On ultrasound, which finding is associated with an arachnoid cyst?

• A. Enlargement of the posterior fossa with a normal cerebellar vermis
• B. Hydrocephalus with vermian herniation
• C. Agenesis of the corpus callosum
• D. Small posterior fossa with cerebellar hypoplasia

Answer: (A)

An arachnoid cyst in the posterior fossa shows up as a discrete, CSF-filled sac within the arachnoid space that enlarges the posterior fossa while the cerebellar vermis remains normal. This preserved vermis helps distinguish it from other posterior fossa conditions that distort or reduce the vermis, such as anomalies in the Dandy-Walker spectrum. On ultrasound you’d see the posterior fossa expanded with an anechoic cyst adjacent to the cerebellum, but the vermis is not hypoplastic or displaced. In contrast, hydrocephalus with vermian herniation indicates downward displacement due to pressure, agenesis of the corpus callosum is a midline forebrain defect, and a small posterior fossa with cerebellar hypoplasia points to cerebellar development problems—none of which match the typical appearance of an arachnoid cyst.