Ventricles and CSF
Ventricles, cisterns, CSF flow and meninges: hydrocephalus and intracranial bleeds
CSF spaces outline the brain on CT and MRI: their size shows hydrocephalus or atrophy, and the layer a collection lies in shows which vessel bled. CPSP asks about each ventricle's relations, the CSF pathway, the cisterns and the three intracranial bleeds.
Lateral ventricles and their walls
| Part | Key relation | Imaging point |
|---|---|---|
| Frontal horn | Roof corpus callosum; medial wall septum pellucidum; lateral wall head of caudate | Huntington disease flattens the caudate bulge: box-shaped horns |
| Body | Thalamus in the floor | Caudothalamic groove: germinal matrix of preterm babies (see below) |
| Atrium (trigone) | Junction of body, occipital and temporal horns; holds the glomus of the choroid plexus | Glomus often calcifies; fetal atrial width 10 mm or more = ventriculomegaly; adult intraventricular meningioma |
| Temporal horn | Hippocampus in its floor and medial wall; optic radiation (Meyer loop) curves over its roof | Dilates early in hydrocephalus |
| Occipital horn | Optic radiation in its lateral wall | Dependent part in a supine patient: pus layers here in ventriculitis |
- Germinal matrix haemorrhage: in babies born before about 32 weeks the germinal matrix at the caudothalamic groove is a highly cellular zone with fragile, thin-walled capillaries and poor autoregulation, so swings in blood pressure make them rupture. Ultrasound through the anterior fontanelle shows an echogenic focus with blood in the ventricle; post-haemorrhagic hydrocephalus may follow weeks later.
- Pyogenic ventriculitis: fever and drowsiness after an external ventricular drain or meningitis; layering debris in the occipital horns that is bright on diffusion and thin smooth ependymal enhancement.
- A cavum septi pellucidi between the two leaves of the septum is normal in fetuses and preterm babies and persists in some adults. Its absence on a fetal scan suggests callosal agenesis or septo-optic dysplasia.
CSF pathway
- Made by the choroid plexus at about 500 mL a day; total volume about 150 mL, so it turns over three to four times daily.
- Lateral ventricles, interventricular foramina (Monro), third ventricle, cerebral aqueduct (from the mesencephalon), fourth ventricle, then out through the median aperture (Magendie) and two lateral apertures (Luschka), which let a tuft of choroid plexus project into the cerebellopontine angle.
- Absorbed through arachnoid granulations into the dural venous sinuses.
| Block at | Dilated | Example |
|---|---|---|
| Foramen of Monro | One or both lateral ventricles | Colloid cyst: hyperdense round mass in the anterior roof of the third ventricle; positional headache, sudden collapse. Subependymal giant cell astrocytoma (SEGA): growing, intensely enhancing mass at the foramen of Monro in tuberous sclerosis (facial angiofibromas, calcified subependymal nodules); regular MRI screening catches it before hydrocephalus |
| Aqueduct | Lateral and third, normal fourth | Congenital aqueduct stenosis (commonest cause of congenital obstructive hydrocephalus), tectal plate tumour, pineal mass |
| Fourth ventricle exits | All four ventricles | Dandy–Walker: large posterior fossa, cyst continuous with the fourth ventricle, small upturned vermis, raised torcular |
| Arachnoid granulations | All ventricles and sulci (communicating) | Meningitis, subarachnoid haemorrhage |
- Hydrocephalus or atrophy? Hydrocephalus: dilated temporal horns, rounded frontal horns, effaced sulci, and periventricular low density around the horns: raised pressure forces CSF across the ependyma into the white matter. Chronic compensated hydrocephalus may lack this sign. Atrophy: sulci widen in proportion to the ventricles.
- Evans index = maximal frontal horn width / maximal inner skull width; above 0.3 is enlarged. Normal pressure hydrocephalus: gait, incontinence, dementia, with large ventricles, wide Sylvian fissures and tight high-convexity sulci.
- Hydranencephaly (no cortical mantle, from bilateral ICA occlusion in utero) differs from severe hydrocephalus, where a thin cortex remains.
