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Available online at www.sciencedirect.com Surgical Neurology 70 (2008) 135 – 152 www.surgicalneurology-online.com Aneurysm-Rainbow Team/Helsinki Microneurosurgical management of aneurysms at A3 segment of anterior cerebral artery Martin Lehecka, MDa , Reza Dashti, MDa , Juha Hernesniemi, MD, PhDa,⁎, Mika Niemelä, MD, PhDa , Timo Koivisto, MD, PhDb , Antti Ronkainen, MD, PhDb , Jaakko Rinne, MD, PhDb , Juha Jääskeläinen, MD, PhDb a Department of Neurosurgery, Helsinki University Central Hospital, 00260 Helsinki, Finland b Department of Neurosurgery, Kuopio University Hospital, 70211 Kuopio, Finland Received 17 December 2007; accepted 1 March 2008 Abstract Background: Aneurysms originating from the A3 segment of anterior cerebral artery (A3A) form about 5% of all IAs. They are the most common among distal anterior cerebral artery aneurysms. There are relatively few reports on management of A3As. In this article, we review the practical anatomy, preoperative planning, and avoidance of complications in the microsurgical dissection and clipping of A3As. Methods: This review, and the whole series on IAs, is mainly based on the personal microneurosurgical experience of the senior author (JH) in 2 Finnish centers (Helsinki and Kuopio), which serve, without patient selection, the catchment area in Southern and Eastern Finland. Results: These 2 centers have treated more than 10000 patients with IAs since 1951. In the Kuopio Cerebral Aneurysm Database of 3005 patients and 4253 IAs, there were 163 patients carrying 174 A3As, forming 5% of all patients with IAs, 4% of all IAs, and 15% of all ACA aneurysms. Ninetyseven (60%) patients presented with ruptured A3As with ICH in 27 (28%) and IVH in 26 (27%). Ninety-four (58%) patients had multiple aneurysms. Conclusions: Aneurysms of A3 segment of ACA are often small, even when ruptured, with relatively wide base, and they are frequently associated with ICHs of IVHs. Our data suggest that A3As rupture at smaller size than IAs in general. The challenge is to select appropriate approach, to locate the aneurysm deep inside the interhemispheric fissure, and to clip the neck adequately without obstructing branching arteries at the base. Unruptured A3As also need microneurosurgical clipping even when they are small. © 2008 Elsevier Inc. All rights reserved. Keywords: Aneurysm; Anterior cerebral artery; Clipping; Distal; Callosomarginal artery; Microanatomy; Microsurgical technique; Pericallosal artery; Subarachnoid hemorrhage Abbreviations: A1, proximal segment of anterior cerebral artery; A1A, aneurysm of the A1 segment; A2, A2 segment of anterior cerebral artery; A2A, aneurysm of A3 segment of anterior cerebral artery; ACA, anterior cerebral artery; ACoA, anterior communicating artery; ACoAA, anterior communicating artery aneurysm; AdistA, aneurysm distal to A3 segment of anterior cerebral artery; AIFA, anterior internal frontal artery; CMA, callosomarginal artery; CSF, cerebrospinal fluid; CTA, computed tomographic angiography; DACA, distal anterior cerebral artery; DSA, digital subtraction angiography; IA, intracranial aneurysm; ICA, internal carotid artery; ICG, indocyanine green; ICH, intracerebral hematoma; ICP, intracerebral pressure; IVH, intraventricular hemorrhage; MCA, middle cerebral artery; MIFA, middle internal frontal artery; MRI, magnetic resonance imaging; PIFA, posterior internal frontal artery; SAH, subarachnoid hemorrhage; STA, superficial temporal artery. ⁎ Corresponding author. Tel.: +358 50 427 0220; fax: +358 9 471 87560. E-mail address: [email protected] (J. Hernesniemi). 0090-3019/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.surneu.2008.03.019 136 M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 1. Introduction Aneurysms of the ACA can be classified into 5 different groups as follows: A1As or proximal anterior cerebral artery aneurysms; ACoAAs; A2As or proximal pericallosal aneurysms; A3As or classical pericallosal aneurysms; and AdistAs or distal pericallosal artery aneurysms (Table 1, Fig. 1) [5]. The last 3 groups, also called DACA aneurysms, are further divided into 7 subgroups according to microneurosurgical criteria (Fig. 2). 1.1. A3 segment aneurysms The A3As are located at the A3 segment of the ACA at the genu of corpus callosum, often at the origin of the CMA. They have also been called pericallosal artery–callosomarginal artery junction aneurysms or loco classico pericallosal artery aneurysms. The A3As are the most common of the DACA aneurysms (see previously), forming 2% to 7% of all intracranial aneurysms, with relatively small series on their management [9,11,21,23,35,36,42,47,55,66,71,85,89,90]. The A3As are small and often associated with ICH when ruptured [21,27,36,40,56,66,68,71]. The A3As are difficult to reach as they lie deep in the interhemispheric fissure closely attached to the surrounding brain tissue. Their location deep inside the narrow interhemispheric fissure, wide base, frequent involvement with vascular anomalies of the region, as well as branches originating from their base make their intraoperative identification and microneurosurgical clipping challenging [3,4,9,21,23,35,36,40,42,45,47,51,56,66,71,82, 85,88,90]. 1.2. Purpose of review Fig. 1. Illustration demonstrating segments and branches of ACA and location of A3As. Eastern Finland. These 2 centers have treated more than 10000 aneurysm patients since 1951. The data presented in our series of articles represent 3005 consecutive patients harboring 4253 IAs from the Kuopio Cerebral Aneurysm Database (1977-2005). The aim is to present a consecutive, nonselected, population-based series of IAs. This database is not reflective of the personal series of the senior author (JH) alone. This review, and the whole series on intracranial aneurysms, is intended for neurosurgeons who are subspecializing in neurovascular surgery. The purpose is to review the practical anatomy, preoperative planning, and avoidance of complications in the microsurgical dissection and clipping of A3As. 1.3. Authors The microneurosurgical technique in this review is mainly based on the personal experience of the senior author (JH) in 2 Finnish centers (Helsinki and Kuopio), which serve, without selection, the catchment area in Southern and Table 1 Five categories of ACA aneurysms (see Fig. 1) Category Location A1A ACoAA A2A A3A AdistA A1 segment, between ICA bifurcation and ACoA Anterior communicating artery A2 segment and its frontobasal branches, between ACoA and genu of corpus callosum A3 segment, curving around genu of corpus callosum A4 and A5 segments or distal cortical branches such as CMA Fig. 2. Microsurgical division of distal anterior cerebral artery aneurysms with emphasis on A3As. M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 Table 2 Patients