Prevalence:

• 1 in 30,000 births.

Ultrasound diagnosis:

• 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.

Associated abnormalities:

• 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.

Investigations:

• 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.

Follow up:

• Ultrasound scans every 4 weeks to monitor possible development of severe ventriculomegaly.

Delivery:

• Standard obstetric care, but delivery in a hospital with neonatal intensive care.
• Cesarean section if the fetal head circumference is >40 cm.

Prognosis:

• Isolated: neurodevelopmental delay in >50% of cases.
• Severe ventriculomegaly: mortality rate >50% and neurodevelopmental delay in most survivors.

Recurrence:

• Isolated: 3-5%.
• Part of trisomies: 1%.