Aneurysm–Rainbow Team/HelsinkiMicroneurosurgical management of distal middle cerebral artery aneurysms
Introduction
Middle cerebral artery aneurysms can be classified into proximal, bifurcation, or distal type (Table 1). The proximal MCA aneurysms or M1As are located in the main trunk (M1), between the bifurcation of the internal carotid artery (ICA) and the main bifurcation of MCA [9]. The MbifAs are located in the main bifurcation of MCA [10]. The MdistAs, originating from branches of MCA distal to the main bifurcation or the peripheral branches, are the focus of the present article.
Distal middle cerebral artery aneurysms are the least frequent of MCA aneurysms [18], [35], [36], [43], [56]. Microneurosurgical treatment of MdistAs is demanding. It is difficult to localize them, the small ones in particular, because they lie deep in the sylvian cistern, among the distal branches of the MCA. Intraoperative navigation may be further complicated by the presence of SAH and ICH. Furthermore, they can be mycotic, inflammatory, or dissecting [29], [39], [43]. The lack of collateral circulation makes occlusion more challenging, necessitating bypass and revascularization techniques [27], [35], [36]. Since the first definition by Poppen [30] in 1951, there are only few reports on management of MdistAs [18], [29], [35], [36], [56].
The purpose is to review the practical anatomy, preoperative planning, and avoidance of complications in the microsurgical dissection and clipping of MdistAs. This review, and the whole series on intracranial aneurysms, is intended for the neurosurgeons who are subspecializing in neurovascular surgery.
This review is mainly based on the personal microneurosurgical experience of the senior author (JH) in 2 Finnish centers (Helsinki and Kuopio), which serve, without selection, the catchment area in the southern and eastern Finland. These 2 centers have treated more than 10 000 patients with aneurysm since 1951.
The data presented in our series of articles represent 3005 consecutive patients harboring 4253 intracranial aneurysms from the Kuopio Cerebral Aneurysm Database (1977-2005). The aim is to present a consecutive, nonselected, population-based series of intracranial aneurysms without any selection bias. This database is not reflective of the personal series of the senior author (JH) alone.
Section snippets
Occurrence of MdistAs
Distal middle cerebral artery aneurysms are the least frequent of the MCA aneurysms reported to form 1.1% to 5% of them [18], [35], [36], [43], [56]. Four MdistAs form 2% of the series reported by Yaşargil [56]. In the largest series reported to date, Horiuchi et al [18] reported 9 MdistAs. Table 2, Table 3, Table 4, Table 5present clinical data of patients with MdistA in a consecutive and population-based series of 3005 patients with 4253 intracranial aneurysms from 1977 to 2005 in the Kuopio
Microsurgical anatomical considerations of MdistAs
Distal middle cerebral artery aneurysms originate from the M2s or more distal branches of MCA. Distal middle cerebral artery aneurysms are located in the lateral and distal part of the sylvian fissure, between the frontal and temporal lobes, where diverse vascular anatomy may affect the outcome of surgery. The sylvian fissure varies in shape and volume, and previous SAHs may toughen the arachnoid in and on the fissure [8], [55]. The venous anatomy of the sylvian fissure is complex and varies
Imaging of MdistAs
Digital subtraction angiography is still the present criterion standard in many centers. Multislice helical CTA is the primary modality in our centers for several reasons: noninvasive and quick imaging, comparable sensitivity and specificity to DSA in aneurysms larger than 2 mm [20], [46], [53], [54], disclosure of calcifications in the walls of arteries and aneurysm, and quick reconstruction of 3-dimensional (3D) images (Fig. 3).
For intraoperative navigation toward MdistAs, 3D CTA or DSA
Microsurgical strategy with MdistAs
We divide MdistAs into (a) aneurysms on M2 or M2-M3 junction and (b) those distal to M2-M3 junction. In the proximity of the main MCA bifurcation, the approach suitable for the by far more prevalent MbifAs will do [10]. To expose more MdistAs, the approach must be more occipital over the sylvian fissure.
The more distal MdistAs are, the more difficult it is to locate them in the sylvian cistern and sulci, among the distal branches of the MCA, and, in particular, in the presence of SAH or ICH.
General principles
The sylvian fissure is opened appropriately over the site of the aneurysm. We prefer to identify and follow M2s or M3s in proximal to distal direction. After finding the aneurysm, the rest of the dissection and subsequent clipping are often straightforward. With the help of temporary clipping, the base of the aneurysm is dissected free, and usually, one microclip is placed on the base. It may not be advisable to dissect the dome completely before applying a pilot clip. The parent artery and the
Associated aneurysms
Distal middle cerebral artery aneurysms are particularly often associated with other aneurysms. At least one additional aneurysm was found in 51 (74%) of the 69 patients with MdistA and in 10 (56%) of the 18 patients with ruptured MdistA (Table 4). Our strategy is to clip all aneurysms that can be exposed through the same craniotomy (see video 1). This may not be advisable if the clipping of the ruptured aneurysm is difficult or the brain is swollen due to acute SAH [34].
Giant MdistAs
Distal middle cerebral artery aneurysms that occasionally grow large or giant though the hemodynamic stress are seen less than in M1As and MbifAs. In the series of 1704 MCA aneurysms (Table 3), 5 (6%) of the 78 MdistAs were large or giant (15 mm or more) as compared with 16 (7%) of the 241 M1As [9] and 207 (15%) of the 1385 MbifAs [10]. Large size further complicates clipping in terms of preserving the parent artery.
Fusiform, dissecting, and mycotic MdistAs
Fusiform, dissecting, and mycotic aneurysms are longitudinal dilations of the cerebral arteries, in sharp contrast to saccular aneurysms of the arterial forks. We prefer exosurgery over endosurgery when there is SAH or, in particular, large ICH. In some fusiform aneurysms, it may be possible to find the so-called beer belly, which is tangentially clipped with a small straight or curved clip under temporary occlusion of the parent artery. In distal aneurysms with retrograde flow, it is often
Bypass operations and arteriotomies
End-to-side or end-to-end STA-MCA bypass should be considered, particularly in proximally located MdistAs, when the exclusion of the aneurysm neck from the parent and branching arteries cannot be performed [18], [29], [39], [40].
Acknowledgments
We thank Mr Ville Kärpijoki for excellent technical assistance.
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