A 55-year-old man presents with sudden-onset thunderclap headache. CT brain is performed 2 hours after onset and is negative for haemorrhage. What is the MOST appropriate next investigation to exclude subarachnoid haemorrhage (SAH)?

• A Repeat CT brain at 24 hours
• B MRI brain with FLAIR sequence immediately
• C Lumbar puncture (LP) for xanthochromia (performed at least 12 hours after headache onset) ✓
• D CT angiography of the cerebral vessels

Explanation

When CT brain is negative within 6–12 hours of thunderclap headache, the next investigation is lumbar puncture (LP) performed at least 12 hours after symptom onset to allow breakdown of haemoglobin to oxyhaemoglobin and then bilirubin (xanthochromia). Xanthochromia on spectrophotometry is highly sensitive for SAH when CT is negative. LP performed too early (<12 hours) may miss xanthochromia. MRI-FLAIR is increasingly used but is less well validated for very early SAH than CT. CT angiography may identify an aneurysm but does not confirm haemorrhage. LP remains the gold standard confirmatory investigation when CT is negative.

Reference: Bailey & Love's Short Practice of Surgery, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.