Aneurysms

Abstract

Intracranial aneurysms are focal outpouchings of the wall of the cerebral arteries. They mainly develop close to the circle of Willis, an anastomotic network linking the internal carotid arteries and the vertebrobasilar system, which surrounds the sellar region. Therefore, intracranial aneurysm may present as an intra-, supra-, or laterosellar mass lesion. In this area, unruptured aneurysms may exert mass effect on the pituitary gland or the stalk or the cranial nerves, or may be discovered incidentally on brain or sellar MRI. They might be misdiagnosed or missed on preoperative MRI, but it is important to be aware that intracranial aneurysms may coexist with a pituitary adenoma (Fig. 60.1). They may be fortuitously associated with any nonsecreting or secreting pituitary adenoma, but acromegaly carries an increased risk of harboring intracranial aneurysms, with a reported incidence ranging from 4 to 17 %. In this setting, development of intracranial aneurysm might be due to the arterial hypertension frequently observed in this disease. Aneurysms in acromegalic patients are said to be usually located on the anterior circulation, are small (mean diameter <7 mm), and seem not to be more prone to rupture than aneurysms observed in the general population. It is of critical importance to evoke the diagnosis of sellar aneurysm preoperatively, because unruptured aneurysms should benefit from dedicated care and also because cases of aneurysms accidentally discovered during transsphenoidal surgery have been reported, with variable outcomes. Management of intracranial unruptured aneurysms principally depends on their risk of spontaneous rupture, which is influenced by the size and location of the aneurysm. First, the larger the aneurysm, the higher the risk of rupture. Risk is considered minimal, <1 % per year, when aneurysm diameter is smaller than 7 mm, while it may reach 30 % at 5 years for giant aneurysms larger than 25 mm. Second, the location is crucial because it is fundamental in determining whether an intracranial aneurysm is intradural or extradural, especially in the sellar region. The origin of the ophthalmic artery divides the two subgroups. On one hand, extradural aneurysms rise before the origin of this artery. They are intracavernous and thus virtually never cause subarachnoid hemorrhage, but behave as would any cavernous lesion, compressing the adjacent structures or destroying the bony skull base. On the other hand, intradural aneurysms arise after the origin of the ophthalmic artery and may rupture in the subarachnoid space. In the sellar region, such aneurysms precisely arise from the superior hypophyseal artery and extend medially, from the supraclinoid segment of the internal carotid artery (so-called carotid-ophthalmic aneurysm) and usually develop upward, or grow downward from the anterior communicating artery. Note that intracranial aneurysms are multiple in about 20 % of cases and that carotid-ophthalmic aneurysms are often bilateral.