Ventricles and CSF
Ventricles, CSF circulation, hydrocephalus, raised pressure and meninges
Ventricles and CSF for FCPS Part 1: ventricles, CSF production, flow, absorption and composition, hydrocephalus types and raised pressure. Covers CSF in meningitis, herniation syndromes, extradural and subdural haematomas, CSF leaks and post-LP headache.
One of the most tested topics in past papers
Ventricles and their origin
| Cavity | Position | Develops from |
|---|---|---|
| Lateral ventricles (two) | Cerebral hemispheres; separated in the midline by the septum pellucidum | Telencephalon |
| Third ventricle | Between the two thalami | Diencephalon |
| Cerebral aqueduct (of Sylvius) | Midbrain | Mesencephalon |
| Fourth ventricle | Between pons and medulla in front, cerebellum behind | Rhombencephalon |
| Central canal | Lower medulla and spinal cord | Neural tube |
CSF pathway
| Step | From | Through | To |
|---|---|---|---|
| 1 | Lateral ventricles | Interventricular foramina (Monro) | Third ventricle |
| 2 | Third ventricle | Cerebral aqueduct (narrowest point) | Fourth ventricle |
| 3 | Fourth ventricle | Two lateral foramina of Luschka, one median foramen of Magendie | Subarachnoid space (cisterna magna, basal cisterns) |
| 4 | Subarachnoid space | Around brain and cord, up over the convexity | Arachnoid granulations |
| 5 | Arachnoid granulations | One-way valve-like flow driven by pressure | Superior sagittal sinus (venous blood) |
Production and composition
- Made mainly by the choroid plexus of all four ventricles by active secretion (ependymal cells with tight junctions form the blood–CSF barrier). About 500 mL/day; total volume about 150 mL, so it is replaced three to four times daily.
- Normal opening pressure lying down: about 7–18 cmH₂O (above 25 cmH₂O is raised in adults).
- Compared with plasma, CSF has far less protein (15–45 mg/dL, 0.15–0.45 g/L), less glucose (about 60% of blood: 45–80 mg/dL, 2.5–4.4 mmol/L), more chloride and magnesium, less potassium and calcium, and fewer than 5 lymphocytes per µL.
- It floats the brain (effective weight falls from about 1,400 g to about 50 g), removes metabolites and keeps a stable ionic environment.
CSF in meningitis
| Type | Appearance | Cells | Protein | Glucose |
|---|---|---|---|---|
| Bacterial | Turbid | Neutrophils, very high | High | Very low |
| Viral | Clear | Lymphocytes | Normal or slightly raised | Normal |
| Tuberculous | Clear, fibrin web on standing | Lymphocytes | Very high (often above 100 mg/dL) | Low |
| Cryptococcal | Clear | Lymphocytes, may be few | Raised | Low; India ink, antigen test |
- Subarachnoid haemorrhage: uniformly bloodstained CSF and xanthochromia (bilirubin) from about 12 hours.
- CSF rhinorrhoea after an anterior fossa fracture is confirmed by beta-2 transferrin, a desialated form found only in CSF (and perilymph); beta-trace protein is an alternative. Glucose testing is unreliable.
Hydrocephalus
| Type | Mechanism | Causes |
|---|---|---|
| Obstructive (non-communicating) | Block inside the ventricular system | Aqueduct stenosis (commonest congenital cause), posterior fossa tumours, colloid cyst of third ventricle at the foramen of Monro, Dandy–Walker malformation, Chiari II with myelomeningocele |
| Communicating | CSF leaves the ventricles but is not absorbed or cannot flow over the brain | Subarachnoid haemorrhage (blood clogs arachnoid granulations), meningitis; rarely overproduction (choroid plexus papilloma) |
| Normal pressure hydrocephalus | Communicating type in the elderly with intermittent pressure rises | Triad: magnetic gait, urinary incontinence, dementia; improves after CSF drainage; ventriculoperitoneal shunt |
- Ten days after SAH, drowsiness with dilatation of all four ventricles means communicating hydrocephalus from blocked absorption. Other causes of decline after SAH are rebleeding, vasospasm and hyponatraemia.
- Tuberculous meningitis produces a thick basal exudate that blocks CSF flow through the basal cisterns, causing hydrocephalus (all four ventricles dilated), and traps cranial nerves (VI palsy). Dexamethasone is added to anti-TB treatment.
- Infants: enlarging head, bulging fontanelle, separated sutures, "setting sun" eyes. Adults: morning headache, vomiting, papilloedema, VI palsy.
Raised intracranial pressure
- The earliest fundoscopic sign of papilloedema is loss of spontaneous venous pulsation, followed by disc hyperaemia, blurred margins and haemorrhages.
- Idiopathic intracranial hypertension: young, often overweight women; headache, transient visual obscurations, papilloedema, raised opening pressure with normal CSF and normal imaging including venography. Drugs include tetracyclines (doxycycline, minocycline), vitamin A derivatives and growth hormone.
- Cryptococcal meningitis in advanced HIV causes very high CSF pressure, often with normal-sized ventricles. Repeated therapeutic lumbar punctures to lower pressure substantially reduce death and blindness.
