The precise incidence of cerebral aneurysms is unknown. From autopsy studies, the incidence is estimated at 5 % (Stehbens 1972, 1990).

Cerebral aneurysms are commonly of a saccular type (berry aneurysm) located at the bifurcation of the arteries of the circle of Willis. The most frequent sites are the internal carotid artery (ICA; 30–35 %). Among them, those in the area of the junction of the posterior communicating artery (PcomA) account for more than the half. Also occurring very frequently are aneurysms involving the sector of the anterior cerebral artery (ACA; 33–34 %) and those of the middle cerebral artery (20 %).

Aneurysms in the posterior circulation are less common (10 %). Of these, half are basilar artery aneurysms (Locksley 1966a, b; Weir 1987; Nakstad et al. 1988).

Multiple cerebral aneurysms frequently occur together, with an incidence that varies from 20 to 40 % (Locksley 1966a, b; Nakstad et al. 1988; Osborn 1999; Rinne et al. 1994). Multiple aneurysms occur more frequently among women than men. It is thinkable that in those patients in whom there is a tendency to develop multiple aneurysms, these do not appear always simultaneously but in different period of life. This could also explain the appearance of the in the literature called “de novo” aneurysms sometimes even years after the diagnosis of a first aneurysm (Wermer et al. 2005). Furthermore, the presence of multiple aneurysms suggest that at least in these cases we are dealing not with a focal but with a diffuse progressing vascular disease of the cerebral vessels which needs to be monitored (Bruneau et al. 2011) (Fig.11.14c–f).

Saccular aneurysms vary in size. In in vivo studies, the majority have a diameter of 5–7 mm, though smaller and larger aneurysms also occur; those larger than 25 mm are termed giant aneurysms. Saccular aneurysms can be round or more elongated. The surface may be regular or irregular with estroflexion (blebs), which in the case of a subarachnoid hemorrhage (SAH) can indicate the point of rupture. Pathological studies show that the neck and sac have no internal elastic lamina and no tunica media.

Intracranial cerebral vessels and intracranial aneurysms have some unique features. The arteries are composed of four layers, represented by the adventitia, media, internal elastic lamina, and intima. There is no external elastic lamina. The arteries have thin walls and are located in the subarachnoid space, where they have relatively little external support. Typical aneurysms of the intracranial cerebral vessels are saccular and thus different from aneurysms affecting the aorta or other extracranial arteries, which are commonly fusiform (Stehbens 1990; Powell 1991). In the early 1930s, some authors (Forbus 1930) showed in histological studies a congenital gap defect of the media in the wall of the cerebral artery, which was interpreted as a locus minoris resistentiae, where aneurysms could develop. Later, however, it was demonstrated that these gaps are very frequent in normal individuals without aneurysms. Today, it is commonly accepted that aneurysms are acquired lesions linked to a degenerative process involving the connective tissue of the media and internal elastic lamina, in which atherosclerosis probably plays an important role (Glynn 1940; Carmichel 1950; Crompton 1966a, b; Stehbens 1959, 1972, 1989, 1990). Others (Pope et al. 1984; Pope 1989; Ostergaard and Oxlund 1987; Chyatte et al. 1990) have demonstrated a reduction in reticular fibers owing to a decrease in type III collagen in the tunica media of cerebral arteries of patients with aneurysms; this is perhaps due to an undetected metabolic disorder of connective tissue proteins. In a few of these patients, genetic disorders could be demonstrated (Chyatte and Mjerzejewski 1991).

These degenerative changes lead to mural fragility, upon which hemodynamic factors, such as hypertension, variations in the circle of Willis, and arteriovenous malformation (AVM), could act further toward the formation of aneurysms. For aneurysms linked to AVM, see Chap. 12.

There are other more uncommon pathological conditions in which cerebral aneurysm can occur. Among them are a few systemic diseases of the connective tissue, such as fibromuscular dysplasia, Ehlers–Danlos syndrome, Marfan syndrome, neurofibromatosis, polycystic kidney, and aortic coarctation. In some of these patients, hypertension is also present as an adjuvant factor (Schwartz and Baronofsky 1960; Handa et al. 1970a; Stehbens 1989; Osborn 1999; Benyounes et al. 2011).

