CSF flow and hydrocephalus: where the flow fails
Trace cerebrospinal fluid from the choroid plexus to the dural venous sinuses, then use the point of failure to separate communicating, non-communicating and normal pressure hydrocephalus.
Chapters (11)
Trace CSF production and flow and distinguish communicating, non-communicating and normal pressure hydrocephalus by where the flow fails.
The short version
- CSF flows from the lateral ventricles to the third ventricle, the cerebral aqueduct, the fourth ventricle, the subarachnoid space and the arachnoid granulations.
- A block inside the ventricular system is non-communicating hydrocephalus: the ventricles upstream of the block dilate and those downstream do not.
- Failed reabsorption is communicating hydrocephalus: all four ventricles dilate.
- Normal pressure hydrocephalus is a communicating form in older adults with big ventricles, normal or only slightly raised pressure, and gait, cognitive and bladder symptoms.
- Large ventricles from brain shrinkage, as in Alzheimer disease, are hydrocephalus ex vacuo, and the CSF is not the problem.
The case: big ventricles, normal pressure
An elderly woman shuffles as if her feet are stuck to the floor. She is forgetful, and she has started to leak urine. Her scan shows huge ventricles, yet her lumbar puncture pressure is normal. How can that be?
This lesson on CSF flow and hydrocephalus answers it in two steps: first trace the flow of cerebrospinal fluid (CSF), then classify hydrocephalus by where that flow fails.[3]
How CSF is made: 500 mL a day, 150 mL at a time
CSF is made mainly by the choroid plexus, which sits in the ventricles. Its epithelial cells, a specialised kind of ependymal cell, secrete it from blood plasma.[1][2][7]
An adult makes about 500 mL a day yet holds only about 150 mL at a time, so the fluid is replaced several times daily.[2][5]
- Made per day: about 500 mL
- Present at one time: about 150 mL
- 500 ÷ 150 ≈ 3.3
- So the whole volume is replaced about three to four times a day, matching "several times daily"
The CSF route, from lateral ventricles to arachnoid granulations
The route starts in the lateral ventricles. Through the interventricular foramina CSF reaches the third ventricle, then the cerebral aqueduct, then the fourth ventricle.[1][5]
From the fourth ventricle it leaves by the median aperture (the foramen of Magendie) and two lateral apertures (the foramina of Luschka) into the subarachnoid space. Finally it is reabsorbed through the arachnoid granulations into the dural venous sinuses.[1][2][10]
- Lateral ventriclesCSF is made by the choroid plexus and starts here.
- Interventricular foramina to the third ventricleThe lateral ventricles drain into the third ventricle.
- Cerebral aqueduct to the fourth ventricleThe aqueduct is the narrow link between the third and fourth ventricles.
- Median and lateral aperturesThe foramen of Magendie and the foramina of Luschka open into the subarachnoid space.
- Arachnoid granulationsCSF is reabsorbed into the dural venous sinuses.
What hydrocephalus is, and the inside-or-after split
Hydrocephalus is too much CSF in the ventricles. It comes from blocked flow or poor reabsorption, and only rarely from overproduction, as in a choroid plexus papilloma.[3][5]
The simplest split is this: is the block inside the ventricular system, or after it? That one question separates the two main types of hydrocephalus.[3][5]
Non-communicating (obstructive) hydrocephalus: the block is inside
Something blocks the path inside the ventricles. The ventricles upstream of the block dilate, and those downstream do not.[3][5]
The classic site is the cerebral aqueduct, because it is so narrow. Congenital aqueductal stenosis or a tumour can block it. With the aqueduct blocked, the lateral and third ventricles swell and the fourth does not.[3][8]
Communicating hydrocephalus: reabsorption fails
CSF leaves the ventricles freely but is not reabsorbed, so all four ventricles dilate.[3][8]
Think of blood after a subarachnoid haemorrhage, or scarring after meningitis, clogging the arachnoid granulations.[3][8][10]
| Feature | Non-communicating | Communicating |
|---|---|---|
| Where flow fails | Inside the ventricular system | After the ventricles: reabsorption |
| Which ventricles dilate | Those upstream of the block | All four |
| Examples from the lesson | Aqueductal stenosis, tumour | Subarachnoid haemorrhage, meningitis |
Normal pressure hydrocephalus: the triad, the shunt and the ex vacuo caution
Normal pressure hydrocephalus (NPH) is a form of communicating hydrocephalus in older adults, with enlarged ventricles and a normal or only slightly raised CSF pressure.[4][6]
Its classic triad is gait disturbance, cognitive impairment and urinary incontinence: wobbly, wacky and wet. The gait usually comes first, short-stepped and magnetic, as if stuck to the floor.[4][6][9]
It is one of the few potentially reversible causes of dementia: draining the CSF with a shunt can improve the symptoms, the gait most of all.[4][6][9]
That resolves the opening case: the stuck-to-the-floor gait, the forgetfulness, the leaking urine, big ventricles and a normal pressure add up to normal pressure hydrocephalus.[4][9]
Ventricles are also large when the brain has shrunk, as in Alzheimer disease. That is hydrocephalus ex vacuo, and the CSF is not the problem.[3][4][9]
Practice question
Original · USMLE styleA 4-year-old girl is brought to the physician because of 3 weeks of morning headaches and vomiting. Fundoscopic examination shows bilateral papilledema. MRI of the brain shows dilated lateral and third ventricles. The fourth ventricle is normal in size.
