Subarachnoid haemorrhage

A subarachnoid haemorrhage (SAH), a type of intracerebral haemorrhage, denotes the presence of blood within the subarachnoid space. 

Epidemiology

Patients tend to be older middle age, typically less than 60 years of age ². Although subarachnoid haemorrhage only accounts for 3% of stroke it results in 5% of stroke deaths ². 

Causes include ¹:

• trauma (with associated cerebral contusion) : traumatic subarachnoid haemorrhage
• spontaneous
  • ruptured berry aneurysm : 85% ¹
  • non-aneurysmal (perimesencepalic) haemorrhage : 10%
  • arteriovenous malformation (AVM)
  • dural arteriovenous fistula (DAVF)
  • spinal AVM
  • venous infarction ¹
  • intradural arterial dissection
  • pituitary apoplexy
  • cocaine use

Risk factors include ²:

• family history
• hypertension
• heavy alcohol consumption
• abnormal connective tissue
  • autosomal dominant polycystic kidney disease (ADPKD)
  • Ehlers-Danlos disease type IV
  • neurofibromatosis type 1 (NF1) 
• female gender: 1.6 x baseline risk
• black race: 2.1 baseline risk
• Japanese or Finnish descent

Clinical presentation

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 the 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. 

Radiographic features

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

CT

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 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 ^{ref needed}. 

MRI

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

Differential diagnosis

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

• pseudo-subarachnoid haemorrhage
• bacterial meningitis
• tuberculous meningitis
• granulomatous meningitis
  • neurosarcoidosis

See also

• cerebral vasospasm (post SAH)