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SO Nervous System > CSF & Intracranial Pressure > Flashcards

CSF & Intracranial Pressure Flashcards

1
Q

Where is the CSF located?

A

In the ventricles and the subarachnoid space.

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2
Q

Describe the meninges

A

There are 2 Types

  1. Dura Mater (pachymeninges): thick, rigid membrane surrounding the brain and the spinal cord. Ordinarily adherent, the layer can split to form the venous sinuses,
    • The Thick periosteal layer and an inner-meningeal layer.
  2. Leptomeninges: Arachnoid (thin CT that encloses the csf in the subarachnoid space) outer layer and the Pia (adherent to the brain and spinal cord) inner layer
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3
Q

What’s in the subdrual, subarachnoid?

A

Subdural Space: between the periosteal and meningeal layers of the dura mater. Adherent but small veins run through here, putting this area at risk for bleeds/blood clots, and because the skulls so rigid this can compress the brain

Subarachnoid space: between Arachnoid and pia mater. Arteries run through this larger space ⇒ can get aneurysms/bleeding, or meningitis can occur!

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4
Q

Virchow-Robin Space?

A

Where the pia mater invaginates into the brain or spinal cord around blood vessels entering or leaving the brain, forming a perivascular space.

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5
Q

What is a tentorial notch and the falx, and how are they formed?

A

The inner dura (meningeal layer) duplicates to form the ‘falx’ and the tentorium.

Falx: Fold of meningeal layer of dura mater that descends vertically in the longitudinal fissure, seperating the cerebral hemispheres of the human brain.

Tentorium: separates the cerebellum from the inferior portion of the occipital/temporal lobes.

This creates the tentorial notch; a space with the spinal cord comes through. This space can be filled and put pressure on the SC. :(

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6
Q

How is the CSF produced?

A

In the Choroid Plexus in the lateral ventricles (to a lesser degree in the 3rd and 4th ventricles) involving two processes.

  1. Ultrafiltration across choroidal capillary wall: depends on hydrostatic pressure in the capillaries which pushes fluid out into interstitial fluid of the CP
  2. Active Secretive by choroidal epithelium: Due to tight junctions CSF molecules are actively secreted out
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7
Q

Describe the structure of the Choroid Plexus?

A

Fenestrared capillary network surrounded by a single row of epithelial cells.

Epithelial Cells:

  • Tight junctions between cells
  • Have numerous vesicles, lysosomes
  • Ventricular surface of epithelial cells has a microvilli brush border.
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8
Q

How does the CSF circulate?

A
  1. Produced in all ventricles, especially the lateral.
  2. Hydrostatic pressure gradient created from production in the Choroid Plexus pushes CSF through the system, from lateral ventricles
  3. Through the Foramen of Munroe
  4. To the 3rd ventricle in the midline
  5. Through the cerebal aquaduct (upper brainstem)
  6. To the 4th ventricle (lower posterior fossa)
  7. Fluid pushed out through 3 foramena to subarachnoid space where its mainly located and circulates all the way arounf the brain and spinal cord.
  8. In the brain it is absorbed by the arachnoid villi
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9
Q

Where is the CSF on this CT scan?

A

White on CT

Dark on MRI

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10
Q

What is the volume, rate-of-production and turnover-per-day of the csf?

A
  • Total Volume = 150ml (depends on age as brain shrinks)
    • 12-25ml in ventricles
    • Most of CSF is in Sub-arachnoid space
  • Rate of CSF relatively constant
    • 0.35ml/min
    • 600ml/day
  • Turnover CSF 3-4x per day
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11
Q

Where is the CSF absorbed?

A

Absorbed in the arachnoid villi and granulations.

Arachnoid Villi:

  • herniations of arachnoid mater through dura amter into lumen of superior sagittal sinus (major vein in the midline)
  • Absorb CSF by unidirectional/one-way “bulk flow”
  • Function as ‘one-way valves’ that allow flow of CSF into veins
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12
Q

CSF absorbtion depends on _______ in the ________.

Regulated by______?

A

CSF absorbtion depends on hydrostatic pressure in the subarachnoid space.

It’s not regulated by any transport process!

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13
Q

How do you get a sample of CSF fluid?

A

Via a lumbar puncture.

Performed using local anaesthesia, with the patient lying in the left lateral lying-down position with neck, trunk hips and knees flexed (to open up the intervertebral space, and the needle is inserted in the L3/4 intervertebral space.

When the needle is in the subarachnoid spacem the CSF pressure can be measured (manometer) and a sample of the CSF can be removed.

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14
Q

What is the CSF tested for and what is the normal composition?

A

Tested for the presense of cells (leukcytes, RBCs, tumour cells) and protein + glucose measured.

Meningitis ⇒ murky fluid
Post Haemmorrhage ⇒ yellow fluid
SHOULD BE CRYSTAL CLEAR

**learn these figures!

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15
Q

How does the CSF compinsation change in disease?

Meningitis

Subarachnoid Haemorrhage

A

Meningitis:

  • increased WBC
  • Increased protein (murky)
  • Decreased glucose (only in bacterial men.)

