Occasionally they are familial. Approximately two thirds rupture before death and one third of these will die within 72 hours of the initial haemorrhage. A further third die or become disabled due to complications of rupture. Massive haemorrhage can lead to coma and death within minutes. They almost always occur at points of bifurcation of the main intracranial arteries where there are frequently gaps in the muscle coat and where there is a tendency for the formation of fibrous pads within the intima.
Haemodynamic stress is also greatest at points of bifurcation.
There is an increased tendency for saccular aneurysms to occur in association with arteriovenous malformations, where there is increased regional blood flow. They are more common in women than men. Smoking and alcohol intake increase the incidence of rupture. These usually occur in the intracranial part of the internal carotid artery and the basilar artery and are usually seen in elderly patients with advanced cerebral atherosclerosis. They may result in transient ischaemic attacks, infarction due to thrombosis, or haemorrhage.