Prevalence:

• 1 in 5,000 births.

Ultrasound diagnosis:

• The cisterna magna is >10 mm in the transverse cerebellar view.
• Vermis: normal.
• Differential diagnosis: Blake’s pouch cyst (expansion of the 4^th ventricle into the cisterna magna resulting in a unilocular, avascular cyst in the posterior fossa; vermis normal in size with upward rotation), arachnoid cyst (cyst in the cisterna magna with mass effect on surrounding structures; normal vermis).

Associated abnormalities:

• It is usually an isolated finding, but in up to 10% of cases there is ventriculomegaly.

Investigations:

• Detailed ultrasound examination, including neurosonography.
• Fetal brain MRI may be useful if other brain abnormalities are suspected.

Follow up:

• Ultrasound scans every 4 weeks to monitor the size of the cisterna magna and possible development of ventriculomegaly.

Delivery:

• Standard obstetric care and delivery.

Prognosis:

• Normal neurodevelopment.

Recurrence:

• No increased risk of recurrence.