A 6-month-old infant presents with increasing head circumference (crossing two centile lines over 2 months), bulging anterior fontanelle, 'setting sun' sign, and prominent scalp veins. CT brain shows markedly enlarged lateral and third ventricles with a small posterior fossa. Which type of hydrocephalus does 'setting sun' sign indicate and what is its mechanism?
- A Communicating hydrocephalus — impaired CSF reabsorption at arachnoid villi
- B External hydrocephalus — enlarged subarachnoid spaces causing sundowning
- C Non-communicating (obstructive) hydrocephalus — 'setting sun' sign caused by forced downward gaze from compression of superior colliculi (Parinaud's phenomenon) by dilated suprapineal recess ✓
- D Pseudo-tumor cerebri — 'setting sun' sign is pathognomonic of raised ICP from any cause
Explanation
The 'setting sun' sign (forced downward conjugate gaze deviation with visible sclera above iris) in hydrocephalus results from stretching and downward displacement of the dorsal midbrain periaqueductal region by the dilated suprapineal recess of the third ventricle, compressing the superior colliculi and causing Parinaud's syndrome (dorsal midbrain syndrome) — loss of upward gaze. This is classically seen in non-communicating hydrocephalus with third ventricle dilatation (e.g., aqueductal stenosis). The finding indicates severe raised ICP requiring urgent VP shunting. External hydrocephalus does not cause setting sun sign.
Reference: Ghai Essential Pediatrics, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.