A 4-month-old infant presents with bulging fontanelle, sunset sign (downward gaze paresis), irritability, and increasing head circumference crossing two percentile lines over 6 weeks. MRI shows massive dilatation of lateral and third ventricles with normal fourth ventricle size and no evidence of aqueductal CSF flow. The primary anatomical site of obstruction is:
- A Sylvian aqueduct (aqueduct of Sylvius) ✓
- B Foramina of Luschka and Magendie (communicating hydrocephalus)
- C Foramen of Monro (bilateral obstruction)
- D Arachnoid granulations (impaired CSF absorption)
Explanation
The imaging pattern — massive dilatation of lateral and third ventricles with a normal-sized fourth ventricle — is the hallmark of aqueductal stenosis (obstruction at the cerebral aqueduct/Sylvian aqueduct). This is the most common cause of obstructive (non-communicating) hydrocephalus in infants, accounting for ~65–70% of congenital hydrocephalus. The Sylvian aqueduct (between third and fourth ventricle) when stenosed traps CSF above, dilating lateral and third ventricles while sparing the fourth. Foramen of Monro obstruction dilates only one or both lateral ventricles. Communicating hydrocephalus (Luschka/Magendie/arachnoid granules) dilates all ventricles including the fourth.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.