Which of the following would distinguish hydrocephalus due to aqueductal stenosis when compared to that due to Dandy walker malformation?

Ans. is 'b' i.e., Posterior fossa volume To under stand this question you are required to know the basics of CSF production and drainage. o Both Aqueductal stenosis and Dandy Walker cause non communicating hydrocephalus but the site of obstruction is different. o Let us see the causes of hydrocephalus in Aquedutal stenosis and Dandywalker malformation. o Aqueductal stenosis o In aqueductal stenosis the aqueduct connecting the 3'd and 4th ventricle is stenosed which leads to hydrocephalus with the dilatation of ventricular system prior to the aqueduet i.e. lateral ventricle, third ventricle. o The post fossa is normal in size. o Dandy Walker Malformation o In Dandy walker malformation there is cystic dilatation of the fouh ventricle in the post fossa with obstruction at the foramina of Lushcka and Magendie. o So the ventriclular system in this condition will also be dilated as in aqueductal stenosis but here the post fossa is also enlarged (in contrast to normal post fossa size in queductal stenosis).