with ACA aneurysms in a consecutive and population-based series of 3005 patients with 4253 IAs from 1977 to 2005 in the Kuopio Cerebral Aneurysm Database Whole series Patients with primary SAH Patients without primary SAH ACA aneurysms A1As ACoAAs A2As A3As AdistAs Ruptured ACA aneurysms A1As ACoAAs A2As A3As AdistAs Fusiform ACA aneurysms Fusiform A1A Fusiform ACoAA Fusiform A2A Fusiform A3A Fusiform AdistA No. of patients No. of aneurysms 3005 2365 640 1145 23 (2%) 898 (78%) 35 (3%) 163 (14%) 26 (2%) 855 12 (1%) 715 (84%) 21 (2%) 97 (11%) 10 (1%) 6 2 3 1 0 0 4253 3325 928 1179 23 (2%) 921 (78%) 35 (3%) 174 (15%) 26 (2%) 855 12 (1%) 715 (84%) 21 (2%) 97 (11%) 10 (1%) 6 2 3 1 0 0 1.4. Occurrence of A3As The incidence of A3As is 2% to 7% of all IAs or 69% to 82% of all DACA aneurysms [9,21,23,35,36,42,47,55,66, 71,85,89,90]. Tables 2 to 5 present the clinical data on the 163 A3A patients in the consecutive and population-based series of 3005 patients with 4253 IAs from 1977 to 2005 in the Kuopio Cerebral Aneurysm Database. Of the 3005 patients, 1145 (38%) had 1179 ACA aneurysms (Table 2). There were 163 patients with 174 A3As, 4% of all the 4253 IAs, 15% of all the 1179 ACA aneurysms, and 74% of all the Table 3 Characteristics of 174 A3As Ruptured No. of aneurysms Median aneurysm size (mm) Aneurysm size Small (b7 mm) Medium (7-14mm) Large (15-24mm) Giant (≥25mm) Aneurysm side Right Left ICH Temporal Frontal Parietal IVH Preoperative hydrocephalus 137 Table 4 Intracerebral hematoma, IVH, and acute hydrocephalus associated with aneurysm rupture on different ACA segments A1As ACoAAs A2As A3As AdistAs Ruptured 12 715 21 97 10 aneurysms ICH only 3 (25%) 33 (5%) 6 (29%) 13 (13%) 2 (20%) ICH with IVH 0 (0%) 74 (10%) 5 (24%) 14 (14%) 2 (20%) component IVH only 2 (17%) 137 (19%) 2 (10%) 12 (12%) 0 (0%) Preoperative 5 (42%) 317 (44%) 7 (33%) 32 (33%) 1 (10%) hydrocephalus 235 DACA aneurysms. Most of the A3As were located anterior to the genu of the corpus callosum at the origin of the CMA. The left side (n = 101, 58%) slightly dominated over the right side (n = 73, 42%). There were no fusiform A3As. Giant A3As are extremely rare [11-13,18,21,36,39,46,49,53, 54,67,71,74,77], only one in our series (Table 3). 1.5. Ruptured and unruptured A3As In our series, 855 (73%) of the 1179 ACA aneurysms presented with SAH, of which 97 (11%) were A3As (Table 2). Of our 174 A3As, 97 (58%) were ruptured and 77 (42%) unruptured (Table 3). Their size distribution is presented in Table 3. Of the 97 ruptured A3As, 59 (61%) were smaller than 7 mm, suggesting that even small unruptured A3As would require occlusive therapy. 1.6. Intracerebral hematoma and IVH Ruptured DACA aneurysms bleed frequently into the adjacent brain [19,36,40,68,71]. Of the 97 patients with ruptured A3A, ICH was present in 27 (28%) and IVH in 26 (27%) (Table 3, Fig. 3A). Bleeding into the frontal lobe often extends into the ventricle (Table 4). 1.7. Associated aneurysms Unruptured Total 97 77 174 6 (range, 2-40) 3 (range, 1-11) 6 (range, 1-40) 59 (61%) 33 (34%) 61 (79%) 16 (21%) 4 (4%) 1 (1%) 0 (0%) 0 (0%) 37 (38%) 60 (62%) 27 (28%) 1 24 2 26 (27%) 32 (33%) 36 (47%) 41 (53%) - Data are given in number of aneurysms. 120 (69%) 49 (28%) 4 (2%) 1 (0.6%) 73 (42%) 101 (58%) - The DACA aneurysms are often associated with other aneurysms [9,21,36,47,56,88,90]. In our series, 94 (58%) of the 163 patients had at least one associated aneurysm (Table 5), most frequently on the MCA. Multiple A3As occurred in 24 (15%) patients, in 17 on the opposite A3, and in 4 on the same A3, and 3 had associated A3As on the both A3s (Table 5). Table 5 Patients with an A3A and possible associated aneurysms Ruptured Unruptured Total Patients with A3A Patients with single aneurysm Patients with multiple aneurysms Associated A3As Same pericallosal artery Opposite pericallosal artery Both pericallosal arteries Associated aneurysms at other sites Data are given in number of patients. 97 62 (64%) 35 (36%) 10 2 6 2 25 66 7 (11%) 59 (89%) 14 2 11 1 45 163 69 (42%) 94 (58%) 24 4 17 3 70 138 M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 2. Microsurgical anatomical considerations of A3As 2.3. A3 segment and its braches 2.1. Anterior cerebral artery The A3 segment starts at the junction of the rostrum and the genu of the corpus callosum, curves around the genu, and terminates at the start of the horizontal part of the ACA [14] (Fig. 2). All A3 to A5 segments coursed in the callosal sulcus in 60%, at least one segment was found in the cingulate sulcus in 33% and in 7%, the A3 to A5 segments, were located in the cingulate sulcus, not involving the corpus callosum at all [78]. The A3 segment gives origin to several cortical branches as follows: the AIFA; MIFA; the PIFA; and most important, the CMA [51]. High variation in the origin and size of these branches makes it impossible to define a The microneurosurgical anatomy of ACA and its branches has been well described [2,3,16,24,29,41,51,52, 78,88]. The ACA, the smaller of the 2 terminal branches of the ICA, arises at the medial end of the sylvian fissure lateral to the optic chiasm. It traverses the optic chiasm or the optic nerve, ascends in front of the lamina terminalis in the lamina terminalis cistern. Before entering the interhemispheric fissure, it is connected to the opposite ACA via the ACoA (Fig. 1). In the interhemispheric fissure, the left and right ACA trunks run parallel along the corpus callosum, in the pericallosal cistern. Both trunks give origin to several cortical, subcortical, and callosal branches, some of which may cross over to the opposite side. Cortical branches of the ACA supply the anterior two thirds of the medial aspects of the cerebral hemisphere as well as the superior portions of the superior frontal, precentral, and postcentral gyri [51]. The ACA is divided into 5 segments, A1 to A5, according to Fischer [14] (Table 1, Fig. 1). The A1 segment is located between the ICA bifurcation and the ACoA. The A2 segment extends from the ACoA to the region between the rostrum and the genu of the corpus callosum. The A3 segment curves around the genu of corpus callosum and ends at the rostral part of the body of the corpus callosum. The A4 and A5 segments follow the superior surface of the corpus callosum with a virtual plane of division at the level of the coronary suture. Traditionally, the ACA has been divided into a proximal part (A1) and a distal part (A2-A5), the latter also called the pericallosal artery [16,24,29,51,80,89]. 