Written by Sam Parnell
The prevalence of subarachnoid hemorrhage (SAH) for patients presenting with sudden onset headache after a negative head CT is less than 1%, and the time, resources, and risks associated with lumbar puncture (LP) are not insignificant. Therefore, LP may not routinely be needed to rule out SAH after a negative head CT, even if the scan is performed more than 6 hours from symptom onset.
Why does this matter?
The presentation of sudden onset “thunderclap” headache is concerning for SAH as well as other acute neurologic emergencies (reversible cerebral vasoconstriction syndrome, cerebral venous sinus thrombosis, cerebral artery dissection, pituitary apoplexy, etc.). I was taught that the standard approach to diagnose SAH was CT head and then LP if the CT is negative. Evidence suggests that a negative result from a modern CT scanner within 6 hours of symptom onset effectively rules out SAH. But what about patients who have a negative head CT completed more than 6 hours from symptom onset? We covered one approach that advocated shared decision making recently. Is an LP still required for all of these patients?
Working up SAH can be such a headache…
The original study establishing the high sensitivity of head CT within 6 hours for SAH was published by Perry et al. in BMJ in 2011. In this study, a CT performed more than 6 hours from symptom onset had sensitivity of 87.5% for SAH compared to 100% for CT performed within the first 6 hours. Even so, only 0.8% of patients with a negative head CT were ultimately diagnosed with SAH, and CT more than 6 hours from symptom onset still yields a negative likelihood ratio of 0.07. Potential causes of false-negative head CT results for SAH include:
Older-generation CT scanners (second generation or older)
CT slices > 5 mm when imaging through the base of the brain
Artifact owing to metal in the surrounding bone or soft tissue
Interpreting radiologist is not an attending physician
Hematocrit level < 30%
In addition, lumbar puncture is an imperfect test to confirm SAH. No combination of xanthochromia or RBCs in the CSF has sensitivity and specificity greater than 90% to diagnose SAH, and traumatic taps (which occur in 10-30% of patients) can further muddy the waters and lead to false positive results. Furthermore, LP is a time and resource intensive procedure that has significant risks including post-LP headache and rare but serious complications such as epidural hematomas and infections.
Some patients may still need a LP after a negative head CT, especially if there are high-risk clinical features or other considerations such as meningitis or encephalitis. However, considering the baseline low prevalence of SAH among headache patients, the very low posttest probability of SAH with a negative head CT (even after 6 hours), and the inherent risks of LP, most patients likely do not need a LP after a negative head CT to rule out SAH. Instead, shared decision making and consideration of alternative diagnostic modalities such as CTA and MRI should be considered after a negative head CT for select high risk patients with sudden onset headache.
Lumbar Puncture Should Not Be Routinely Performed For Subarachnoid Hemorrhage After A Negative Head CT. Ann Emerg Med. 2021 Jun;77(6):643-645. doi: 10.1016/j.annemergmed.2020.11.018.