Herniation syndromes
| Type | What moves | Result |
|---|---|---|
| Subfalcine | Cingulate gyrus under the falx | Compresses the ACA: contralateral leg weakness |
| Uncal (transtentorial) | Medial temporal lobe over the tentorial edge | Ipsilateral III compression: dilated pupil first; ipsilateral peduncle gives contralateral weakness; PCA compression gives occipital infarction |
| Kernohan notch | Opposite peduncle pushed against tentorium | Weakness on the same side as the mass (false localising) |
| Central (downward) | Diencephalon and brainstem | Duret haemorrhages from torn paramedian basilar perforators |
| Tonsillar | Cerebellar tonsils through foramen magnum | Medullary compression: Cushing reflex, apnoea; neck stiffness and head tilt early |
- The pupil dilates before eye movements fail because the parasympathetic fibres from the Edinger–Westphal nucleus run on the surface of III and are compressed first.
- Lumbar puncture with raised pressure from a mass (papilloedema, focal signs) lowers spinal pressure and can precipitate tonsillar herniation. Image first.
Meninges and spaces
| Layer | Features |
|---|---|
| Dura mater | Outer endosteal and inner meningeal layers, separating to enclose venous sinuses; folds are falx cerebri, tentorium cerebelli, falx cerebelli, diaphragma sellae. Sensory supply: mainly trigeminal (all three divisions) above the tentorium; vagus and C1–C3 in the posterior fossa |
| Arachnoid | Avascular; granulations project into sinuses |
| Pia mater | Follows the brain surface; forms denticulate ligaments and filum terminale |
| Space | Bleed | Features |
|---|---|---|
| Extradural | Extradural haematoma (middle meningeal artery, temporal fracture) | Lens-shaped, does not cross sutures, lucid interval |
| Subdural | Subdural haematoma (torn bridging veins) | Crescent, crosses sutures; elderly, alcoholics, infants |
| Subarachnoid | SAH (berry aneurysm) | Blood in sulci and cisterns |
- An extradural clot stops at the sutures because the outer dura is firmly attached to bone there; it can cross the midline over the sagittal sinus.
- The spinal dural sac ends at S2. A caudal epidural needle advanced too far up the sacral canal can puncture it; in neonates the sac ends lower.
Post-lumbar puncture headache
- CSF leaks through the dural hole, the brain sags when upright, and traction on pain-sensitive structures causes a headache within minutes of sitting up that eases lying flat.
- A fine, atraumatic (pencil-point) needle is the main preventive measure. Bed rest afterwards does not prevent it. An epidural blood patch treats persistent cases.
Remember CSF is made by choroid plexus, flows out through Luschka and Magendie, and is absorbed by arachnoid granulations into the superior sagittal sinus.
Exam trap Blood or pus in the subarachnoid space blocks absorption: SAH and meningitis cause communicating hydrocephalus with all four ventricles enlarged.
Exam trap A normal-sized ventricle on CT does not exclude dangerously high pressure in cryptococcal meningitis.
How FCPS asks about this
- Commonest cause of congenital obstructive hydrocephalus: aqueduct stenosis.
- Structure separating the lateral ventricles: septum pellucidum; aqueduct develops from the mesencephalon.
- Earliest fundoscopic sign of papilloedema: loss of venous pulsation.
- Nasal clear fluid after fracture: beta-2 transferrin confirms CSF.
- Leg weakness from frontal tumour with midline shift: subfalcine herniation compressing the ACA.
- Postural headache after LP: prevented by a fine pencil-point needle.
- Teenager on acne antibiotic with papilloedema and normal imaging: doxycycline-induced intracranial hypertension.
Test yourself on this topic
5 of the 65 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 17-year-old girl taking an oral antibiotic for acne for four months has daily headaches, brief episodes of blurred vision and bilateral papilloedema. MRI and MR venography are normal, and the CSF opening pressure is 32 cmH₂O with normal CSF contents. Which of the following drugs is most likely responsible?
Answer: Oral doxycycline
This is idiopathic (secondary) intracranial hypertension: raised CSF pressure with normal imaging and normal CSF, causing headache, transient visual obscurations and papilloedema that can damage vision. Tetracyclines (doxycycline, minocycline), vitamin A derivatives such as isotretinoin, and excess vitamin A are recognised triggers; the mechanism is thought to involve impaired CSF absorption at the arachnoid granulations. The drug should be stopped, and acetazolamide reduces CSF production. Tetracyclines and isotretinoin should not be combined.
Drug-induced raised ICP
- Headache, papilloedema, normal MRI
- High opening pressure, normal CSF
- Drugs: tetracyclines, isotretinoin, vitamin A
- Stop drug; acetazolamide
Remember Acne treatment plus papilloedema: tetracycline.
Exam trap Exclude venous sinus thrombosis with venography before calling it idiopathic.
Why the other options are wrong
- Topical clindamycin Topical use gives negligible systemic levels.
- Topical benzoyl peroxide A topical agent with no effect on CSF pressure.
- Oral cetirizine Not linked to raised intracranial pressure.