2.2. A2 segment The A2 segment originates at the junction of the A1 and the ACoA (Fig. 1). It ascends in the lamina terminalis cistern, in front of the lamina terminalis, enters the interhemispheric fissure and the callosal cistern with a course toward the genu of the corpus callosum. The A2 terminates at the junction of the rostrum and the genu of the corpus callosum where the A3 segment starts [51,87]. Inside the interhemispheric fissure, A2s were side by side in 18%, the left A2 anteriorly in 48%, and the right A2 anteriorly in 34% [52]. The free margin of the anterior falx is well above the corpus callosum. The A2 segment is entirely below the free margin that allows free shift and crossover of its branches across the midline [51]. This means that both A2 segments can be reached from a unilateral approach [21,88]. The rostrum and the genu of corpus callosum are mainly supplied by the subcallosal artery that usually originates from the ACoA, courses posteriorly toward the region of lamina terminalis, and ascends along the rostrum in the midline [78]. In 25% of cases, this artery extends beyond the genu of corpus callosum and continues along the body of corpus callosum [78]. Fig. 3. A: Intracerebral hematoma with IVH component related to ruptured anterior A3A. B: Dislocation of the pericallosal arteries (arrow) because of the ICH. M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 139 standard vascular pattern for the A3s [29,51,80]. Some of the cortical branches may have a common trunk of origin, may arise from different segments, or may be totally absent [29,51,80]. The 3 internal frontal arteries (see previously) supply the medial and lateral surfaces of the superior frontal gyrus as far posteriorly as the paracentral lobule [61]. In addition to the cortical branches, there are also thin arteries originating from the A3 to A5 segments that directly supply the superficial surface of the corpus callosum, called callosal and cingulocallosal arteries [78]. 2.4. Callosomarginal artery The CMA is the major branch of the distal ACA with diameter of 1.8 to 1.9 mm, as thick as the ACA at the same level [51,80]. Like other cortical branches of the ACA, the CMA cannot be defined by its branches because the usual branches of that region can arise directly from the ACA trunk as well. The CMA has been defined as the artery that courses in or near the cingulate sulcus and gives rise to 2 or more major cortical branches [43]. The CMA is totally absent in 9% to 18% [24,29,51,80]. It originated most frequently (73%) at the A3, but origins from the A2 or the A4 were also observed [51]. After its origin, the CMA courses in or near the cingulate sulcus and often gives rise to several cortical branches. According to the course, 3 types of hemispheres have been described: (1) no CMA; (2) atypical CMA, lacking long course in the cingulate sulcus and oriented directly toward the cortex; and (3) typical CMA, running parallel to the pericallosal artery in the cingulate sulcus for a relatively long distance [80]. The origin of the CMA is the most frequent site for the DACA aneurysms [9,21,36,47,56,71]. Fig. 5. Superior sagittal sinus (SSS) and its relation to venous lacuna, meningeal sinus, and bridging veins. 2.5. Anatomical anomalies of ACA The important anomalies involving the ACA are the azygos ACA; bihemispheric ACA; triplication of ACA; and crossover branches of the ACA [3,16,34,38,51,87]. The azygos ACA is a single trunk distal to the A1 segments, so it is the supplier of the both hemispheres. In the bihemispheric ACA, one A2 is hypoplastic and the larger A2 gives origin to most cortical branches (Fig. 4). In case of the triplication of ACA, the middle A2, also called the hemispheric type of the medial callosal artery [78], is prominent and supplies the corpus callosum. Crossover branches, found in 26% to 64% of hemispheres, are sent from distal ACA to the contralateral hemisphere where they supply a small medial area [51,69]. Severing of crossover branches may cause injury in the contralateral side to the approach. Association between anomalies of the ACA and DACA aneurysms was seen in large angiographic series [22]. Clinical observations on the association are mainly based on case reports [74]. In clinical series, anomalies of the ACA occurred in 7% to 35% of the patients with DACA aneurysms, more often than in the anatomical studies [4,23,35,71,88]. In anatomical studies, the azygos ACA was seen in 0.2% to 4%, the bihemispheric ACA in 0.2% to 12%, and the triplication of ACA in 3% to 13% of patients [3,16,29,38,51,69,79,80,87]. 2.6. Interhemispheric fissure Fig. 4. Bihemispheric ACA with A3A (arrow), note the hypoplastic A2 on the left (small arrows). The A3As are located in the midline, inside the interhemispheric fissure, where the cerebral hemispheres are partially separated by the falx. The depth of falx varies, but its free margin is well above the corpus callosum. Importantly, the A2 and A3 segments are below the free margin that allows free shift and crossover of their branches across the midline [51]. On the other hand, only the most anterior portion of the CMA is below the free margin. The cingulate gyri may be so adherent to each other that during 140 M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 the surgical exposure they may be mistaken for the corpus callosum [88]. The interhemispheric fissure is so narrow that only limited amount of CSF can be removed. 2.7. Venous structures The veins draining the lateral convexity of the frontal lobe are either ascending to the superior sagittal sinus or descending to the superior sylvian vein. The ascending veins are frontopolar, anterior frontal, middle frontal, posterior frontal, precentral, and central veins [48,60]. The ascending veins of the medial frontal surface join the ascending convexity veins along the superior rim of the frontal lobe to form subdural bridging veins that empty to the superior sagittal sinus [29,48,50,60,65]. The bridging veins, with a diameter of 1 to 4 mm, have a short free course (5-10 mm) in the subdural space before entering the superior sagittal sinus [60]. Bridging veins may enter the sagittal sinus directly, but they may also first join a meningeal sinus within the dura, with a course of 5 to 30 mm to the lateral angle of the sagittal sinus [1,48]. A single meningeal sinus may drain several cortical veins. The dura may also contain enlarged venous spaces called lacunae, extending laterally up to 3 cm from the midline. The lacunae predominantly drain meningeal veins before entering the sagittal sinus [48]. Most of the cortical veins pass beneath the lacunae and enter directly into the superior sagittal sinus, but a few have a common access to the sinus. The lacunae are largest in the posterior frontal region. Arachnoid granulations often protrude into the floor and walls making them adherent to the cortical surface (Fig. 5). The bridging veins pose a serious obstacle to the surgical approaches into the interhemispheric space. All bridging veins should be left intact if possible, and sufficient working space should be searched in between them. The venous pattern varies highly, but often, a corridor of some centimeters can be found [48]. The larger the damaged vein, the higher the risk of venous infarction. Aplastic superior sylvian vein requires special attention because of high risk of venous infarction because of reduced venous collateral flow [50]. Inside the interhemispheric fissure, one may encounter right and left anterior pericallosal veins side by side on the corpus callosum, varying highly in size. The pericallosal veins drain the genu and rostrum of the corpus callosum and empty into the anterior end of the inferior sagittal sinus. They should be left intact if possible, but there are often good venous collaterals. 