Subarachnoid haemorrhage
Sudden rupture of a blood vessel, most often a berry aneurysm, into the subarachnoid space causes an instantaneous, maximal-at-onset thunderclap headache as blood irritates the meninges and abruptly raises intracranial pressure, making the pattern of onset itself the diagnostic clue.
First principles
The headache is maximal at onset because the event itself is instantaneous
Unlike migraine or tension headache, which build up, a subarachnoid haemorrhage is a sudden mechanical rupture, with arterial blood under systemic pressure forced into the CSF-filled subarachnoid space in seconds. The headache is therefore severe from the very first second, classically described as being hit or the worst headache of a patient's life, because pain onset tracks the vascular event itself rather than a gradually building process. This thunderclap pattern, more than the severity alone, is the key discriminating feature.
Educational content pending clinical review. Not medical advice.