Abnormal Cerebellum
Karen Y. Oh, MD
DIFFERENTIAL DIAGNOSIS
Common
Chiari II Malformation
Dandy-Walker Continuum: Classic
Dandy-Walker Continuum: Variant
Arachnoid Cyst
Mega Cisterna Magna
Mega Cisterna Magna (Mimic)
Less Common
Cerebellar Hypoplasia
Rhombencephalosynapsis
Joubert Syndrome
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
Assess cerebellar size and shape
Measure transcerebellar diameter
Gestational age ∽ transcerebellar diameter until about 3rd trimester
Use normative tables for exact percentiles
Absent or partially absent cerebellum/vermis
Consider Dandy-Walker spectrum if vermian defect present
May also be associated with posterior fossa cyst
If vermis not actually absent but distorted/compressed, consider arachnoid cyst in posterior fossa
Cerebellum appears small
Confirm small cerebellum versus relatively large posterior fossa
If cerebellum actually normal, could be mega cisterna magna or posterior fossa arachnoid cyst
Cerebellum is flattened and “banana-shaped”
Consider Chiari II malformation
Look for associated neural tube defect
Helpful Clues for Common Diagnoses
Chiari II Malformation
“Banana sign” in posterior fossa
Flattened cerebellum
Curved around midbrain
“Lemon sign”
Refers to appearance of calvarium secondary to flattened frontal bones and dolichocephaly
Typical appearance often resolves by 3rd trimester
Nonspecific finding, can even be seen in normals
Very small or absent cisterna magna
Due to caudal displacement of posterior fossa structures (tonsillar herniation) through foramen magnum
Ventriculomegaly
Usually mild but can progress during pregnancy
If typical intracranial findings identified, search carefully for the associated open neural tube defect (ONTD)
Can be subtle at times despite dramatic intracranial abnormalities
May be skin covered
Posterior ossification sites of vertebral bodies are splayed
Check sagittal view to visualize overlying sac (seen in 80%)
May have associated scoliosis/kyphosis at level of ONTD
Look for clubfoot, additional anomalies
Dandy-Walker Continuum: Classic
Absent vermis
Large posterior fossa cyst
4th ventricle open to posterior fossa cyst
Elevated torcular Herophili (confluence of superior sagittal, straight, & occipital sinuses)
Look for other associated anomalies (seen in 70-90%)
Callosal dysgenesis
ONTD
Heterotopias
Chromosomal abnormalities
Cardiac anomalies
Dandy-Walker Continuum: Variant
Milder form of Dandy-Walker malformation
Absent or hypoplastic inferior vermis
4th ventricle open to cisterna magna
“Keyhole” appearance of 4th ventricle on axial ultrasound
Posterior fossa should be normal size
Avoid over diagnosis
Vermis not always completely formed until 17 weeks
Overly coronal oblique scan plane can simulate vermian defect
Arachnoid Cyst
One third of cases occur in posterior fossa
Vermis is intact
Cyst does not communicate with 4th ventricle
Cerebellum often appears distorted by mass effect from cyst
Mega Cisterna Magna
Enlarged cisterna magna measuring > 10 mm
Make sure this is a true measure
Should see cavum septi pellucidi
Can give overall impression of relatively small cerebellum
Actual cerebellar diameter normal for gestational age
Usually an incidental finding of no clinical consequence
May be part of multiple findings seen with trisomy 18
Mega Cisterna Magna (Mimic)
An overly coronal image gives false impression of mega cisterna magna
Helpful Clues for Less Common Diagnoses
Cerebellar Hypoplasia
Cerebellar axial diameter less than expected for gestational age
Normally formed cerebellar hemispheres present
Folia appear normal (prominent sulci seen with atrophy, not hypoplasia)
Look for other anatomic abnormalities
Associated with chromosomal anomalies
Rhombencephalosynapsis
Congenital fusion of cerebellar lobes
Agenesis of vermis
Cerebellum appears small and “ball-like”
Typical bilobed appearance is absent
Abnormal 4th ventricle shape (“pointed” appearance on postnatal MR)
Look for other associated intracranial abnormalities
Consider fetal MR for optimal visualization of intracranial structures
Joubert Syndrome
Cerebellar cleft at midline between hemispheres
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