Lecture 37: CSF, BBB and ICP Flashcards
Describe the anatomy between the skull and brain:
Meninges (Epidural space) - Dura mater (pachymeninges) (Subdural space) - Leptomeninges --> Arachnoid (Subarachnoid space) --> Pia
What is the tentorial notch?
It is a space within the tentorium that allows the brainstem to connect to the brain… images?
Where is CSF located?
Fluid in the ventricels and in the subarachnoid space around brain and spinal cord
Where is CSF produced?
- Mostly produced by choroid plexus in lateral ventricles
Two processes
- Ultrafiltration across choroidal cap. wall
- Active secretion by choroid epithelium
Describe the structure of choroid plexus epithelial cells:
- Tight junctions between cells
- Contain numerous vesicles, lysosomes
- Ventricular surface of epithelial cells has brush border
- Fenestrated cap network surrounded by row of epithelial cells.
What is the turnover of CSF per day?
Total volume = 150ml, mostly in subarachnoid space
CSF produced constantly around 600ml/day therefore turned over 3-4x per day
Describe CSF abrosption:
Absorbed in arachnoid villi and arachnoid granulations IN THE LUMEN OF SUP. SAG. SINUS
- Absorb CSF by unidirecitonal BULK FLOW
i. e one way valves that allow CSF into veins
What does CSF absorption depend on and what is it regulated by?
CSF absorption depends on hydrostatic pressure in subarachnoid space
Not regulated by any transport processes
What is the CSF composed of?
Clear and colourless is norm
- NO NEUTROPHILS, RBC
- Some WBC, low protein and glucose
What does CSF glucose depend on?
Blood glucose and rate of brain glucose metabolism
How does CSF change in meningitis and subarachnoid heamorrhage?
Meningitis
- Increase WBC
- Increase protein
- +/- dec. glucose
Subarachnoid heamorrhage
- Increase RBC
- Xanthocrhomia (yellow b/c RBC breakdown byproduct)
What is the function of CSF?
- Homeostasis, - maintains constant environment for neurons and microglia
- Mechanical cushion
- Counters sudden increases in ICP
- Conduit for some hormones
Describe the flow of CSF:
Left ventricle:
- Through the foramen of Munroe
- 3rd ventricle
- Cerebral aqueduct
- 4th Ventricle
- Around the spinal cord and brain
What is the function of the BBB?
- Ion balance
- Transport of essential substrates
- Barrier, prevents entry of pot. harmful molecules
- Selective transport of metabolites across endothelial cells
What does the BBB consist of?
- Specialised endothelial cells
- Thick basement membrane
- Astrocyte foot processes on cap
- Pericytes
Describe the structural features of the BBB:
- Tight intracellular junctions
- Thick basement membrane
- Many mitochondria
- Astrocytic foot processes
Pinocytotic vessels are uncommon
How are metabolites / key compounds transported across the BBB?
Diffusion
- Lipid soluble substances i.e O2, CO2, Alcohol
Active transport
- Glucose, some AA, vitamins, nucleosides
Ion channels
What factors impact transport across the BBB:
Mw Lipid solubility Ionisation Protein binding Specific transport mechanism
What types of disease processes can impact the BBB?
- Disruption of tight junctions, basement membrane, endothelial-astrocyte interactions
- Altered function of specific transport mechanisms
- angiogenesis lacking BBB features
How can brain tumors interrupt the BBB?
- Abnormal blood vessels that can be leaky, ISF can accumulate (oedema)
How can meningitis interrupt the BBB?
- Inflammation causes BBB break down
- WBC and protein in CSF
How is ICP measured?
Lumbar puncture
Intracranial pressure monitoring
Normal is 65-195 mm of CSF
What is the monro-kellie doctrine?
Intracranial contents: Brain, Blood, CSF.
Intracranial volume is fixed.
Increase CSF volume by one component bust be accompanied by a decrease in another. Otherwise ICP increases.
What are the compensatory mechanisms for ICP increases?
- CSF displaced into spinal cord
- Cerebral veins collapse
- Increase in CSF absorption
- Lumbosacral dura distensible
What are the causes of increased ICP?
Increase in volume of brain tissue i.e Space occupying lesion, oedema
Increase in CSF (hydrocephalus)
- Obstruction of CSF flow, Decrease absorption, increased production (Rare)
Increased cerebral blood flow
- Obstruction of venous outflow
- Loss of vascular autoregulation
What is cushings triad?
- Art. hypertension
- Slow HR
- Slow RR
What is the mechanism of cushings signs / triad?
- Reduction in blood flow to medulla
- Direct distorting of medulla
What is cerebral herniation and what can it cause?
Displacement of brain tissue from
- One intracranial compartment to another (midline shift)
- Through foramen magnum into SC
Can cause compression of
- Brain
- Cranial nerves
- Blood vessels
What is a transtentorial herniation?
Herniation of medial temporal lobe through tentorial notch -> COmpression of midbrain, oculomotor nerve, post. cerebral art.
What is tonsillar herniation?
Herniation of inf. cerebellum into spinal canal
What is subfalcine herniation?
Herniation of cingulate gyrus beneath falx
What is an upward herniation?
Herniation of sup. cerebullum through tentorial notch
Describe cerebrovascular autoregulation:
Cerebral perfusion pressure depends on MAP and ICP (CPP = MAP -ICP)
Autoregulation maintains constant cerebral blood flow over range of CPP
Neurons release vasoactive factors to dilate small art.
When autoregulation is lost, cerebral blood flow is proportional to arterial blood pressure
What systemic factors impact intracranial pressure?
- Arterial blood pressure
- Venous pressure (inc VP = inc ICP)
- -> Increased intrathoracic pressure increases venous pressure
- -> Lying down increases VP
Increased PaCO2 or decreased PaO2 increases CBF ad ICP
Decrease temp decreases CBF and ICP
