Cerebrospinal Fluid

What is the function of CSF?

Cerebrospinal fluid is a specialised extracellular ultrafiltrate of plasma, which surrounds the central nervous system. It has four main roles:

  • Regulation of intracranial pressure
  • Mechanical protection of the brain
  • Maintenance of a stable chemical environment
  • Control of respiration via central chemoreceptors

How much CSF is there in the brain and spinal cord, and how is it produced?

  • Volume of CSF is 100 – 150ml
    • 300 – 500 ml produced per day
      • 3ml/min
    • 125ml in the subarachnoid space, the remainder in the ventricles

CSF is produced by specialised choroid plexus cells via a process of ultrafiltration and active transport through fenestrated capillaries.

Choroid plexus is found in the 3rd, 4th and lateral ventricles, and CSF production depends on cerebral perfusion pressure. If CPP drops below 70mmHg then CSF production decreases.


CSF circulation

Starting in the lateral ventricles, the Cerebrospinal fluid flows, with the help of ciliary movements of ependymal cells, via the foramen of Monro into the third ventricle. It then passes through the aqueduct of Sylvius to the fourth ventricle.

Then it flows through the central foramen of Magendie and the lateral foramina of Luschka to the subarachnoid space around the medulla and spinal cord. It then either passes down to the spinal cord or up around the cerebral hemispheres.

It is reabsorbed by the arachnoid villi and drains into the dural sinuses, with a small amount being absorbed directly into spinal veins. Hence reabsorption is determined at least in part by the difference between CSF pressure and venous pressure.


What problems can occur with CSF circulation?

Problems with the circulation of CSF can be categorised depending on the amount of CSF produced, the presence of obstruction to flow, and the amount reabsorbed.

Production
  • Choroid plexus tumour can produce too much
  • Poor cerebral perfusion can produce too little
Obstruction
  • Tumour
  • Bleeding
  • Abscess
  • Meningeal disease
Reabsorption
  • Infection
  • Haemorrhage
  • Increased venous pressure

All three of the above categories can also be caused by congenital anatomical abnormalities.


How does CSF differ to plasma?

mmol/litrePlasmaCSF
Sodium145140
Calcium2.01.2
Potassium4.0-4.53
Glucose64
Chloride (only one that’s higher in CSF)100120
Bicarbonate22-2822-28
Osmolality280-300290
PaCO25.36.6
pH7.35-7.457.28-7.32
Protein70 g/l0.2 – 0.4 g/l
White cell count4000 – 5000 per mm cubed0 – 5

How can analysis of CSF be used to aid diagnosis?

  • Pressure
  • Visual observation
  • Glucose
    • 66% of fasting plasma glucose normally
      • below 33% = bacterial, fungal, malignancy
      • too high = viral meningitis
  • Lactate
    • high lactate differentiates bacterial and fungal meningitis from viral
  • Red blood cells
    • Bleeding
    • Stroke
    • Traumatic tap
  • WCC and differential
    • Increased in infection, allergy, leukaemia, demyelnation, haemorrhage and traumatic tap
  • Culture and gram stain
    • Acid fast
    • Syphillis serology
  • Protein Count
    • Higher in bacterial meningitis or tuberculosis
    • Demyelinating disease
    • Subarachnoid haemorrhage
    • Tumours
    • Bloody tap

Find all of this and more in our Primary Toolkit

Free version available here