ICP: the pressure exerted by the cranium onto brain tissue, CSF and intracranial circulating blood volume

Aetiology

Causes of increased ICP

• Mass effect e.g. tumour - distort surrounding brain
• Brain swelling due to e.g. ischaemia, acute liver failure
• Increase in central venous pressure e.g. venous sinus thrombosis
• Problems with CSF flow
  • Hydrocephalus - obstruction (obstructive hydrocephalus) or decreased absorption (communicating hydrocephalus)
  • Increased production of CSF - choroid plexus papilloma

Pathophysiology

• ICP is constantly fluctuating but at rest 7-15 mmHg
  • Can be negative in vertical position
• The Monroe-Kellie Doctrine is a compensatory mechanism for expanding masses
  • Immediate: decrease in CSF volume by moving it out of FM, decrease in blood volume by squeezing sinuses
  • Delayed: decrease in ECF
• CSF is secreted by the choroid plexus → ventricular system → subarachnoid space (Magendie and Luschka) → venous system (arachnoid granulations)
  • Any obstruction in this flow will lead to hydrocephalus and increased ICP
• Cerebral perfusion pressure (CPP) = MAP - ICP
• Cerebral blood flow = CPP/cerebral vascular resistance
• Autoregulation of CBF over wide range of BP, CBF remains constant, exact mechanism is unknown
  • Pressure autoregulation - arterioles dilate or constrict in response to changes in BP or ICP
  • Metabolic autoregulation - arterioles dilate in response to chemicals e.g. CO2
  • Other theories include myogenic theory (direct reaction of smooth muscle to stretch)

Clinical features of raised ICP

Early signs

• Decreased level of consciousness
• Headache
• Pupillary dysfunction +/- papilloedema
• Changes in vision