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The pertinent investigations for subarachnoid haemorrhage are:

  • a CT scan: the investigation of choice:
    • if positive, perform angiography
    • if negative, consider MRI if available or lumbar puncture
  • lumbar puncture is considered if CT scan is normal - a lumbar puncture must not be performed if there are features of raised intracranial pressure. If performed within 6-12 hours then CSF is uniformly blood-stained. If performed between 12 hours and 2 weeks after initial headache then the supernatant is xanthochromic.
  • attribute to a non-haemorrhagic "thunderclap" headache if both CT and CSF are negative.
  • if angiography is negative despite a xanthochromic CSF, then MRI is indicated

Last reviewed 06/2020