- 1 in 1,000 births.
- Expansion of the 4th 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).
- 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.
- 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.
- 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.
- Standard obstetric care and delivery.
- Neurodevelopment: good in 90% of cases, mild impairment in 10%.
- Small risk of postnatal hydrocephalus with the need to shunt.
- Isolated: no increased risk of recurrence.
- Part of trisomies: 1%.