Posterior Fossa Arachnoid Cyst with Progressive Hydrocephalus: A Case of Successful Prenatal Diagnosis and Postnatal Management

Abstract Posterior fossa arachnoid cysts are uncommon fetal findings that are usually incidental and stable; however, lesions in critical locations, such as the cerebellopontine angle, may cause progressive neurological compromise. We report a case in which serial prenatal imaging enabled timely intervention and a favorable long-term outcome. A 25-year-old primigravida with an Rh-negative pregnancy was referred at 28 weeks' gestation. Earlier scans at 16 and 19 weeks performed elsewhere were reported as normal. Ultrasound with targeted neurosonography at 28 weeks identified a 1.9 × 2.7 cm avascular cyst in the left posterior fossa, causing mild midline shift and obliteration of the cisterna magna without ventriculomegaly. Fetal magnetic resonance imaging confirmed a cerebellopontine arachnoid cyst with brainstem displacement and hypoplasia of the left cerebellar hemisphere. Serial imaging demonstrated progressive cyst enlargement with severe bilateral ventriculomegaly and obstructive hydrocephalus by 33 + 4 weeks. Following multidisciplinary discussion, antenatal corticosteroids were administered, and elective delivery was performed at 34 weeks. Postnatally, the neonate underwent endoscopic cyst fenestration with Ommaya reservoir placement. Recovery was uneventful, with radiological improvement, no recurrent seizures, and normal neurodevelopment at 3 years of age. This case emphasizes the importance of vigilant prenatal neurosonographic surveillance and multidisciplinary management in strategically located arachnoid cysts.