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1.3 Intracranial aneurysms, AV malformations, and other vascular malformations
Introduction and conservative treatment: Werner Weber, Uwe Dietrich
Interventional treatment: Werner Weber
Surgical treatment: Maximilian J.A. Puchner
Intensive care: Hans-Georg Bone
1.3.1 Aneurysms
1.3.1.1 Clinical picture
Intracranial aneurysms are bulges, usually saccular, in the arteries at the base of the brain that occur at vascular branching points, which represent a site of predilection for this disease. The vascular branching points must be regarded as a weak point at which the fibers of the tunica media—which is thinner in the intracerebral vessels than in the extracranial ones—diverge in order to divide into two vessels. This histological aspect means that the wall structure of aneurysms differs from that in normal cranial vessels. The tunica media is absent and the internal elastic lamina is very thin. The pathogenesis of the disease has not been fully explained. In addition to congenital factors such as hereditary connective-tissue diseases, factors such as degenerative vascular diseases, infection (in what are known as mycotic aneurysms, which are typically located mainly in the peripheral cerebral vessels) and hemodynamic factors (hypertension and downstream arteriovenous malformations) also need to be taken into account. In addition, there is an association of aneurysm with cystic renal degeneration.
The circle of Willis is the vascular circuit that connects the posterior and anterior circulation as well as linking the right and left halves of the brain circulation, and the circle has many variants. The connecting segments linking the two carotid flow areas are called the anterior communicating branch (usually unpaired, but with many variants, including even aplasia) and the posterior communicating branch, linking the carotid flow area with the posterior circulation (paired, with widely varying caliber and even paired or unpaired aplasia; hypoplasia of the segment of the posterior cerebral artery between the tip of the basilar artery and the orifice of the posterior communicating branch may also be seen here). It is not possible to describe all of the possible variants here, but to understand the frequency of aneurysms in these locations it is important to know that due to hypoplasia and aplasia, individual vascular segments in the circle of Willis may be subject to greater pressure at the remaining points; this is one reason for the frequency of aneurysms there. The most frequent location for aneurysms is in the anterior communicating branch, followed by the internal carotid artery/posterior communicating branch and the middle cerebral artery (together 60–70%). Aneurysms in the posterior circulation are much rarer; the most frequent there are aneurysms at the tip of the basilar artery, at about 10% (Fig. 1.3-1).
In addition to saccular aneurysms, there are also fusiform ones that affect the entire circumference of the vessel segment involved. These are caused by vascular wall dissection with bleeding into the wall and formation of what are known as “false aneurysms.” Vascular dissections occur either spontaneously or due to trauma (Fig. 1.3-6). On the basis of autopsy studies, the incidence of aneurysms is estimated at around 3–5% of the population. The incidence of intracranial aneurysmal bleeding varies regionally, and in Germany it is estimated at 10 cases per 100,000 population per year. The probability of bleeding increases with age; the patients’ average age is approximately 50, and women are more often affected. The morbidity and mortality rates for these initial bleeds are very high. The outcome for the affected patients can be roughly estimated using the “rule of thirds”: one-third of the patients die due to the acute effects of hemorrhage and complications during the course (spasm and hydrocephalus); one-third become handicapped as a result of the hemorrhage and are unable to return fully to normal everyday life; and one-third of the patients are able to return to their normal lives. The indications for the treatment of ruptured or space-occupying aneurysms (cranial nerve paralysis or other compression syndromes, e.g., in the brainstem with large fusiform basilar aneurysms) are based on the symptoms. Apart from emergency operations when there are acute symptoms of constriction, ruptured aneurysms should be treated promptly (within 24 h) in order to prevent an early second bleed. The indication for treatment of unruptured and asymptomatic aneurysms is much less clear. The bleeding risk in unruptured aneurysms