Anterior Communicating Artery Aneurysm Morphology and the Risk of Rupture

Literature Review

Anterior Communicating Artery Aneurysm Morphology and the Risk of Rupture Wu Cai1,2, Chunhong Hu2, Jianping Gong1, Qing Lan3

Key words Anterior communicating artery - Intracranial aneurysm - Morphology - Rupture -

Abbreviations and Acronyms ACA: Anterior cerebral artery ACoA: Anterior communicating artery ACoAA: Anterior communicating artery aneurysm ICA: Internal carotid artery MCAA: Middle cerebral artery aneurysm SAH: Subarachnoid hemorrhage From the 1Department of Radiology, Second Affiliated Hospital of Soochow University, Suzhou; 2Department of Radiology, First Affiliated Hospital of Soochow University, Suzhou; and 3Department of Neurosurgery, Second Affiliated Hospital of Soochow University, Suzhou, People’s Republic of China To whom correspondence should be addressed: Chunhong Hu, M.D., Ph.D. [E-mail: [email protected]] Citation: World Neurosurg. (2018) 109:119-126. https://doi.org/10.1016/j.wneu.2017.09.118

- BACKGROUND:

Recently, with improvements in computed tomography angiography and digital subtraction angiography, the assessment of certain morphologic traits of anterior communicating artery aneurysms (ACoAA) has drawn great attention. The determination of specific factors associated with rupture would provide much-needed guidance for the treatment of unruptured intracranial aneurysms, such as surgical clipping or endovascular coiling. Morphologic factors include, but are not limited to, aneurysm size, number, shape, dome direction, neck/dome ratio, and relationship of the aneurysm to the surrounding vessels. However, the results of previous investigations concerning morphologic parameters have yielded inconsistent results.

- METHODS:

This review presents and analyzes the literature on the morphology of ACoAAs and risk of rupture.

- RESULTS:

This literature review reveals that the strongest predictors of ACoAA rupture are size ratio, direction of the dome, and fenestration. These were the only factors that were either unanimously or near unanimously found to be predictive of rupture across multiple studies.

- CONCLUSIONS:

The size ratio, direction of the dome, and fenestration should be examined most meticulously when deciding when to treat an ACoAA.

Journal homepage: www.WORLDNEUROSURGERY.org Available online: www.sciencedirect.com 1878-8750/$ - see front matter ª 2017 Elsevier Inc. All rights reserved.

INTRODUCTION Intracranial aneurysms result from the bulging of arterial walls secondary to several factors including flow, vessel morphology, and genetics. Incidence rates are estimated at 2%e5%,1-4 and rupture and subsequent hemorrhage may lead to severe consequences. Mortality and disability rates associated with aneurysm rupture are relatively high, and rupture also accounts for approximately 22%e25% of deaths attributed to cerebrovascular disease. Anterior communicating artery (ACoA) aneurysms (ACoAAs) are the most common site of intracranial aneurysms, accounting for about 30%e37% of intracranial aneurysms overall.5 In addition, the ACoA is the most common location of intracranial aneurysm rupture, accounting for 40% of aneurysm-related subarachnoid hemorrhages (SAHs).6-9

With the rapid development of medical imaging equipment and technology, unruptured aneurysms are readily detectable and easily characterized.10-13 However, only 1%e2% of detected aneurysms (10e30 per 100,000 per year) progress to rupture and spontaneous SAH.14-16 Treatment of ruptured aneurysms such as craniotomy, clipping, and endovascular embolization is risky and costly.17-19 Thus, careful consideration of the heavy economic burden and potential iatrogenic complications of the various treatments is necessary. To develop treatment algorithms for unruptured aneurysms, factors that contribute to a higher risk of aneurysm rupture must be determined first. Once an aneurysm is identified as being high risk for rupture, surgical clipping or endovascular embolization could then be used at an early stage. Likewise, unruptured aneurysms identified as low risk for rupture could be treated conservatively. Because it has long been believed that geometric characteristics may be used to identify dangerous aneurysms at risk for

