When an aneurysm leaks (releases a small amount of blood), it can result in a sudden and severe headache. These headaches are called sentinel or warning headaches. Typically, a brain aneurysm ruptures without warning, so very few people get these sentinel headaches before a rupture.
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Honor Society of Nursing (STTI) answered
A sentinel headache is due to the leakage of blood from a brain aneurysm. Most often there is no warning prior to an aneurysm rupturing but occasionally a small leakage of blood occurs which produces a very sudden and very severe headache. If recognized, this could lead to diagnosis of the aneurysm prior to rupture. This is why a sudden, severe headache should be considered a medical emergency.