- Oral metronidazole Not a recognised cause.
Ten days after a subarachnoid haemorrhage, a woman becomes drowsy. CT shows enlargement of all four ventricles. Why has this happened?
Answer: Blocked CSF absorption
CSF is absorbed into the dural venous sinuses through the arachnoid granulations. Blood and protein in the subarachnoid space after a haemorrhage, meningitis or surgery can block this absorption. CSF can still flow out of the ventricles, so all four enlarge: communicating hydrocephalus. It needs a drain or shunt if consciousness falls.
Communicating hydrocephalus
- Absorption at arachnoid granulations fails
- After SAH, meningitis
- All four ventricles enlarge
- Treat: drain or shunt
Remember Four big ventricles: absorption problem.
Exam trap A drop in consciousness after SAH can also be rebleeding, vasospasm or low sodium.
Why the other options are wrong
- Choroid plexus overproduces CSF Overproduction is rare (choroid plexus papilloma).
- Aqueduct compressed by a tumour Would enlarge only the lateral and third ventricles.
- Brain atrophy after the bleed Atrophy develops slowly, without drowsiness.
- Low pressure from a CSF leak Would not enlarge the ventricles.
Which is the earliest fundoscopic sign of papilloedema?
Answer: Loss of venous pulsation
Papilloedema is optic disc swelling from raised intracranial pressure, usually bilateral, with normal visual acuity early and an enlarged blind spot. Signs: loss of venous pulsation (absent in some normal people), disc hyperaemia, blurred margins, elevated disc, haemorrhages. Lumbar puncture is contraindicated until a mass lesion is excluded by imaging.
Papilloedema
- Raised ICP, bilateral disc swelling
- Early: loss of venous pulsation, blurred margins
- Image before LP
Remember Swollen disc: scan before you tap.
Why the other options are wrong
- Cupping of the optic disc Glaucoma.
- Cherry-red spot Central retinal artery occlusion, Tay–Sachs.
- Cotton wool spots only Hypertensive or diabetic retinopathy.
- Optic disc drusen Benign deposits.
A 40-year-old woman with headaches, vomiting and papilloedema has a lumbar puncture in the OPD before any scan. Twenty minutes later she becomes deeply unconscious and stops breathing. Later imaging shows a large cerebellar tumour. What is the mechanism of her collapse?
Answer: Tonsils herniated, compressing the medulla
With a posterior fossa mass, pressure is high above the foramen magnum. Removing CSF from below creates a pressure gradient that pushes the cerebellar tonsils down through the foramen magnum (coning). The tonsils compress the medulla, where the respiratory and cardiovascular centres lie, causing apnoea and death. This is why imaging is needed before lumbar puncture when raised intracranial pressure is suspected.
Tonsillar herniation
- Tonsils through foramen magnum
- Compresses medulla
- Apnoea, cardiac arrest
- Scan before LP if ICP raised
Remember LP with a posterior fossa mass can cause coning.
Exam trap Neck stiffness and head tilt can be early signs of tonsillar herniation, mimicking meningitis.
Why the other options are wrong
- Uncus herniated, compressing the midbrain Seen with supratentorial masses; pupil dilates first.
- Cingulate gyrus herniated under the falx Compresses the anterior cerebral artery; rarely causes apnoea.
- Upward herniation of the vermis Can occur with posterior fossa masses but compresses the midbrain, not the medulla.
- Spinal cord injury from the needle Needle at the lumbar level is below the cord.
Why is lumbar puncture for suspected subarachnoid haemorrhage with a normal CT delayed until 12 hours after headache onset?
Answer: Bilirubin takes about 12 hours to form
Red cells released into the subarachnoid space are broken down, and their haemoglobin is converted to oxyhaemoglobin and then, by haem oxygenase in macrophages and the meninges, to bilirubin. Bilirubin gives the yellow colour (xanthochromia) measured by spectrophotometry and takes about 12 hours to appear. It distinguishes true haemorrhage from a traumatic tap, where blood is fresh and no bilirubin has formed. Blood mixes through the CSF within minutes, not hours.
Xanthochromia
- Red cells lysed in CSF
- Haem to bilirubin by haem oxygenase
- Detectable after ~12 hours
- Separates SAH from traumatic tap
Remember Wait 12 hours for the yellow to appear.
Exam trap A falling red cell count across tubes is unreliable; xanthochromia is the better marker.
Why the other options are wrong
- CSF pressure is too high before then Pressure is not the reason for waiting.
- Blood takes 12 hours to reach the lumbar sac CSF mixes rapidly; blood reaches the lumbar sac quickly.
- Red cells cannot be counted earlier Cells can be counted at any time.
- Meningeal irritation is absent before then Meningism may appear early or late, and is not the reason for the timing.
Where this sits in the CPSP syllabus
- Paper 1: Neuroanatomy, Ventricles, CSF circulation and hydrocephalus; meninges
- Surgery & Allied, Paper 2: Cranium and brain, Ventricles, CSF circulation and hydrocephalus; meninges
- Medicine & Allied, Paper 2: Central nervous system, Ventricles, CSF circulation and hydrocephalus; meninges
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