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

Both aqueductal stenosis and Dandy-Walker malformation lead to hydrocephalus, but the site of CSF blockage is different in each, and this changes what the scan looks like.

1. Third ventricle size: In aqueductal stenosis the block sits just below the third ventricle, so it enlarges. In Dandy-Walker malformation the whole ventricular system upstream of the fourth ventricle can also back up, so the third ventricle can be large here too. This does not reliably tell the two apart.
2. Posterior fossa volume: In Dandy-Walker malformation the fourth ventricle itself is cystic and the vermis is underdeveloped, and this cyst physically enlarges the posterior fossa. In aqueductal stenosis the blockage is above the fourth ventricle, so the posterior fossa keeps its normal size. This difference is consistent and specific.
3. Lateral ventricular size: Both conditions cause the lateral ventricles to dilate because CSF cannot drain past the block. This feature is shared, not distinguishing.
4. Head circumference: This grows with any significant hydrocephalus regardless of its cause, so it does not point to which condition is present.

The posterior fossa is the structure whose size actually differs between the two conditions: enlarged in Dandy-Walker malformation, normal in aqueductal stenosis.

Let's summarize:

• Aqueductal stenosis blocks CSF above the fourth ventricle, sparing the posterior fossa.
• Dandy-Walker malformation involves a cystic fourth ventricle that enlarges the posterior fossa.

So posterior fossa volume is the feature that distinguishes hydrocephalus due to aqueductal stenosis from that due to Dandy-Walker malformation.