Investigations

• diagnose a subarachnoid haemorrhage if the non-contrast CT head scan shows blood in the subarachnoid space (1)
  
  
• a CT scan: the investigation of choice:
  • if positive, perform angiography
  • if negative, consider MRI if available or lumbar puncture
  • if a CT head scan done within 6 hours of symptom onset and reported and documented by a radiologist shows no evidence of a subarachnoid haemorrhage:
    • do not routinely offer a lumbar puncture
    • think about alternative diagnoses and seek advice from a specialist
  • if a CT head scan done more than 6 hours after symptom onset shows no evidence of a subarachnoid haemorrhage, consider a 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
  • NICE state that allow at least 12 hours after symptom onset before doing a lumbar puncture to diagnose a subarachnoid haemorrhage (1)
    • if the CT head scan is done more than 6 hours after symptom onset, the evidence showed that diagnostic accuracy is reduced and false-negative results are more likely
    • when a lumbar puncture is indicated, then it should be done at least 12 hours after symptom onset, when bilirubin formation is sufficient to be detected reliably
      
    • diagnose a subarachnoid haemorrhage if the lumbar puncture sample shows evidence of elevated bilirubin (xanthochromia) on spectrophotometry
    • think about alternative diagnoses if the lumbar puncture sample shows no evidence of elevated bilirubin (xanthochromia) on spectrophotometry
      
• 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

Reference:

• NICE (November 2022).Subarachnoid haemorrhage caused by a ruptured aneurysm: diagnosis and management