A 3-year-old boy presents with progressive macrocephaly, developmental delay, and limited upward gaze. MRI brain sagittal view shows marked enlargement of the structure marked **D** with a narrowed cerebral aqueduct. The child also exhibits convergence-retraction nystagmus on attempted upward gaze and lid retraction. Which of the following is the most likely pathological process responsible for this clinical presentation?

Explanation

## Why option 1 is correct Aqueductal stenosis is the most common cause of congenital obstructive hydrocephalus in pediatrics. The narrowed cerebral aqueduct of Sylvius (the narrowest part of the CSF pathway between the 3rd and 4th ventricles) obstructs CSF flow, causing enlargement of the structure marked **D** (the fourth ventricle). The clinical triad of macrocephaly, developmental delay, and the specific finding of limited upward gaze with convergence-retraction nystagmus and lid retraction constitute Parinaud syndrome—a dorsal midbrain syndrome caused by compression of the tectal plate (which carries the superior and inferior colliculi and vertical gaze centers). This constellation of findings is pathognomonic for aqueductal stenosis with secondary tectal compression. (Sutton Radiology; Harrison 21e) ## Why each distractor is wrong - **Option 2 (Communicating hydrocephalus)**: Communicating hydrocephalus results from impaired CSF absorption at the arachnoid granulations and does NOT cause aqueductal narrowing or Parinaud syndrome. The aqueduct remains patent. This is the mechanism of normal pressure hydrocephalus (NPH), not congenital obstructive hydrocephalus with tectal compression. - **Option 3 (Choroid plexus papilloma)**: While choroid plexus papillomas do cause hydrocephalus, they do so by increased CSF production (secretory hydrocephalus), not by aqueductal obstruction. They would not produce the characteristic Parinaud syndrome from tectal plate compression seen in this case. - **Option 4 (Posterior fossa mass without aqueductal obstruction)**: A posterior fossa mass could compress the fourth ventricle, but if the aqueduct remained patent, CSF would still flow freely and Parinaud syndrome would not develop. The narrowed aqueduct and Parinaud findings are essential to the diagnosis of aqueductal stenosis. **High-Yield:** Aqueductal stenosis → obstructive hydrocephalus + Parinaud syndrome (up-gaze palsy, convergence-retraction nystagmus, light-near dissociation, lid retraction) from tectal plate compression; treat with endoscopic third ventriculostomy (ETV) in children >1–2 years. [cite: Sutton Radiology; Harrison 21e]