subarachnoid haemorrhage

Last edited 12/2022 and last reviewed 12/2022

Subarachnoid haemorrhage (SAH) accounts for about 5% of strokes with an annual incidence of 2 to 20 per 100,000 per year

  • in around 80% of people, the bleed arises from rupture of an intracranial arterial aneurysm

  • aneurysmal subarachnoid haemorrhage is slightly more common in women than men, and can occur across a wide range of ages with a median age at presentation between 50 and 60 years
    • prevalence of aneurysms in the general population is roughly 2-5%, (2) greater in those with family history of aneurysms, and/or personal history of Ehlers-Danlos or polycystic kidney disease
    • factors associated with the risk of rupture include
      • hypertension,
      • tobacco use,
      • excessive alcohol use,
      • sympathomimetic drugs,
      • Black race,
      • Hispanic ethnicity,
      • and aneurysmal size > 10 millimeters (mm)
    • aneurysmal SAH is more common in women and in patients 40-60 years old (2)

  • occasionally, the arachnoid layer gives way and a subdural haematoma develops

  • main symptom of subarachnoid haemorrhage is a sudden and severe 'thunderclap' headache
    • 'thunderclap' headache (a sudden severe headache, typically peaking in intensity within 1 to 5 minutes) is a red-flag symptom of subarachnoid haemorrhage (1)
      • thunderclap headache is associated with other conditions or causes such as migraine, cough, coitus or exertion
      • most people with a thunderclap headache do not have a subarachnoid haemorrhage, but this should not deter further investigation if subarachnoid haemorrhage is suspected

    • other possible symptoms include neck stiffness, altered consciousness or seizures
    • is more easily diagnosed in people presenting with severe symptoms, unconsciousness or sudden onset acute headache but some people with subarachnoid haemorrhage present with less severe or non-specific symptoms and signs

  • urgent investigation to confirm a diagnosis of subarachnoid haemorrhage facilitates treatment to prevent rebleeding from the ruptured aneurysm and reduces disability and death

When carrying out an initial assessment in a person who presents with unexplained acute severe headache:

  • have a high index of suspicion for subarachnoid haemorrhage

  • take a careful history to establish the rate of onset and time to peak intensity of the headache

On initial presentation, the one-year mortality of untreated SAH is up to 65% (2)

  • with appropriate diagnosis and treatment, mortality can be reduced to 18%

Refer people with suspected subarachnoid haemorrhage seen outside of acute hospital settings to an emergency department immediately for further assessment

Refer the person for an urgent non-contrast CT head scan if review in secondary care by a senior clinical decision-maker confirms unexplained thunderclap headache, or other signs and symptoms that suggest subarachnoid haemorrhage. Be aware that the diagnostic accuracy of CT head scans is highest within 6 hours of symptom onset.

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