Intracranial Pressure

Adult ICP is normally around 12-15cmH2O (9-12mmHg)

Cerebral perfusion pressure = Mean Arterial Pressure (MAP) – ICP

i.e. BP has a major influence on CPP.

In the treatment of any head injury, the aim is for a CPP of >60mmHg (MAP >80mmHg).

Raised ICP

In any inflammation or space occupying lesion (including haematoma), the ventricles and vessels can become squashed to accommodate the rise in pressure.  However, once this process is exhausted (i.e. no CSF space in the cranial cavity), the rise in pressure will sharply increase.  Raised ICP can be caused by a number of problems:

  • Raised ICP can be caused by a number of pathologies
    • Mass lesions e.g. tumours, abscesses, haematomas, focal oedema (secondary eg. to trauma, infarction, tumour)
    • Generalised oedema e.g. meningoencephalitis; trauma; metabolic (e.g. water intoxication)
    • Obstruction to CSF circulation (obstruction/impaired absorption/central venous obstruction) -> Hydrocephalus

Features

  •  Seizures
    • Usually focal onset +/- generalisation
  • Focal symptoms
    • Progressive loss of function
    • Weakness, numbness
    • Dysphasia
    • Cranial nerve neuropathy
  • Focal signs
    • Unilateral/bilateral 6th nerve palsies
    • Contralateral (then bilateral) 3rd nerve palsy (usually pupil dilation first)
  • Headache worse on lying/straining
  • Nausea/vomiting
  • Papilloedema
  • Bradycardia, hypertension
  • Impaired conscious level and mental state
    • Irritable, coma, stupor
  • Occasionally, behavioural change; features of stroke/TIA

Complications

Intracranial shifts and herniations occur in different areas of the brain (at points of weak boundaries).  In the process, areas of brain may be ‘stripped’ of tissue.  This is called decrtication, and can cause lasting damage.

  1. Uncal- (transtentorial)- can affect fibres of CN III (pupil dilation and failure to constrict and down and out appearance).  It may also cause indentation of the cerebral peduncles (kernohan’s notch) causing ipsilateral hemiparesis.
  2. Central herniation- the diencephalon and parts of the temporal lobes are squashed under the tentorium cerebelli.  It can stretch branches of the basilar artery and cause fatal bleeding if they rupture.  (Radiologically- there is obliteration of the suprasellar cistern).
  3. Cingulate (subfalcine)- the innermost part of the frontal lobe is pushed under the falx cerebri.  It is usually not as serious, but can put pressure on anterior arteries, causing a further rise in ICP (increased likelihood of a more central herniation).
  4. Transcalvarial- ‘external herniation’ either as a result of a fracture or as a therapeutic surgical procedure (may be for raised ICP). (craniectomy)
  5. Upward cerebellar- increased pressure in the posterior fossa can push the cerebellum up.  The midbrain may also be pushed down and through the tentorial notch.
  6. Tonsillar/ Downward cerebellar (coning)/ Chiari malformation- movement of the cerebellar tonsils through the foramen magnum can cause compression of the lower brainstem and upper spinal cord.  Since the brainstem controls respiratory and cardiac function, this herniation is often fatal (coning).  For sx, see Chiari malformation

 

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