Stroke
Sudden focal neurological deficit from disrupted blood supply to the brain (ischaemic in the majority from vessel occlusion, haemorrhagic in the rest from vessel rupture), and telling the two apart with an urgent CT head is the single decision that determines whether thrombolysis can be given.
First principles
Two opposite mechanisms produce the same sudden deficit
Ischaemic stroke occurs when a vessel is occluded by thrombus or embolus, cutting off oxygen and glucose to the territory it supplies, so neurons there fail within minutes. Haemorrhagic stroke occurs when a vessel ruptures and blood expands within the fixed cranial vault, causing mass effect, raised intracranial pressure and direct tissue destruction. Both produce a sudden focal deficit but for opposite reasons: one is too little blood, the other is blood in the wrong place, which is why the two cannot be distinguished clinically and imaging must come before any treatment that assumes ischaemia.
Educational content pending clinical review. Not medical advice.