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rupture, continued attention has been placed on the contribution of ACoAA morphology toward the risk of rupture.20-23 The association between morphologic factors and rates of rupture has become a common focus of research. Despite numerous investigations pertaining to the relationship between geometric characteristics of aneurysms and rupture, only a few consistent conclusions have resulted.24 In this analysis, we discuss the various morphologic factors that have previously been tied to ACoAA rupture, citing evidence for the strength or weakness of each factor as a predictor of rupture. MORPHOLOGIC PARAMETERS OF ACoAA Previously evaluated morphologic parameters of ACoAA include various continuous and categorical variables derived primarily from computed tomography angiography or digital subtracted angiography. Lin et al.25 reported on 1 categorical and 7 continuous morphologic parameters of ruptured and unruptured ACoAA. Subsequently, Cai

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Figure 1. (A and B) The 13 continuous variables related to anterior communicating artery aneurysm morphology as determined by three-dimensional computed tomography angiography. (Reprinted with permission from Ref. 26). AA, aneurysm angle; D, aneurysm neck diameter; H, maximum vertical height of aneurysm; Hmax, maximum height of aneurysm;

et al.26 investigated 9 continuous and 4 categorical computed tomography angiographyebased morphologic variables while simultaneously also integrating almost all previously reported morphologic parameters. Further work by additional investigators increased the investigated number of morphologic parameters to 18, including 13 continuous and 5 categorical variables.27-30 In review of further literature,25-33 reported morphologic attributes of ACoAA can be summarized as follows (Figure 1): 1) aneurysm size (S), defined as maximum aneurysm diameter; 2) maximum height of aneurysm (Hmax), defined as furthest distance from the center of the aneurysm neck to the aneurysm dome; 3) maximum vertical height of aneurysm (H), defined as maximum vertical distance from the aneurysm neck to the aneurysm dome; 4) aneurysm width (W), defined as maximum diameter perpendicular to H; 5) aneurysm neck diameter (D), defined as the maximum cross-sectional diameter of the neck of the aneurysm; 6) aspect ratio (AR), computed as the ratio between the maximum vertical height of the aneurysm and the diameter of the aneurysm neck (AR¼H/D); 7) size ratio (SR), defined as the ratio between the maximum height of the

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LA1, left A1 segment of anterior cerebral artery; LA2, left A2 segment of anterior cerebral artery; LFA, left flow angle; LPDA, left parent-daughter angle; RA1, right A1 segment of anterior cerebral artery; RA2, right A2 segment of anterior cerebral artery; RFA, right flow angle; RPDA, right parent-daughter angle; S, aneurysm size; W, aneurysm width; VA, vessel angle.

aneurysm and the average vessel diameter of all of the vessels related to the aneurysm (L_A1v, L_A2v, R_A1v, R_A2v), i.e., the vessel diameter of a particular branch of L_A1v was determined by averaging the diameter of the cross section of this vessel just proximal to the neck of the aneurysm (L_A11) with the diameter of the cross section at 1.5  L_A11 from the neck of the aneurysm (L_A12), or L_A1v ¼ (L_A11þ L_A12)/2, SR¼ Hmax/ (L_A1vþ L_A2vþ R_A1vþR_A2v)/4; 8) bottleneck factor (BF), defined as the ratio between the aneurysm width and neck diameter (BF¼W/D); 9) height/width ratio (HWR), calculated as maximum vertical height divided by width (HWR¼H/W); 10) aneurysm angle (AA), defined as the angle between the aneurysm neck and maximum height of the aneurysm; 11) vessel angle (VA), defined as the angle formed between the main vessel of the aneurysm and the plane of the aneurysm neck; 12) flow angle (FA), defined as the angle between the maximum height of the aneurysm and the main blood vessel; 13) parent-daughter angle (PDA), defined as the angle between the A1 artery and the ipsilateral A2 artery; 14) direction of the aneurysm dome, either anterior or posterior (this was determined by drawing a straight line parallel to the anterior cranial fossa and through the

