18 VENTRICULAR SYSTEM OF THE BRAIN
# 19 VENTRICULAR SYSTEM OF THE BRAIN
The ventricular system is a connected set of fluid-filled cavities inside the brain that produce, circulate and finally hand off cerebrospinal fluid (CSF) to the venous system. For MRCS Part A, this topic is almost entirely about three things: knowing the flow pathway in order, recognising where it gets blocked (and what type of hydrocephalus that produces), and interpreting CSF analysis from a lumbar puncture. Get those three right and you'll bank every ventricular question in the paper.
Anatomy of the four ventricles
The ventricles are remnants of the lumen of the embryonic neural tube. They are lined by ependyma and contain CSF produced by the choroid plexus.
➡ Two lateral ventricles — one in each cerebral hemisphere. C-shaped, with a frontal horn, body, occipital horn and temporal horn. They are the largest of the ventricles and contain the bulk of the choroid plexus.
➡ Third ventricle — a narrow midline slit between the two halves of the thalamus, sitting above the hypothalamus. The two thalami are often joined across it by the interthalamic adhesion (massa intermedia).
➡ Cerebral aqueduct of Sylvius — not a ventricle but the slender canal running through the midbrain that links the third and fourth ventricles. It is the narrowest segment of the system and therefore the most vulnerable to obstruction.
➡ Fourth ventricle — a tent-shaped cavity between the pons and medulla (anteriorly) and the cerebellum (posteriorly). Its floor forms the rhomboid fossa.
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CSF flow pathway
👩⚕️ Mnemonic — "Lateral Monsters Travel Aqueducts For Liberation"
Lateral ventricles → foramen of Monro (interventricular foramen) → Third ventricle → Aqueduct of Sylvius → Fourth ventricle → Foramina of Luschka (lateral, x2) and Magendie (midline, x1) → subarachnoid space → arachnoid granulations → superior sagittal sinus (and other dural venous sinuses) → systemic venous return.
👩⚕️ Luschka = Lateral. Magendie = Midline. Both start with the same letter as their location.
CSF — production, volume, function
| Parameter | Value |
|---|---|
| Site of production | Choroid plexus (70–80%); rest from ependyma and brain interstitial fluid |
| Daily production | ~500 mL/day |
| Total volume at any time | ~150 mL (≈25 mL in ventricles, ≈125 mL in subarachnoid space) |
| Turnover | 3–4 times per day |
| Site of absorption | Arachnoid granulations → superior sagittal sinus |
| Composition | Clear, colourless. Glucose ≈ 2/3 plasma. Low protein. Few cells. |
CSF cushions the brain mechanically, provides buoyancy (the brain effectively "floats", reducing its in-situ weight from ~1400 g to ~50 g), and supports homeostasis by clearing metabolic waste.
👩⚕️ High-yield trap: CSF production is essentially pressure-independent. It keeps being made even when pressure rises — which is exactly why an untreated obstruction inexorably dilates the ventricles.
Hydrocephalus
Hydrocephalus is an abnormal accumulation of CSF within the ventricular system, usually with ventricular dilatation and raised ICP. Classify by where the problem is:
| Type | Site of problem | Common causes |
|---|---|---|
| Obstructive (non-communicating) | Block within the ventricular system | Aqueductal stenosis (commonest congenital cause), colloid cyst of third ventricle, posterior fossa tumour, tectal plate glioma |
| Communicating | CSF flows freely through the ventricles; problem is at arachnoid granulations or in the subarachnoid space | Post-subarachnoid haemorrhage, post-meningitic scarring, leptomeningeal carcinomatosis |
| Normal pressure hydrocephalus (NPH) | A form of communicating hydrocephalus with normal opening pressure on LP | Often idiopathic; sometimes post-SAH or post-meningitis |
| Ex vacuo | Not true hydrocephalus — ventricles enlarge passively because the brain has shrunk | Alzheimer's, chronic ischaemia, alcohol-related atrophy |
Localising the block by the pattern of dilatation
Everything upstream of the block dilates; everything downstream stays normal.
- Lateral ventricle(s) dilated, third and fourth normal → foramen of Monro (e.g. colloid cyst)
- Lateral and third dilated, fourth normal → aqueduct of Sylvius (commonest)
- All four ventricles dilated → fourth ventricle outlets or communicating hydrocephalus
Normal pressure hydrocephalus — "Wet, Wobbly, Wacky"
The classic NPH triad, in the order they usually appear:
- Wobbly — gait apraxia (magnetic, shuffling gait). Usually the earliest and most prominent feature.