Aneurysms can occur in infectious diseases, arteritis, and cases of cardiac myxoma. Other factors reported to promote aneurysm formation are smoking, consumption of alcohol, and oral contraceptives (Hillbom and Kaste 1982; Lindergard et al. 1987; Juvela et al. 1993). The incidence of aneurysms is reported to be high in some families (Edelsohn et al. 1972; Hashimoto 1977; Crompton 1979; Norrgard et al. 1987).

Finally intracranial aneurysms can occur following a traumatic lesion of the arterial wall leading to dissection and to formation of a pseudoaneurysm (Birley and Trotter 1928; Benoit and Wortzman 1973; Jacobson et al. 1984; Bozzetto-Ambrosi et al. 2006; De Andrade et al. 2008; Nakstad et al. 2008). In the past, angiography was the method of choice to examine patients with craniocerebral trauma. Aneurysms could be easily detected. Today the diagnosis is performed with CT. With this method, in spite of the improved quality, small aneurysms can escape to the routine CT diagnosis. As reported also by others (Jussen et al. 2012) in patients with severe trauma especially in whom CT has shown an intracranial hemorrhage, a study of the vessels with CT angiography or conventional angiography is mandatory to exclude a vascular lesion especially an aneurysm of the cerebral or meningeal arteries. As far as it concerns aneurysms due to spontaneous dissection, see Chap. 16.

Aneurysms are typical in patients of adult age, and there is predominance among women. Aneurysms in pediatric patients are rare; they are more frequent in boys. In 90 % of cases, the aneurysm presents with subarachnoid hemorrhage (SAH) (10 cases of SAH/year/100,000 people). On CT the bleeding in the subarachnoid space and in the ventricular system is commonly easily recognizable. Involvement of the adjacent parenchyma can occur especially in aneurysms of the middle and the anterior cerebral artery. Extension to the subdural space is sometimes recognizable in aneurysm near the skull basis. It is the merit especially of Dr. Symonds in the years 1923 and 1924 to identify some typical symptoms of the SAH and correlate these with a rupture of a cerebral aneurysm. Today this clinical condition with its typical sudden onset of headache, stiff neck, vomiting, and frequent disturbances of consciousness (drowsiness, confusion, and coma) is commonly easily recognized and correctly diagnosed. In some cases due to the very high increase of the intracranial pressure, death can ensue within minutes and hours. In other patients, the bleeding can be minimal (warning leak) and the mild clinical symptoms can be misinterpreted and the diagnosis of SAH can be eluded. Particular clinical aspects, which occur especially in severe SAH, are pulmonary complications and a myocardial dysfunction including arrhythmias, troponin elevation, myocardial infarction, and the syndrome called “neurogenic stunned myocardium” (Kono et al. 1994; Urbaniak et al. 2007; Frontera et al. 2008). All these cardiac complications can lead to cerebral infarct (Mayer et al. 1999; Gaita et al. 2002; Jain et al. 2004; Lee et al. 2006b). In other cases, the patient presents with neuropathy, particularly oculomotor palsy, in PcomA and basilar artery aneurysms. Trigeminal neuralgia can also occur in large aneurysms in this location. Visual symptoms, due to compression of the chiasma and/or optic nerve, and sometimes hydrocephalus, due to compression of the third ventricle, can be present in large aneurysms involving the anterior communicating artery (AcomA) and in carotid ophthalmic lesions. Symptoms as a result of brain compression can occur in large basilar artery and middle cerebral artery (MCA) aneurysms.

The typical cavernous sinus syndrome is present in intracavernous aneurysms.

In exceptional cases, aneurysms are present with ischemia. This occurs especially in nonruptured, partially thrombosed aneurysms (Antunes and Correll 1976; Stewart et al. 1980). Ischemia can be due to the distal embolization or involvement of perforators adjacent to the aneurysm.

In addition to aneurysms in patients presenting with related symptoms, an increasing number of asymptomatic aneurysms today are diagnosed with CT and MRI in patients who underwent the examination for other reasons. The clinical problems and therapeutic considerations with such aneurysms are discussed in Sect. 11.10.