In patients with thunderclap headache and normal neurological exam, a normal brain computed tomography (CT) scan can rule out aneurysmal subarachnoid hemorrhage (SAH), according to new research.

A negative brain CT scan within six hours of thunderclap headache onset in patients with a normal neurological exam is highly sensitive in ruling out aneurysmal SAH when the CT scan is technically adequate and is interpreted by an experienced radiologist, the researchers say.

“I think the most important finding is our extremely low calculated miss rate—one to two in 1,000 patients—when applying the ‘six-hour rule’ in the appropriate clinical setting,” corresponding author Dr. Nicole M. Dubosh, of Beth Israel Deaconess Medical Center in Boston, told Reuters Health by email.

“Clinicians must always consider their testing threshold in the workup of any diagnosis, and it is important to recognize that this miss rate is actually lower than many of those that are commonly accepted for other life-threatening diagnoses,” she advised.

Dr. Dubosh and colleagues conducted a comprehensive literature search of original studies of adults presenting with a history concerning for spontaneous SAH and who had noncontrast brain CT scans using modern scanners with 16-slice technology or greater within six hours of headache onset. Their goal was to determine the sensitivity of brain CT to exclude SAH in neurologically intact patients.

Of the 882 titles they reviewed, five articles comprising an estimated 8,907 patients met their inclusion criteria in a meta-analysis. Among these, 13 cases had a missed SAH (incidence 1.46 per 1,000) on brain CT within six hours, the researchers reported in Stroke.

The overall CT sensitivity was 0.987, the specificity was 0.999; and the pooled likelihood ratio of a negative CT was 0.010.

“We decided to perform this meta-analysis because a growing body of evidence strongly supports the extremely high sensitivity of CT alone in diagnosing early aneurysmal subarachnoid hemorrhage, despite the variability in the five studies we included. Synthesizing all the best available evidence is important for the practicing clinician in real-time decision making,” said Dr. Dubosh.

“I hope that by eliminating the need for lumbar puncture in the early workup of aneurysmal subarachnoid hemorrhages, we will have a decrease in the potential complications associated with this procedure and the additional tests that traumatic taps necessitate,” she said.

Dr. Dubosh hopes that the application of these results will lead to decreased spending on unnecessary testing and emergency department stays for these patients.