Common questions
What is the route of CSF flow?
CSF flows from the lateral ventricles through the interventricular foramina to the third ventricle, then the cerebral aqueduct and the fourth ventricle. It leaves through the median and lateral apertures into the subarachnoid space and is reabsorbed at the arachnoid granulations.
What is the difference between communicating and non-communicating hydrocephalus?
In non-communicating (obstructive) hydrocephalus a block inside the ventricular system dilates the ventricles upstream of it. In communicating hydrocephalus CSF leaves the ventricles but is not reabsorbed, so all four ventricles dilate.
What is the triad of normal pressure hydrocephalus?
Gait disturbance, cognitive impairment and urinary incontinence, remembered as wobbly, wacky and wet. The gait usually comes first and is short-stepped and magnetic.
What is hydrocephalus ex vacuo?
It is enlargement of the ventricles because the brain has shrunk, as in Alzheimer disease. The CSF is not the problem.
How much CSF does an adult make each day?
About 500 mL a day, mainly from the choroid plexus, while only about 150 mL is present at one time.
Transcript
Read the full narration, by chapter664 words
0:00The case: big ventricles, normal pressure
An elderly woman shuffles as if her feet are stuck to the floor. She is forgetful, and she has started to leak urine. Her scan shows huge ventricles, yet her lumbar puncture pressure is normal. How can that be? Today: trace the flow of CSF, and tell the types of hydrocephalus apart by where the flow fails.
0:21How CSF is made: 500 mL a day, 150 mL at a time
CSF is made mainly by the choroid plexus, which sits in the ventricles. Its epithelial cells, a specialised kind of ependymal cell, secrete it from blood plasma. An adult makes about 500 mL a day, yet holds only about 150 at a time, so it is replaced several times daily. The route starts in the lateral ventricles.
0:43The route: lateral ventricles to the arachnoid granulations
Through the interventricular foramina it reaches the third ventricle, then the cerebral aqueduct, then the fourth ventricle. From the fourth ventricle it leaves by the median aperture, the foramen of Magendie, and two lateral apertures, the foramina of Luschka, into the subarachnoid space. Finally it is reabsorbed through the arachnoid granulations into the dural venous sinuses. Hydrocephalus is too much CSF in the ventricles.
1:12What hydrocephalus is, and the inside-or-after split
It comes from blocked flow or poor reabsorption, and only rarely from overproduction, as in a choroid plexus papilloma. The simplest split is this: is the block inside the ventricular system, or after it? Non-communicating,
1:30Non-communicating: block inside, upstream dilates
or obstructive: something blocks the path inside the ventricles. The ventricles upstream of the block dilate, and those downstream do not. The classic site is the cerebral aqueduct, because it is so narrow. Congenital aqueductal stenosis or a tumour can block it. With the aqueduct blocked, the lateral and third ventricles swell and the fourth does not. Communicating: CSF leaves the ventricles
1:57Communicating: reabsorption fails, all four dilate
freely, but is not reabsorbed, so all four ventricles dilate. Think of blood after a subarachnoid haemorrhage, or scarring after meningitis, clogging the arachnoid granulations. Normal pressure hydrocephalus is a form of
2:12Normal pressure hydrocephalus: the triad and the shunt
communicating hydrocephalus in older adults, with enlarged ventricles and a normal or only slightly raised CSF pressure. Its classic triad is gait disturbance, cognitive impairment, and urinary incontinence: wobbly, wacky and wet. The gait usually comes first, short-stepped and magnetic, as if stuck to the floor. It is one of the few reversible causes of dementia, because draining the CSF with a shunt can improve the gait. One caution.
2:45The caution: ex vacuo
Ventricles are also large when the brain has shrunk, as in Alzheimer disease. That is hydrocephalus ex vacuo, and the CSF is not the problem. So our woman, with the stuck-to-the-floor gait, the forgetfulness, the leaking urine, big ventricles and a normal pressure, has normal pressure hydrocephalus.
