A congenital anomaly of the pia-arachnoid layer with splaying of the cerebellar hemispheres is known as what?

• A. Arachnoid cyst
• B. Agenesis of corpus callosum
• C. Dandy-Walker malformation
• D. Hydranencephaly

Answer: (A)

Arachnoid cysts are congenital fluid-filled sacs that arise from splitting or duplication of the arachnoid membrane, leaving a CSF-filled space. When such a cyst sits in the posterior fossa, it can mass effect the surrounding cerebellar tissue and cause the cerebellar hemispheres to appear splayed apart on imaging. This description—congenital anomaly of the pia-arachnoid layer with a CSF-filled cavity causing separation of the cerebellar hemispheres—fits an arachnoid cyst best.

In contrast, agenesis of the corpus callosum involves absence of the connecting bridge between the hemispheres rather than a CSF-filled cyst causing mass effect. Dandy-Walker malformation centers on vermian hypoplasia with a large posterior fossa cyst that communicates with the fourth ventricle and often an enlarged posterior fossa, a different pattern. Hydranencephaly describes near-complete absence of the cerebral hemispheres replaced by fluid rather than a cystic arachnoid lesion.