- Preterm cranial ultrasound uses the anterior fontanelle.
- A fourth ventricular ependymoma in a child squeezes out through the foramina of Luschka and Magendie.
- CSF seeding (drop metastases): medulloblastoma, ependymoma and germinoma shed cells that spread through the CSF, coating the cord surface ("sugar-coating") and forming nodules on the cauda equina roots. Image the whole spine before surgery, because postoperative blood mimics seeding.
Cisterns
| Cistern | Site and contents |
|---|---|
| Suprasellar | Five-pointed star above the sella; contains the optic chiasm and circle of Willis |
| Ambient | CSF band around the side of the midbrain; effaced early in uncal herniation |
| Quadrigeminal | Behind the tectum, below the splenium; contains the great cerebral vein; site of lipoma (fat, about −90 HU) |
| Interpeduncular and prepontine | In front of the midbrain and pons; basilar artery |
| Cisterna magna | Behind the medulla below the cerebellum; absent in Chiari II |
- Blood only in the cisterns in front of the midbrain and upper pons, with normal angiography, is perimesencephalic (venous) haemorrhage with a good outlook; a basilar tip aneurysm must first be excluded.
Meninges and the spaces between them
| Collection | Layer | Shape and limits | Source |
|---|---|---|---|
| Extradural | Between skull and dura | Biconvex, stopped at sutures (dura fixed there); can cross the midline | Middle meningeal artery; at the vertex a superior sagittal sinus tear pushes the sinus down |
| Subdural | Between dura and arachnoid | Crescentic, crosses sutures, stopped at the falx and tentorium (inner dural folds) | Bridging veins; elderly, alcoholics, anticoagulants |
| Subarachnoid | Between arachnoid and pia | Fills sulci and cisterns | Cerebral arteries run here: berry aneurysm |
- Ageing subdural blood: hyperdense when acute, isodense at about 1–3 weeks (look for effaced sulci and shift), hypodense when chronic. Dark swirls inside an acute clot mean active bleeding.
- Extra-axial signs: a CSF cleft between mass and cortex, a broad dural base, buckled grey–white junction.
| Enhancement | Pattern | Cause |
|---|---|---|
| Pachymeningeal (dura–arachnoid) | Smooth, follows the inner table, does not enter sulci | Intracranial hypotension, after surgery |
| Leptomeningeal (pia–arachnoid) | Follows the brain surface into sulci, over cerebellar folia and basal cisterns | Meningitis, carcinomatosis (breast, lung, melanoma) with headache and multiple cranial nerve palsies; CSF cytology may need repeating |
- Superficial siderosis: repeated slow bleeding into the subarachnoid space (for example from a dural tear after old spinal surgery) deposits haemosiderin on the pial surface: a thin black line on susceptibility MRI coating the brainstem, cerebellar folia and eighth nerves. It causes progressive sensorineural deafness and ataxia; search the whole spine for the source.
- Arachnoid cyst: commonest in the middle cranial fossa; CSF on every sequence, suppresses on FLAIR, no diffusion restriction (an epidermoid stays bright on diffusion).
- After meningitis in infants, CSF-like sterile subdural effusions are common; one that restricts diffusion or enhances is an empyema.
- A growing skull fracture in an infant follows a dural tear with CSF pushing into the fracture.
- Spinal meninges: a meningocele contains only meninges and CSF (no neural tissue), so leg function is normal. The dural sac ends at S2; in spinal anaesthesia the arachnoid is the last layer before CSF; denticulate ligaments are pia anchoring the cord to the dura.
- Dura above the tentorium is supplied by the trigeminal nerve (mainly V1, also V2 and V3), so supratentorial dural irritation refers pain to the forehead; posterior fossa dura by C1–C3 and the vagus.
Visual pathway near the ventricles
| Lesion | Field defect |
|---|---|
| Optic tract | Contralateral homonymous hemianopia |
| Meyer loop (temporal lobe) | Contralateral superior quadrantanopia |
| Parietal optic radiation | Contralateral inferior quadrantanopia |
Remember Biconvex and stopped by sutures: extradural; crescentic and stopped by the falx: subdural.
