Joubert syndrome

Autosomal recessive ciliopathy causing congenital malformation of the midbrain-hindbrain junction, classically presenting with infantile hypotonia, developmental delay, abnormal breathing pattern (episodic hyperpnoea/apnoea), and oculomotor apraxia.

Multisystem associations

Exam like to test haha.

• Eyes: retinal dystrophy/coloboma (vision loss risk)
• Kidneys: nephronophthisis, cystic dysplasia (CKD risk)
• Liver: congenital hepatic fibrosis, portal hypertension
• Polydactyly can point towards related ciliopathies (Joubert spectrum phenotypes)

Key imaging features

MRI brain

• Molar tooth sign is a pattern, not a diagnosis: look for the trio
  • Vermian hypoplasia/dysplasia with deep interpeduncular fossa
  • Thickened, elongated, horizontally oriented superior cerebellar peduncles
  • Narrowed isthmus at the pontomesencephalic junction ("batwing" 4th ventricle often accompanies)
• Cerebellar vermis: underdeveloped and malformed rather than simply "absent"
• Brainstem: small/abnormal superior cerebellar peduncles insertion and midbrain configuration changes
• Variable supratentorial findings: corpus callosum dysgenesis, ventriculomegaly, cortical malformations can occur (not mandatory)

Prenatal imaging

• Mid-trimester US: vermian abnormality, enlarged posterior fossa CSF spaces can be subtle and non-specific
• Fetal MRI: better at demonstrating vermian dysplasia and brainstem configuration suggestive of Joubert spectrum

Top differentials

• Dandy-Walker malformations: enlarged posterior fossa with elevated tentorium and cystic 4th ventricle, not the brainstem peduncle configuration
• Rhombencephalosynapsis: vermis absent with fused cerebellar hemispheres, no molar tooth pattern
• Pontocerebellar hypoplasia: global cerebellar and pontine hypoplasia without the characteristic peduncle/interpeduncular configuration
• "Isolated" vermian hypoplasia: beware overcalling Joubert if the brainstem configuration is not there