- Wet — urinary incontinence.
- Wacky — subcortical dementia. Usually the latest feature.
Imaging shows ventriculomegaly out of proportion to cortical atrophy. It is one of the few treatable causes of dementia: ventriculoperitoneal (VP) shunting is the definitive treatment, and gait is the symptom most likely to improve.
👩⚕️ Trap: Untreated raised-ICP signs (headache, vomiting, papilloedema, reduced GCS) do not occur in NPH — opening pressure is normal by definition.
Lumbar puncture and CSF analysis
Lumbar puncture is performed in the L3/4 or L4/5 interspace — below the termination of the spinal cord at L1/2 in adults — to sample CSF from the subarachnoid space.
👩⚕️ Absolute contraindication: suspected obstructive hydrocephalus or a posterior fossa mass with raised ICP — removing CSF below a block risks tonsillar herniation (coning).
Normal CSF values
| Parameter | Normal |
|---|---|
| Opening pressure | 10–20 cm H₂O |
| Appearance | Clear, colourless |
| Glucose | ≈ 2/3 of plasma glucose (2.5–4.5 mmol/L) |
| Protein | 0.15–0.45 g/L |
| White cells | <5 /mm³, all lymphocytes |
| Red cells | 0 |
Interpreting pathological CSF
| Appearance | Opening pressure | WCC | Protein | Glucose | |
|---|---|---|---|---|---|
| Bacterial meningitis | Cloudy / turbid | ↑↑ | ↑↑ neutrophils | ↑↑ | ↓↓ |
| Viral meningitis | Clear | Normal / mildly ↑ | ↑ lymphocytes | Normal / mildly ↑ | Normal |
| TB / fungal meningitis | Fibrin web | ↑ | ↑ lymphocytes | ↑↑ | ↓ |
| Subarachnoid haemorrhage | Bloody / xanthochromic | ↑ | RBCs present; xanthochromia from bilirubin >12 h post-bleed | ↑ | Normal |
| Normal pressure hydrocephalus | Clear | Normal | Normal | Normal | Normal |
👩⚕️ Easy mark: Low glucose + neutrophils + cloudy fluid = bacterial meningitis. Normal glucose + lymphocytes + clear fluid = viral.
👩⚕️ Xanthochromia (yellow CSF from bilirubin breakdown of haemoglobin) is the gold-standard finding for SAH on LP and takes >12 hours to develop after the bleed — which is why LP for suspected SAH should be performed at least 12 h after symptom onset.
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Test yourself
The most common site of obstruction in hydrocephalus is?
- ((Foramen of Monro::Connects lateral ventricles to the third; colloid cysts block here but it is not the commonest site.))
- ((Foramina of Luschka and Magendie::Fourth ventricle outlets; post-meningitic scarring blocks them but less commonly than the aqueduct.))
- ((Subarachnoid space::Causes communicating hydrocephalus (e.g. post-SAH), not obstructive — and not the commonest site.))
- ((Interventricular septum::Septum pellucidum is not part of the CSF pathway.))
- ((Cerebral aqueduct of Sylvius::☑️ Narrowest segment of the system — commonest site of obstruction and of congenital hydrocephalus.))
👩⚕️ Aqueductal stenosis is the single commonest cause of congenital hydrocephalus; in adults, think tectal or pineal tumours.
The majority of cerebrospinal fluid is produced by which structure?
- ((Arachnoid granulations::Site of CSF absorption into dural venous sinuses, not production.))
- ((Choroid plexus::☑️ Produces ~70–80% of CSF (~500 mL/day); found in all four ventricles, most in the laterals.))
- ((Ependyma::Contributes a small amount via interstitial fluid; the bulk comes from choroid plexus.))
- ((Superior sagittal sinus::Receives reabsorbed CSF — plays no role in production.))
- ((Subarachnoid space::A reservoir and transit pathway, not a production site.))
👩⚕️ Choroid plexus Creates, arachnoid granulations Absorb.
A 72-year-old man presents with gait disturbance, urinary incontinence and dementia. CT shows ventriculomegaly out of proportion to cortical atrophy. What is the most likely diagnosis?
- ((Alzheimer disease::Cortical atrophy with ventricular dilatation proportional to atrophy (ex vacuo) — no NPH triad.))
