Numerous reports have documented a familial occurrence of saccular aneurysms, lending support to the idea that genetic factors play a role in their development. Numerous genetic associations have been reported, none entirely convincing. The number of first-degree relatives found to harbor an unsuspected aneurysm has been approximately 4 percent in most series. This low rate, the finding that half of the discovered aneurysms are small, and the complications of surgery make routine screening of siblings, children, and parents of patients with ruptured aneurysms impractical, according to the Magnetic Resonance Angiography in Relatives of Patients with Subarachnoid Hemorrhage Study Group. However, because aneurysms of the familial variety tend to be larger at the time of rupture and more numerous than in patients who have sporadic ones, there are exceptions to this statement (Ruigrok et al), and there is little question that, in practice, the close relatives of patients with ruptured aneurysms ask for, and are accommodated, screening for aneurysms. From a survey in Scotland, the lifetime risk of hemorrhage was only 4.7 percent for a first-degree relative and 1.9 percent for a second-degree relative (Teasdale et al). From several series, it is apparent that the risk is highest for individuals with two or more first-degree relatives and negligible for one second-degree relative.
Although hypertension is more frequently present than in the general population, aneurysms most often occur in persons with normal blood pressure. Pregnancy does not appear to be associated with an increased incidence of aneurysmal rupture, although there is always theoretical concern about the possibility of inducing bleeding during the straining of natural delivery. Atherosclerosis, although present in the walls of some saccular aneurysms, probably plays no part in their formation or enlargement.
Approximately 90 to 95 percent of saccular aneurysms lie on the anterior part of the circle of Willis (see
Fig. 33-25). The four most common sites are (1) the proximal portions of the anterior communicating artery, (2) at the origin of the posterior communicating artery from the stem of the internal carotid, (3) at the first major bifurcation of the middle cerebral artery, and (4) at the bifurcation of the internal carotid into middle and anterior cerebral arteries. Other sites include the internal carotid artery in the cavernous sinus, at the origin of the ophthalmic artery, the junction of the posterior communicating and posterior cerebral arteries, the bifurcation of the basilar artery, and the origins of the cerebellar arteries. Aneurysms of the carotid artery that rupture in the cavernous sinus give rise to an arteriovenous fistula (see further on).
There are several types of aneurysms other than saccular, for example, mycotic, fusiform, diffuse, and globular. The mycotic aneurysm is caused by a septic embolus that weakens the wall of the vessel in which it lodges, almost always at a site in a distal cerebral vessel, well beyond the circle of Willis. These lesions are discussed separately in a later section of this chapter. The others are named for their predominant morphologic characteristics and consist of enlargement or dilatation of the entire circumference of the involved vessels, usually the internal carotid, vertebral, or basilar arteries. Fusiform deformities are also referred to as arteriosclerotic aneurysms, as they frequently show atheromatous deposition in their walls, but it is likely that they are at least partly developmental in nature. Some are very large (so-called giant aneurysms) and press on neighboring structures or become occluded by thrombus, but they rupture only infrequently (as discussed further on).