A 60-year-old hypertensive patient presents with sudden severe 'thunderclap' headache. CT brain is negative for haemorrhage. Lumbar puncture shows xanthochromia (yellow CSF supernatant) at 12 hours. The most important next investigation is:

• A Repeat CT brain with contrast
• B MRI brain with FLAIR sequence
• C EEG to rule out seizure disorder
• D CT angiography of cerebral vessels to identify aneurysm ✓

Explanation

Xanthochromia on lumbar puncture (performed at ≥12 hours after headache onset) confirms subarachnoid haemorrhage (SAH) in a CT-negative presentation (sensitivity of CT for SAH decreases from 98% at <6 hours to ~85–90% at 24 hours). Once SAH is confirmed, CT angiography (CTA) is the first-line imaging to identify the causative aneurysm (present in ~85% of non-traumatic SAH). Digital subtraction angiography (DSA) is the gold standard if CTA is negative but clinical suspicion remains high. Early aneurysm identification determines surgical (clipping) or endovascular (coiling) treatment to prevent re-bleed (highest risk in first 24 hours).

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

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Written and medically reviewed by the StethoPrep medical team.