A 2-month-old infant is noted to have a rapidly enlarging head circumference crossing two centile lines over 8 weeks, bulging anterior fontanelle, and sun-setting sign. CT scan shows enlarged lateral and third ventricles with normal fourth ventricle and cerebral aqueduct. Which is the MOST LIKELY anatomical site of CSF obstruction?

• A Foramen of Monro (interventricular foramen)
• B Foramina of Luschka and Magendie (outlet foramina of fourth ventricle)
• C Arachnoid granulations (communicating hydrocephalus)
• D Cerebral aqueduct (aqueduct of Sylvius) ✓

Explanation

The pattern of enlarged lateral and third ventricles with a normal-sized fourth ventricle points to obstruction at the cerebral aqueduct (aqueduct of Sylvius), as CSF cannot flow from the third ventricle to the fourth ventricle. This is the most common cause of congenital obstructive (non-communicating) hydrocephalus and is called aqueductal stenosis. Obstruction at the foramen of Monro would cause asymmetric hydrocephalus (one lateral ventricle enlarged). Obstruction at the foramina of Luschka/Magendie would cause enlargement of all four ventricles (fourth included). Communicating hydrocephalus affects all ventricles equally.

Reference: Ghai Essential Pediatrics, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.