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.

Dr. James R. Brorson, associate professor of neurology at the University of Chicago Medicine, told Reuters Health by email, “Through a carefully performed meta-analysis, this study underscores the growing evidence for the very high sensitivity of plain head CT scanning for acute subarachnoid hemorrhage (within six hours of onset).”

“By assembling several previously published studies in a systematic review, the authors rather starkly present data that suggest that the current standard practice of requiring lumbar puncture after negative CT when SAH is suspected may no longer be necessary,” he said.

Dr. Brorson suggested that doctors may begin to omit lumbar puncture after negative CT in selected cases.

“It has been thought that small ‘sentinel’ bleeds, or bleeding in patients with low hematocrit, may sometimes produce false-negative CT scans. It would be interesting to know whether patient-specific factors like these might still sometimes consistently produce false-negative scans,” he added.

Dr. Brorson, who was not involved in the study, cautioned that “a danger in emphasizing these results might be if they are extrapolated by practitioners to patients outside the six-hour time window covered in this study, possibly leading to missed cases of subarachnoid hemorrhage first presenting for scanning at later times after the ictus.”

Regarding further study, Dr. Dubosh would like to explore the sensitivity of a noncontrast head CT beyond six hours.

The authors reported no funding or disclosures.