ACoAA, and then drawing a straight line perpendicular to this line to form 4 equal quadrants, with anterior defined as the 2 quadrants anterior to the second line and posterior defined as the 2 quadrants posterior to the second line); 15) shape of the aneurysm, with regular shape defined as simple saccular aneurysms and irregular shape defined as saccular aneurysms with additional daughter domes or blebs; 16) number of aneurysms, divided into single or multiple aneurysms; 17) variation of the A1 segment, including dominance of the A1 segment (defined as >33% of the difference between the A1 segment diameters34), as well as hypoplasia and absence of an A1 segment; and 18) ACoA fenestration, defined as the ACoA trunk dividing into 2 branches and then converging into only 1 trunk. ANEURYSM SIZE, WIDTH, AND NECK DIAMETER It has been widely reported that aneurysm size and width linearly correlate with the risk of rupture of ACoAA.12,27,29,30,32 Moreover, Choi et al.32 reported that ACoAA size >7 mm was more likely to rupture. However, rupture of smaller anterior circulation aneurysms account for

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more cases of SAH, perhaps because of the overall greater prevalence of smaller aneurysms. Bijlenga et al.35 found that aneurysms of the ACoA or distal anterior cerebral artery (ACA) between 4 and 7 mm have similar odds of rupture to posterior circulation aneurysms. Given these findings, size was not deemed an independent risk factor for rupture of ACoAA, posing a dilemma for clinicians to make treatment decisions for small unruptured ACoAAs. Although numerous studies9,13,14,23,36-38 have concluded that aneurysm neck diameter is a major risk factor for aneurysm rupture overall, investigators solely looking at ACoAAs have reported no significant difference in neck diameter between those that are ruptured or unruptured.25,26,39 Pathologically, aneurysm neck diameter is in part related to abnormal hemodynamics. The size of the entrance to the aneurysm, which is indirectly reflected by the neck diameter of the aneurysm, in part results from changes in shear stress caused by blood flow. In addition, the impact from abnormal blood flow may further result in rupture of the aneurysm. Because some studies fail to show a correlation between aneurysm neck diameter and rupture, these findings suggest that aneurysm rupture must be affected by many other factors in addition to shear stress from abnormal hemodynamics. Overall, the findings suggest that size and width may predict ACoAA rupture, but neck diameter likely does not. MAXIMUM HEIGHT OF THE ANEURYSM Maximum height of the aneurysm reflects overall size and is the most commonly used morphologic factor to predict aneurysm rupture. Various studies9,13,14,23,36-38 have shown that size of the aneurysm is a major risk factor for aneurysm rupture overall. Although ISUIA (International Study of Unruptured Intracranial Aneurysms) showed that the probability of rupture is small when the maximum height of an ACoAA is <7 mm, in most subsequent studies, the relationship between maximum aneurysm height and risk of rupture is still controversial, with variations across regions and ethnicities. Cai et al.26 and Maiti et al.28 found no significant difference in maximum

MORPHOLOGY AND RUPTURE OF ACOAA

aneurysm height between ruptured and unruptured ACoAAs. In subsequent studies, the focus has turned toward the ratio of maximum aneurysm height to diameter of the parent artery of the aneurysm, which is defined as the size ratio. This morphologic parameter of the aneurysm integrates 2 single factors in determining the risk of aneurysm rupture. These findings suggest that maximum aneurysm height is not a good predictor of ACoAA rupture. MAXIMUM VERTICAL HEIGHT OF THE ANEURYSM Cai et al.26 and Maiti et al.28 in addition determined that the maximum vertical height of the aneurysm was not significantly different between ruptured and unruptured ACoAAs. Moreover, maximum vertical height of the aneurysm is only used as a morphologic measurement index in most studies, and it has been used for calculation of the ratio of maximum vertical height of the aneurysm to aneurysm neck diameter, which defines the aspect ratio. Thus, maximum vertical height of the aneurysm tends not to be separately analyzed.25,26,31,32 This situation is probably because the aneurysm neck diameter and maximum vertical height of the aneurysm are interrelated rather than independent factors, and their interaction with each other affects the rupture rate of ACoAA. There are few data regarding the morphologic parameter maximum vertical aneurysm height. In many studies, this variable tends not to be separately analyzed. The few studies that have examined it have not found it to be predictive of ACoAA rupture. Maximum vertical height of the aneurysm is a weak predictor of rupture. ASPECT RATIO The aspect ratio is related to blood flow hemodynamics and plays an important role in determining aneurysm rupture risk. The aspect ratio has been evaluated in many studies, but there is no consensus for its measurement method or threshold value.40-42 Beck et al.40 reported a higher risk of rupture when the aspect ratio of an aneurysm exceeds

