SBA 3374

Fundamentals

What is cerebrospinal fluid (CSF)?

  • Specialized transcellular fluid that surrounds the brain and spinal cord
  • Circulates within the cerebral ventricular system and the subarachnoid space

What is the normal volume of cerebrospinal fluid (CSF)?

  • Overall volume is between 100-150ml
    • 2⁄3 within the ventricles
    • 1⁄3 within the subarachnoid space around the spinal cord (35ml)
  • Equates to ~10% of intracranial volume

What is the pressure of cerebrospinal fluid (CSF)? 

  • CSF pressure is gravitational and varies with position
    • In the lateral position normal pressure is 5-20 cm of H2O
    • In the sitting position:
      • Pressure in the lumbar region rises to 20-50 cmH20
      • Pressure in the cervical region may be sub-atmospheric

Where is CSF produced and through which processes?

  • Produced by the four choroid plexuses:
    • Located in the two lateral, third, and fourth ventricles
    • Highly vascular invaginations of pia mater
    • Covered by specialised ciliated ependymal cells
Location of the choroid plexuses
  • Produced by a combination of:
    • Filtration of plasma through the fenestrated capillaries
    • Active transport of solutes
  • Control of substances entering is regulated by the blood-CSF barrier (distinct from the BBB)
Structure of the choroid plexus

How much CSF is normally produced?

  • Normal rate of production is 0.3-0.4ml/min or 20ml/hour or 500ml/day
  • Results in effective replacement of CSF volume 3x daily
  • Production is largely independent of ICP:
    • Raised ICP is compensated by increased absorption of CSF reducing total volume
    • Decreased when CPP <70 mmHg due to reduction in choroid plexus blood flow

How and where is CSF reabsorbed?

  • Reabsorbed by the arachnoid granulations:
    • Villi arising from the arachnoid mater
    • Project into venous sinuses and veins
  • Reabsorption occurs throughout the brain and spine
    • 90% by villi of the sagittal and sigmoid Dural sinuses
    • 10% by spinal villi
  • Reabsorption due to differences in pressure between CSF and veins
    • Pressure of CSF typically 15 cm H2O and venous blood typical 8cm H2O
    • Removal of CSF increases with rising intracranial pressure.
Structure of the arachnoid granulations