To Head CT Scan or Not: The Clinical Quandary in Suspected Subarachnoid Hemorrhage; a Validation Study on Ottawa Subarachnoid Hemorrhage Rule.

Introduction: The Ottawa Subarachnoid Hemorrhage rule (OSR) is a clinical decision tool identified for ruling out subarachnoid hemorrhage (SAH) in those patient above 15 years of age who present to the emergency department (ED) with acute onset atraumatic headache.

Objective: The primary objective of this study was to externally validate the OSR in a single national health service (NHS) setting in the UK and secondly, to compare it with our current practice without using a decision rule.

Method: A retrospective review of computerized medical records was done for all patients registered with headaches from January to December 2016. The data were manually charted on a data sheet from individual patient records. Patients fulfilling the preset inclusion and exclusion criteria as per the OSR were enrolled in the analysis. According to the OSR, if patient had any of the 6 criteria enlisted (age > 40 years, neck stiffness/pain, witnessed loss of consciousness, onset during exertion, thunderclap headache, limited neck flexion on examination), further diagnostic decision was required. All patients were followed up for 6 months on the computer system as it gets highlighted if the patient is represented again to the ED or is deceased.

Results: A total of 737 ED visits with acute headache were reviewed for potential eligibility. Out of these, 649 were estimated to be eligible. On excluding 485 patients that could not meet the predetermined inclusion criteria and 19 patients as per the exclusion criteria, 145 (19.7%) patients were included in the analysis. There were 5 cases of SAH, yielding an incidence of 3.4 % (95% CI 1.3 % - 8.3 %). The sensitivity for SAH was 100% (95% CI, 46.3 % - 100 %); specificity of 44.2 % (95% CI, 36 % - 53 %); positive predictive value of 6.02 % (95% CI 2.2 % - 14.1 %); and negative predictive value of 100% (95% CI, 92.7 % - 100%).

Conclusion: Although being poorly specific, the OSR is a highly sensitive, simple tool for ruling out SAH in alert patients with a headache in ED settings.