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.


  • Incidence: 8.00 cases per 100,000 person-years
  • Peak incidence: 40-50 years
  • Sex ratio: more common in females 2:1
Condition Relative
Subarachnoid haemorrhage1
Idiopathic intracranial hypertension0.25
Central venous thrombosis0.11
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+


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

Clinical features

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


  • 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).
    • 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

After spontaneous SAH is confirmed, the aim of investigation is to identify a causative pathology that needs urgent treatment:
  • CT intracranial angiogram (to identify a vascular lesion e.g. aneurysm or AVM)
  • +/- digital subtraction angiogram (catheter angiogram)


Management of a confirmed aneurysmal subarachnoid haemorrhage
  • supportive
    • bed rest
    • analgesia
    • venous thromboembolism prophylaxis
    • discontinuation of antithrombotics (reversal of anticoagulation if present)
  • vasospasm is prevented using a course of oral nimodipine
  • intracranial aneurysms are at risk of rebleeding and therefore require prompt intervention, preferably within 24 hours
    • most intracranial aneurysms are now treated with a coil by interventional neuroradiologists, but a minority require a craniotomy and clipping by a neurosurgeon


Complications of aneurysmal SAH:
  • re-bleeding
    • happens in around 10% of cases and most common in the first 12 hours
    • if rebleeding is suspected (e.g. sudden worsening of neurological symptoms) then a repeat CT should be arranged
    • associated with a high mortality (up to 70%)
  • hydrocephalus
    • hydrocephalus is temporarily treated with an external ventricular drain (CSF diverted into a bag at the bedside) or, if required, a long-term ventriculoperitoneal shunt
  • vasospasm (also termed delayed cerebral ischaemia), typically 7-14 days after onset
    • ensure euvolaemia (normal blood volume)
    • consider treatment with a vasopressor if symptoms persist
  • hyponatraemia (most typically due to syndrome inappropriate anti-diuretic hormone (SIADH))
  • seizures


If SAH is untreated the prognosis is often poor: around 50% within 1 month of the haemorrhage, and of those who survive the first month, 50% will remain dependent for help with activities of daily living. However, survival improves to 85% in patients with confirmed SAH admitted to a specialist neurosurgical unit.

Important predictive factors in SAH:
  • conscious level on admission
  • age
  • amount of blood visible on CT head