When having a brain aneurysm, do blood vessels become 100% occluded?

No: Aneurysms are due to weakness in the vessels wall, usually at branch points of the vessel. They are similar to blebs in the sidewall of a tire. The bleb grows under increasing pressure and on-going weakness of the wall. In an aneurysm, blood continues to flow, some entering the outpouched aneurysm and exiting, and some may clot along the wall. This latter event is seen in larger aneurysms>smaller.

Rarely: Aneurysms do tend to cause occlusion by increasing turbulence promoting endothelial recruitment of platelets, but the primary concern is rupture of the aneurysm if it is large enough, not clot.

No: An aneurysm is an abnormal dilation or bulging of the wall of the blood vessel. If that dilated segment becomes very fragile and thin it can rupture due to the pressure of the blood and bleeding occurs into the brain. This bleeding can cause spasm of surrounding vessels. But most of the vessels do not become "occluded".

Initially, a sufficiently large aneurysm will rupture, and blood suddenly bursts outwards into the subarachnoid spaces and even into the ventricular system. Commonly, vasospasm emerges in other vessels due to local bleeding. Although scattered strokes may occur, blood vessel occlusion does not play a dominant role in devastating morbidity/ mortality.