Subarachnoid haemorrhage

A subarachnoid haemorrhage (SAH) is an intracranial haemorrhage that is defined as the presence of blood within the subarachnoid space, i.e. deep to the subarachnoid layer of the meninges.

The most common cause of SAH is head injury and this is called traumatic SAH. In the absence of trauma, SAH is termed spontaneous SAH. The rest of this note focuses on spontaneous SAH.

Causes of spontaneous SAH include:

• intracranial aneurysm (saccular 'berry' aneurysms)
  • accounts for around 85% of cases
  • conditions associated with berry aneurysms include hypertension,adult polycystic kidney disease, Ehlers-Danlos syndrome and coarctation of the aorta
• arteriovenous malformation
• pituitary apoplexy
• mycotic (infective) aneurysms

Classical presenting features include:

• headache
  • usually of sudden-onset ('thunderclap' or 'hit with a baseball bat')
  • severe ('worst of my life')
  • occipital
  • typically peaking in intensity within 1 to 5 minutes
  • there may be a history of a less-severe 'sentinel' headache in the weeks prior to presentation
• nausea and vomiting
• meningism (photophobia, neck stiffness)
• coma
• seizures
• ECG changes including ST elevation may be seen
  • this may be secondary to either autonomic neural stimulation from the hypothalamus or elevated levels of circulating catecholamines

Investigation

• non-contrast CT head is the first-line investigation of choice
  • acute blood (hyperdense/bright on CT) is typically distributed in the basal cisterns, sulci and in severe cases the ventricular system.
• if CT head is done within 6 hours of symptom onset and is normal
  • new guidelines suggest not doing a lumbar puncture
  • consider an alternative diagnosis
• if CT head is done more than 6 hours after symptom onset and is normal
  • do a lumber puncture (LP)
  • timing wise the LP should be performed at least 12 hours following the onset of symptoms to allow the development of xanthochromia (the result of red blood cell breakdown).
  • xanthochromia helps to distinguish true SAH from a 'traumatic tap' (blood introduced by the LP procedure)
  • xanthochromia is due to red blood cells in the CSF undergoing haemolysis: haemoglobin → oxyhaemoglobin → bilirubin. Spectrophotometry detects bilirubin and oxyhaemoglobin
  • as well as xanthochromia, CSF findings consistent with subarachnoid haemorrhage include a normal or raised opening pressure
• if the CT shows evidence of a SAH
  • referral to neurosurgery to be made as soon as SAH is confirmed