Hydrocephalus on imaging

Finding

Hydrocephalus is too much fluid inside the brain's cavities, which swell and press on the brain. CT or MRI shows their size and where flow is blocked.

Updated Nov 13, 2026

What it is

Cerebrospinal fluid is made in the ventricles, flows from the lateral ventricles through the third ventricle, the aqueduct and the fourth ventricle into the subarachnoid space, and is absorbed over the brain surface. Hydrocephalus is ventricular enlargement from a block along this path (obstructive or non-communicating) or from poor absorption (communicating, as after subarachnoid haemorrhage or meningitis). On imaging the key is to show that the ventricles are big out of proportion to the sulci, which separates hydrocephalus from atrophy; to find the level of the block from which ventricles are dilated; and to look for signs that it is acute. Normal pressure hydrocephalus in older adults is a separate pattern with its own clinical triad.

What it looks like

Ventricles enlarged out of proportion to the sulci; in acute cases the sulci are effaced.
Rounded, ballooned frontal horns and a rounded third ventricle; temporal horns visibly dilated when they are normally slit-like.
Acute or pressurised hydrocephalus: a smooth low-density rim around the frontal horns (CSF pushed through the ventricle lining).
Evans index: widest frontal horn width divided by the widest inner skull diameter on the same slice; above 0.3 means ventriculomegaly.
Level of block: lateral ventricles only (foramen of Monro, for example a colloid cyst); lateral and third with a normal fourth (aqueduct); all four (fourth ventricle outlet or posterior fossa mass).
Normal pressure hydrocephalus: Evans index above 0.3, tight sulci at the vertex with wide Sylvian fissures (disproportionate subarachnoid spaces) and a narrow callosal angle on coronal images.
In infants with an open fontanelle, cranial ultrasound shows ventricular size without radiation.

Why it looks that way

Fluid builds up behind a block, so the ventricles upstream of it dilate and those downstream stay normal.
In atrophy (ex vacuo), brain volume is lost everywhere, so the sulci widen along with the ventricles and the temporal horns enlarge with the hippocampi shrinking.
In communicating hydrocephalus absorption fails at the surface, so all four ventricles enlarge.

Pattern	Ventricles	Sulci	Usual cause
Obstructive at foramen of Monro	One or both lateral ventricles	Effaced	Colloid cyst, tumour
Obstructive at the aqueduct	Lateral and third; fourth normal	Effaced	Aqueductal stenosis, pineal or midbrain mass
Obstructive at the fourth ventricle	All four	Effaced	Posterior fossa tumour, bleed, cyst
Communicating	All four	Normal or effaced	Subarachnoid haemorrhage, meningitis
Normal pressure hydrocephalus	Lateral enlarged	Tight at vertex, wide Sylvian fissures	Usually idiopathic, older adults
Atrophy (ex vacuo)	Enlarged in proportion	Widened everywhere	Ageing, neurodegeneration

Differential and pitfalls

Atrophy versus hydrocephalus: wide sulci in proportion to the ventricles suggest atrophy; effaced sulci and dilated temporal horns suggest hydrocephalus.
In Colombia, neurocysticercosis is an important cause: an intraventricular cyst may be almost invisible on CT and is better seen on MRI.
A shunted patient with symptoms can have a malfunction even if the ventricles look the same; always compare with the previous scan.
Normal pressure hydrocephalus is an imaging pattern plus a clinical triad; ventriculomegaly alone in an older adult is not enough.
Posterior fossa bleed or tumour with a dilated temporal horn is an emergency: obstructive hydrocephalus can follow within hours.

Next step

Acute hydrocephalus (falling consciousness, dilated temporal horns, periventricular low density): call neurosurgery for possible ventricular drainage.
No lumbar puncture in obstructive hydrocephalus or when the cause is unclear on CT.
Find the cause: MRI to look for tumour, cyst, aqueductal stenosis or a Chiari malformation once stable.
Shunt malfunction suspected: CT or rapid MRI compared with the last scan, plus a shunt series of X-rays to look for breaks or disconnections.
Suspected normal pressure hydrocephalus: refer to neurology or neurosurgery for gait testing before and after drainage of CSF (tap test).

Test yourself

Think of the answer, then tap the question to see it.

On CT, how do you tell hydrocephalus from cerebral atrophy?

In hydrocephalus the ventricles are enlarged out of proportion to the sulci, which are normal or effaced; in atrophy the sulci widen in proportion to the ventricles.

The lateral and third ventricles are dilated and the fourth is normal. Where is the block?

At the cerebral aqueduct, between the third and fourth ventricles (for example aqueductal stenosis or a tectal mass).

What does a smooth low-density rim around the frontal horns indicate?

Interstitial oedema from CSF forced through the ventricular lining: a sign of acute or pressurised hydrocephalus.

Which imaging features support normal pressure hydrocephalus?

Ventriculomegaly with Evans index above 0.3, tight high-convexity sulci with wide Sylvian fissures, and a narrow callosal angle, in a patient with the gait, urine and cognitive triad.

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Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.

Sources: Hydrocephalus (StatPearls (NCBI Bookshelf), 2023); Idiopathic Normal Pressure Hydrocephalus (StatPearls (NCBI Bookshelf), 2026).

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