V. Subarachnoid Hemorrhage and Intracranial Aneurysms

Aneurysms are abnormal localized dilatations in arteries. The most common non-traumatic cause of clinically significant subarachnoid hemorrhage is rupture of a berry aneurysm.

A. Characteristics of berry aneurysms (saccular or congenital aneurysms; most common type)

1. Prevalence:  Aneurysms occur in about 1-5% of the population (most of these are asymptomatic).

2. Structure: The walls are composed of adventitia and intima only; the muscular and elastic coats are absent.

3. Most common sites: In adults, aneurysms most commonly occur at bifurcations of vessels at the base of the brain, especially in the anterior circulation 

4. Sequelae: Rupture is the most common sequel, with subarachnoid hemorrhage occurring most often. In addition, aneurysm enlargement may exert local pressure on adjacent structures.