25 Internal Carotid Artery Bifurcation Aneurysms
Abstract
Internal carotid artery aneurysms represent about 5% of all intracranial aneurysms. They are often not located symmetrically at the bifurcation; instead, they may preferentially sit on the origin of the proximal anterior cerebral artery or, less frequently, along the origin of the middle cerebral artery. Treatment is indicated for ruptured aneurysms and for unruptured aneurysms larger than 7 mm. However, treatment may also be indicated for patients with unruptured aneurysms smaller than 7 mm if young and with known risk factors for development and rupture of intracranial aneurysms. When treatment is indicated, an individualized approach that takes into consideration various patient- and aneurysm-related factors leads to excellent outcomes in the vast majority of patients.
Introduction
Internal carotid artery (ICA) bifurcation aneurysms (ICABifAs) are those located at the bifurcation of the ICA where the artery divides into the anterior cerebral artery (ACA), A1 segment, and the middle cerebral artery (MCA), M1 segment. Often these aneurysms are not symmetrically located at the apex of the bifurcation, but may preferentially sit on the origin of the ACA (more frequently) or less commonly on the MCA origin.
ICABifAs are relatively uncommon and, in large series encompassing both ruptured and unruptured aneurysms, they represent about 5% of all aneurysms. In our series of consecutive 1,230 patients with unruptured aneurysms evaluated over an 8-year interval, 74 patients or 6% harbored an ICABifA. ICABifAs represent about 15% of all aneurysms involving the ICA and do not display side preference. Patients with ICABifAs tend to bleed at a younger age than patients with ruptured aneurysms in other locations. In contrast to older adults, the ICA bifurcation is one of the most common sites of aneurysms in the very young, representing up to 27 to 40% of all intracranial aneurysms (IAs) in patients younger than 20 years ( 1 , 2, 3 in algorithm ).
Aneurysms of the ICA bifurcation are associated with other IAs in up to 25% of patients. Bilateral ICABifAs are only seen in 6% of cases. Giant ICABifAs occur in 7% of all ICABifAs, while fusiform ICABifAs are rare and, when encountered, they often include the distal supraclinoid ICA and the proximal portion of the M1 tract. In ruptured cases, intracerebral hemorrhage (ICH) is present in 19% (usually located in the basal frontal lobe; ▶ Fig. 25.1 ) and intraventricular hemorrhage in 23%. Especially in very large and giant cases, ICH may occur in the absence of associated SAH.
Major controversies in decision making addressed in this chapter include:
Whether or not treatment is indicated.
Open versus endovascular treatment for ruptured and unruptured ICABifAs aneurysms.
ICABifAs dome projection and microsurgery clipping nuances.
Role of new endovascular devices (e.g., neck reconstruction devices and intrasaccular flow disrupters).
Whether to Treat
Treatment of ruptured ICABifAs is indicated except in patients with very poor neurological condition who fail to improve after systemic and neurological stabilization and support ( 2, 3 in algorithm ). Because of their relative rarity, there are no natural history studies specifically focused on ICABifAs and for the same reason, these aneurysms were underrepresented in the large multicenter observational cohort studies published so far. Therefore, there are no definitive data to guide treatment of unruptured ICABifAs. In general, treatment is indicated for symptomatic aneurysms and for unruptured ICABifAs larger than 7 mm. We feel that because of the increased hemodynamic stress at the bifurcation of a large vessel like the ICA, ICABifAs may have a higher risk of rupture than aneurysms at bifurcation of smaller vessels. Thus, we consider and recommend treatment even for patients with unruptured aneurysms smaller than 7 mm especially if young and with known risk factors for development and rupture of IAs such as smoking and hypertension ( 2, 3 in algorithm ). Treatment is also considered in patients with small ICABifAs and with family history of aneurysmal SAH ( 2, 3 in algorithm ) both because of a possible higher risk of rupture than the general population and because of the psychological burden associated with the knowledge of harboring an IA in these subjects. Conservative management is considered in elderly patients with small unruptured ICABifAs, those with very small (≤3 mm) incidental aneurysms without risk factors, in patients considered at high risk for treatment, and in those patients with small unruptured aneurysms and a projected life expectancy of less than 3 to 5 years.
