Distribution and Clinical Manifestations of Congenital Aneurysm Rupture

Saccular (berry) aneurysms are the most common cause of spontaneous SAH. Nonsaccular aneurysms include fusiform (dolichoectatic), dissecting, infectious (mycotic), neoplastic, and post-traumatic lesions. The dolichoectatic and dissecting aneurysms that are associated with SAH are usually in the posterior circulation. Mycotic infectious and neoplastic aneurysms usually are found in distal branch arteries. Perimesencephalic hemorrhage, which is confirmed by the presence of blood in the spaces around the brainstem detected by computed tomography (CT), is an alternative diagnosis to a ruptured aneurysm.

Saccular aneurysms are found in approximately 2% to 5% of adults, and in most cases, persons live their entire lives without having symptoms secondary to these aneurysmal lesions. The locations of saccular aneurysms are at sites with a predilection for hemodynamic stress, namely at the bifurcations of major intracranial arteries. Approximately 85% of berry aneurysms arise adjacent to the circle of Willis; the most common locations are the anterior communicating artery, bifurcation of the middle cerebral artery, or the junction of the internal carotid artery and posterior communicating artery.

The absence of a second layer of internal elastic lamina in intracranial arteries probably plays a role in the evolution of the aneurysm. The wall of a saccular aneurysm contains intima, media, and adventitia, and the thickness of the wall may be very thin, particularly at the dome. Although the aneurysms often are quite large, the neck of the aneurysm (area adjacent to the parent artery) may be quite small. There is growth of the aneurysm during adulthood.

SAH and aneurysms affect men and women of all ethnic groups. The frequency of aneurysmal SAH is low in children. Although a ruptured aneurysm may cause SAH in adults of any age, the peak age for the illness is the sixth decade. The risk of subarachnoid hemorrhage is increased in individuals who smoke or have hypertension; the smoking association is especially strong in women and in those taking oral contraceptives. The use of sympathomimetic drugs may incite rupture of the aneurysm, presumably through a sudden surge in blood pressure. The risk of SAH also may be increased during periods of increased physical activity or emotional stress. These globular shaped lesions are categorized as small (<10 mm in diameter), large (10-25 mm), or giant (>25mm) (see Plate 9-40). The risk of hemorrhage increases with aneurysmal enlargement. Approximately 25% of patients will have more than one aneurysm.

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