- 1 in 5,000 births.
- The cisterna magna is >10 mm in the transverse cerebellar view.
- Vermis: normal.
- Differential diagnosis: Blake’s pouch cyst (expansion of the 4th 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).
- It is usually an isolated finding, but in up to 10% of cases there is ventriculomegaly.
- Detailed ultrasound examination, including neurosonography.
- Fetal brain MRI may be useful if other brain abnormalities are suspected.
- Ultrasound scans every 4 weeks to monitor the size of the cisterna magna and possible development of ventriculomegaly.
- Standard obstetric care and delivery.
- Normal neurodevelopment.
- No increased risk of recurrence.