A 3-month-old boy presents with progressive increase in head circumference crossing percentiles, bulging anterior fontanelle, and episodes of vomiting. On axial CT brain at the midbrain level, the structure marked **D** is narrowed. Imaging shows enlarged lateral and third ventricles with a normal-sized fourth ventricle. Which of the following is the most likely diagnosis and the primary reason for the imaging pattern observed?

Explanation

## Why "Aqueductal stenosis causing obstructive hydrocephalus with impaired CSF flow from the third to fourth ventricle" is right The cerebral aqueduct of Sylvius (marked **D**) is the critical narrow channel connecting the third and fourth ventricles. Stenosis at this level is the most common cause of congenital obstructive (non-communicating) hydrocephalus, accounting for 6–66% of pediatric cases. The clinical presentation—progressive macrocephaly with bulging fontanelle and vomiting in an infant—combined with the pathognomonic imaging finding of enlarged lateral and third ventricles with a NORMAL or SMALL fourth ventricle is the hallmark of aqueductal obstruction. This pattern reflects blocked CSF flow at the aqueduct, causing upstream ventricular dilation while the fourth ventricle remains unaffected. (Gray's Anatomy 42e Ch 22; Harrison 21e Ch 444) ## Why each distractor is wrong - **Communicating hydrocephalus with impaired CSF absorption at the arachnoid granulations**: In communicating hydrocephalus, CSF flows freely through the aqueduct and into the fourth ventricle, so ALL ventricles (lateral, third, AND fourth) enlarge symmetrically. The normal fourth ventricle size here rules this out. - **Choroid plexus papilloma causing overproduction of CSF in the lateral ventricles**: While papillomas do cause hydrocephalus by CSF overproduction, they do not produce the selective enlargement of lateral and third ventricles with sparing of the fourth ventricle. This pattern requires an obstruction at the aqueduct itself, not upstream overproduction. - **Dandy-Walker malformation with hypoplasia of the cerebellar vermis and fourth ventricle**: Dandy-Walker malformation is a posterior fossa malformation characterized by cerebellar vermis hypoplasia and fourth ventricle enlargement (not sparing), often with an enlarged foramen of Magendie. The imaging pattern here—normal fourth ventricle—is incompatible with Dandy-Walker. **High-Yield:** Aqueductal stenosis = enlarged lateral + 3rd ventricles + NORMAL/SMALL 4th ventricle; communicating hydrocephalus = ALL ventricles enlarged equally. [cite: Gray's Anatomy 42e Ch 22; Harrison 21e Ch 444]