A 55-year-old presents with sudden-onset thunderclap headache reaching peak intensity within 60 seconds. CT head is negative. The next diagnostic step and the gold standard for subarachnoid haemorrhage diagnosis when CT is negative is:
- A Repeat CT head with thin slices in 6 hours
- B MRI brain FLAIR immediately; LP is not required if MRI is negative
- C Lumbar puncture for xanthochromia (spectrophotometry) at >12 hours after headache onset; CT angiography for aneurysm if LP positive ✓
- D Digital subtraction angiography (DSA) immediately as first investigation
Explanation
When CT head (performed within 6 hours) is negative in a patient with thunderclap headache, lumbar puncture for xanthochromia remains the gold standard for diagnosing SAH due to its higher sensitivity than CT beyond 6-12 hours. Xanthochromia (yellow discolouration due to oxyhaemoglobin and bilirubin from haem degradation) is best detected by spectrophotometry, not naked-eye inspection, and develops 2-4 hours after SAH, persisting for up to 2 weeks. If LP confirms SAH, CT angiography (CTA) is performed to identify the causative aneurysm, with DSA reserved if CTA is non-diagnostic.
Reference: Bailey & Love's Short Practice of Surgery, 27th ed.
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Written and medically reviewed by the StethoPrep medical team.