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1.6. Raghavan et al.43 also concluded that ruptured aneurysms have higher aspect ratios. These studies examined all types of aneurysms. When ACoAAs were examined on their own, the impact of aspect ratio on aneurysm rupture was less compelling if present at all. Although Lin et al.25 reported that the aspect ratio significantly differed between ruptured and unruptured ACoAAs, binary logistic analysis showed that the aspect ratio was not associated with ACoAA rupture. Furthermore, Cai et al.26 and Maiti et al.28 found no significant difference in the aspect ratio between ruptured and unruptured ACoAAs. On the other hand, some studies44,45 have shown a correlation between the aspect ratio and aneurysm rupture of middle cerebral artery aneurysms (MCAAs) and posterior communicating artery aneurysms. These findings suggest that the usefulness of the aspect ratio in predicting ACoAA rupture is controversial at best. The lack of predictive ability of the aspect ratio after multivariate analyses in multiple studies seems to overshadow the single study in which it was shown to be a significant predictor. SIZE RATIO The size ratio is a comprehensive parameter that includes the diameter of the parent artery and plays a vital role in determining the growth and rupture potential of intracranial aneurysms.46 Aneurysms with higher size ratios may show more complex flow, multiple vortices, and lower aneurysmal wall shear.45 Dhar et al.44 concluded that size ratio was the strongest correlative morphologic parameter in predicting aneurysm rupture. Lin et al.25 and Cai et al.26 further confirmed this theory to be true for ACoAAs as well, and their binary logistic analysis in addition showed that size ratio was the strongest predictive factor of ACoAA rupture. Several studies have shown that all types of aneurysms with a higher size ratio have a higher propensity to rupture.25,26,42,44 45 Furthermore, Lv et al. and Jiang et al.27 also reported that size ratio was an independent risk factor for rupture of small posterior communicating artery aneurysms as well as multiple other

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intracranial aneurysms in the anterior circulation. Overall, the findings suggest that size ratio is a strong and unanimous predictor of ACoAA rupture. BOTTLENECK FACTOR AND HEIGHT/ WIDTH RATIO The bottleneck factor and height/width ratio have been shown to be significantly different between ruptured and unruptured aneurysms for multiple types of anterior circulation aneurysms, and these morphologic factors may also accurately determine the propensity of intracranial aneurysms to rupture.27 Although Wang et al.30 reported that bottleneck factor and height/width ratio were positively associated with aneurysm rupture, binary logistic regression models failed to show a significant correlation between these factors and ACoAA rupture. Elsharkawy et al.47 reported that the bottleneck factor and height/width ratio were associated with rupture of MCAA. However, additional multivariate analyses showed a significant correlation between MCAA rupture and the height/width ratio only, not the bottleneck factor. Although the height/width ratio may offer some predictive value for rupture in other aneurysms, the evidence suggests that it is not a good predictor of rupture for ACoAAs. The bottleneck factor is also likely not a good predictor of rupture for ACoAAs as well. ANEURYSM, VESSEL, FLOW, AND PARENT-DAUGHTER ANGLES Dhar et al.44 reported an association between aneurysm angle (AA) and rupture of aneurysms of all types. However, in subsequent studies in which ACoAAs were examined alone,25,26,28 AA was not significantly different between ruptured and unruptured aneurysms, and further binary logistic analyses also failed to show a correlation between AA and rupture of ACoAAs. Vessel angle may be related to the direction of blood flowing into an aneurysm. A larger vessel angle theoretically leads to a larger impact from blood flowing into the aneurysm and thus leads to a greater risk of rupture. However, Cai et al.26 reported no significant difference in vessel angle

