Neuro Physiology - Cerebrospinal Fluid Flashcards
What is the function of CSF?
Specialist extracellular ultrafiltrate of plasma which surrounds the CNS with four main roles
1. Regulation of ICP
2. Mechanical protection of the brain, acting as a shock absorber, and reducing the effective weigh tof the brain from 1.5kg to 50g thorugh buoyancy, reducing brain inertia and protecting against deformation from acceleration/deceleration
3. Maintenence of a stable chemical environment
4. Control of respiration via central chemoreceptors
How is CSF produced?
Volume of CSF is 100-150ml
300-500ml produced per day (3ml/min)
125ml in subarachnoid space, remainder in the ventricles
Produced by specialised choroid plexus cells (in 3rd/4th/lateral ventricles) through ultrafiltration and active transport through fenestrated capillaries.
Unlike the BBB which has tight junctions to prevent exposure of brain cells to chemicals in the blood.
If CPP drops below 70mmHg then CSF production decreases.
Describe the circulation of CSF
Starting in the lateral ventricles, CSF flows, with the help of the ciliary movements of ependymal cells, via the foramen of Monro into the third ventricle.
Via aqueduct of Sylvius to the fourth ventricle
Through central foramen of Magendie and the lateral foramina of Luschka to the subarachnoid space
Either down the spinal cord or up around the cerebral hemispheres
Reabsorbed by the arachnoid villi, draining nto the dural sinuses, with a small amount absorbed directly into** spinal veins**.
Reabsorption is determined to some extent by the difference between CSF pressure and venous pressure.
What problems can occur with CSF circulation?
Production
Choroid plexus tumour can produce too much
Poor perfusion can produce too little
Obstruction
Tumour, bleeding, abscess, meningeal disease
Reabsorption
Infection, haemorrhage, increased CVP
All of the above can be caused by congenital anatomical abnormalities
Structure the answer logically - categorise into the amount produced, presence of any obstruction to flow, and the amount reabsorbed
How does CSF differ from plasma?
PICTURE/TABLE
Most ions have lower concentrations - Cloride is higher in CSF
Osmolality is relatively constant at 290 compared to plasma with a range of 280-300
PaCO2 is higher in CSF
How can analysis of CSF be used to aid diagnosis?
Pressure, visual observation
Glucose
66% of plasma glucose normally, below 33% bacterial/fungal/malignancy
too high viral meningitis
High lactate suggests bacterial/fungal meningitis over viral
RBC bleeding, stroke, traumatic tap
WCC & differential infection, allergy, leukaemia, demyelination, haemorrhage and traumatic tap
Culture and gram stain acid fast or syphillis serology
Protein count higher in bacterial meningitis or tuberculosis, demyelinating disease, SAH, tumours, bloody tap
