Subarachnoid Haemorrhage – SAH
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A subarachnoid haemorrhage (SAH) describes bleeding in the brain into the subarachnoid space. A subarachnoid haemorrhage is considered a type of hemorrhagic stroke.  The bleeding occurs as the result of rupture of aneurysm (80%) and AV malformations (15%). In the remainder of cases, no cause can be identified.
  • Cerebral aneurysms are present in about 2% of the population
  • Previously thought to be congenital, now they are believed to be acquired, but their cause is unknown
  • Trauma is also a major cause, but is not considered true SAH.
The classical sign of a subarachnoid haemorrhage is a sudden onset (“thunderclap”) intense headache (“feels like I’ve been hit on the back of the head”). Most patients are aged under 60 years, with a mean age of onset at 50.

Early diagnosis and management reduces long term disability and death – the diagnosis is time critical.

Diagnosis is usually made with a plain CT of the brain, but in patients whose symptoms started more than 6 hours ago the blood cannot always be seen, and if the CT is negative – lumbar puncture should be performed. The presence of blood confirms the diagnosis.

Epidemiology and Aetiology

  • Incidence is about 8 per 100 000
    • About 5% of all strokes
  • Risk factors
    • These are very similar to the risk factors for all causes of stroke, and include:
    • ↑BP
    • Smoking
    • Known aneurysm – Or disease that causes aneurysm – e.g. polycystic kidney disease, co-arctation of the aorta, Ehlers-Danlos Syndrome
    • Family history – increases risk by 3-5x
    • Increased alcohol intake – increases risk by 2x
    • Female gender has slightly increased risk (1.6x)
    • Afro-Caribbean ethnicity increases risk (2x)
    • Also higher incidence in Finland and Japan
  • Incidence is declining in the last 40 years
    • Probably due to declining rates of smoking and better control of hypertension
  • Mean age of onset is 50
  • 85% cases of SAH are due to arterial aneurysms
    • 10% from non-aneurysms
    • 5% from other vascular malformations

The role of cerebral aneurysms

  • About 2% of the population have a cerebral aneurysm – most of these are completely asymptomatic
  • 90% of aneurysms are <1cm – these have a low risk of bleeding and are unlikely to cause localised effects
    • Some larger aneurysms (about 5% of all aneurysms) can cause mass effects by pressing on local brain structures with can lead to focal neurological symptoms, or seizures
    • The bigger the aneurysm the bigger the risk of bleeding
  • Up to 90% of aneurysms are located in the anterior circulation (internal carotid, middle cerebral arteries)

Signs and Symptoms

You should have a low threshold of suspicion for SAH in patients with sudden onset headache. Early treatment prevents rebelled and is associated with lower risk of long-term disability and death. Making the diagnosis is time critical. Patients seen outside of the emergency department should be referred to hospital immediately for assessment.

  • Sudden onset severe headache, often at the back of the head
    • Known as thunderclap headache
    • The headache reaches its maximum severity within 1-5 minutes of onset
    • Most patients with thunderclap headache do not have SAH. Other causes include migraine, cough, coitus headache
  • Neck stiffness – Kernig’s sign may be present after 6 hours (due to chemical meningitis), or in massive bleeds it may occur sooner due to herniation of the cerebellar tonsils (due to raised ICP).
    • Sometimes the chemical meningitis can cause vomiting and extensor plantar responses after 24hrs
  • Photophobia
  • Nausea and vomiting
  • Impaired consciousness (drowsiness / coma) – usually occurs very shortly after the onset of symptoms, but can occur several hours later.
  • Cranial nerve signs
  • Hemiplegia
    • CNS deficits can become permanent within minutes. If it lasts more than several hours it is highly unlikely to ever resolve.
  • Sentinel headache – is experienced by about 6% of patients, and is a prodromal headache thought to be the result of a small leak before rupture of an aneurysm or malformation.


Mortality (%)
Neck stiffness, cranial nerve lesions
 ± Drowsiness
± Hemiplegia
Prolonged coma


  • Overall mortality is 35-50%
    • About 30% die within a few days, and another 10-15% within a few weeks.
  • Most patients die within 1 month
  • After 1 month, 90% of patients will survive >1 year
  • Prognosis is worse with aneurysm, better with AMV, and best when no lesion is detected (presumably the lesion was small, and has healed in these instances).


  • The period of haemorrhage is actually very short – and it normally stops bleeding by itself.
  • After the initial haemorrhage, patients are at risk from vasospasm. This can causes ischaemia, which can result in secondary brain damage and further neurological signs.
    • 25% of patients will have signs of TIA / stroke after SAH as a result of vasospasm
    • Brain oedema and risk of vasospasm is greater between 72hrs and 10 days
  • Secondary acute hydrocephalus often occurs
  • Re-bleeding is VERY COMMON and usually occurs within 7 days. The risk thereafter is about 3% / year


Diagnosis is by CT, or if this is normal with a high sense of suspicion, CSF.

CT – is able to detect >90% of lesions within 48 hours of onset of symptoms.

  • Often star shaped lesion on CT – or the blood fills in giral patterns around the brain the ventricles
  • If CT is negative, but SAH is highly suspected, consider lumbar puncture, or CT angiogram of the brain.
Subarachnoid haemorrhage on seen on CT scan. Hyperdense material is seen filling the subarachnoid space - most commonly - as in this example - around the circle of Willis. This is the white-ish area in the centre of the image in a star-like shape.
Subarachnoid haemorrhage on seen on CT scan. Hyperdense material is seen filling the subarachnoid space – most commonly – as in this example – around the circle of Willis. This is the white-ish area in the centre of the image in a star-like shape.


Lumbar puncturecontraindicated in raised ICP – so be careful! – Several features:

  • Blood – detected via the presence of bilirubin. Previously, people would use the level of RBCs as an indicator, however, it is unreliable to use the rule: if blood remains constant in 3 separate samples = SAH, if blood declines = tap trauma.
  • Xanthochromia – yellow appearance of CSF if it is left to stand for a few hours


  • Get specialist help! – call the Medical Registrar, the Neurosurgeon on call, and depending on GCS, call the anaesthetist. (Patients are unable to maintain a competant airway at GCS <8)
  • Stablise the patient haemodynamically
  • Many patients require ICU treatment
  • Neurosurgical interventions are limited, but in severe cases, a drain may be placed.
  • After the acute presentation, the patient may have their aneurysm clipped or coiled to prevent further bleeds


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Dr Tom Leach

Dr Tom Leach MBChB DCH EMCert(ACEM) FRACGP currently works as a GP and an Emergency Department CMO in Australia. He is also a Clinical Associate Lecturer at the Australian National University, and is studying for a Masters of Sports Medicine at the University of Queensland. After graduating from his medical degree at the University of Manchester in 2011, Tom completed his Foundation Training at Bolton Royal Hospital, before moving to Australia in 2013. He started almostadoctor whilst a third year medical student in 2009. Read full bio

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