Exam trap Dilated lateral and third ventricles with a normal fourth ventricle mean an aqueduct block, not a fourth ventricle outflow block.
Exam trap An isodense subdural haematoma can be missed on CT; look for effaced sulci and midline shift.
How FCPS asks about this
- Commonest cause of congenital obstructive hydrocephalus: aqueduct stenosis (normal fourth ventricle).
- Periventricular low density in acute hydrocephalus: CSF forced across the ependyma.
- Preterm bleed at the caudothalamic groove: fragile germinal matrix vessels.
- Enhancement following sulci and cerebellar folia: leptomeningeal (pia-arachnoid) disease such as carcinomatosis.
- Enhancing mass at the foramen of Monro in tuberous sclerosis: subependymal giant cell astrocytoma.
- Biconvex collection stopped at sutures: extradural haematoma from the middle meningeal artery.
- Deafness and ataxia with a black rim on brainstem surfaces: superficial siderosis.
Test yourself on this topic
5 of the 52 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 48-year-old man has a sudden headache while lifting. CT shows subarachnoid blood confined to the cisterns in front of the midbrain and upper pons, without extension into the Sylvian or interhemispheric fissures. CT and catheter angiography show no aneurysm. What is the most likely source of the bleeding?
Answer: Venous bleeding around the brainstem
Blood centred in the interpeduncular and prepontine cisterns in front of the midbrain and pons, sparing the fissures, with normal angiography is perimesencephalic non-aneurysmal subarachnoid haemorrhage. It is thought to come from rupture of small veins around the brainstem, so the bleeding is low-pressure and limited. These patients rarely rebleed or develop vasospasm and usually do well. Aneurysmal bleeds spread more widely, into the fissures, and an anterior communicating aneurysm fills the interhemispheric fissure.
Perimesencephalic SAH
- Blood in front of midbrain and pons
- Spares Sylvian and interhemispheric fissures
- Normal angiography
- Probably venous; good prognosis
Remember Blood only in front of the brainstem with a normal angiogram: perimesencephalic bleed.
Exam trap A basilar tip aneurysm gives the same blood pattern, so good-quality angiography is essential.
Why the other options are wrong
- Ruptured anterior communicating aneurysm Fills the interhemispheric fissure and is seen on angiography.
- Ruptured middle cerebral aneurysm Fills the Sylvian fissure and is seen on angiography.
- Arteriovenous malformation Would be seen on angiography and usually bleeds into brain.
- Traumatic cortical contusion Needs trauma and lies in the cortex, not the basal cisterns.
Cranial ultrasound of a baby born at 30 weeks, and later MRI of a healthy 25-year-old, both show a CSF-filled midline space between two thin leaves separating the frontal horns, lying in front of the columns of the fornix. What is this space?
Answer: Cavum septi pellucidi
The septum pellucidum is made of two thin leaves stretching from the corpus callosum to the fornix, separating the frontal horns. In the fetus the leaves are apart, enclosing a fluid space called the cavum septi pellucidi, which is present in all preterm babies and usually closes from back to front after birth. It persists in a minority of adults as a normal variant. If the space extends backward behind the columns of the fornix, that posterior part is the cavum vergae.
Cavum septi pellucidi
- Space between the two leaves of the septum
- In front of the fornix columns
- Present in all preterm babies
- Persists as a normal variant in some adults
Remember Midline CSF slot between the frontal horns: cavum septi pellucidi.
Exam trap Absence of the fetal cavum on antenatal scans is abnormal and suggests callosal agenesis or septo-optic dysplasia.
Why the other options are wrong
- Cavum vergae Lies behind the fornix columns, as a backward extension of the cavum.
- Third ventricle Lies lower, between the thalami, below the fornix.
- Interhemispheric arachnoid cyst Lies above the corpus callosum between the hemispheres.
- Cistern of the lamina terminalis Lies in front of and below the third ventricle, outside the septum.