2.8. Location and orientation of A3As Fig. 6. Inferior A3A (A), anterior A3A (B), and superior A3A (C) as seen on sagittal view of CTA. The A3As originate from the A3 segment at the genu of corpus callosum, in most cases at the origin of the CMA. The genu of the corpus callosum is semicircular or oval in sagittal section with a mean anteroposterior diameter of 11.2 mm [29]. Consequently, the A3A may occur at 3 principal locations in the sagittal view as follows: (a) inferior to the M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 141 Fig. 7. Unruptured inferior A3A (arrow) as seen on (A) preoperative 2-dimensional CTA, (B) preoperative 3-dimensional CTA, (C) preoperative MRI, and (D) postoperative CTA (see Supplementary video A3A-1). genu of the corpus callosum, often at the A2-A3 junction (inferior A3A); (b) anterior to the genu of the corpus callosum, on the central part of the A3 (anterior A3A); and (c) superior to the genu of the corpus callosum, close to the A3-A4 junction (superior A3A) (Figs. 2 and 6). Anterior A3As are the most frequent and the superior A3As the least. For each of the 3 locations, the interhemispheric approach should be so directed that the genu does not obstruct the view to the A3A base. Inferior A3As are usually directed forward, anterior A3As forward or upward, and superior A3As upward (Fig. 6A-C). 3. Imaging of A3A Digital subtraction angiography is still the present gold standard in many centers. Multislice helical CTA is the primary modality in our center as noninvasive, safe, and quick imaging method with a comparable sensitivity and specificity as DSA in aneurysms larger than 2 mm [17,25,26,64,73,81,83,84]. Computed tomographic angiography provides more information on the vascular anomalies of the ACA region, and it allows the disclosure of calcifications in the arterial walls and quick reconstruction of 3-dimensional images [28]. The CTA data are used for quick reconstruction of 3-dimensional images that show, for example, the surgeon's view of A3A, the relationship with the corpus callosum. Intracerebral hematomas are usually frontal, either unilateral or bilateral, sometimes with an intraventricular component. For intraoperative navigation toward the A3As, 3dimensional CTA or DSA reconstructions should be evaluated for the bilateral angioarchitecture of the A2-A3 region and its variations and anomalies; the number of pericallosal arteries; the correct parent artery (right or left) and its diameter; distance of the A3A from the A1-A2 junction; orientation of the dome; branching arteries at the 142 M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 Fig. 8. Ruptured anterior A3A (arrow) as seen on (A) preoperative sagittal CTA, (B) preoperative coronal CTA, (C) preoperative 3-dimensional CTA, and (D) postoperative CTA. A3A neck; location of the A3A with respect to the genu of the corpus callosum; and distances to the A3 segment and the A3A from the cortical surface (Figs. 3, 7-9). In addition to DSA and CTA, in unruptured A3As, T2-weighted MRI images may be helpful in preoperative planning (Fig. 7C). In the workstation, 3-dimensional CTA images can be rotated accordingly to show the surgeon's view and to design a suitable bony exposure. Intracerebral hematoma may dislocate the interhemispheric fissure. 4. Microsurgical strategy with A3As The A3As are rather unusual, and it may be difficult to gain experience in their microneurosurgical treatment [4,9,21,23,35,36,40,42,45,47,56,66,71,85,88,90]. Nevertheless, clipping is shown to be an efficient and long-lasting treatment of distal ACA aneurysms [37]. The A3As pose certain unique problems and require a different surgical trajectory than other anterior circulation aneurysms. The A3As are difficult to reach. The optimal and properly directed trajectory (inferior vs anterior vs superior A3A) requires detailed knowledge of the microsurgical anatomy of the A2-A4 region and careful study of images. The anterior interhemispheric approach requires tedious dissection deep in the narrow space, with poor control of the parent A3. The technique of clipping ruptured or unruptured A3A is almost the same. The only real difference is the lack of working space in acute SAH that makes the whole procedure more difficult. Yaşargil [88] listed special features of A3As as follows: (1) lack of working space in the interhemispheric space and pericallosal cistern; (2) dense adhesion between the cingulate gyri making separation and finding the aneurysm difficult; (3) sclerotic wall and broad base of the aneurysm; (4) origin of branching arteries at the neck and attachment of the dome to the opposite pericallosal artery; (5) difficult preoperative decision on the parent A3; (6) attachment or embedding of the dome in the pial layer of the cingulate gyrus; and (7) M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 143 Fig. 9. Unruptured anterior A3A (small arrow) and unruptured superior A3A found on the same pericallosal artery as seen on CTA in (A) sagittal view, (B) coronal view, (C,D) 3-dimensional reconstructions. location of the aneurysm at the bifurcation of an azygos pericallosal artery. We wish to add the following: (8) saving the bridging venous system; (9) frontal expansive ICH; (10) frequent intraoperative ruptures; (11) difficult proximal control, particularly in inferior A3As below the genu of the corpus callosum; (12) small size of aneurysm making correct clip placement difficult; and (13) difficult dissection and orientation inside the interhemispheric fissure because of thick blood clots in acute SAH. 4.1. Neuroanesthesiological principles A general review of our neuroanesthesiological principles has been published previously [59]. 4.2. Intracerebral hematoma Ruptured A3As are frequently associated with ICH, 28% in the Kuopio series (Table 3), and the narrow CSF space and adhesions may add to this. The ICH is usually frontal and can be located on either side (Fig. 3A-B). We remove massive ICHs when feasible. Frontal ICHs are likely to cause late cognitive deficits [70]. 