Non-contrast CT Head

The first investigation should be a non-contrast CT scan of the head [11,18].


If there is abnormal neurology or a reduced conscious level this should take place as soon as possible after the index episode. Ideally, this would hold true for all patients as the sensitivity of CT scanning is higher the closer it is performed to the index episode.

The largest studies on ED patients reveal an overall sensitivity of 92-93% though this may be substantially higher, perhaps even 100%, if performed in the first 6 hours [43-45] The sensitivity drops off to about 85% at 3 days and 50% at 1 week [7] as blood diffuses away from the site of haemorrhage.

Interpretation of the results of the CT scan may also be subject to spectrum bias (patients who are neurologically well will likely have smaller bleeds harder to see on CT) and the fact that neuroradiology reporting is superior to that of a general radiologist [46].

CT appearances of SAH

The distribution of blood on the initial CT Head scan can be helpful in distinguishing aneurysmal SAH (Fig 4) from perimesencephalic haemorrhage (Fig 5) [18,19,37].

Fig 4: aneurysmal SAH (click to enlarge) Fig 5: Perimesencephalic haemorrhage (click to enlarge)

Ruptured aneurysms are most often found in the anterior communicating artery (blood in interhemispheric fissure) (Fig 4) followed by the internal carotid artery, middle cerebral artery (blood in the Sylvian fissure) and vertebrobasilar circulation.

About 20% of patients with SAH will have multiple aneurysms so the CT pattern of blood is important in identifying the probable culprit [37].

Patients with perimesencephalic patterns of SAH (blood localised to the midbrain cisterns) do very well [20] with no specific treatment. However, non-contrast CT brain appearances are not unique [20] and CT angiography (CTA) is required in these patients to exclude a ruptured vertebrobasilar aneurysm [47,48].

Learning Bite

All patients with CT-proven SAH should undergo CT or formal angiography to identify the aneurysm responsible or confirm the absence of such in cases of perimesencephalic haemorrhage [47-51].

NICE NG228 states that if a CT head scan is done within 6 hours of symptom onset and is reported and documented by a radiologist as showing no evidence of a subarachnoid haemorrhage:

Do not routinely offer a lumbar puncture.

Think about alternative diagnoses and seek advice from a specialist.

However, if the CT head scan is done more than 6 hours after symptom onset and shows no evidence of a subarachnoid haemorrhage, consider a lumbar puncture.

Learning bite

A negative CT scan alone within 6 hours of onset of symptoms can be enough to exclude SAH.