A baby born at 27 weeks has a sudden fall in haematocrit and a bulging fontanelle on day 2. The bleed began in the floor of the right lateral ventricle, at the groove between the head of the caudate and the thalamus, and has burst into the ventricle. Why does bleeding start at this site?
Answer: Fragile germinal matrix vessels
The germinal matrix is a highly cellular, very vascular layer beneath the ependyma that produces neurons and glia, and before about 32 weeks it is most prominent over the head of the caudate at the caudothalamic groove. Its capillaries are thin-walled, lack supporting tissue and lie in a region of venous convergence. In the preterm baby, who cannot regulate cerebral blood flow well, swings in blood pressure rupture these vessels. Bleeding may stay in the matrix or burst into the ventricle, and the matrix involutes by term.
Germinal matrix haemorrhage
- Preterm under 32 weeks
- Site: caudothalamic groove
- Fragile capillaries, poor autoregulation
- Can extend into the ventricle
Remember Preterm baby, bleed at the caudothalamic groove: germinal matrix.
Exam trap Post-haemorrhagic hydrocephalus can develop weeks later, so repeat scans are needed.
Why the other options are wrong
- Watershed between ACA and MCA Explains term watershed injury, not subependymal bleeding.
- Weak point in the choroid plexus Choroid plexus bleeding occurs mainly in term infants.
- Rupture of the vein of Galen Vein of Galen malformations cause heart failure, not this bleed.
- Venous sinus thrombosis Causes venous infarcts, not focal caudothalamic bleeding.
A small tuft of choroid plexus normally protrudes from each lateral recess of the fourth ventricle into the cerebellopontine angle cistern. Through which openings does it pass?
Answer: Foramina of Luschka
The fourth ventricle drains CSF into the subarachnoid space through three openings. The paired lateral apertures (foramina of Luschka) lie at the ends of the lateral recesses and open into the cerebellopontine angle cisterns, and choroid plexus often protrudes through them. The single median aperture (foramen of Magendie) opens into the cisterna magna. The foramina of Monro join the lateral ventricles to the third ventricle.
Fourth ventricle outlets
- Luschka: lateral, into CPA cistern
- Magendie: median, into cisterna magna
- Choroid plexus may protrude through Luschka
- Outlets lead to subarachnoid space
Remember Luschka is lateral, Magendie is medial.
Exam trap Protruding choroid plexus at the CPA can be mistaken for an enhancing nodule or tumour.
Why the other options are wrong
- Foramen of Magendie Opens in the midline into the cisterna magna.
- Foramina of Monro Join the lateral ventricles to the third ventricle.
- Cerebral aqueduct Joins the third and fourth ventricles.
- Central canal of the medulla Continues down into the spinal cord, not into a cistern.
A routine head CT in a 55-year-old woman shows symmetrical, dense, rounded calcifications within the lateral ventricles at the junction of the body, occipital horn and temporal horn on each side. Where in the ventricular system are these normal calcifications?
Answer: Atrium (trigone) of lateral ventricle
The choroid plexus of the lateral ventricle is largest in the atrium (trigone), where it forms a bulky tuft called the glomus. This glomus commonly calcifies with age and appears as symmetrical dense foci in the atria on CT, a normal finding in adults. The frontal and occipital horns contain no choroid plexus. Calcification of the choroid plexus in a young child, or very large asymmetric calcification, is unusual and deserves a second look.
Choroid plexus glomus
- Largest tuft in the atrium (trigone)
- Calcifies with age
- Symmetrical dense foci on CT
- None in frontal or occipital horns
Remember Paired dense dots in the trigones of an adult: normal choroid plexus.
Exam trap Do not mistake glomus calcification for intraventricular haemorrhage; haemorrhage is less dense and layers.
Why the other options are wrong
- Frontal horn of lateral ventricle Contains no choroid plexus.
- Foramen of Monro The plexus passes through here but its glomus lies in the atrium.
- Roof of the third ventricle Contains a thin plexus that rarely calcifies visibly.
- Fourth ventricle Has choroid plexus but this does not lie at the junction of the horns.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Brain, Ventricles, cisterns, CSF flow and meninges