4.3. Acute hydrocephalus In case of acute hydrocephalus, 33% in the Kuopio series (Table 3), ventricular drainage can be applied to reduce ICP and to lower the risk of brain damage, in most cases after immediately securing the ruptured aneurysm. Before or after clipping, additional CSF can be released, through a small callosotomy into the lateral ventricle. 4.4. Approach and craniotomy The A3As are approached through the anterior interhemispheric approach [86,88]. The exposure depends on the course of the pericallosal arteries, the location of A3A in 144 M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 relation to the genu of the corpus callosum, projection of the dome, and possible ICH. For a right-handed neurosurgeon, the right-sided approach is more convenient because both A3s can be reached under the inferior margin of the falx. A Table 6 Videos on microsurgical management of IAs in this and previous publications of the series Aneurysm and publication Video Proximal MCA aneurysms (M1As) [8] Craniotomy and exposure (LSO) (M1A-1, Hernesniemi) Clipping of temporally oriented M1A (M1A-2, Hernesniemi) Clipping of frontally oriented M1A (M1A-3, Hernesniemi) Clipping of giant M1A (M1A-4, Hernesniemi) Clipping of intertruncal-type MbifA (MbifA-1, Hernesniemi) Clipping of inferior-type MbifA (MbifA-2, Hernesniemi) Clipping of lateral-type MbifA (MbifA-3, Hernesniemi) Clipping of insular-type MbifA (MbifA-4, Hernesniemi) Contralateral approach to bilateral MbifAs (MbifA-5, Hernesniemi) Clipping of MdistA (MdistA-1, Hernesniemi) Clipping of MdistA (MdistA-2, Hernesniemi) Craniotomy and clipping of A1A (A1A-1, Hernesniemi) Craniotomy and clipping of fusiform A1A (A1A-2, Hernesniemi) Contralateral approach to A1A (A1A-3, Hernesniemi) Clipping of downward ACoAA (AcoAA-1, Hernesniemi) Clipping of forward ACoAA 1 (AcoAA-2, Hernesniemi) Clipping of forward ACoAA 2 (AcoAA-3, Hernesniemi) Clipping of intertruncal ACoAA (AcoAA-4, Hernesniemi) Clipping of backward ACoAA (AcoAA-5, Hernesniemi) Clipping of large complex ACoAA (AcoAA-6, Hernesniemi) Clipping of double ACoAA (AcoAA-7, Hernesniemi) Interhemispheric approach for high ACoAA (AcoAA-8, Hernesniemi) Clipping of fusiform ACoAA (AcoAA-9, Hernesniemi) Craniotomy for interhemispheric approach (A2A-1, Hernesniemi) Clipping of proximal A2 trunk A2A (A2A-2 Hernesniemi) Clipping of distal A2 trunk A2A (A2A-3, Hernesniemi) Clipping of distal frontopolar artery A2A (A2A-4, Hernesniemi) MCA bifurcation aneurysms (MbifAs) [7] Distal MCA aneurysms (MdistAs) [6] Proximal ACA aneurysms (A1As) [5] Anterior communicating artery aneurysms (ACoAAs) A2 segment and frontobasal aneurysms of ACA (A2As) Table 6 (continued) Aneurysm and publication Video A3 segment ACA aneurysms (A3As) in this issue Clipping of unruptured inferior A3A (A3A-1, Hernesniemi) Clipping of unruptured inferior A3A (A3A-2, Hernesniemi) Clipping of unruptured anterior A3A (A3A-3, Hernesniemi) Clipping of ruptured anterior A3A (A3A-4, Hernesniemi) LSO indicates lateral supraorbital approach. left-sided ICH or left-sided–associated anterior circulation aneurysms may require a left-sided approach. Importantly, the inferior A3As require a more anterior bone flap than the anterior A3As or the superior A3As. With wrong angle of approach, the genu will obstruct neurosurgeons' view toward the A3A base and prevent proper clip placement. We measure position of the A3A in relation to the outside cranium for the exact head positioning and bone flap placement. Usually the shortest route is selected. Some authors have used the bifrontal basal anterior interhemispheric approach [4,72]. We feel that the unilateral approach is less invasive and quicker and provides equal exposure to the A3As, deep in the interhemispheric fissure. The interhemispheric approach has been presented on video in our previous article on A2As (Table 6). The patient is in supine position with the head fixed in a head frame and elevated about 20° above the heart level. The head should be in neutral position with the nose pointing exactly upward. The head is slightly flexed or extended according to the location of the A3A with respect to the genu of corpus callosum. In the correct head position, the trajectory is almost vertical. Tilting the head to either side increases the chance that the bone flap is placed too laterally from the midline. This would make the entrance into the interhemispheric fissure and navigation there more difficult. If intraoperative DSA is considered, the frame pins should be placed accordingly. It is our practice to adjust the position of the fixed head and body during the operation when needed [20]. After minimal shaving, an oblique skin incision with its base frontally is made just behind the hairline, over the midline, extending more to the side of the planned bone flap. Location, curvature, and extent of skin incision depend on the hairline, dimensions of the frontal sinuses, and the orientation of the A3A. A 1-layer skin flap is reflected frontally with spring hooks. Bicoronal skin incision is unnecessary because strong retraction with hooks often allows anterior enough exposure of the frontal bone. The bone flap is placed slightly over the midline to allow better retraction of the falx medially. The superior sagittal sinus may deviate laterally from the sagittal suture, more often to the right, and as far as 11 mm [75]. The size of the bone flap depends both on the surgeon's experience and on the presence of ICH. We usually use a 3 to 4 cm diameter flap. A too small flap may not provide sufficient room for working M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 between the bridging veins. In most patients, only one burr hole in the midline over the superior sagittal sinus at the posterior border of the bone flap is needed. Through this 145 hole, bone can be detached from the underlying dura. One has to be careful with the underlying sagittal sinus, particularly in the elderly with very adherent dura. The Fig. 10. Intraoperative picture of the interhemispheric fissure with CMA and the tightly attached (A) cingulate gyri (CG), (B) rolled cottons used as expanders, (C) pericallosal artery (PerA) in between the CG, and (D) white color of the genu of corpus callosum (CC) (see Supplementary video A3A-2). 146 M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 bone flap is removed using a side cutting drill. High-speed drill can be used to smooth the edges or to enlarge the opening if necessary. The dura is opened under a surgical microscope as a C-shaped flap with its base at the midline. The incision is first made in the lateral region and then extended toward the midline in the anterior and the posterior direction to prevent opening of the superior sagittal sinus. The dural opening should be planned so that possible meningeal sinuses and lacunae are left intact. Bridging veins may be attached to the dura for several centimeters along the midline. Careful dissection and mobilization of these veins is necessary. It is usually during the opening of the dura that unwelcome damage to the bridging veins takes place. Dural edges are elevated with multiple stitches extended over the craniotomy dressings. 