Here's a review from CCJM. I found this section of interest, on the initial diagnostic approach:
Emergency physicians should have a low threshold for ordering noncontrast computed tomography (CT) of the head in patients with even mild symptoms suggesting aneurysmal subarachnoid hemorrhage. Failure to order CT is the most common diagnostic error in this situation.6 CT performed within 6 hours of headache onset is nearly 100% sensitive for this condition,7 but the sensitivity falls to 93% after the first 24 hours and to less than 60% after 5 days.8 In patients who have symptoms highly suggestive of aneurysmal subarachnoid hemorrhage but a normal CT, lumbar puncture is the next diagnostic step.
There are two alternatives to CT followed by lumbar puncture: ie, noncontrast CT followed by CT angiography,9,10 and magnetic resonance imaging followed by magnetic resonance angiography. In patients with suspicious clinical symptoms but negative CT results, CT followed by CT angiography can rule out aneurysmal subarachnoid hemorrhage with a 99% probability.9,10 However, CT followed by lumbar puncture remains the standard of care and carries a class I recommendation in the American Heart Association guidelines for ruling out subarachnoid hemorrhage.5