Lecture 34: CSF and ICP

Question 1

Two types of meninges? and their spaces?

Answer 1

1. Dura mater (pachymeninges) - Thick, rigid membrane surrounding the brain and spinal cord
2. Leptomeninges (arachnoid and pia mater) are thin connective tissue layers.

Subarachnoid space containing the CSF where menigitis and anurysms begin.

Subdural space containing veins that can bleed leading to subdural haematoma

(+perivascular spaces where the pia invaginates into the cortex)

Question 2

Dura mater forms the ______ and the ______?

Answer 2

The falx cerebri runs between the hemispheres

The tentorium seperates the lobes from the cerebellar and the posterior fossa structures. Clinically important due to compression of the brainstem in increased ICP as they are rigid membranes

Question 3

CSF production?

Answer 3

Mostly by the choroid plexus in the lateral ventricles

involving 2 processes in series:

1. Ultrafiltration across choroidal capillary wall
2. active sevretion by choroidal epithelium

Question 4

Choroid plexus structure?

Answer 4

Fenesrated capillary network surrounded by a row of epithelial cells

Choroid plexus epithelial cells have TJ and contain numerous vesicles and lysosomes. Ventricular surface of epithelial cells has a microvilli brush boarder.

Question 5

CSF circulation and flow?

Answer 5

• Produced in the lateral ventricles
• hydrostatic presssure pushes it out the foramina of monroe
• from the 3rd ventricles it flows through the cerebral (sylvian) aqueduct to the 4th ventricle
• then to the subarachnoid space via a midline and two lateral foramina

Question 6

CSF volume (distribution), production rate?

Answer 6

total volume = 150mL with 12-25mL in ventricles and most in SAS

0.35Ml/min = 600mL/day = 3-4x a day turnover

Question 7

CSF absorption?

Answer 7

CSF absorption by the arachnoid villi and arachnoid granulations

Arachnoid villi are:

• Herniations of arachnoid mater through the dura mater into the lumen of superior sagittal sinus
• Absorb CSF by uni-directional bulk flow
• function as a one-way valve that allows flow of CSF into veins

Depends on hydrostatic pressure in subacachnoid space and is not regulated by a transport process

Question 8

Sampling the CSF?

Answer 8

Lumbar puncture with the patient on their left side in a verticle upright position. Insertion of a needle into the L3/4 space angled slightly towards the head to go under the styloid process. Pressure is measured first before a sample is taken.

Question 9

CSF composition?

Answer 9

Clear and colourless:

• WBC < 5 x 10⁶ /L with no neutrophils
• No RBC (yellow colour from billiruben)
• Protein < 0.45g/L
• Glucose > 2.5mmol/L but varies dependent on blood glucose and brain glucose metabolism

Question 10

CSF changes in meningitis or SA heamorrhage?

Answer 10

Meningitis :

• increase in WBC (bacteria = neutrophils , viral = lymphocytes)
• increase in protein
• +/- decrease in glucose (eg. in bacterial meningitis)

SA haemorrhage

• increase in RBC
• xanthochromia - yellow colour > 12-24h earlier

Question 11

CSF function?

Answer 11

1. homeostasis - constant environment for neurons and glia
2. mechanical cushion for brain
3. Counters sudden increases in ICP (eg. when you cough)
4. Conduit for some hormones

Question 12

BBB consists of and it functions to…?

Answer 12

• Specialised endothelial cells
• Thick BM
• Astrocytic processes on capillaries
• many mitochondria

1. Regulates ionic balance in the brain
2. facilitates transort of essential substrates into the brain
3. Prevents entry of potetially harmful molecules

Question 13

Factors affecting passage of molecule across the BBB?

Answer 13

1. Molecular weight
2. Lipid solubility
3. Ionisation
4. Protein binding
5. Specific transport mechanism - facilitated diffusion or AT

Question 14

Disorders of the BBB?

2 exaples?

Answer 14

1. Disruption of tight junctions, BM or endothelial-astrocyte interactions
2. Altered function of specific transport mechanism
3. New vessles lacking features of BBB (can be found in tumours)

Brain tumours:

• abnormal BV that can leak
• interstitial fluid accumulates

Meningitis:

• Inflammatory response causes BBB breakdown
• WBC and protein in the CSF

Question 15

ICP? Monro-Kellie doctrine?

Answer 15

Intracranial pressure = 65-250 mm of CSF (or water) = 5-15 mm Hg

Intracranial volume is fixed and increase in volume of one component MUST be accompanied by a decrease in another, if not - ICP will inrease.

Question 16

Compemsatory mechanism if ICP increases?

Answer 16

• CSF displaced into spinal canal
• Cerebral veins collapse
• Increase CSF absorption
• Lumbrosacral dura is slightly distensible

Question 17

Causes of increased ICP?

Answer 17

1. increased volume of brain tissue (tumour or oedema)
2. Increased volume of CSF (hydrocephalus)
3. increase in cerebral blood volume (obstruction of venous outflow or loss of vascular autoregulation)

Question 18

Cushing’s Signs and mechanism?

Answer 18

Cushings triad: Immediate pre death signs - must lower ICP asap

• arterial hypertension
• slow heart rate
• slow respiratory rate

Mechanism:

• Reduction in blood flow to medulla
• direct distortion of medulla

Question 19

Cerebral herniations?

Answer 19

Displacement of brain tissue from:

• One intracranial compartent to another
• through foramen magnum into the spinal canal

Cause compression of:

• Brain
• CNs
• BVs

Question 20

Name two clinically important herniation?

Answer 20

Transtentorial herniation

• Brain tissue being forced through the tentorial notch putting pressure on the midbrain.
• Can cause hamorrhages into the brainstem or occlusion of posterior cerebral arteries infarcting occiptial lobe = blind
• 3rd canial nerve = dilitation of one pupil

Cerebellar tonsil herniation

• Where the inferior cerebellar parts are forced through the foramen magnum by increased pressure in posterior fossa
• medulla and upper spinal cord compression = cushing’s triad

Question 21

5 cerebral herniations summary?

Answer 21

Question 22

Cerebrovascular autoregulation? CPP?

Answer 22

Cerebral perfusion pressure (CPP) = MAP - ICP

• Autoregulation maintains constant cerebral blood flow over a wide range of cerebral perfusion pressures (60-150mm Hg)
• Vasoactive factors released by neurons mediate this via small cerebral arteries
• Loss of autoregulation leads to blood flow being proportional to blood pressure.

Question 23

System fators affecting ICP?

Answer 23

1. Arterial BP (but autoregulation)
2. Increased venous pressure > increased ICP
3. Increased intrthoracic p > inc venous p > Inc ICP
4. posture: lying > inc venous p > inc ICP
5. inc PaCO2 > inc CBF > Inc ICP
6. dec PaO2 > inc CBF > inc ICP
7. inc temp > inc CBF > Inc ICP

Question 24

Answer 24

BP rising and HR falling is cushings triad

pupil dilated due to occulomotor nerve compression

= Extradural haemorrhage (middle meningeal artery bleeding)

• commonly present with head injury and either wake up or appear fine but an hour or so later deteriorate due to inc ICP

Question 25

Answer 25

(probably bacterial) **meningitis** **-** Do a lumbar puncture to determine if bacterial or viral

Question 26

Answer 26

**Thunderclap headache - instantaneous** **Subarachnoid haemorrhage most likely from an aneurysm** -CT scan and can most likely see the blood on this rather than doing a lumbar puncture that takes longer