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are outpouchings of blood vessels which occur usually where blood
vessels divide. In the brain, when they rupture, they may cause
a subarachnoid hemorrhage, which is bleeding around the surface of
the brain, and under a membrane known as the arachnoid membrane.
Aneurysms are usually located near the base of the brain, and arise
from a protective circle of circulation known as the circle if Willis.
Occasionally aneurysms are located more peripherally in the brain,
and may be associated with infection. These are known as mycotic
aneurysms. Roughly 50 % of patients who suffer an aneurysmal
rupture will die.
Occasionally, aneurysms will leak
a bit, before a subsequent major hemorrhage, This is known
as a sentinel hemorrhage.
Aneurysms may also present with mass
effect, such as compression of the brainstem. This is seen
in the case of giant aneurysms.
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Glossary 