4.5. Removal of ICH In case of large ICH and lack of space (Fig. 3A-B), a small cortical incision is made accordingly, and hematoma is partially evacuated to gain space. This may risk the rerupture of A3A that would be difficult to control through the ICH cavity. In removing the ICH clot, before or after clipping, only minor force should be applied not to sever perforating arteries. Intracerebral hematoma in the immediate vicinity of the aneurysm should be left in place until the proximal and the distal control have been obtained. In acute SAH, thick blood clots inside the interhemispheric fissure make dissection and visualization of the A3A and the parent artery difficult. Successive irrigation with saline (“water dissection”) and gentle suction can be used to flush the clots out and to provide better room for further dissection. Only the blood clots that obstruct the approach trajectory are removed. Too extensive removal can easily damage the surrounding brain tissue. 4.6. Cerebrospinal fluid drainage The callosal cistern is shallow and not much CSF can be removed during interhemispheric approach. In unruptured A3As, this space will usually do. In acute SAH, CSF can be released by a puncture to the lateral ventricle at the lateral border of craniotomy. The alternative is puncturing the corpus callosum by closed bipolar forceps medial to the pericallosal artery followed by opening of the forceps to create a small channel for CSF release. Partial removal of ICH may also provide enough room. 4.7. Interhemispheric dissection toward A3A For the interhemispheric approach, the images should be evaluated for several microsurgical aspects such as depth of the free margin of the falx; depth of the genu of the corpus callosum; depth and course of the pericallosal and the callosomarginal arteries; correct parent A3; location of the A3A (inferior, anterior, superior); size and orientation of the dome; and possible dislocations because of ICH. Computed tomographic angiography should be carefully reviewed for calcifications in the arteries and the A3A wall. Calcified plaques in the parent artery will affect temporary clipping, and those at the dome risk intraoperative rupture and incomplete closure of the neck. Upon entering the interhemispheric fissure, bridging veins may obstruct the view, preventing even the slightest retraction of the frontal lobe. The veins are likely to restrict the working space, and one may have to work between them. It may help to dissect some of them for a few centimeters from the brain surface that is very difficult. Cutting a few small branches may allow safe displacement of the major trunk. One may have to sacrifice a smaller vein, at the risk of venous infarction though. Extensive and long-lasting use of retractors, preventing venous flow, may have the same result as severing of bridging veins. We use a handheld syringe to expose and to expand planes and to clean the blood clots for further dissection, that is, the water dissection technique of Toth [44]. Arachnoid membranes and strands are cut sharply by microscissors that can be also used as a dissector when closed. Use of retractors is kept at minimum, and they are not routinely used at the beginning of the approach. Instead, bipolar forceps in the right hand and suction in the left, with cottonoids of different sizes as expanders, are used as microretractors [20]. When the interhemispheric fissure is widely opened and the frontal lobe mobilized, the retractor may be used to retain some space for clipping, but otherwise avoided. Rolled cottons, placed inside the interhemispheric fissure at the anterior and the posterior margin of the approach, provide a more gentle retraction than classical, mechanical retractors (Fig. 10B). Inside the interhemispheric fissure, after clearing the arachnoid adhesions, dissection is directed along the falx toward the genu of the corpus callosum. It is important to be aware of the microscope's angle and the exact head position. With a wrong angle of approach, one gets easily lost inside the interhemispheric fissure with no good landmarks to guide toward the aneurysm. At the inferior border of the falx, the dissection plane is identified between the cingulate gyri attached to each other (Fig. 10A). The pericallosal artery may be found already in the cingulate sulcus, but in most cases the dissection must be continued deeper toward the corpus callosum, identified by its white color and transverse fibers (Fig. 10C-D). Taking the attached cingulate gyri as the corpus callosum or other paired arteries as the pericallosal arteries leads to serious problems of navigation. Once inside the callosal cistern, the both pericallosal arteries are visualized, realizing that they can be on either side of the midline. The artery that leads to the A3A is identified and followed to the proximal direction toward the aneurysm. A tedious and careful dissection is performed in the deep and narrow proximal interhemispheric fissure. The aim is to identify the proximal part of the parent A3 trunk. Landmarks of help are the origin of the callosomarginal artery, the genu of the corpus callosum, and possible vascular anomalies recognized in the images. Because the A3A is approached along the distal parent A3, the dome with its M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 possible rupture site is likely to get in the way, obstructing the view to the proximal parent A3, the site for temporary clipping. Premature rupture at this phase, with little room and no proximal control, may cause great difficulties. The dome often extends more to one side and is embedded in the pial layer of the cingulate gyrus. This may allow traversing along the opposite gyrus to get proximal control of the parent A3, often the most difficult part of the interhemispheric approach. The direction of the A3A may change because of retraction, and blood may obscure the anatomy, making identification of the aneurysm difficult. Strong retraction is likely to cause intraoperative rupture. We do not use partial callosal resection to enlarge exposure in the infracallosal region [10]. With appropriate head positioning and well-placed bone flap, it is possible to obtain adequate visualization of the A3As without any callosal resection [31]. In acute SAH, the cingulate gyri are usually very tightly attached to each other (see Supplementary video A3A-4). Dissection of the proximal parent artery can cause great difficulties, and it can easily damage both cingulate gyri. In such situations, at the expense of poor proximal control, the aneurysm is approached directly, and pilot clip is placed at the neck as soon as possible. With the pilot clip in place, dissection of the dome is continued. 