There has been a fair bit of back and forth about the validity of early CT in the setting of “thunderclap headache” to obviate a lumbar puncture in the search for aneurysmal subarachnoid hemorrhage. David Newman and Kevin Klauer debated this subject a few years ago – and that was in all-comers, not simply those in the few hours following onset.
This most recent meta-analysis and systematic review gathers together all the published literature regarding early CT and the incidence of SAH on follow-up. Including 8,907 patients from five publications, based on a few assumptions from retrospective studies, there were up to 13 missed cases of aneurysmal SAH occurring despite a negative CT within 6 hours of onset. Worst-case sensitivity based on these data, then, was 0.987 (95% CI 0.971-0.994).
The prevalence of SAH in patients presenting with true thunderclap headache is estimated at ~10%. The post-test odds, then, after a negative CT, are on the order of 0.1% – in line with David Newman’s posit of requiring 1000 LPs to catch one missed SAH. The problem, then, lies in taking the next step in Bayesian reasoning – how likely is the positive LP to be true SAH? If prevalence has dropped to 1 in 1000 after a negative CT, and the specificity of LP for SAH is only 65%, even a positive result barely budges the likelihood of disease.
How do you consent a patient for an invasive procedure in a setting in which a positive result has only the tiniest fraction of a chance of being real – and the treatments based on findings of follow-up examinations may be more likely to harm the patient than the magnitude of benefit associated with detection of a true positive?
“Sensitivity of Early Brain Computed Tomography to Exclude Aneurysmal Subarachnoid Hemorrhage: A Systematic Review and Meta-Analysis”