Thunderclap Headache – CT Normal, LP Next — SCE Neurology MCQ

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EasyHeadache & Facial PainThunderclap Headache – CT Normal, LP NextSCE Neurology

A 60-year-old man with a sudden severe headache ('worst headache of my life') has a normal CT head performed 4 hours after onset. What is the next investigation?

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Correct answer: A — Lumbar puncture performed ≥12 hours after headache onset to look for xanthochromia — a normal CT does not exclude SAH; LP sensitivity is highest at 12 hours to 2 weeks

Thunderclap headache (sudden onset, reaching maximum intensity within seconds to minutes) requires exclusion of subarachnoid haemorrhage. CT head is ~98% sensitive for SAH within 6 hours but sensitivity drops over time. If CT is normal (or performed >6 hours after onset), LP must be performed ≥12 hours after onset to allow sufficient time for red blood cell lysis to produce xanthochromia (a yellow discolouration of CSF from bilirubin). Spectrophotometry is used to detect xanthochromia. If both CT and LP are negative, other causes of thunderclap headache should be considered (RCVS, cerebral venous thrombosis, dissection). A: Normal CT does not exclude SAH. C: MRI can help but LP is the standard next step. D: Repeat CT misses SAH. E: CTA detects aneurysms but doesn't confirm SAH.

Reference: RCP Stroke Guidelines; NICE CG176 Head Injury; ABN Headache Guidelines