Subarachnoid Haemorrhage

  • Increased RBC
  • “xanthochromia” (yellow discolouration due to RBC breakdown)
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16
Q

What are the 4 Functions of the CSF

A
  1. Homeostasis: maintains constant environ. for neurons + glia, by faciliting entry/exit of metabolites
  2. Mechanical Protection: cushions the brain from impact
  3. Counters sudden increases in intracranial pressure: so when you strain or cough, CSF is forced from the intracranial cavity → spinal cord, reducing the pressure to normal
  4. Conduit for hormones****(not important)
17
Q

What is the function of the Blood-brain-barrier (BBB)

A

BBB and the blood-CSF barrier preserve homeostasis of neurons and glia.

  1. Regulation of ionic balance in brain
  2. Facilitates entry of essential subsrates (eg; O2, glucose) into brain
  3. Prevents entry of potentially harmful substrates

Substrates ‘selectively’ transported across the endothelial cells

18
Q

What consists of the BBB, and how is this different to other parts of the body?

A
  • Specialised endothelial cells
    • ​Tight junctions between epithelial cells
    • Lots of mitochondria
  • Thick BM
  • Astrocytic processes on capillaries
19
Q

How do you transport across the BBB

A
  1. Diffusion (only lipid-soluble substances): O2, CO2 and alcohol
  2. Active transport (energy-dependent): glucose, some aa, vitamins, nucleosides
  3. Ion Channels
20
Q

What affects transport over the BBB?

A
  • Molecular weight
  • lipid solubility (increases rate)
  • Ionisation
  • Protein binding (inhibits)
  • Specific transport mechanism
    • Facilitated diffusion
    • Active Transport
21
Q

What are some disease processes involving the BBB?

A
  • Disruption of tight junctions
  • Disruption of the BM
  • Disruption of endothelial-astrocyte interactions
  • Altered function of specific transport mechanism
  • New Blood vessels lacking features of BBB

Brain tumours: abnormal blood vessels, that can be ‘leaky’, causing interstital fluid to accumulate (oedema)

Meningitis: inflammatory response causes BBB breakdown, WBC and protein in the CSF.

22
Q

How do you measure the intracranial pressure?

A

Via a Lumbar Puncture or intracranial pressure monitoring (drill hole into brain, also good for releasing brain pressure during haemorrhage)

Normal CSF Pressure: 65-195mm of CSF(or water) = 5-15mmHg

23
Q

What is the Monro-Kellie Doctrine?

A

The 3 components of the intracranial contents

  1. Brain 1300-1500mL
  2. Blood 75mL
  3. CSF 75mL

Intracranial volume is fixed (bc of solid skull), so these volumes are compressable but they are displacable!!

“Increase in volume of one component must be accompanied by decrease in another; otherwise the ICP increases”- MK-Doctrine

24
Q

What are the 4 compensatory mechanism if the ICP increases?

A
  1. CSF dsplaced into the spinal cord
  2. Cerebral veins collapse
  3. Increase in CSF absorption
  4. Lumbrosacral dura distensible
25
Q

causes of increased ICP

A
  1. Increase in brain tissue volume (spacial lesion like a tumor, increas H2O content due to oedema)
  2. Increase in CSF volume (‘hydrocephalus’, big ventricles)
    - obstruction of CSF flow
    - decr. CSF absorption
    - Increase CSF production (rare)
  3. Increase in cerebral blood volume
    - obstruction of venous outflow
    - loss of vascular autoreg
26
Q

What is the ‘Cushings Triad’

A

Increased Intracranial Pressure leads to reduced blood-flow to the medulla and medullary disortion. This leads to…

  1. Arterial hypertension
  2. Slow heart rate
  3. Slow respiratory rate

Cushing triad is seen at the very last stages of increased ICP before death

27
Q

What’s a cerebral herniation and what can it cause compression of?

A
  • Displacement of brain tissue from
  • one intracranial compartment to another
  • Through foramen magnum into spinal cord
  • Herniations can cause compression of
  • brain
  • cranial nerves
  • blood vessels

  • eg; medial tentorial herniation into the upper spinal cord ⇒ comatosed patient*
  • ***the tentorial notch is a tight space, has the reticular activating group*
28
Q

Transtentorial Herniation?

A

Herniation of medial temporal lobe through tentorial notch → compression of midbrain, oculomotor nerve, posterior cerebral artery

29
Q

Tonsillar Herniation

A

Herniation of inferior cerebellum into spinal canal

30
Q

SubfalcineHerniation

A

Herniation of cingulate gyrus beneath falx

31
Q

Upward Herniation

A

Herniation of superior cerebellum through tentorial notch

32
Q

What is cerebrovascular autoregulation

A

It’s the CPP ‘cerebral perfusion pressure’ (pressure that will perfuse the brain) that depends on MAP (mean arterial pressure) and ICP (intracranial pressure)
CPP = MAP - ICP

Autoregulation maintains constant cerebral blood flow over a wide range of CPPs (60-150mmHg).

Does this by vasoactive factors released by neurons mediate constriction or dilatation of small cerebral arteries.

**when there’s loss of autoregulation cerebral blood flow is propportional to arterial BP

33
Q

What are some factors that affect ICP?

A
  • Arterial BP
  • inc. venous BP → inc. ICP
  • inc. intrathoracic pressure → inc. venour BP → inc. ICP
  • Posture:
    • lying → inc. venous P → increased ICP
  • inc. PaCO2 → inc CBF → inc ICP
  • decr. PaO2 → inc. CBF → inc ICP
  • decr. temp → decr. CBF → decr ICP

SO Nervous System (32 decks)

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