Neurology – subarachnoid haemorrhage assessment after negative CT.
a. Ottawa SAH rule
Applies to alert patients aged at least 15 years with new severe atraumatic headache reaching maximum intensity within 1 hour.
Positive criteria: age 40 years or older, neck pain/stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache peaking within 1 second, or limited neck flexion on examination.
Do not apply with new neurological deficit, prior aneurysm/SAH, known brain tumour, or chronic recurrent headaches.
If no criteria are present, SAH can be ruled out with very high sensitivity in the validated population.
b. Further investigation options
Investigation
Pro
Con
Lumbar puncture
Can diagnose SAH via xanthochromia.
Invasive; some centres lack spectrophotometry access.
CT angiography
Can diagnose aneurysm and alternative vascular causes such as dissection.
Incidental aneurysm rate around 1-2%, which may not explain the headache.
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