A 6-month-old infant presents with increasing head circumference (>95th centile) crossing two percentile lines over 2 months, sunset sign, and bulging anterior fontanelle. CT scan shows dilated lateral and third ventricles with normal-sized fourth ventricle. What does this suggest, and what is the next diagnostic step?
- A Communicating hydrocephalus; perform lumbar puncture to check opening pressure
- B Aqueductal stenosis causing obstructive (non-communicating) hydrocephalus; MRI of brain ✓
- C Dandy-Walker malformation; no further imaging needed
- D Subdural hematoma; CT angiography
Explanation
Dilation of the lateral and third ventricles with a normal-sized fourth ventricle points to obstruction at the level of the cerebral aqueduct (Sylvius). This is the classic radiological pattern of aqueductal stenosis, the most common cause of congenital obstructive hydrocephalus. MRI of the brain provides superior resolution of the aqueduct and surrounding posterior fossa structures, distinguishes aqueductal stenosis from other causes (mass lesions, Chiari malformation), and guides surgical planning (endoscopic third ventriculostomy vs VP shunt). Dandy-Walker shows a dilated fourth ventricle communicating with a posterior fossa cyst.
Reference: Ghai Essential Pediatrics, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.