Anatomical Considerations
Local anatomical considerations are of critical importance in planning surgical steps and understanding the potential risks of treatment. The arteries and/or arterial segments that need to be recognized during surgery for ICABifAs include the following: (1) the distal supraclinoid ICA; (2) the origin of the ACA; (3) the origin of the MCA in the proximal sylvian fissure; (4) the medial lenticulostriate (MLS) arteries originating from the proximal portion of the ACA; (5) the recurrent artery of Heubner; (6) the lateral lenticulostriate (LLS) arteries arising from the proximal MCA; and for larger aneurysms or aneurysms projecting posteriorly (7) branches of the anterior choroidal artery (AChA); and (8) the posterior communicating artery (PCoA). These anatomical relationships are illustrated in ▶ Fig 25.2 .
Distal Internal Carotid Artery and Internal Carotid Artery Bifurcation
After exiting the cavernous sinus, the ICA becomes intradural, with its supraclinoid segment, crossing the carotid cistern inferomedially to the anterior clinoid process. The supraclinoid portion is divided into three segments, based on the origin of its major branches: the ophthalmic segment, which extends from the ophthalmic artery to the origin of the PCoA; the communicating segment, which spans from the PCoA to the origin of the AChA, and the choroid segment, extending from the origin of AChA to ICA bifurcation.
Below the anterior perforating substance, the ICA bifurcates into its two terminal branches: the proximal M1 segment of the MCA and the proximal A1 segment of ACA. The ICA bifurcation is usually the highest point of the circle of Willis. From the ICA bifurcation itself, there are two to six small perforating branches (usually one half the size of the lenticulostriate arteries) that run from the apex of the bifurcation to the anterior perforated substance.
Proximal ACA, Medial Lenticulostriate Arteries, and Recurrent Artery of Heubner
The proximal ACA after its origin from the ICA bifurcation is directed anteriorly and medially and gives origin to a variable number of perforating branches, the MLS arteries, which then take a posterior recurrent course to enter the anterior perforated substance. These small perforating arteries are often behind the sac of ICABifAs.
The recurrent artery of Heubner is of variable caliber and arises from the lateral wall of the A2 immediately distal to the A1/A2 junction or less commonly from the junction itself. It then courses as a recurrent course (hence the name) parallel to the A1 (and usually posterior to it) to enter the anterior perforated substance behind and medial to the ICA bifurcation.
Proximal MCA and Lateral Lenticulostriate Arteries
The proximal MCA arises as the larger of the two terminal branches of the ICA. From its origin, it courses laterally, parallel to the sphenoid ridge. As it passes below the anterior perforated substance, it gives the LLS arteries.
The LLS arteries originate from the lateral part of M1 and enter the posterolateral part of the anterior perforated substance, crossing the putamen to irrigate the anterior-to-posterior part of the internal capsule as well as the head and the body of the caudate nucleus.
Classification
Similarly to aneurysms in other locations, ICABifAs can be classified into small, large, and giant based on their size. Specifically to this location is a classification based on the orientation of the aneurysm fundus. This is of importance since the direction of the fundus (along with the size of the aneurysm) will determine the risk of treatment and it influences choice of treatment. According to the predominant dome projection, ICABifAs can be classified as follows:
Superior projection (the most common), with their dome projecting into the anterior perforated substance and the basal frontal lobe.
Anterior projection, with the dome projecting into the lateral fronto-orbital gyrus or the base of the olfactory tract.
Posteroinferior projection, with their dome projecting into the carotid and interpeduncular cistern, or the ambient and crural cistern. This is the most difficult orientation for surgical clipping.
Workup
Clinical Evaluation
In patients with ruptured aneurysms, clinical evaluation is straightforward and not dissimilar to ruptured aneurysms in other locations. Careful clinical evaluation has a critical role in patients with unruptured ICABifAs. In these cases, many factors must be considered before recommending treatment and choosing the most appropriate treatment. Great attention should be paid to the actual signs and symptoms that have led to the discovery of the aneurysm. Presence of risk factors associated with aneurysm formation, growth, and rupture such as smoking habit, history of hypertension, and family history of IAs and aneurysmal SAH should be investigated. Comorbidities that may increase the risk of treatment and/or affect life expectancy should also be considered in the decision of whether or not to treat.
Understanding individual patient attitude toward the knowledge of having an IA is very important. In an anxious patient, treatment may be considered, if the risk of treatment is very low, to relief anxiety and, in turn, improve quality of life ( 2, 3, 4 in algorithm ). Patient preference of type of treatment, factors such as depression, and low pain threshold are also very important in our decision-making process.
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