Lecture 34: CSF and ICP Flashcards
Two types of meninges? and their spaces?
- Dura mater (pachymeninges) - Thick, rigid membrane surrounding the brain and spinal cord
- 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)
Dura mater forms the ______ and the ______?
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
CSF production?
Mostly by the choroid plexus in the lateral ventricles
involving 2 processes in series:
- Ultrafiltration across choroidal capillary wall
- active sevretion by choroidal epithelium
Choroid plexus structure?
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.
CSF circulation and flow?
- 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
CSF volume (distribution), production rate?
total volume = 150mL with 12-25mL in ventricles and most in SAS
0.35Ml/min = 600mL/day = 3-4x a day turnover
CSF absorption?
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
Sampling the CSF?
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.
CSF composition?
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
CSF changes in meningitis or SA heamorrhage?
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
CSF function?
- homeostasis - constant environment for neurons and glia
- mechanical cushion for brain
- Counters sudden increases in ICP (eg. when you cough)
- Conduit for some hormones
BBB consists of and it functions to…?
- Specialised endothelial cells
- Thick BM
- Astrocytic processes on capillaries
- many mitochondria
- Regulates ionic balance in the brain
- facilitates transort of essential substrates into the brain
- Prevents entry of potetially harmful molecules
Factors affecting passage of molecule across the BBB?
- Molecular weight
- Lipid solubility
- Ionisation
- Protein binding
- Specific transport mechanism - facilitated diffusion or AT
Disorders of the BBB?
2 exaples?
- Disruption of tight junctions, BM or endothelial-astrocyte interactions
- Altered function of specific transport mechanism
- 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
ICP? Monro-Kellie doctrine?
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.
Compemsatory mechanism if ICP increases?
- CSF displaced into spinal canal
- Cerebral veins collapse
- Increase CSF absorption
- Lumbrosacral dura is slightly distensible
Causes of increased ICP?
- increased volume of brain tissue (tumour or oedema)
- Increased volume of CSF (hydrocephalus)
- increase in cerebral blood volume (obstruction of venous outflow or loss of vascular autoregulation)
Cushing’s Signs and mechanism?
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
Cerebral herniations?
Displacement of brain tissue from:
- One intracranial compartent to another
- through foramen magnum into the spinal canal
Cause compression of:
- Brain
- CNs
- BVs
Name two clinically important herniation?
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
5 cerebral herniations summary?
Cerebrovascular autoregulation? CPP?
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.
System fators affecting ICP?
- Arterial BP (but autoregulation)
- Increased venous pressure > increased ICP
- Increased intrthoracic p > inc venous p > Inc ICP
- posture: lying > inc venous p > inc ICP
- inc PaCO2 > inc CBF > Inc ICP
- dec PaO2 > inc CBF > inc ICP
- inc temp > inc CBF > Inc ICP
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
(probably bacterial) meningitis
- Do a lumbar puncture to determine if bacterial or viral
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
