Prevalence:

• 1 in 1,000 births.

Ultrasound diagnosis:

• Expansion of the 4^th ventricle into the cisterna magna resulting in a unilocular, avascular cyst in the posterior fossa – ‘key-hole’ sign in the transverse cerebellar view.
• Vermis: normal size with mild to moderate upward rotation.
• Cisterna magna: normal.
• Differential diagnosis: mega cisterna magna (>10 mm; normal vermis), arachnoid cyst (cyst in the cisterna magna with mass effect on surrounding structures; normal vermis).

Associated abnormalities:

• It is usually an isolated finding.
• Risk of chromosomal abnormalies, mainly trisomy 21, in up to 5% of cases but usually in the presence of other suggestive markers.

Investigations:

• Detailed ultrasound examination, including neurosonography.
• Fetal brain MRI may be useful if other brain abnormalities are suspected.
• Invasive testing and array is recommended in non-isolated cases.

Follow up:

• Ultrasound scans every 4 weeks to monitor the size of the cyst and possible compression resulting in ventriculomegaly.
• Spontaneous resolution by 24-26 weeks in 50% of cases.

Delivery:

• Standard obstetric care and delivery.

Prognosis:

• Neurodevelopment: good in 90% of cases, mild impairment in 10%.
• Small risk of postnatal hydrocephalus with the need to shunt.

Recurrence:

• Isolated: no increased risk of recurrence.
• Part of trisomies: 1%.