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MORPHOLOGY AND RUPTURE OF ACOAA

between ruptured and unruptured ACoAAs, and binary logistic analysis also failed to show an association with rupture of ACoAAs. Lin et al.25 in addition reported that the vessel angle was not associated with ACoAA rupture. Flow angle reflects the relationship between an aneurysm and the structure of its surrounding vessels, which determines the direction of the aneurysm dome.25,26 Anterior circulation aneurysms have larger flow angles, and Lin et al.25 used binary logistic analysis to show that ruptured ACoAAs tended to have larger flow angles. However, Shao et al.29 and Cai et al.26 reported that flow angle was not significantly different between ruptured and unruptured ACoAAs. On the other hand, some studies25,39,44 have shown significantly larger flow angles in ruptured versus unruptured aneurysms for several other types of aneurysms, including MCAAs. The parent-daughter angle also reflects the relationship between an aneurysm and its surrounding vessels.25,26 Lin et al.25 reported that ruptured ACoAAs tended to have smaller parent-daughter angles, and binary logistic analysis confirmed an association with rupture. Although Cai et al.26 also reported a significant difference in the parent-daughter angle between ruptured and unruptured ACoAAs, binary logistic analysis did not confirm an association with rupture. On the other hand, other studies have concluded that smaller parent-daughter angles are predictive of MCAA rupture.44 The various angles that aneurysms take have been studied relatively extensively in ACoAAs. Although there is no definite evidence that any of the noted angles can predict ACoAA rupture, the flow and parent-daughter angle probably have the highest potential to be able to do so. There is strong evidence that the aneurysm and vessel angle are unable to predict rupture in ACoAAs. DIRECTION OF ANEURYSM DOME The direction of the aneurysm dome reflects aneurysm hemodynamics, including shear stress on the wall, wall pressure, blood flow velocity, and impact force, which all play important roles in the growth and potential rupture of an aneurysm.48,49 Shao et al.29 reported a higher

proportion of anteriorly directed aneurysms among ruptured ACoAAs compared with posteriorly directed aneurysms. Shao et al. also noted that anteriorly directed aneurysms tended to have larger size ratios. Matsukawa et al.39 further confirmed a higher rate of rupture for anteriorly directed ACoAAs. Although Cai et al.26 were able to confirm that anteriorly directed aneurysms were more likely to rupture, binary logistic analysis did not show a significant correlation with rupture. The direction of the aneurysm dome is a categorical variable, because the dome can be directed either anteriorly or posteriorly. There is strong evidence across 3 distinct studies that the direction of the dome can predict ACoAA rupture, with an anterior direction leading to significantly higher odds of rupture.

SHAPE AND NUMBER OF ANEURYSMS Irregular aneurysm shape is seen in a higher proportion of ruptured aneurysms and is believed to be caused by unstable blood flow patterns.30 Various 25,32,39,41,49 studies have reported that the presence of aneurysm blebs is associated with rupture of ACoAAs. However, Cai et al.26 did not observe a significant correlation between irregular shape and rupture, although they noted the caveat that the ACoAAs in their study had fewer and smaller blebs compared with ACoAAs in previous studies. Furthermore, Elsharkawy et al.47 reported that irregular aneurysm shape was independently correlated with rupture of MCAAs. Recently, the association between rupture risk and the presence of multiple intracranial aneurysms has been investigated.27,34 Lin et al.25 reported that unruptured ACoAAs were more likely to be accompanied by additional aneurysms. However, Cai et al.26 did not observe a significant predictive effect of multiple aneurysms (vs. a single aneurysm) on ACoAA rupture. The predictive value of aneurysmal shape and number of aneurysms is controversial. For both variables, there are studies that show a predictive relationship and studies that show no such relationship. Therefore, both shape and number of aneurysms should be considered as mid

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MORPHOLOGY AND RUPTURE OF ACOAA

Table 1. Findings of Studies Examining the Morphologic Parameters of Anterior Communicating Artery Aneurysms Study

Year

Number of Patients

Factor(s) Predictive of ACoAA Rupture (P Value) (OR)

Factor(s) Not Predictive of ACoAA Rupture (P Value) (OR)

