Subarachnoid haemorrhage

Subarachnoid haemorrhage (SAH) is one of the types of extra-axial intracranial haemorrhage and denotes the presence of blood within the subarachnoid space.

Patients tend to be older middle age, typically less than 60 years old 2. Subarachnoid haemorrhage accounts for 3% of stroke and 5% of stroke deaths 2.

Patients typically present with a thunderclap headache, usually the worst headache of their lives. It is often associated with photophobia and meningism. In a substantial number of patients (almost half 2), it is associated with collapse and loss of consciousness, even in those patients who subsequently regain consciousness and have a good grade.

Focal neurological deficits often present either at the same time as a headache or soon thereafter 2.

Patients can be graded into 5 groups based on their clinical presentation, using the commonly employed Hunt and Hess grading system, which is predictive of outcome.

Three distinct patterns of subarachnoid haemorrhage have been described each with their own aetiology and treatment/prognostic implications 4:

  1. suprasellar cistern with diffuse peripheral extension.
  2. perimesencephalic and basal cisterns.
  3. isolated cerebral convexity.

Causes include 1:

Risk factors

Risk factors include 2:

Although MRI is thought to be more sensitive to the presence of subarachnoid blood than CT, as well as having greater sensitivity to the wide range of causative lesions, logistics and limited access mean that in the vast majority of cases a CT of the brain is obtained as the first investigation.

A description of the radiographic features of each causative underlying lesion is clearly beyond the scope of this article; these are discussed separately (see above).


The sensitivity of CT to the presence of subarachnoid blood is strongly influenced by both the amount of blood and the time since the haemorrhage.

The diagnosis is suspected when a hyperattenuating material is seen filling the subarachnoid space. Most commonly this is apparent around the circle of Willis, on account of the majority of berry aneurysms occurring in this region (~65%), or in the Sylvian fissure (~30%) ref needed.

Small amounts of blood can sometimes be appreciated pooling in the interpeduncular fossa, appearing as a small hyperdense triangle, or within the occipital horns of the lateral ventricles 5.

Subarachnoid haemorrhages are grouped into four categories according to the amount of blood by the Fischer scale. This scale has been updated to the modified Fisher scale, which more accurately correlates the risk of vasospasm.


MRI is sensitive to subarachnoid blood and is able to visualise it well in the first 12 hours typically as a hyperintensity in the subarachnoid space on FLAIR 3.

Susceptibility weighted sequences are also exquisitely sensitive to blood products.

MR angiography and MR venography are also able to detect a causative aneurysm or another source of bleeding, although in general MRI suffers from poor availability (compared to CT), longer scan times and greater difficulty in transferring and looking after patients who are often unstable and intubated.

DSA: angiography

Digital subtraction catheter angiography remains the gold standard for diagnosis and characterisation of vascular abnormalities and in many centres, even if the causative lesion is identified on MRA or CTA and it is thought to require surgical management, a catheter study is carried out. The benefit of DSA is two-fold:

  1. higher spatial resolution: better able to delineate small vessels and characterise vascular morphology (e.g. aneurysm neck and incorporation of adjacent vessels).
  2. temporal resolution: contrast can be seen to wash into and out of vascular malformations, giving important information in regards to the feeding vessels (e.g. arteriovenous malformations (AVM) or dural arteriovenous fistulas (DAVF))

Additionally, depending on the cause, endovascular therapy (e.g. aneurysm coiling) may be appropriate.

Treatment will vary according to the underlying cause, however, regardless of the source of subarachnoid blood, a number of treatment principles and potential complications are encountered:

  • elevated intracranial pressure
    • often require ICP monitoring
    • hydrocephalus may require extraventricular drain placement
  • cerebral vasospasm
    • triple H therapy (Haemodilution, Hypertension, Hypervolaemia)
    • calcium channel blockers (e.g. nimodipine)
    • endovascular intervention (e.g. intra-arterial delivery of vasodilating agents (such as NO) and/or balloon angioplasty)
  • hyponatremia
  • coronary spasm
  • neurogenic pulmonary oedema

Prognosis varies greatly depending on:

  • cause of subarachnoid
  • grade of subarachnoid
  • presence of other injuries/pathologies/co-morbidities

A small amount of traumatic subarachnoid haemorrhage or small peri mesencephalic bleeds has an excellent prognosis with little if any significant long-term sequelae. A grade V aneurysmal subarachnoid, on the other hand, has a dismal prognosis, despite aggressive treatment.

It is important to realise that apparent hyperdensity in the subarachnoid space is not pathognomonic of subarachnoid haemorrhage. Other diagnostic possibilities include:

Stroke and intracranial haemorrhage
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Article information

rID: 2119
Section: Pathology
Synonyms or Alternate Spellings:
  • Subarachnoid haemorrhage (SAH)
  • SAH
  • Subarachnoid hemorrhage
  • Subarachnoid hemorrhage (SAH)

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Cases and figures

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    Figure 1: subarachnoid haemorrhage
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    Case 1: from left PCOM anrurysmal rupture
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    Figure 2: gross pathology
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    Case 2: perimesencephalic haemorrhage
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    Case 3: traumatic SAH
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    Case 4: from ACOM berry aneurysmal rupture
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    Case 5
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    Subarachnoid bleed
    Case 6: traumatic
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    Non-aneurysmal pe...
    Case 7: non-aneurysmal perimesencephalic SAH
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    Traumatic blood
    Case 8: with concurrent subdural haemorrhage
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    Open skull fractu...
    Case 9: with open skull fracture
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    Case 10: traumatic
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    Case 11
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    Case 12
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    Case 15
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