Subarachnoid haemorrhage (SAH) is a type of extra-axial intracranial haemorrhage and denotes the presence of blood within the subarachnoid space.
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Epidemiology
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.
Risk factors
Risk factors include 2:
family history
heavy alcohol consumption
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abnormal connective tissue
female gender: ~1.5x baseline risk 10
African descent: ~2x baseline risk ref
Japanese or Finnish descent 12
Clinical presentation
Patients typically present with a thunderclap headache, described as a sudden-onset headache that is the worst headache of their life. It is often associated with photophobia and meningism. Focal neurological deficits often present either at the same time as a headache or soon thereafter 2.
In a substantial number of patients (almost half 2), it is associated with collapse and decreased or loss of consciousness, even in those patients who subsequently regain consciousness and have a good grade.
Patients can be graded variably depending on which subarachnoid haemorrhage grading system is used locally. Most commonly either using the Hunt and Hess grading system or the WFNS grading system.
Pathology
Three distinct patterns of subarachnoid haemorrhage have been described, each with its own aetiology and treatment/prognostic implications 4:
suprasellar cistern with diffuse peripheral extension
perimesencephalic and basal cisterns
isolated cerebral convexity
Aetiology
Causes include 1:
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spontaneous
ruptured berry aneurysm: 85% 1
ruptured mycotic aneurysm
sympathomimetic drugs (e.g. cocaine)
anticoagulation therapy 9
Radiographic features
Although MRI is thought to be more sensitive, non-contrast CT is frequently performed first due to wider availability. As well as being more sensitive to haemorrhage, MRI has greater sensitivity to the wide range of causative lesions.
A description of the radiographic features of each causative underlying lesion is beyond the scope of this article; these are discussed separately (see above).
CT
The sensitivity of non-contrast 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 hyperdense 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.
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 on unenhanced CT by the Fisher scale. This scale has been updated to the modified Fisher scale, which more accurately correlates the risk of vasospasm.
MRI
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.
In aneurysm-associated subarachnoid haemorrhage, diffusion weighted imaging may demonstrate early ischaemic changes (within 0-3 days) in more than half of all patients 8. Additionally, delayed ischaemia detected on DWI, associated with vasospasm developing 4-21 days after ictus, may develop in about half of all patients 8.
Angiography (DSA)
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:
higher spatial resolution: better able to delineate small vessels and characterise vascular morphology (e.g. aneurysm neck and incorporation of adjacent vessels)
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 or dural arteriovenous fistulas)
Additionally, depending on the cause, endovascular therapy (e.g. aneurysm coiling) may be appropriate.
Treatment and prognosis
Treatment will vary according to the underlying cause.
Complications
Potential complications encountered are:
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elevated intracranial pressure
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often require ICP monitoring
serial non-invasive screening is possible in equivocal cases using sonographic indices of elevated ICP
hydrocephalus may require extraventricular drain placement
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cerebral vasospasm causing delayed cerebral ischaemia
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)
neurogenic pulmonary oedema
Prognosis varies greatly depending on:
cause of subarachnoid haemorrhage
grade of subarachnoid haemorrhage
presence of other injuries/pathologies/co-morbidities
A small amount of traumatic subarachnoid haemorrhage or small perimesencephalic blood 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.
Differential diagnosis
It is important to realise that apparent hyperdensity in the subarachnoid space is not pathognomonic of subarachnoid haemorrhage. Other diagnostic possibilities include:
cortical laminar necrosis: affects the cortex, not the cisterns
post-thrombectomy iodine extravasation vs haemorrhagic transformation of ischaemic stroke extending to cisterns differentiated on DECT iodine map
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