Lecture 37: CSF + intracranial pressure Flashcards
Where is CSF produced and how
CSF is produced by the choroid plexus: a fenestrated capillary network surrounded by epithelial cells with tight junctions, vesicles, lysosomes and microvilli on ventricular surface
Mainly in lat ventricles (but also 3,4)
Its made by
1. ultrafiltration of the choroidal capillary wall and
2. active secretion by choroidal epithelium
Describe the route of circulation of CSF
- Choroid plexus of Lateral ventricle
- interventricular foramen of Monro
- 3rd ventricle (cp)
- Cerebral aqueduct of sylvius
- 4th ventricle (cp)
- 2 Foramina of Luschka + Foramen of Magendle
- Sub arachnoid space to circulate around brain and spinal cord
- Reabsorbed by arachnoid villi + granulations at lumen of superior sagittal sinus
Describe a normal volume of CSF, and its rate of production
Volume= 150 mL with 25mL in ventricles
Rate of CSF production is fairly constant, with turnover 3-4x per day
What factors affect the reabsorption of CSF by arachnoid villi (herniations through dura mater
Depends on hydrostatic pressure in the subarachnoid space instead of transport processes.
The villi absorb CSF by uni-directional bulkflow which allow CSF into the veins but not veins into CSF (one way valve)
Why is a lumbar puncture taken and how
Patient lies down with vertical back/ curled up to widen intervertebral space.
Needle is angled headwards between spinous processes between L3/L4.
- While lying down (not sitting up) CSF pressure can be taken with manometer.
- Allows abnormal coloured CSF to be seen (normal is colourless)
- Invasive agents can be cultured.
What is the normal composition of CSF- cellular,, proteins, glucose
WBC should be less than 5e6, with no neutrophils, RBCS.
Protein <0.45g/L
Glucose should be more than 2.5 mmol however it is dependent on blood glucose (diabetes) and rate of brain glucose metabolism
What are changes in CSF composition in Meningitis and Subarachnoid haemorhhage
- Meningitis
Increase WBC: neutrophils for bacterial, lymphocytes for viral as well as proteins. Same or reduced glucose - SubAH
Increased in RBCS. Xanthochromia: yellow discolouration due to RBC lysis products (24hrs+)
What is the overall function of the CSF
As part of the BBB it maintains constant environment for neurons and glia.
It acts as mechanical cushion for brain
Counters sudden increases in intracranial pressure during coughing/ straining by dispersing into SC
Conduit for some hormones.
What is the function of the BBB and how does this relate to its structure
- Ion channels regulate ionic balance in brain
- transport via energy dependent carrier mediated= so lots of mitochondria in endothelial cells. (lipid soluble diffusion also)
To Selectively facilitates transport of essential substrates into brain:
- Tight junctions between specialised endothelial cells, thick basement membrane, astrocytic processes on capillaries: Prevents entry of harmful molecules:
Compare the intercellular junctions, pinocytotic vesicles, basement membrane, mitochondria, astrocytic processes of systemic endothelium and BBB endothelial cells
Sys 1. fenestrated 2, pino is common 3. thin BM 4. little mitochondria 5. absent astrocytic processes
BBB
- Tight junctions
- pino is uncommon
- Thick BM
- Lots of mitochondria
- Astrocytic processes present
what types of molecules find it hard to cross the BBB
Increased molecular weight, protein binding, ionisation, and reduced lipid solubility.
How do brain tumours and meningitis affect the BBB
- Brain tumour leads to systemic blood vessels being grown which are leaky leading to interstitial fluid accumulation (oedema)
- Meningitis leads to inflammatory response which breaks down the BBB allowing white cells and protein into CSF
What is normal Intracranial pressure (ICP)
65-195 mm of CSF (or water)
~5-15 mmHg.
What is the Monro-Kellie doctrine about ICP
The intracranial contents are
- brain 1300-1500mL
- blood 75mL
- CSF 75mL in a fixed space by skull.
therefore if volume of one component increases, it must be accompanied by decrease in another, otherwise there will be ICP increase
What are the 4 mechanisms to compensate for ICP increase
- CSF displaced into the Spinal canal
- Cerebral veins collapse
- increase in CSF absorption = sub arachnoid pressure increase
- Slightly distensible lumbrosacral dura.
What are the 4 main causes of ICP increase
- Increase in volume of brain tissue:
a) space occupying lesion (tumour)
b) increased water content: oedema - Increase in volume of CSF (hydrocephalus) from obstruction of CSF flow, reduced CSF absorption, increased CSF production (rare)
- Increase in cerebral blood volume:
a) obstruction of venous outflow - increased venous pressure - lying down- increased intrathoracic pressure
b) loss of vascular autoregulation - Cerebral blood flow changes
a) increase in PaCO2 or decrease in PaO2 (more mild) : dilate to increase ICP
b) increase temperature (fever)
How do you determine cerebral perfusion pressure and what are the mechanisms of autoregulation, what happens when this is lost
CPP= Mean ArtP - ICP
As bp increases during the normal range of 60-150mmHg, vasoactive factors released by neurons mediate progressive constriction of small cerebral arteries (dilation if pressure is dropping). This is to keep cerebral blood flow constant.
-If >150mmHg then max constriction fails to compensate (vcvsa dilatation <60mmHg) so blood flow is proportional to arterial blood pressure
What are Cushings signs and why important
Signs of increased intracranial pressure when things getting bad.
Due to reduction in blood flow to the nuclei in medulla
- Arterial hypertension
- Bradycardia
- Hypoventilation
What is a cerebral herniation and what can it cause
Displacement of brain tissue from one intra cranial compartment to another or through foramen magnum into the spinal canal.
Causes compression of brain, cranial nerves and blood vessels+ midline shift
What are transtentorial, tonisilar, Subfalcine and upward herniation
- TransTentorial: herniation of medial temporal lobe through tentorial notch:
Compression of midbrain, CN3, post cerebral artery - Ton: herniation of inferior cerebellum into spinal canal
- Subf: herniation of cingulate gyrus beneath falx
- herniation of superior cerebellum through tentorial notch
How does a subarachnoid haemorrhage present compared to subdural haematoma
Thunderclap headache. Photophobia. Tachycardic : subarachnoid
Subdural: increase in ICF, dilated pupil, cushings signs
