Draw a graph to show the relationship between intracranial pressure (ICP) and intracranial volume?

  • The relationship between ICP and intracranial volume can be demonstrated using the intracranial compliance curve
  • Compliance reflects the ability to compensate for changes in volume without subsequent rises in ICP – with reducing compliance comes large rises in ICP
  • The curve can be described in three stages:
Intracranial compliance curve demonstrating the relationship between intracranial volume and intracranial pressure (ICP)
Stage 1
(Initial stage)
  • Characterised by high compliance and low ICP
  • Compensatory mechanisms act to prevent rises in ICP associated with increased intracranial volume:
    • CSF moves into the spinal subarachnoid space (primary mechanism)
    • Blood displaces into extracranial vessels
    • Brain itself has minimal compressibility therefore is very limited in scope for compensation
  • Reserve volume varies with age due to ongoing cerebral atrophy:
    • 60–80 mL in young persons and approximately 100–140 mL in geriatric population
Stage 2
(Transition stage)
  • Characterised by low compliance and low ICP
  • Gradual depletion of compensatory reserve as intracranial volume rises with slow increase in ICP
Stage 3
(Ascending stage)
  • Characterised by low or null compliance and high ICP
  • Compensatory mechanisms stop working and small changes in volume elicit high increases in pressure
  • Results in a high risk of cerebral ischaemia and herniation