ICP

• ICP - pressure within the skull
• ICP is the pressure of tissues inside cranial cavity - conducted to spinal CSF
• Normal ICP is 5-10 mmHg
• Small changes in ICP can lead to headache or nauseas
• Large rises in ICP can lead to impaired perfusion/ischaemia
• Cerebral perfusion pressure = mean arterial pressure - intracranial pressure
• Raised ICP will reduce cerebral perfusion pressure and may compromise vital perfusion
• As cerebral perfusion pressure decreases, the ability to transfer nutrients and oxygen is diminished
• Monro-Kellie hypothesis

  • volume of cranium is fixed
  • contents of cranium are incompressible
  • any increase in volume of one component must be compensated by decrease in another
• in cerebral atrophy brain volume decreases but is replaced by fluid
• an intact blood-brain barrier means that fluid mobement between intravascular and interstitial spaces is minimal
• CSF acts

  • to cushion brain
  • to regulate ICP
  • as a transfer medium
  • delivery of nutrients
  • elimination of metabolic products
  • circulation of neurotransmitters
• CSF is produced by the choroid plexus
• why do we get a headache after a lumbar puncture?

  if csf escapes from a LP site a low pressure headache can result as the brain sags, pulling on its meningeal attachments - worse on sitting/standing
• why is a lumbar puncture contraindicated in raised ICP?

  when the ICP is markedly raised, if CSF is allowed to escape via an LP this can result in the brain suddenly being pulled down into the foramen magnum
• Puncture

  A spinal needle is inserted into the lumbar spine (between L4 and L5 vertebrae) at an angle.
• Aspiration

  A few milliliters of cerebrospinal fluid (CSF) are removed for testing.
• manometer: an instrument for measuring the pressure acting on a column of fluid, consisting of a U-shaped tube of liquid in which a difference in the pressures acting in the two arms of the tube causes the liquid to reach different heights in the two arms. Used in lumbar puncture
• Yellowish or cloudy CSF on lumbar puncture could suggest cranial haemorrhage or it can be sent for further analysis
• analysing CSF

  • look for bilirubin and other blood products
  • check pressure
• cerebrospinal fluid is about 150ml total divided into cranial and spinal (*75ml each)
• Composition of cerebrospinal fluid is similar to interstitial fluid

  • no cells
  • virtually no protein - no buffering
• Most specific CSF marker is B2 transferrin
• short term ICP regulation is achieved by adjusting volume of intracranial

  • blood - veins readily change calibre
  • CSF - easily moved between cranial and spinal spaces
• if ICP increases (e.g. cerebral vasodilation) pressure on ventricles moves CSF to the spinal space to decrease ICP
• Rapid, major changes to the cerebral blood volume may exceed ability to compensate, resulting in marked (high pressure) headache. Seen commonly with vasodilator drugs e.g. GTN, sildenafil
• Volume of CSF is balanced between production and reabsorption
• Production of CSF is is energy-dependent (Na+/K+ ATPase) and has a constant rate of ~500 ml/day (20 ml/hr)
• CSF is replenished 3-4 times per day
• reabsorption of CSF is variable - rate increases as ICP rises
• CSF production is constant but reabsorption varies with ICP
• ventricular system

  A) lateral ventricles

  B) foramina of monro

  C) 3rd ventricle

  D) aqueduct of sylvius

  E) 4th ventricle

  
• CSF is produced mainly by the choroid plexus in lateral and 3rd ventricles
• CSF circulates into the 3rd ventricle via foramina of monro, flows into 4th ventricle via aqueduct, circulates over surface of brain and spinal cord and is reabsorbed through arachnoid granulations
• hydrocephalus

  • csf blockage - rise in ICP
  • in childhood leads to cranial expansion
  • will progress to cerebral damage
  • communicating and non-communicating
• communicating hydrocephalus

  • blocked CSF reabsorption at arachnoid granulations
  • CSF circulation normal
• Non-communicating

  • blocked CSF circulation
  • tumour, haemorrhage etc.
  • commonly at aqueduct
• pressure from hydrocephalus can be relieved via a shunt
• the brain is clinically dependent on oxygen and glucose
• hypoxic brain damage starts after ~ 3 minutes