Lin et al25

2013

79

SR (P ¼ 0.03) (OR, 1.28), FA (P ¼ 0.04) (OR, 1.05), PDA (P ¼ 0.04) (OR, 0.95), Nm (P ¼ 0.04)

D (P ¼ 0.60) (OR, 0.99), AR (P ¼ 0.26) (OR, 3.97), AA (P ¼ 0.10) (OR, 1.02), VA1 (P ¼ 0.09) (OR, 1.00)

Cai et al26

2015

80

SR (P ¼ 0.006) (OR, 411.08)

D (P ¼ 0.899) (OR, 1.081), Hmax (P ¼ 0.059) (OR, 0.148), H (P ¼ 0.532) (OR, 1.416), AR (P ¼ 0.388) (OR, 0.177), AA (P ¼ 0.761) (OR, 0.993), VA (P ¼ 0.132) (OR, 0.982), FA (P ¼ 0.247) (OR, 1.017), PDA (P ¼ 0.075) (OR,0.939), DoD (P ¼ 0.092) (OR, 3.749), Sh (P ¼ 0.170) (OR, 2.620), Nm (P ¼ 0.107) (OR, 3.466), VA1 (P ¼ 0.430) (OR, 0.459)

Maiti et al28

2016

30

29

Shao et al

2016

503

SR (P ¼ 0.005) (OR, 1.46)

S, Hmax, H, D, AR, FA, VA, AA, VA1, Sh, Nm

31

Cai et al

2014

112

AR (P ¼ 0.008) (OR, 2.127), FA (P ¼ 0.003) (OR, 1.102), PDA (P ¼ 0.005) (OR, 0.891)

D, SR, AA,

Choi et al32

2016

255

S (P ¼ 0.044), F (P ¼ 0.026) (OR, 4.135), AR (P ¼ 0.009) (OR, 3.138), Sh (P < 0.001) (OR, 5.998), DoD (P ¼ 0.023) (OR, 2.802)

VA1 (P ¼ 0.247)

Wang et al33

2015

57

SR (P ¼ 0.162) (OR, 0.032), FA (P ¼ 0.444) (OR, 0.982), PDA (P ¼ 0.143) (OR, 1.125), VA1 (P ¼ 0.154) (OR, 0.006)

2013

932

S

Matsukawa et al

2013

140

S (P ¼ 0.035) (OR, 3.2), Sh (P < 0.001) (OR, 2.2), DoD (anterior) (P ¼ 0.0012) (OR, 6.0)

Flores et al51

2013

156

VA1 (P ¼ 0.02e0.04)

Bijlenga et al35 39

D, W, Hmax, H, AR, SR, AA, VA, FA, PDA, BF

D (P ¼ 0.46), Nm (P ¼ 0.081), FA (P ¼ 0.77), VA1 (P > 0.05)

ACoAA, anterior communicating artery aneurysm; OR, odds ratio; SR, size ratio; FA, flow angle; PDA, parent-daughter angle; Nm, number of aneurysms; D, aneurysm neck diameter; AR, aspect ratio; AA, aneurysm angle; VA1, variation of the A1 segment; Hmax, maximum height of aneurysm; H, maximum vertical height of aneurysm; VA, vessel angle; DoD, direction of aneurysm dome; Sh, shape; W, aneurysm width; BF, bottleneck factor; S, aneurysm size; F, fenestration.

predictors of ACoAA rupture, at best, and certainly not as strong predictors.

VARIATION OF THE A1 SEGMENT AND FENESTRATION The ACoA complex is composed of the ACoA and the A1 and A2 segments of the bilateral ACAs. Anatomic variations of the ACoA complex are common, and these correspond to locations in which aneurysms often occur.26,50 Many studies have reported an association between ACoAAs and imbalanced growth of the A1 segment, which has been termed dominance of A1 segment. Additional risk factors for ACoAA rupture include hypoplasia and absence of the A1 segment of the ACA as well as a smaller angle between the A1 and A2 segments of the ACA.25,26 On the other hand, the shape and bifurcation angle of ACoAAs contribute toward reducing energy consumption of arterial flow and shear stress of blood flow on the

