Hydrocephalus questions are plumbing questions. The enlarged ventricles are upstream. The normal ventricle is just downstream of the blockage.
Before you scroll: A 5-year-old boy has progressive headache and vomiting. MRI shows enlargement of both lateral ventricles and the third ventricle. The fourth ventricle is normal in size. Which structure is most likely obstructed?
CSF flows lateral ventricles to foramen of Monro to third ventricle to cerebral aqueduct to fourth ventricle to Luschka and Magendie to subarachnoid space. If the lateral and third ventricles are enlarged but the fourth is normal, the block must sit between the third and fourth ventricles. Big lateral + big third + normal fourth = cerebral aqueduct obstruction.
Ventricular plumbing
Which Ventricles Swell?
Find the transition point. Everything upstream enlarges; the next compartment stays normal or small.
Foramen of Monro
Lateral to third
LocationBetween each lateral ventricle and the third ventricle
PatternLateral ventricle enlargement, often unilateral if one side is blocked
Classic trapColloid cyst or intraventricular tumor near anterior third ventricle
Not this openerThe third ventricle is enlarged, so CSF reached it
Cerebral aqueduct
Third to fourth
LocationNarrow channel through the midbrain
PatternBoth lateral ventricles and third ventricle enlarged; fourth normal
Classic trapCongenital aqueductal stenosis or pineal region compression
Board ruleNormal fourth ventricle means block is above it
Luschka and Magendie
Fourth to subarachnoid space
LocationFourth ventricle exits
PatternFourth ventricle also enlarges with upstream ventricles
Mass trapPosterior fossa mass or fourth-ventricle outlet obstruction
Board ruleIf fourth is big too, look at the exits
Arachnoid granulations
Reabsorption failure
LocationCSF absorption into venous sinuses
PatternCommunicating hydrocephalus, all ventricles can enlarge
Clinical trapPost-meningitis or subarachnoid hemorrhage scarring
Board ruleNo focal transition point in the ventricular plumbing
Board trap: Foramen of Monro does not explain an enlarged third ventricle. A Monro block traps CSF before it enters the third.
Flow switchboard
Read the Ventricles
Pick an imaging pattern. The readout walks the CSF route and names the obstruction.
Which blockage matches this ventricular pattern?
Pick a clue.CSF route: lateral ventricles to Monro to third to aqueduct to fourth to Luschka/Magendie to arachnoid granulations.
Board trap: Arachnoid granulations are after the ventricular system, so the pattern is communicating, not a normal fourth ventricle transition.
Two-gate discriminator
Decision Tree
A ventricle question is a downstream-normal question.
1
Are the lateral ventricles enlarged but the third ventricle normal?
Yes. CSF failed before the third.
No. The third is enlarged too.
2
Is the fourth ventricle normal while the lateral and third ventricles are enlarged?
Yes. Block is between third and fourth.
No. Fourth is enlarged too or all ventricles communicate.
3
Is there a focal outlet block or a reabsorption failure after infection or hemorrhage?
Focal fourth-outlet obstruction.
Reabsorption failure, communicating pattern.
Make it stick
Hooks and Images
Turn the ventricles into a drain pipe. Upstream swells; downstream stays normal.
PIPE
Normal means downstream
If a ventricle is normal, the block is just upstream of it. That is the whole game.
tap to reveal
AQUA
Aqueduct is the narrow neck
Third swollen, fourth normal means the narrow midbrain aqueduct is closed.
tap to reveal
ABSORB
Granulations are the drain
Arachnoid granulations fail after meningitis or blood in the subarachnoid space. Pipes open, drain blocked.
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CSF flow map · tap to expand
Hydrocephalus · tap to expand
Ventriculomegaly MRI · tap to expand
Board walkthrough
Prove It
One vignette at a time. Choices shuffle. The bank does not repeat until it is exhausted.
Vignette 1Never-repeat tracking ready
Exam tools: right-click or long-press to cross out a choice. Double-click or double-tap to highlight one. Tools switch off after you answer.
