial atrial diverticulum
- Superior interhemispheric fissure
  - Most common: Cyst associated with callosal dysgenesis, holoprosencephaly; neurocysticercosis
  - Less common: Arachnoid cyst, atretic cephalocele, Aicardi syndrome
- Posterior interhemispheric fissure
  - More common: Holoprosencephaly, medial atrial diverticulum
  - Less common: Epidermoid cyst
- Anteroinferior interhemispheric fissure
  - Neurocysticercosis
  - Dermoid cyst (more common in midline)
  - Epidermoid cyst (less common in midline)
Two morphologically distinct types of interhemispheric CSF-containing cysts
- Interhemispheric cyst associated with callosal dysgenesis or holoprosencephaly
- Parasagittal cyst unassociated with callosal dysgenesis
  - Arachnoid cyst, medial atrial diverticulum
  - Tumor-associated cysts (macroadenoma, meningioma)

Helpful Clues for Common Diagnoses

Pineal Cyst
- Glial-lined intrapineal cyst located in pineal recess
- Common (23% of healthy adults)
- Multiple small (< 2 mm) or larger confluent cysts
- Usually isointense with CSF on T1-, T2WI; FLAIR variable
- Wall thickness < 2 mm
- Smooth rim-enhancement typical
- Thick/nodular enhancement may be indistinguishable from pineocytoma
  - Truly cystic pineocytomas are rare
  - Some pineal cysts may have variant appearance, may even hemorrhage (cyst apoplexy)

Helpful Clues for Less Common Diagnoses

Callosal Dysgenesis
- 3rd ventricle open dorsally
- Two types of agenesis with interhemispheric cyst
  - Type 1 (most common): Cyst is diverticulum of lateral ventricle, density/signal like CSF, ependymal-lined
  - Type 2: Multilocular/septated cysts within/adjacent to midline that do not communicate with ventricles, typically hyperdense/hyperintense to CSF
Neurocysticercosis
- “Racemose” cysts > solitary cysts
  - Convexity sulci
  - Anteroinferior interhemispheric fissure
  - Suprasellar/basal, quadrigeminal cisterns
Arachnoid Cyst
- Only 5% of ACs occur in parasagittal region/interhemispheric fissure
  - Usually are convexity ACs that extend medially
  - Most are small, unilateral, asymptomatic
- Large/symptomatic ACs in interhemispheric fissure rare
  - Typically not associated with callosal dysgenesis
  - May also “straddle” falx, extend equally on each side
  - Do not communicate with ventricular system
  - May cause progressive lower extremity weakness

Helpful Clues for Rare Diagnoses

Holoprosencephaly (HPE)
- Alobar HPE
  - Central monoventricle opens to large dorsal CSF-filled cyst
  - Cyst wall comprised of telencephalic roof plate, tela choroidea remnants
- Semilobar HPE
  - May occur with large dorsal CSF space
Medial Atrial Diverticulum
- Local herniation of posteromedial lateral ventricle
- Typically associated with severe, long-standing hydrocephalus
- Massive ventricular enlargement → uni- or bilateral pulsion diverticulae of inferomedial atrial wall
- CSF-filled pouch herniates medially into quadrigeminal cistern
  - Large medial atrial diverticulae may extend inferiorly through incisura into posterior fossa
Atretic Cephalocele
- T2 hyperintense subscalp mass extends through midline calvarial defect
- ± Primitive falcine vein
Dermoid Cyst
- Congenital inclusion cyst
- Fat & calcification
- Location
  - Midline > off-midline
  - Frontonasal, sella/parasellar, quadrigeminal cistern
- Look for fatty droplets in cisterns, sulci, ventricles
Epidermoid Cyst
- 4-9x more common than dermoid cyst BUT
  - Off-midline > midline
  - Rarely arises in interhemispheric fissure
- May adhere to surrounding structures like ACA, make resection difficult
- Resembles CSF on CT, MR
  - Often very slightly hyperintense to CSF
  - Doesn’t suppress on FLAIR
  - Restricts on DWI
  - Insinuates/infiltrates along subarachnoid cisterns
Tumor-Associated Cysts
- Most common with pituitary macroadenoma, meningioma
- Trapped pools of CSF (subarachnoid space) or interstitial fluid (perivascular spaces)
Aicardi Syndrome
- X-linked dominant
- Associated with broad spectrum of cerebral malformations (e.g., Dandy-Walker continuum)
- Classic triad
  - Infantile spasms
  - Chorioretinal lacunae
  - Agenesis CC ± interhemispheric cyst
- Choroid plexus cysts, papillomas