Does a normal CT scan within 6?h rule out subarachnoid haemorrhage?

Clinical Scenario
A normally fit and well 26-year-old man presents to the emergency department with a sudden onset headache. It came on 2?h ago, and is the worst he has ever had. He has taken paracetamol without success. The headache made him feel very unwell, but he has no neurological symptoms.

His Glasgow Coma Scale (GCS) is 15 and clinical examination is normal. You are concerned that he may have had a subarachnoid haemorrhage (SAH) and want to rule this out.

He has a CT scan within 6?h of the onset of the headache. It is reported as normal. You wonder if this excludes a diagnosis of SAH.

Comment(s)
Headache is a common presentation to the emergency department, comprising approximately 2% of all attendances. Of these, 7% will have a SAH.

Cerebrospinal fluid analysis has been regarded as essential to successfully exclude a SAH if the CT scan is normal. This dogma has not gone unchallenged, especially when the scan is performed within 12?h.

Being able to rule out SAH in the emergency department using CT scan would be beneficial to patients. It would reduce inpatient admissions to carry out and await results from a lumbar puncture, which is an invasive procedure carrying risks of infection, pain, bleeding and dural puncture headache.

The evidence reviewed, with one exception, supports the use of a CT scan without lumbar puncture if patients present with an acute severe headache, no neurological deficit, and a normal level of consciousness.

For the scan to be diagnostic it must be done within 6?h of the onset of headache and must be reported by an experienced radiologist who regularly reports CT brain scans.

In patients presenting with an absence of headache or with atypical features such as neck pain or stiffness, back pain or loss of consciousness, lumbar puncture is still indicated in the event of a negative CT scan.

All patients with a negative scan more than 6?h after the onset of their headache should have a lumbar puncture after 12?h.

Major international guidelines continue to recommend a lumbar puncture after negative CT brain scans irrespective of their timing.

Clinical decision rules, which may give physicians the confidence to discharge more patients with a negative CT brain scan, are currently undergoing validation.

Clinical Bottom Line
CT scan alone is sensitive enough to rule out subarachnoid haemorrhage in patients presenting with lone acute severe headache, normal level of consciousness, and no neurological features,

if performed within 6?h of onset with a third generation CT scanner with thin slices, and reported by a radiologist experienced in reporting CT brain scans.

Evidence Based Medicine -OCT 2015.

G Mohan.