Brain
Hydrocephalus: Localize the Block
Fellowship-level neuroradiology teaching by E. Brooke Schrickel, MD. Open this topic in the interactive reading room →
Watch · concise explainerVisual summary
Core clinical idea
Big ventricles isn't the question — 'are these ventricles under pressure?' is. Read hydrocephalus as plumbing: find the level of obstruction, then judge acuity from the fastest-moving signs, because the danger is acute obstructive hydrocephalus where a tap or a missed lesion can kill.
Bottom line
Don't report big ventricles — report whether they're under pressure and where the block is; temporal horns, transependymal flow, and the third-ventricle floor decide acuity.
Core workstation questions
- Are the temporal horns dilated and ventricular margins blurred (acute), or sharp margins with frontal-horn predominance (chronic/atrophy)?
- Where is the transition point — and is there a mobile ball-valve lesion (colloid cyst, intraventricular tumor, cyst, neurocysticercosis)?
- Is a segment trapped (entrapment) that neurosurgery needs to know about now?
- On sagittal, is the third-ventricle floor concave/flat (reassuring) or convex/depressed (worrying)? Is there a hyperdynamic aqueductal jet?
- Is the extra-axial space tight against the inner table (pressure) or open with big sulci (atrophy/ex-vacuo)?
- In a child, is that fontanelle truly bulging — did I call and correlate?
- Are the ventricles out of proportion to the sulci, or are ventricles and sulci enlarged together (ex vacuo)?
- Which ventricles are enlarged, and where is the transition point from big to small?
What changes reporting / management
- Ballooned ventricles + tight extra-axial space + blurred (transependymal) margins = active, likely acute hydrocephalus — report with urgency.
- Pressure/obstruction makes a tap dangerous and risks precipitating acute obstructive hydrocephalus — flag it; correlate pre- and post-contrast for an aqueductal lesion that only enhances.
- Treat an entrapped/trapped, dilating segment as the urgent surgical finding.
- Read the third-ventricle floor on sagittal (convex = most worrying) and the aqueductal CSF jet (strong/hyperdynamic supports NPH-type physiology).
- Don't over-read absence of a CSF jet as proof of obstruction.
- Don't call hydrocephalus on a purely atrophic brain — it triggers an inappropriate shunt conversation.
- Localize the block by which ventricles are enlarged: lateral+third big with small fourth = aqueductal level; all four big = downstream resorption block.
- Acute obstructive hydrocephalus from a mass at the aqueduct/dorsal third ventricle is a neurosurgical situation — state it plainly and name the lesion and its level.
- Note shunt/catheter tracks; they reframe a finding as previously-judged/treated hydrocephalus.
Practical traps
- Reporting 'ventriculomegaly' without stating whether it is under pressure.
- Confusing transependymal flow (blurred margins) with non-specific white-matter change (sharp margins).
- Missing a mobile ball-valve lesion or a trapped segment.
- Reciting weak metrics (Evans ratio >=0.3, ventricular angle, ventricular index) as if decisive.
- Calling a 'bulging' fontanelle on pediatric CT without clinical correlation.
- Stable ventricular size does not mean stable pressure — chronic hydrocephalus produces stiff, non-compliant ventricles, so a static-looking scan can be a deteriorating patient.
- Communicating vs non-communicating labels are imperfect (the resorption block is still an obstruction); reason in terms of where outflow fails, not the label.
- Combined obstructive + impaired-resorption patterns are common (e.g., after aneurysm rupture) — don't force a single mechanism.
Teaching pearls
- Temporal horns are the acuity marker; frontal horns mark chronicity.
- Blurred ventricular margins = transependymal flow = acuity; sharp margins favor white-matter change.
- Tight extra-axial space + ballooned ventricles = real hydrocephalus, likely acute.
- Third-ventricle floor: concave good, flat watch, convex worry.
- Watch for ball-valve lesions and entrapment — those are the urgent calls.
- Evans ratio and ventricular angle are weak; read pressure qualitatively.
- A soft fontanelle can look bulging on CT — call and correlate before you scare anyone.
- If you want it fixed, call it hydrocephalus; if you don't, call it ventriculomegaly.
- Ventricles out of proportion to the sulci = think active hydrocephalus.
- Temporal horns move fast (acute marker); frontal horns move slowly (chronic marker).
Teaching visuals
Source lectures
- Imaging of Hydrocephalus: Diagnosis
- Imaging of Hydrocephalus: Pathophysiology
Educational material for radiology residents and neuroradiology fellows. Nothing here drives individual patient care, and it contains no patient data.