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Does a normal CT scan rule out a subarachnoid haemorrhage?
  1. Simon Carley,
  2. Paul Wallmann
  1. Department of Emergency Medicine, Manchester Royal Infirmary, Oxford Road, Manchester M13 9WL, UK
  1. Correspondence to: Kevin Mackway-Jones, Consultant (kevin.mackway-jones{at}

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Report by Simon Carley Specialist Registrar Search checked by Paul Wallmann Specialist Registrar

Clinical scenario

A 24 year old man who has been previously well presents to the emergency department complaining of headache. He describes the headache as the worst he has ever had. It came on suddenly approximately two hours previously and has not resolved with paracetamol. It was so severe as to cause him to collapse when it started. He has no other neurological symptoms and clinical examination reveals no neurological signs. You are concerned that he may have had a sub-arachnoid haemorrhage and arrange a CT scan. The scan is reported as normal. You wonder if this rules out the diagnosis of subarachnoid haemorrhage in your patient.

Three part question

[In patients presenting with a history of sudden severe headache] is [CT scanning alone as good as CT scanning plus lumbar puncture] in ruling out [sub-arachnoid haemorrhage].

Search strategy

Medline 1966–12/00 using the OVID interface. [(exp subarachnoid hemorrhage OR AND (cerebrospinal fluid OR spinal OR exp spinal puncture OR lumbar OR AND (exp tomography, x-ray computed OR OR computed] LIMIT to human, english AND abstracts.

Search outcome

Altogether140 papers found of which 134 were irrelevant and of insufficient quality for inclusion. The remaining six papers are shown in table 3.

Table 3


Emergency physicians need to know if CT is sensitive enough to rule out the diagnosis of subarachnoid bleeding in patients presenting with severe headache. Subarachnoid haemorrhage is an important diagnosis to make, the risk of re-bleeding is high if the initial bleed is missed and it is a condition for which treatment is possible. We must therefore err on the side of caution and seek investigations with a very high sensitivity to rule out the diagnosis. The use of lumbar puncture (LP) as a gold standard in many of these studies can be questioned as it too has a false negative rate, particularly when performed soon after a bleed. The diagnosis of subarachnoid haemorrhage is so important that sensitivity must approach 100% for CT to obviate the need for LP. The current trials found reveal two interesting facts. (1) That CT has a high sensitivity (91–98%) for detecting subarachnoid haemorrhage, though this is not high enough to satisfactorily exclude subarachnoid haemorrhage. (2) That the sensitivity of CT for subarachnoid haemorrhage decreases with time.

The sensitivity given in the more recent trials is approximately 95%. This is not high enough to rule out subarachnoid haemorrhage. It is more sensitive the earlier it is performed, this is the converse of LP. The advantage of CT is that it quick and easy to perform, may be positive in the early stages of subarachnoid haemorrhage and it may give information on the cause or size of the bleed. It may also exclude a space occupying lesion.

Clinical bottom line

Patients with lone acute severe headache should have urgent CT; if this is negative then a LP should be performed.

Report by Simon Carley Specialist Registrar Search checked by Paul Wallmann Specialist Registrar