Cerebrospinal fluid, Intracranial pressure - Physiology and clinical
relevance
Description
- Fluid that envelops the brain and spinal cord and fills the ventricular
spaces
Function
- Buoyancy - prevents weight of brain to compress its own blood supply
- Protection - of brain from head trauma
- Buffer - excess metabolic waste products and neuroendocrine factors
- Cerebral perfusion regulation - buffers intracranial pressure
Production
- Choroid plexus in ventricles
- Circulation and drainage
- Paired lateral ventricle > Interventricular foramen of Monro > Third
ventricle > Cerebral aqueduct of Sylvius > Lateral foramen of Luschka x 2 +
Median foramen of Magendie > Basal cistern > Subarachnoid space brain and
spinal cord
- Drainage via arachnoid villi of superior sagittal sinus (venous channel)
- Intracranial pressure (ICP)
- ICP is a function of brain volume, cerebral blood volume, CSF volume.
Complications of raised ICP
- Direct pressure effects > brain damage
- Hemispheric or brainstem herniation > brainstem compression and
ischaemia > cardiorespiratory arrest > death
Monro-Kelly doctrine
- Any increase in any one of brain, blood or CSF volume will require
decrease of others to maintain ICP.
Values
- CSF production: 150ml/min (usually equals absorption)
- CSF pressure: < 20 cmH2O
- Intracranial pressure: 5-15 cmH2O
CLINICAL RELEVANCE
CSF rhinnorhea and otorrhea
- Abnormal CSF leakage from ear canal or nose indicative of basal skull
fracture
Hydrocephalus
- Excessive CSF volume and pressure resulting in ventricular dilatation
Causes
- Obstruction to flow (commonest) - blockage within ventricular system
(non-communicating) or outside ventricular system (communicating)
- Increased production
- Decreased absorption
Treatment
- Surgical shunting procedure to facilitate drainage e.g. ventriculostomy,
ventricular-peritoneal shunt, ventriculo-atrial shunt.
CSF fluid analysis
Procedure
- Sample typically obtained by puncture of subarachnoid space at level
lower lumbar spine (Lumbar puncture)
Analysis
- Biochemical - protein, glucose, xanthrochromia
- Serological - oligoclonal bands
- Microbiological
Diagnostic uses
- Subarachnoid haemorrhage (red cells)
- Meningitis (bacteria)
- Multiple sclerosis (oligoclonal bands)
Cerebral perfusion pressure (CPP)
- CPP = Mean arterial pressure (MAP) - ICP
- Main determinant of brain perfusion
- Improve perfusion by ^ MAP or v ICP
ICP monitoring
- Insertion of catheter into subarachnoid or ventricular space for
pressure measurement
Conditions that raise ICP
Space occupying lesions (e.g. tumours, haemorrhage)
Hydrocephalus
Acute mountain sickness (cellular oedema)
Benign intracranial hypertension (imbalance CSF production/absorption)
Clinical features
- Early - headache, vomiting
- Mid - Papilloedema
- Late / Severe (brainstem herniation) - decreased conscious state,
pupillary inequality
- Pre-terminal - Cushing's reflex (^BP, v PR), Respiratory depression >
arrest
ICP reduction techniques
- also see hydrocephalus
- Removal of space occupying lesion
- Hyperventilation: ^ alveolar ventilation > v pCO2 > cerebral
vasoconstriction > v blood volume > ICP
- Mannitol or Hypertonic saline: osmotic diuretic > v intracellular water
and volume > v ICP
- Dexamethasone: v cellular swelling > v ICP
- Carbonic anhydrase inhibitor: v bicarbonate diuresis > v ICP
Dural leak
- CSF leakage from subarachnoid space following puncture or tear of dural
covering. Most commonly following lumbar puncture.
- Results in traction on brain and headache.
- Treated by injecting patient's blood over dural defect (blood patch)