A 6-month-old infant has a bulging anterior fontanelle, enlarged head circumference (head circumference >97th centile), sunset sign, and irritability. CT head shows triventricular hydrocephalus with dilated lateral and third ventricles and a normal fourth ventricle. What is the most likely site of obstruction?

• A Foramen of Luschka and Magendie (4th ventricle exits)
• B Foramen of Monro (bilateral)
• C Sylvian aqueduct (aqueduct of Sylvius) ✓
• D Pacchionian granulations (arachnoid villi)

Explanation

Triventricular hydrocephalus (lateral ventricles + third ventricle dilated, fourth ventricle normal) indicates obstruction at the aqueduct of Sylvius (cerebral aqueduct) — the narrow connection between the third and fourth ventricles. This is the most common site of obstructive (non-communicating) hydrocephalus in infants and is often congenital (X-linked aqueductal stenosis, caused by L1CAM gene mutation). Bilateral Foramen of Monro obstruction causes bilateral lateral ventricle dilation with normal third and fourth ventricles. Fourth ventricular outlet obstruction (Luschka/Magendie) causes all four ventricles to dilate. Arachnoid villi obstruction causes communicating hydrocephalus with all ventricles dilated.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.