5. Dissection and clipping of A3As 5.1. General principles Small size, thin wall, and a relatively broad base involving branches make the dissection of A3As tedious in the narrow working space. The proximal and the distal parts of parent arteries as well as all the adjacent branches should be unhurriedly and painstakingly visualized before the final clipping. A small subpial resection is often necessary to allow the mobilization and visualization of the whole A3A dome. 5.2. Dissection under temporary clipping of arteries Temporary clipping facilitates sharp dissection of the A3A and the adjacent arteries. Dissection and preparation of the sites for the temporary clip(s) should be performed with blunttipped bipolar forceps or with microdissector. One temporary clip, usually a small one, curved or straight, is applied proximal to the aneurysm. The proximal clip can be close to the aneurysm, but the distal ones should be in a distance not to interfere with the visualization and the permanent clipping of the A3A neck. In ruptured cases, CMA may require its own temporary clip. When the main part of the base is dissected, a short, straight pilot clip is applied, and the temporary clip is removed. When removed, the temporary clip should be first opened carefully in place to test if any unwanted bleeding occurs. Removal in rush can be followed by heavy bleeding and great difficulties in placing the clip back. Furthermore, while removing the temporary clip, even the slightest resistance should be noted as a possible involvement of a small branch in the clip or its applier. If access to the proximal 147 A3 cannot be achieved, direct pilot clipping is the only choice, usually a small microclip applied to the aneurysm base. Longer clips may involve or kink side branch(es). 5.3. Clipping of A3A base A proper selection of clips with different shapes and lengths of blades and applicators, suiting the imaging anatomy of the A3A, should be ready for use. A limited selection of final clips is needed when temporary clipping of the arteries and bipolar shaping of the aneurysm dome is used. The A3As are generally small with many surrounding small branches. To prevent kinking or occlusion of adjacent branches, the smallest but adequate final clip should be selected. If bipolar reshaping is not considered, the blade of a single occluding clip should be one and a half times the width of the base. We prefer inserting first a pilot clip to the A3A dome, preferring Sugita clips for their wide opening distance and blunt tips. The pilot clip is later changed for a smaller and lighter final clip, after reshaping of the dome by bipolar coagulation. In the interhemispheric approach, one has to insert the pilot clip at a rather early phase of dome dissection, if the proximal parent artery cannot be visualized, or a premature rupture occurs. Adequate dissection, proper sizes of clips, and painstaking checking that the clip blades are well placed up to their tips are required to preserve the adjacent branches. If the first clip slides, exposing some of the neck, another clip may be applied proximal to the first one for final closure (“double clipping”). Removal of the retractors and cottonoids may lead to kinking of the parent artery or compression of the perforators by the clip blades or the clip itself. The flow has to be checked once more and papaverine applied. 5.4. A3A rupture before clipping The A3As may rupture while entering the interhemispheric fissure or dissecting the aneurysm base. The risk is high because most A3As are oriented so that their dome is encountered before visualization of the proximal artery. The inferior A3As are particularly problematic for difficult proximal control. Control should be first attempted via suction and compressing the bleeding site with cottonoids. We always keep a second suction prepared and functional. Sudden and short hypotension by cardiac arrest, induced by intravenous adenosine, can be used to facilitate quick dissection and application of a pilot clip in case of uncontrolled bleeding [59]. A pilot clip may be inserted to a ruptured secondary pouch if visible. Otherwise, temporary clips are inserted proximally on the parent A3, possibly also on A3 branches such as CMA, to allow dissection of the base and final clipping. A small and thin-walled A3A may rupture at its neck during dissection. In such case, under temporary clipping, reconstruction of the base involving a part of the parent A3 in the clip should be attempted. 5.5. Intraoperative verification of clipping We use micro-Doppler to check the patency of the proximal and distal arteries and branches, but unexpected occlusions are sometimes seen in postoperative angiography 148 M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 [33]. We routinely use intraoperative noninvasive ICG infrared angiography [57,58]. It helps the orientation during dissection, and it visualizes wall thickness and plaques, perforating arteries, and incomplete neck occlusion. The ICG angiography reduces the need for invasive intraoperative angiography for clipping control, but DSA is still required in giant and complex aneurysms. trajectory is too anterior, one may become lost down in the interhemispheric fissure, not identifying the genu. In addition to the head position, the angle of the microscope should be checked as well to prevent the wrong approach angle. The corpus callosum and both A3s are identified and followed along the curved surface of the genu toward the A2-A3 junction where the A3A lies. 5.6. Resection of A3A dome When appropriate, not risking the branching arteries or other structures, we resect the aneurysm dome for final check of closure and for research purposes [15,76]. This policy teaches one to dissect domes more completely and avoid closure of the perforators. 6.1.5. Clipping The parent artery should be visualized if possible, without damaging the cingulate gyri. This may be easier from behind the A3A than in front of it (Fig. 11). If proximal control cannot be obtained, pilot clip is placed at the dome, and under its control, the final dissection is carried out. The pilot clip is exchanged for the smallest possible final clip. Care is taken not to occlude branch(es) at the A3A base. 6. Considerations for individual A3A location 6.2. Anterior A3As 6.1. Inferior A3As 6.2.1. Planning The anterior A3As are located at the midsection of the A3 segment, anterior to the genu of corpus callosum (Fig. 8A-D, see Supplementary videos A3A-3 and A3A-4). The genu does not obstruct the base of the anterior A3As as much as in the inferior A3As, so it is somewhat easier to obtain proximal control. The anterior A3As are usually oriented forward and upward. 