Neurology for MRCPI Part 2

A 68-year-old man presents with a sudden severe occipital headache reaching maximum intensity in seconds, followed by vomiting and neck stiffness. CT head 8 hours after onset is reported as normal. What is the most appropriate next step?

1. MRI brain with diffusion-weighted imaging Not the standard next step. DWI is for ischaemia; specific blood-sensitive sequences can help but the established pathway after a negative CT beyond six hours is a lumbar puncture.
2. Immediate lumbar puncture now, at 8 hours Too early. Xanthochromia takes around 12 hours to develop reliably, so an early tap risks an uninterpretable result and repeat procedure.
3. Discharge with analgesia, since the CT is normal Wrong and dangerous. A missed subarachnoid haemorrhage carries a high risk of catastrophic rebleeding, and CT alone at this timing is not sufficient to exclude it.
4. Start nimodipine and admit for observation without further investigation Wrong. Nimodipine reduces delayed cerebral ischaemia in CONFIRMED subarachnoid haemorrhage; it is not a substitute for making the diagnosis.
5. Lumbar puncture at least 12 hours after headache onset, examining the CSF for xanthochromia correct Correct. CT sensitivity for subarachnoid haemorrhage falls with time, and a normal scan at 8 hours does not exclude it. Waiting until at least 12 hours after onset allows bilirubin to form from breakdown of red cells, which is what distinguishes true subarachnoid blood from a traumatic tap.

The point: CT within six hours of onset, read by a competent reporter, approaches 100% sensitivity for subarachnoid haemorrhage. Beyond six hours sensitivity falls and a lumbar puncture at 12 hours or more is needed. Xanthochromia by spectrophotometry, not the three-tube test, distinguishes it from a traumatic tap.

Source: NICE NG228 — Subarachnoid haemorrhage caused by a ruptured aneurysm NICE · tier 1, national regulator or guidance