Thunderclap headache (worst of life)
Recommended: CT Head without contrast → LP if CT negative is the preferred imaging study for thunderclap headache (worst of life). Non-contrast CT within 6 hours has high sensitivity for subarachnoid hemorrhage.
Recommended study
CT Head without contrast → LP if CT negative
Non-contrast CT within 6 hours has high sensitivity for subarachnoid hemorrhage. LP for delayed presentation or negative CT with persistent suspicion. CTA / DSA after positive findings.
If the default doesn't apply
Beyond 6 hours from onset, CT negative
Lumbar puncture for xanthochromia
No contrast
SAH confirmed on CT
CT
CTA Head (aneurysm localization)
IV contrast
Pregnancy, stable
MRI
MRI Brain + MRA
No contrast
Watch-outs
Sentinel headache history
A 'less-severe' headache days-to-weeks prior may have been a sentinel bleed — image even if symptoms resolved.
Negative CT after 6 hours
Sensitivity drops sharply with time; LP for xanthochromia is still required to exclude SAH unless an alternative diagnosis is established.
Pearls
- Peak intensity within seconds defines 'thunderclap' — distinguishes from migraine and tension headache.
- Modern multidetector CT within 6 hours approaches 100% sensitivity in a competent reader.
- Broaden the differential when SAH is excluded: RCVS, cervical artery dissection, pituitary apoplexy, cerebral venous sinus thrombosis — image the vessels if symptoms persist.