3:08Practice question
Your turn. A 4-year-old girl has had three weeks of morning headaches and vomiting. Examination shows papilloedema. MRI shows dilated lateral and third ventricles. The fourth ventricle is normal in size. Which of the following is the most likely site of obstruction? A, arachnoid granulations. B, cerebral aqueduct. C, foramen of Magendie. D, foramina of Luschka. E, interventricular foramina. Pause here and decide.
3:46Answer and explanation
A is out: blocked reabsorption is communicating hydrocephalus, and all four ventricles would be dilated. C is out: the foramen of Magendie is an outlet of the fourth ventricle, so outlet obstruction would enlarge the fourth ventricle too. D is out: the foramina of Luschka are also outlets of the fourth ventricle, so obstructing them would enlarge the fourth ventricle as well. E is out: blocking both interventricular foramina would dilate only the lateral ventricles and leave the third ventricle normal, because it sits downstream. So the answer is B. Blocking the cerebral aqueduct dilates everything upstream, the lateral and third ventricles, and spares the fourth. Three things to keep.
4:32Recap and next lesson
One: CSF flows from the lateral ventricles to the third, the aqueduct, the fourth, the subarachnoid space, and the arachnoid granulations. Two: a block inside the system is non-communicating, and only the ventricles upstream dilate. Failed reabsorption is communicating, and all four dilate. Three: normal pressure hydrocephalus is big ventricles with normal pressure, and gait, cognition and bladder are affected. Next, the internal capsule and the corticospinal tract.
Sources
Numbers in the text, e.g. [1], point here- OpenStax. Anatomy and Physiology 2e, 13.2 The Central Nervous System. openstax.org/books/anatomy-and-physiology-2e/pages/13-2-the-central-nervous-system
- Cerebrospinal fluid. Wikipedia. en.wikipedia.org/wiki/Cerebrospinal_fluid
- Hydrocephalus. Wikipedia. en.wikipedia.org/wiki/Hydrocephalus
- Normal pressure hydrocephalus. Wikipedia. en.wikipedia.org/wiki/Normal_pressure_hydrocephalus
- CSF physiology review, Neurochem Res 2024. europepmc.org/article/PMC/PMC10991002
- Idiopathic NPH, Neurodegener Dis 2026. europepmc.org/article/PMC/PMC13461114
- Choroid plexus secretion: europepmc.org/article/PMC/PMC13167087
- Hydrocephalus pathways and imaging: europepmc.org/article/PMC/PMC13198784 · europepmc.org/article/PMC/PMC12727080 · europepmc.org/article/PMC/PMC12517981
- NPH and the Hakim-Adams triad, Europe PMC: europepmc.org/article/PMC/PMC13702639 · europepmc.org/article/PMC/PMC13701216
- CSF reabsorption and arachnoid granulations: europepmc.org/article/PMC/PMC13388259
Full claim ledger: 01_references/ledger.csv. Volumes are typical adult values: about 500 mL a day made, about 150 mL present.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined; printed flow arrows removed): "Cerebrospinal fluid", "Brain section". Gray's Anatomy (1918), plates 717 and 769, Henry Vandyke Carter, via Wikimedia Commons, public domain. Original diagrams: the schematic ventricular chain, enlarged-ventricle overlays, pressure gauge, footprints, shunt and all handwriting are drawn in code. No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-NEURO-014.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
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- EP 02Dorsal column vs spinothalamic tract: where each pathway crosses5:01
- EP 03Spinal cord syndromes compared: rebuild each from three tracts5:06
- EP 04Cranial nerve nuclei: a brainstem map by level and position5:10
- EP 05Brainstem crossed findings: the localisation rule4:50
- EP 06Medial vs lateral medullary syndrome: artery, structures and signs5:30
- EP 07Facial nerve palsy: why the forehead tells you the lesion4:59
- EP 08Stroke syndromes by artery: ACA, MCA and PCA4:41
- EP 09Lacunar stroke syndromes: pure motor, pure sensory and more4:54
- EP 10Visual pathway lesions and field defects4:42
- EP 11Basal ganglia direct and indirect pathways: D1, D2 and movement5:19
- EP 12Cerebellar lesion localisation: vermis vs hemisphere4:42
- EP 13Brain herniation syndromes: what is squeezed, and where4:56
- EP 14CSF flow and hydrocephalus: where the flow fails5:12
- EP 15Corticospinal tract: from cortex to anterior horn4:59
- EP 16Hypothalamic nuclei and their functions5:24