wall. Several studies51 have shown that the ratio of the diameters of the A1 and A2 segments of the ACA influence the rupture potential of ACoAAs, with larger A1-A2 diameter ratios being associated with a higher risk of rupture. The higher rupture incidence of ACoAAs compared with other intracranial aneurysms7-9 is mainly attributed to the relatively complex anatomy and hemodynamics of the ACoAA compared with other locations.52 Some studies53 have shown that ACoAAs often occur in the A1 segments of ACAs with thicker diameters, on the A1 dominant side, and at the junction of the A1 and A2 segments. However, investigations by Lin et al.25 and Cai et al.26 found no significant difference in the presence of variations of the A1 segment between ruptured and unruptured ACoAAs. Some reports8,54 suggest that a >50% difference in diameter of the A1 segment with the contralateral side results in larger shear

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stress on the wall, leading to a higher incidence of rupture of the ACoAA. Thus, dominance of the A1 segment of the ACA may be an important risk factor in the development of ACoAAs. In patients without identifiable intracranial aneurysms, but with dominance of an A1 segment, there may be a role for followup observation. Fenestrations are often seen in the ACoA and distal A1 segment of the ACA. De Gast et al.55 found that formation of ACoAAs was frequently associated with ACoA fenestration. In addition, Choi et al.32 reported a significantly higher percentage of ACoA fenestration among ruptured aneurysms, because the medial walls of the fenestrated artery lack the medial layer and have a thin subendothelium. Variations in the A1 segment and fenestrations both have potential as predictors of ACoAA rupture. Multiple studies have shown that A1 variations may be a

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Table 2. The Strength of Evidence Showing Association Between Each Morphologic Parameter and Anterior Communicating Artery Aneurysm Rupture

Morphologic Parameter Size

Strength of Evidence Demonstrating Association with Rupture Mid

Maximum height

Weak

Maximum vertical height

Weak

Width

Mid

Neck diameter

Weak

Aspect ratio

Mid

Size ratio*

Strong

Bottleneck factor

Weak

Height/width ratio

Weak

Aneurysm angle

Weak

Vessel angle

Weak

Flow angle

Mid

Parent-daughter angle

Mid

Direction of aneurysm dome*

Strong

Shape of the aneurysm

Mid

Number of aneurysms

Mid

Variation of the A1 segment Fenestration*

Mid Strong

*The top 3 factors that should be taken into account by a treating physician during daily clinical practice in determining risk of rupture.

significant predictor of ACoAA rupture. Still others have shown that variation in the A1 segment may not be a great predictor of ACoAA rupture. The difficulty with A1 variation as a predictor is the large number of different possible variations that can be observed. Although several of these variations seem to be associated with ACoAA rupture in single studies, additional studies have examined different types of variations. Further still, although some types of variations may be strong predictors of ACoAA rupture, other types of variations may not predict rupture at all. Fenestrations appear to be good predictors of ACoAA rupture. The weakness in the literature lies in the few patients who

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have been examined both because of the limited number of studies as well as the relative rarity of the finding. ACoA VERSUS OTHER SITES Previous research has suggested that ACoAAs are more prone to rupture than are internal carotid artery (ICA) aneurysms or MCAAs but less so than are posterior circulation aneurysms (aneurysms of the posterior communicating artery, basilar artery, or vertebral artery).56 It is not entirely clear what accounts for the varying risk of rupture across these different sites. One factor is likely blood flow. Although the ICA terminus and the tip of the basilar artery almost certainly see more blood flow than the ACoA because of the larger size of these vessels, ICA terminus aneurysms are less likely to rupture than ACoAAs, although basilar tip aneurysms are more likely to rupture than ACoAAs. Certainly, then, blood flow is not the whole story. More turbulent flow probably plays a role as does morphology of the resultant aneurysm, which is likely related to degree of turbulent flow. Previous research has shown that shape may be more important than size in determining rupture risk.43 Further, shape can be divided into regular and irregular, with additional irregular shapes including daughter sacs and bilobulated sacs and other irregularities existing on top of single sac aneurysms. There are numerous potential irregularities that all probably come with different risks of increased rupture. Further, fusiform aneurysms are also more likely to rupture. The closely related wall shear stress also likely has a role in both aneurysm formation and rupture. Previous research57 has shown that both high and low wall shear stress can induce aneurysm growth and even rupture. The ACoA is simply a more common site of aneurysm. This site should have a relatively higher risk of rupture. Findings of studies examining the morphologic parameters of ACoAAs are listed in Table 1. CONCLUSIONS This literature review suggests that the strongest predictors of ACoAA rupture are size ratio, direction of the dome, and fenestration (morphologic parameter

