Prevalence:

• 1 in 100 births. However, only 1% are large enough to be detectable prenatally.

Ultrasound diagnosis:

• 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.

Associated abnormalities:

• Arachnoid cysts are usually isolated. There are rare associations with chromosomal defects, mainly trisomy 18 or 12, and agenesis of the corpus callosum.

Investigations:

• Detailed ultrasound examination, including neurosonography.
• Fetal brain MRI may be useful if ultrasound suggests the presence of other brain abnormalities.

Follow up:

• Ultrasound scans every 4 weeks to monitor the size of the cyst and possible compression resulting in ventriculomegaly.

Delivery:

• Place: hospital with neonatal intensive care and pediatric neurosurgery.
• Time: 38 weeks.
• Method: cesarean section if the fetal head circumference is >40 cm.

Prognosis:

• 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.

Recurrence:

• No increased risk of recurrence.