Congenital Posterior Fossa Malformations: Imaging Patterns and Diagnostic Approach

A practical radiology review of Congenital Posterior Fossa Malformations: Imaging Patterns and Diagnostic Approach, focused on imaging findings, differential diagnosis, reporting points, and high-yield teaching pearls.

Dandy-Walker Malformation

• DWM is characterized by inferior vermian hypoplasia, upward vermian rotation, and cystic dilatation of the 4th ventricle. The key imaging clue is inferolateral displacement of the tela choroidea and the classic "tail sign" representing an unpaired caudal vermian lobule.

Remember

• Large posterior fossa cyst Rotated vermis Increased tegmentovermian angle Frequent supratentorial anomalies

Blake Pouch Cyst

• Blake pouch cyst results from failed perforation of the foramen of Magendie. Unlike DWM, the vermis is normal and the fastigium remains preserved.
• Remember: Normal vermis Enlarged 4th ventricle Infravermian cyst No true vermian dysgenesis

Mega Cisterna Magna

• Mega cisterna magna represents simple enlargement of the retrocerebellar CSF space (>10 mm) with a completely normal cerebellum and vermis.

Remember: Big cisterna magna Normal cerebellum Normal vermis No mass effect

Arachnoid Cyst

• Posterior fossa arachnoid cysts mimic other cystic lesions but produce mass effect on adjacent cerebellum and brainstem.
• Remember: CSF signal lesion Mass effect present May remodel bone Not just enlarged CSF space

Poretti-Boltshauser Syndrome

• A LAMA1-related disorder characterized by cerebellar dysplasia and multiple tiny cerebellar cysts, especially in the superior cerebellum.
• Remember: Multiple tiny cysts Dysplastic cerebellum Square-shaped 4th ventricle Retinal abnormalities

CHARGE Syndrome

• CHARGE syndrome is strongly associated with clival cleft, olfactory hypoplasia, and semicircular canal abnormalities.
• Remember: Inner ear dysplasia Clival cleft Olfactory hypoplasia Vermian hypoplasia

Board pearl

• Clival cleft + abnormal semicircular canals = think CHARGE.

Rhombencephalosynapsis

• Failure of vermian formation leads to fusion of the cerebellar hemispheres and dentate nuclei.
• Remember: Fused hemispheres Absent vermis White matter bridge across midline Aqueductal stenosis common
• One of the easiest posterior fossa diagnoses when recognized.

PHACES Syndrome

• PHACES syndrome combines large facial hemangiomas with cerebrovascular and posterior fossa abnormalities.
• Remember: Facial hemangioma Cerebellar hypoplasia/dysplasia Arterial anomalies Usually ipsilateral

Joubert Syndrome

• Joubert syndrome is the classic ciliopathy producing the molar tooth sign.
• Remember: Molar tooth sign Vermian agenesis/hypoplasia Deep interpeduncular fossa Horizontal SCPs
• If you see a molar tooth, the diagnosis is usually made.

Dystroglycanopathies

• These disorders produce cerebellar dysplasia with cysts and characteristic brainstem abnormalities.
• Remember: Z-shaped brainstem Cerebellar cysts Cerebellar hypoplasia Cobblestone cortex
• Z-shaped brainstem = huge clue.

Tubulinopathies

• Tubulin mutations affect neuronal migration and axonal guidance, producing widespread brain malformations.
• Remember: Lissencephaly/dysgyria Abnormal basal ganglia Brainstem dysplasia Cerebellar hypoplasia
• Always inspect the basal ganglia carefully.

Pontocerebellar Hypoplasia (PCH)

• PCH causes severe volume loss of the pons and cerebellum.
• Remember: Dragonfly cerebellum Tiny pons Tiny cerebellar hemispheres Hypomyelination
• Dragonfly sign = exam favorite.

Chudley-McCullough Syndrome

• Rare syndrome characterized by congenital sensorineural hearing loss and complex brain malformations
• Remember: Hearing loss Cerebellar dysplasia Callosal dysplasia Heterotopia

Pontine Tegmental Cap Dysplasia

• Characterized by ventral pontine flattening with a dorsal pontine protrusion (the "cap").
• Remember: Dorsal pontine cap Flattened ventral pons Absent transverse pontine fibers Cranial nerve abnormalities

Horizontal Gaze Palsy with Progressive Scoliosis

• ROBO3 mutation causes failure of normal brainstem decussation.
• Remember: Horizontal gaze palsy Progressive scoliosis Butterfly medulla Split pons

High-Yield Exam Pearls

• Molar tooth sign → Joubert syndrome
• Dragonfly cerebellum → Pontocerebellar hypoplasia
• Z-shaped brainstem → Dystroglycanopathy
• Fused cerebellar hemispheres → Rhombencephalosynapsis
• Clival cleft + semicircular canal anomaly → CHARGE syndrome
• Superior cerebellar cysts → Poretti-Boltshauser syndrome
• Tail sign + inferolateral tela displacement → Dandy-Walker malformation
• Facial hemangioma + cerebellar anomaly → PHACES syndrome
• Sensorineural hearing loss + cerebellar dysplasia → Chudley-McCullough syndrome