strength of evidence showing association with rupture is given in Table 2). These were the only factors that were either unanimously or near unanimously found to be predictive of rupture across multiple studies. These factors should be examined most meticulously when deciding when to treat an ACoAA. REFERENCES 1. Andreasen TH, Bartek J Jr, Andresen M, Springborg JB, Romner B. Modifiable risk factors for aneurysmal subarachnoid hemorrhage. Stroke. 2013;44:3607-3612. 2. Dupont SA, Wijdicks EF, Lanzino G, Rabinstein AA. Aneurysmal subarachnoid hemorrhage: an overview for the practicing neurologist. Semin Neurol. 2010;30:545-554. 3. Rinkel GJ, Djibuti M, Algra A, van Gijn J. Prevalence and risk of rupture of intracranial aneurysms: a systematic review. Stroke. 1998;29: 251-256. 4. Zacharia BE, Hickman ZL, Grobelny BT, DeRosa P, Kotchetkov I, Ducruet AF, et al. Epidemiology of aneurysmal subarachnoid hemorrhage. Neurosurg Clin North Am. 2010;21:221-233. 5. Andaluz N, Van Loveren HR, Keller JT, Zuccarello M. Anatomic and clinical study of the orbitopterional approach to anterior communicating artery aneurysms. Neurosurgery. 2003;52: 1140-1149. 6. Dehdashti AR, Chiluwal AK, Regli L. The implication of anterior communicating complex rotation and 3-dimensional computerized tomography angiography findings in surgical approach to anterior communicating artery aneurysms. World Neurosurg. 2016;91:34-42. 7. Inagawa T. Site of ruptured intracranial saccular aneurysms in patients in Izumo City, Japan. Cerebrovasc Dis. 2010;30:72-84. 8. Inagawa T. Risk factors for the formation and rupture of intracranial saccular aneurysms in Shimane, Japan. World Neurosurg. 2010;73:155-164. 9. Kassell NF, Torner JC, Haley EC Jr, Jane JA, Adams HP, Kongable GL. The international cooperative study on the timing of aneurysm surgery. Part 1: overall management results. J Neurosurg. 1990;73:18-36. 10. Komotar RJ, Starke RM, Connolly ES. The natural course of unruptured cerebral aneurysms. Neurosurgery. 2012;71:N7-N9. 11. Millon D, Derelle AL, Omoumi P, Tisserand M, Schmitt E, Foscolo S, et al. Nontraumatic subarachnoid hemorrhage management: evaluation with reduced iodine volume at CT angiography. Radiology. 2012;264:203-209. 12. Morita A, Kifino T, Hashi K, Aoki N, Fukuhara S, Hashimoto N, et al. The natural course of unruptured cerebral aneurysms in Japanese cohort. N Engl J Med. 2012;366:2474-2482.

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Conflict of interest statement: This work was supported by grant number SYSD2016088 from the Suzhou Municipal Science and Technology Guiding Project. Received 10 July 2017; accepted 18 September 2017 Citation: World Neurosurg. (2018) 109:119-126. https://doi.org/10.1016/j.wneu.2017.09.118 Journal homepage: www.WORLDNEUROSURGERY.org Available online: www.sciencedirect.com

57. Meng H, Tutino V, Xiang J, Siddiqui A. High WSS or low WSS? Complex interactions of hemodynamics with intracranial aneurysm initiation,

1878-8750/$ - see front matter ª 2017 Elsevier Inc. All rights reserved.

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