Question

Which of the following would distinguish hydrocephalus due to aqueductal stenosis when compared to that due to Dandy-Walker phenomenon

• A. Third ventricle size
• B. Posterior fossa volume
• C. Lateral ventricular size
• D. Head circumference

Hint:

Dandy-Walker malformation enlarges the posterior fossa; aqueductal stenosis does not.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
Both aqueductal stenosis and Dandy-Walker malformation cause hydrocephalus in children, but they block the flow of cerebrospinal fluid (CSF) at different points, so their imaging findings differ. The question asks which feature tells the two apart.

Step 2: Key Concept:
In aqueductal stenosis, the narrow channel between the third and fourth ventricles, the cerebral aqueduct, is blocked. CSF backs up into the lateral ventricles and the third ventricle, which both enlarge, while the fourth ventricle and the posterior fossa (the back, lower part of the skull housing the cerebellum and brainstem) stay normal in size.
In Dandy-Walker malformation, the problem lies in the development of the fourth ventricle and cerebellum. The fourth ventricle is cystically dilated and the cerebellar vermis is partly or fully absent, and this cyst pushes outward, enlarging the posterior fossa itself, often lifting the tentorium and bulging the back of the skull.

Step 3: Detailed Explanation:
Both conditions typically show large lateral ventricles and an enlarged third ventricle on imaging, so the size of these two structures does not reliably separate one from the other. Head circumference simply reflects how severe the hydrocephalus is overall and does not point to its cause.
The one structure that behaves very differently between the two is the posterior fossa. It is enlarged in Dandy-Walker malformation because of the cystic fourth ventricle pushing the bone and tentorium outward, while it remains a normal size in aqueductal stenosis, where the blockage sits above the fourth ventricle and does not disturb the posterior fossa's dimensions.

Step 4: Final Answer:
Posterior fossa volume is the feature that distinguishes the two, option (2).