- 1 in 30,000 births.
- Cystic dilation of the fourth ventricle that fills the posterior fossa and extends into the cisterna magna.
- Hypoplasia or complete agenesis of the cerebellar vermis.
- Chromosomal defects, mainly trisomies 13 or 18, are found in about 30% of cases.
- Genetic syndromes (most common: Walker–Warburg syndrome, Meckel–Gruber syndrome, Aicardi syndrome, Neu–Laxova syndrome) and defects (brain, heart, gastrointestinal and genitourinary) are found in >50% of cases.
- Severe ventriculomegaly is common and postnatally develops in >80% of cases.
- Detailed ultrasound examination, including neurosonography.
- Invasive testing for karyotyping and array.
- Fetal brain MRI at ≥32 weeks’ gestation for the diagnosis of neuronal migration disorders.
- Ultrasound scans every 4 weeks to monitor possible development of severe ventriculomegaly.
- Standard obstetric care, but delivery in a hospital with neonatal intensive care.
- Cesarean section if the fetal head circumference is >40 cm.
- Isolated: neurodevelopmental delay in >50% of cases.
- Severe ventriculomegaly: mortality rate >50% and neurodevelopmental delay in most survivors.
- Isolated: 3-5%.
- Part of trisomies: 1%.