Objective To measure the sensitivity of modern third generation computed tomography in emergency patients being evaluated for possible subarachnoid haemorrhage, especially when carried out within six hours of headache onset.Design Prospective cohort study.Setting 11 tertiary care emergency departments across Canada, 2000-9.Participants Neurologically intact adults with a new acute headache peaking in intensity within one hour of onset in whom a computed tomography was ordered by the treating physician to rule out subarachnoid haemorrhage.Main outcome measures Subarachnoid haemorrhage was defined by any of subarachnoid blood on computed tomography, xanthochromia in cerebrospinal fluid, or any red blood cells in final tube of cerebrospinal fluid collected with positive results on cerebral angiography. ResultsOf the 3132 patients enrolled (mean age 45.1, 2571 (82.1%) with worst headache ever), 240 had subarachnoid haemorrhage (7.7%). The sensitivity of computed tomography overall for subarachnoid haemorrhage was 92.9% (95% confidence interval 89.0% to 95.5%), the specificity was 100% (99.9% to 100%), the negative predictive value was 99.4% (99.1% to 99.6%), and the positive predictive value was 100% (98.3% to 100%). For the 953 patients scanned within six hours of headache onset, all 121 patients with subarachnoid haemorrhage were identified by computed tomography, yielding a sensitivity of 100% (97.0% to 100.0%), specificity of 100% (99.5% to 100%), negative predictive value of 100% (99.5% to 100%), and positive predictive value of 100% (96.9% to 100%). ConclusionModern third generation computed tomography is extremely sensitive in identifying subarachnoid haemorrhage when it is carried out within six hours of headache onset and interpreted by a qualified radiologist.

nvestigation of the neurologically intact (ie, no neurologic deficits) patient with headache is a potentially difficult clinical decision for physicians. Headache accounts for approximately 2% of all emergency department visits, and subarachnoid hemorrhage is one of the most serious diagnoses, accounting for only 1% to 3% of these headaches. [1][2][3][4] Although the decision to evaluate patients with new neurologic deficits is relatively straightforward, it is much more difficult to determine which alert, neurologically intact patients who present with headache alone require investigations-yet such patients account for half of all subarachnoid hemorrhages at initial presentation. 5 This diagnostic dilemma is illustrated by the report that 5.4% of confirmed subarachnoid hemorrhages were misdiagnosed during the patients' initial emergency department assessment. 6 Patients suspected of having subarachnoid hemorrhage are typically evaluated with an unenhanced computed tomography (CT) scan followed by a lumbar puncture if results of the CT scan are negative. Computed tomography is highly sensitive when performed soon after headache onset. 7 Lumbar puncture can be a painful procedure and can result in a headache that may be worse than the original headache. 8 A clinical decision rule is derived from original research and is defined as a decision-making tool that incorporates 3 or more variables from the history, examination, or simple tests. [9][10][11][12] These rules help clinicians with diagnostic or thera-IMPORTANCE Three clinical decision rules were previously derived to identify patients with headache requiring investigations to rule out subarachnoid hemorrhage.OBJECTIVE To assess the accuracy, reliability, acceptability, and potential refinement (ie, to improve sensitivity or specificity) of these rules in a new cohort of patients with headache. DESIGN, SETTING, AND PATIENTS Multicenter cohort study conducted at 10 university-affiliated Canadian tertiary care emergency departments from April 2006 to July 2010. Enrolled patients were 2131 adults with a headache peaking within 1 hour and no neurologic deficits. Physicians completed data forms after assessing eligible patients prior to investigations. MAIN OUTCOMES AND MEASURES Subarachnoid hemorrhage, defined as (1) subarachnoid blood on computed tomography scan; (2) xanthochromia in cerebrospinal fluid; or (3) red blood cells in the final tube of cerebrospinal fluid, with positive angiography findings.RESULTS Of the 2131 enrolled patients, 132 (6.2%) had subarachnoid hemorrhage. The decision rule including any of age 40 years or older, neck pain or stiffness, witnessed loss of consciousness, or onset during exertion had 98.5% (95% CI, 94.6%-99.6%) sensitivity and 27.5% (95% CI, 25.6%-29.5%) specificity for subarachnoid hemorrhage. Adding "thunderclap headache" (ie, instantly peaking pain) and "limited neck flexion on examination" resulted in the Ottawa SAH Rule, with 100% (95% CI, 97.2%-100.0%) sensitivity and 15.3% (95% CI, 13.8%-16.9%) specificity.CONCLUSIONS ...

Objective To identify high risk clinical characteristics for subarachnoid haemorrhage in neurologically intact patients with headache.