Subarachnoid haemorrhage in the ED… pushing the boundary of negative CT out to 24 hours?

On non-contrast head CT, a SAH is usually found by identifying the presence of bright white blood in the subarachnoid space. But over time, the blood becomes isodense and becomes more difficult to find. As such, we are generally happy to exclude SAH if we have a negative non-contrast head CT performed within 6 hours of headache onset. But what about a longer time window… is a CT adequately sensitive to exclude SAH up to 24 hours?

These authors from the UK sought to answer this question by conducting a prospective multicentre cohort study of consecutive patients presenting to the ED with acute headache.

Investigation, diagnosis and management were conducted using their standard practice. All patients were followed up for 28 days to see if they missed any haemorrhages.

3663 patients were enrolled. About 90% got a CT and one third got a lumbar puncture after negative CT. Prevalence of SAH was 6.5% (n=237).

What were the results?

A CT withing 6 hours of headache onset was 97% sensitive with a negative predictive value of 99%. CT within 24 hours had a sensitivity of 94% and a post-test probability of disease of less than 1%. The authors believe this data will inform clinicians and patients about the need (or not) for further investigation after a negative CT.

Although I would very much like to believe these conclusions, there is one major concern with the methodology.

Incorporation bias.

The head CT, the test that they are trying to determine was good or not, formed a crucial part of the gold standard. An alternative title to the paper could have been, “We did a head CT to excluded SAH, it was negative, and we believed it.”

To be fair to the authors, it would be unfeasible and not ethical to have performed angiograms and lumbar punctures on all patients to exclude SAH. And their 28 day follow up was probably a reasonable surrogate to find missed SAH’s.

What should we conclude?

This study provides some evidence that we might be able to push the time window of CT a bit further. But I doubt the quality of this study will be enough to change guidelines. Unfortunately, there still is no right answer, and we will continue to engage in shared decision-making discussions with our patients.

 

Covering:

Trainee Emergency Research Network (TERN). Subarachnoid haemorrhage in the emergency department (SHED): a prospective, observational, multicentre cohort study. Emerg Med J. 2024 Oct 4:emermed-2024-214068. doi: 10.1136/emermed-2024-214068. [Link to article]