original_query: What is Dandy-Walker malformation and what are its key characteristics?
query: What is Dandy-Walker malformation and what are its key characteristics?
answer: Dandy-Walker malformation (DWM), also known as Dandy-Walker syndrome, is a rare congenital brain malformation where the cerebellar vermis (part joining the two hemispheres of the cerebellum) doesn't fully form, leading to an enlarged fourth ventricle and posterior fossa filled with cerebrospinal fluid.  Key characteristics include hypoplasia of the cerebellar vermis and an enlarged fourth ventricle and posterior fossa.