Prevalence:

• 1 in 100,000 births.

Ultrasound diagnosis:

• Transcerebellar diameter <5^th percentile for gestational age.
• The small cerebeum is due to disruption in the white matter of either the vermis (vermian dysplasia) or the cerebellar hemispheres (hemispheric dysplasia).

Associated abnormalities:

• The two most common vermian dysplasias are:
  • Joubert syndrome: autosomal recessive mutation of chromosome 9q34; absence or underdevelopment of cerebellar vermis with cleft between cerebellar hemispheres and communication between the 4^th ventricle and cisterna magna. The condition is associated with neurodevelopmental delay. 
  • Rombencephalosynapsis: sporadic abnormality; complete absence of the vermis with fusion of the cerebellar hemispheres, absent cavum septum pellucidum,  ventriculomegaly and migration disorders. The condition is associated with severe neurodevelopmental delay. 

Investigations:

• Detailed ultrasound examination, including neurosonography.
• Invasive testing for karyotyping and array.
• TORCH test for fetal infections (CMV can cause a hypoplastic cerebellum).
• Fetal brain MRI at ≥32 weeks’ gestation for diagnosis of abnormalities that are not detectable by ultrasound, such as migration anomalies.

Follow up:

• Follow-up should be standard.

Delivery:

• Standard obstetric care, but delivery in a hospital with neonatal intensive care.

Prognosis:

• Death in childhood.

Recurrence:

• Joubert syndrome: 25%.
• Rombencephalosynapsis: no increased risk of recurrence.