- 1 in 100 births. However, only 1% are large enough to be detectable prenatally.
- Unilocular, avascular cyst that does not communicate with the lateral ventricles.
- They are usually found in the midline between the cerebral hemispheres but about 10% are in the posterior fossa behind the vermis.
- Arachnoid cysts are usually isolated. There are rare associations with chromosomal defects, mainly trisomy 18 or 12, and agenesis of the corpus callosum.
- Detailed ultrasound examination, including neurosonography.
- Fetal brain MRI may be useful if ultrasound suggests the presence of other brain abnormalities.
- Ultrasound scans every 4 weeks to monitor the size of the cyst and possible compression resulting in ventriculomegaly.
- Place: hospital with neonatal intensive care and pediatric neurosurgery.
- Time: 38 weeks.
- Method: cesarean section if the fetal head circumference is >40 cm.
- Isolated small cyst: normal neurodevelopment.
- Large and compressing cyst: surgery is required to prevent long term sequelae, including seizures, headache, motor deficit or neurodevelopmental delay.
- No increased risk of recurrence.