Subarachnoid hemorrhage (SAH)

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How the diagnosis is reached

What matters in the history is not that the headache is severe but how fast it arrived. Maximum intensity within seconds during exertion is a thunderclap headache, and that speed is the feature that separates subarachnoid haemorrhage from migraine and meningitis, both of which build over hours. Meningismus and photophobia follow because blood in the subarachnoid space irritates the meninges just as infection does, which is why the two get confused when the tempo is not asked about. The CT timing is the point worth carrying away. Non-contrast CT approaches complete sensitivity inside six hours, because blood has not yet degraded, so a scan this early that is unequivocally positive removes the need for a lumbar puncture; past six hours a negative scan proves much less and the puncture returns to look for xanthochromia. Two separate dangers then drive treatment. Rebleeding is the immediate one and is often fatal, so the aneurysm is secured within 24 to 72 hours, by endovascular coiling where the anatomy permits and surgical clipping where it does not. Delayed cerebral ischaemia from vasospasm is the second, arriving days later, and every patient receives oral nimodipine, a dihydropyridine calcium channel blocker, for 21 days. Its oddity is worth knowing: it improves neurological outcome without measurably changing the vasospasm visible on angiography, so it is almost certainly working on the microcirculation rather than the large vessels.

Key clinical clues

• Thunderclap headache reaching maximal intensity within seconds
• Meningismus and photophobia following exertion
• Non-contrast CT showing blood in the basal cisterns
• CT angiography identifying an anterior communicating artery aneurysm
• Nimodipine given alongside securing the aneurysm to prevent vasospasm-related ischemia

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