- 1 in 100,000 births.
- Transcerebellar diameter <5th 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).
- 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 4th 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.
- 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 should be standard.
- Standard obstetric care, but delivery in a hospital with neonatal intensive care.
- Death in childhood.
- Joubert syndrome: 25%.
- Rombencephalosynapsis: no increased risk of recurrence.