6.1.1. Planning The inferior A3As are located at the junction of the A2 and A3 segments, inferior to the genu of the corpus callosum (Fig. 7A-D; see Supplementary videos A3A-1 and A3A-2). They require more anterior approach than the other A3As so that the genu does not obstruct the view toward the base. Inferior A3As usually point forward and slightly upward, with possible deviation to either side. Proximal control is particularly difficult to obtain in the inferior A3As. Because of their deep location and poor proximal control, inferior A3As are usually more difficult clip than anterior or superior A3As. As a rule, the more proximal the A3A lies, the more difficult the clipping is going to be as one has to work deeper in a narrower corridor. 6.1.2. Head position The head should be in neutral position with the nose pointing exactly upward with slight extension of the neck. No rotation or lateral tilting is allowed. 6.1.3. Skin incision and craniotomy After minimal shaving, an oblique skin incision with its base frontally is made just behind the hairline, over the midline, extending more to the side of the planned bone flap. Location and extent of skin incision depends on the hairline, dimensions of the frontal sinuses, and the orientation of the inferior A3A. A 1-layer skin flap is reflected frontally with spring hooks. Bicoronal skin incision is unnecessary because strong retraction with hooks often allows anterior enough exposure of the frontal bone. Paramedian frontal craniotomy is performed as anterior as possible without opening the frontal sinuses. If this happens, the sinuses should be packed and isolated with fat or muscle grafts and covered with pericranium. 6.1.4. Dissection toward the aneurysm Inside the interhemispheric fissure, the dissection is directed along the falx toward the anterior margin of the genu of the corpus callosum that is the first landmark. If the Fig. 11. Proximal control of the parent pericallosal artery (PerA) can be obtained from below the A3A. M. Lehecka et al. / Surgical Neurology 70 (2008) 135–152 6.2.2. Head position The head should be in neutral position with the nose pointing directly upward with none or only slight flexion and with no rotation or lateral tilting. 6.2.3. Skin incision and craniotomy An oblique skin incision is made behind the hairline in front of the coronal suture with the base frontally. Paramedian craniotomy is located more posteriorly than for the inferior A3As. The approach angle should point directly toward the aneurysm. 6.2.4. Dissection toward the aneurysm Dissection in the interhemispheric fissure is first directed toward the genu of corpus callosum and after both A3s have been identified the appropriate A3 is followed in the proximal direction. The dissection is directed around the aneurysm dome, and proximal A3 is exposed if possible. 6.2.5. Clipping Final, sharp dissection of the base and the dome is performed with the help of temporary clips. If proximal control cannot be obtained, pilot clip is placed at the dome, and under its control, the final dissection is carried out. The pilot clip is exchanged for the smallest possible final clip to prevent kinking. 6.3. Superior A3As 6.3.1. Planning The superior A3As, the least frequent of the A3As, are located at the distal part of the A3 segment, superior to the genu of the corpus callosum (Fig. 9A-D). The genu does not obstruct the approach trajectory and the parent proximal A3 can be usually visualized for temporary control. Superior A3As are often directed upwards, even backwards. 6.3.2. Head position The head should be in neutral position with the nose pointing directly upward, with little flexion and no rotation or lateral tilting. 6.3.3. Skin incision and craniotomy Skin incision is made behind the hairline in front of the coronal suture with the base frontally. The craniotomy is almost the same as in the anterior A3As. The approach angle should point directly toward the aneurysm. 6.3.4. Dissection toward the aneurysm Dissection in the interhemispheric fissure is directed toward the corpus callosum, which is identified along with the both A3s and followed proximally. In superior A3As, it is possible to arrive almost directly at the base of the aneurysm. Exposure of the parent A3 should be easier than in more proximal A3As. Dissection of the aneurysm is continued under proximal control. 6.3.5. Clipping The whole aneurysm base is dissected free, and all the originating branches are visualized. We usually open and 149 coagulate the aneurysm dome and then replace the pilot clip with final clip that should be as small and light as possible. 7. Associated aneurysms The A3As are often associated with other aneurysms [9,21,36,47,56,88,90]. In our series, 58% of our 163 A3A patients, and 36% of the 97 with ruptured A3A had at least one additional aneurysm (Table 5). Multiple A3As were seen in 24 patients. Most A3As can be reached under falx even if they are on the contralateral side to the craniotomy. Bilateral interhemispheric approach is not necessary. Our strategy is to clip all aneurysms that can be exposed through the same approach, that is, in this case only DACA aneurysms. This may not be advisable if clipping of the ruptured aneurysm is difficult or the brain is swollen because of acute SAH [9,62]. This technique of clipping multiple aneurysms at different locations is not recommended at early learning curve. In acute SAH, if there are problems in clipping the ruptured aneurysm, one should not continue with the unruptured one (s). We do not recommend multiple craniotomies for ruptured cases in the acute phase. 8. Giant A3As Giant A2 to A5 (pericallosal) aneurysms of are extremely rare, less than 20 published cases [11-13,18,21,36,39,46,49, 53,54,67,71,74,77]. There was only one giant A3A in our series of 4253 aneurysms (Table 3). Giant A3As may mimic symptoms of frontal tumors. Magnetic resonance imaging and DSA are essential for correct diagnosis. Clipping is considered if A3 and its branches are not heavily involved in the base. Removal of intraluminal thrombus under temporary clipping may be necessary to decompress the aneurysm base for the application of the final clip(s). If direct clipping does not seem possible, bypass and subsequent proximal occlusion of the parent A3 may be considered [30,32,51]. 9. Fusiform A3As Fusiform A3As are extremely rare, none in our series. Wrapping, proximal occlusion, excision, trapping, parent artery occlusion with preoperative bypass, and reconstruction can be considered [63]. 10. Bypass operations and arteriotomies Preoperative bypass, side-to-side A3-A3 bypass, or arterial translation (eg, STA-ACA) may be considered in giant or fusiform A3As [30,32,51]. Acute SAH makes bypasses extremely demanding. 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