Teaching NeuroImages: Giant fetal arachnoid cyst with favorable neurologic outcome

A 29-year-old woman was referred after an ultrasound at 24 weeks' gestation demonstrated a 1.8-cm intracranial cyst. Fetal MRI1 at 29 weeks (figure 1) showed a large cyst anterior to the brainstem measuring 3.4 × 1.7 cm with associated mass effect. Postnatal MRI at age 11 months (figure 2) confirmed the large suprasellar-prepontine arachnoid cyst (SPAC) with mass effect on the brainstem. Neurologic examination at age 17 months revealed no gross neurodevelopmental deficits and no intervention was required. SPACs comprise 5%–12% of arachnoid cysts and are often symptomatic, with signs of hydrocephalus or mass effect, and may require surgery.2