- ((Obstructive hydrocephalus::Presents acutely with raised ICP (headache, vomiting, papilloedema), not the chronic triad.))
- ((Vascular dementia::Imaging shows white-matter ischaemia and lacunes, not isolated ventriculomegaly.))
- ((Normal pressure hydrocephalus::☑️ "Wet, Wobbly, Wacky" with ventriculomegaly disproportionate to atrophy — treatable with VP shunt.))
- ((Subdural haematoma::Causes a crescent-shaped extra-axial collection on CT, not isolated ventriculomegaly.))
👩⚕️ Gait is the earliest NPH symptom and the one most likely to improve after shunting.
Which best distinguishes communicating from obstructive (non-communicating) hydrocephalus?
- ((Presence of papilloedema::Both raise ICP and can cause papilloedema.))
- ((Age at presentation::Both occur at any age — congenital and acquired forms exist for each.))
- ((Whether the obstruction is within or outside the ventricular system::☑️ Obstructive = block inside the ventricles; communicating = problem at arachnoid granulations or subarachnoid space.))
- ((Size of the lateral ventricles::Both dilate the lateral ventricles — size alone is not discriminatory.))
- ((Response to lumbar puncture::LP helps diagnose NPH but is contraindicated in obstructive hydrocephalus (coning).))
👩⚕️ All four ventricles dilated → communicating or fourth-ventricle outlet block. Laterals + third dilated, fourth normal → aqueduct.
A 3-month-old presents with rapidly enlarging head, bulging fontanelle and sunset-sign eyes. MRI shows dilated lateral and third ventricles with a normal fourth ventricle. Where is the block?
- ((Foramen of Luschka::Would also dilate the fourth ventricle — excluded here.))
- ((Arachnoid granulations::Communicating hydrocephalus dilates all ventricles, including the fourth.))
- ((Cerebral aqueduct of Sylvius::☑️ Lateral and third dilated, fourth normal — block sits between third and fourth ventricles. Commonest congenital cause.))
- ((Foramen of Magendie::Would dilate the fourth ventricle — excluded.))
- ((Foramen of Monro::Would spare the third ventricle, which is clearly dilated here.))
👩⚕️ Sunset sign = pressure on the tectal plate impairing upgaze; classic infant hydrocephalus sign.
A patient with suspected meningitis has CSF showing turbid fluid, neutrophils, glucose 1.2 mmol/L (plasma 6.0) and protein 1.8 g/L. Most likely diagnosis?
- ((Bacterial meningitis::☑️ Cloudy CSF, neutrophilic pleocytosis, low glucose (<2/3 plasma) and high protein — classic pyogenic pattern.))
- ((Viral meningitis::Clear fluid, lymphocytes, normal glucose and only mildly raised protein.))
- ((TB meningitis::Lymphocytic with very high protein and low glucose — but indolent course and fibrin web, not turbid fluid.))
- ((Subarachnoid haemorrhage::Bloody or xanthochromic CSF with RBCs, not a neutrophilic pleocytosis.))
- ((Normal pressure hydrocephalus::CSF is completely normal — diagnosis is clinical-radiological.))
👩⚕️ Low glucose is the easiest single discriminator between bacterial and viral meningitis.
Revision summary
- Four ventricles: two lateral, third (midline between thalami), fourth (between brainstem and cerebellum).
- Flow: Lateral → Monro → 3rd → Sylvius (aqueduct) → 4th → Luschka (×2 lateral) + Magendie (midline) → subarachnoid space → arachnoid granulations → superior sagittal sinus.
- CSF: produced by choroid plexus (~500 mL/day, total ~150 mL, turnover 3–4×/day); absorbed at arachnoid granulations.
- Aqueduct of Sylvius is the narrowest part and commonest site of obstruction.
- Obstructive (non-communicating) = block inside ventricles (aqueductal stenosis, colloid cyst, posterior fossa tumour).
- Communicating = block at arachnoid granulations / subarachnoid space (post-SAH, post-meningitis).
- NPH triad = Wet, Wobbly, Wacky with disproportionate ventriculomegaly; treat with VP shunt; gait responds best.
- Localise the block: everything upstream dilates, downstream stays normal.
- CSF analysis: bacterial = cloudy, neutrophils, low glucose, high protein. Viral = clear, lymphocytes, normal glucose. SAH = bloody/xanthochromic (>12 h).
- LP contraindicated in obstructive hydrocephalus / posterior fossa mass — risk of coning.