The most common site of intracranial aneurysms worldwide is the anterior communicating artery which is the junction between the two anterior cerebral arteries (ACAs). The quoted incidence of ACom artery aneurysms ranges from approximately 40% in the International Cooperative Study on the Timing of Aneurysm Surgery study to 45% in the International Subarachnoid Aneurysm Trial study.[1,2] ACom has a complex anatomical relationship with the neighbouring blood vessels and perforators. Therefore, in a surgical or endovascular approach to the ACom complex, attention is paid not only to the aneurysm morphology but also to the anatomy of the neighbouring region. Microsurgical clipping of the ACom is the oldest and the time-tested approach. Although there are recent advances in endovascular technology, the complex anatomical relationship, haemodynamic alterations in the collaterals and unfavourable dome/height aspect ratio preclude solution to all the ACom complex aneurysms by the endovascular route.
CAN EMBRYOLOGICAL DEVELOPMENT OF THE ANTERIOR COMMUNICATING ARTERY BE THE REASON FOR MAXIMUM ANEURYSMS?
Theoretically, the ACom artery is a symmetrical channel of communication between the two ACAs at the junction of segments A1 and A2. However, this is not always true. Often during embryological development, many variations and asymmetries develop in the anterior circulation, leading to the known common congenital anomalies, including multiple AComs, fenestrated AComs and triplication or azygous type of the A2 segment.
Embryological reasons for the development of anterior communicating artery aneurysms can also be attributed to variations in the anatomy of the A1 segment of the ACAs. The most commonly reported variation is the unequal diameter of the A1 segment, which has been reported in as high as 85% of patients with ACom artery aneurysms.[3,4] This inequality in the calibre of the A1 segments leads to a larger diameter of the ACom artery; the higher the difference, the greater the chance of developing an aneurysm.
MICROSURGICAL ANATOMY OF THE ANTERIOR COMMUNICATING ARTERY REGION: A ROADMAP FOR MASTERLY SUCCESSFUL NAVIGATION
The A1 segment, described by Fischer in 1938, also known as the pre-communicating segment, is a portion of the ACA proximal to the origin of the ACom. Arising from ICAs, these segments present with similar diameters in most unaffected individuals, whereas patients with ACom aneurysms are up to 35% more likely to have one segment significantly larger than the other. This was subsequently confirmed in a later study by Rhoton, which revealed that a difference of 1 mm or greater in the size of A1 segments caused the average size of the ACom artery to increase by over 100% (1.2–2.5 mm). The 2018 study on the anterior vasculature of the brain in over 500 adults with no cerebrovascular disease by Shatri et al. showed that 34% of females and 37.8% of males presented with at least one of the several possible variations of the A1 segment.
Post-communicating, infracallosal or the vertical segment of the ACA, synonymous with the A2 segment, is found entering the interhemispheric fissure along the rostrum of the corpus callosum, extending up to the genu. Unlike the A1 segment, the A2 segment does not seem to affect the incidence of ACom aneurysms merely based on size, but rather based on the angle between the ACom and the A2 segment, which have a directly proportional relationship as described by Zhan et al.
The anterior communicating artery
Unlike most vascular structures, the anterior communicating artery serves unique functions in different individuals and is irreplaceable. Either as a connector of the two vital anterior cerebral vessels or as a direct supply of important structures, such as the optic chiasm and hypothalamus, the ACom is a site of extensive anatomical variation. The 2013 study by Kardile revealed that one in three people has a variant version of ACom, with superior ACom being the most frequent variant associated with most aneurysms. Among all the vascular structures in the cranium, ruptured aneurysms of the ACom are the leading cause of spontaneous SAH, accounting for more than a quarter of all such cases. Kirgis et al. and Yasargil described a simple and complex ACom.[8,9] In the simple ACom (found in 40% of trunks), ACom attaches to bilateral A1. In complex ACom (found in 60% of trunks), the trunk includes two branches: the 'Y', 'H' or 'X' type, fenestration deformity or the dimple or 'O' type.
The A1 segment gives rise to medial lenticulostriate arteries and ACom. The medial lenticulostriate arteries supply a portion of the globus pallidus and putamen, whereas the A2 segment has several branches, including the Heubner artery, orbitofrontal artery, frontopolar artery and basal perforating branches. These vessels supply the striatum, optic chiasm and anterior hypothalamus. Among these branches, the recurrent artery of Heubner is the most important from the surgeon's perspective. As described by the German paediatrician Huebner in 1872, this artery is a branch of the A2 segment in over three-fourths of cases. The remainder one-fourths arise from either the A1 segment or the ACom and are occasionally absent. In most cases, this vessel originates within 0.4 cm from the ACom junction. The anterior limb of the internal capsule, globus pallidus and anterior striatum is supplied by the Heubner artery. Hence, injury to this vessel usually causes reversible paresis of the contralateral upper limb and face.
Almost half of the medial lenticulostriate arteries arising from the A1 segment terminate in the anterior perforated substance, with most arising from the proximal half of A1. The remaining medial lenticulostriate supplies to the optic chiasm, optic tract, optic nerve and hypothalamus. While some studies have reported that the ACom has no to four perforators, newer studies have shown at least three perforators. Perforators primarily arise from the superior surface of both the A1 and the ACom regions and usually terminate in the suprachiasmatic region. As in any region with several small arteries, the suprachiasmatic renders another challenge for neurosurgeons to manoeuvre around ACom aneurysms.
Cisterns and Gyral relationship
The A1, A2 and ACom segments are closely related to the three cisterns, namely, the carotid, chiasmatic and lamina terminalis cisterns. These cisterns assist neurosurgeons in determining their proximity to the surrounding vital structures such as the optic nerve, olfactory nerve, optic chiasm and infundibulum. The anterior communicating artery is associated with the lamina terminalis, whereas the optic and olfactory nerves are closely associated with the chiasmatic cistern. The gyrus rectus or straight gyrus is among the medially found pre-central gyri in the frontal lobe. Overlying the basal region of the frontal lobe often requires manipulation during surgery involving or targeting the inferior region, most commonly aneurysms of the anterior communicating artery. Despite being a part of Brodmann area 11 which is responsible for cognition, planning and differentiating between rewards and reprimands, the gyrus rectus is yet to be assigned a discrete function.
THE DIAGNOSTIC CHALLENGE
ACom aneurysms have long been crowned as the most dangerous intracranial aneurysms. With the highest mortality rate among all intracranial aneurysms, one of the several reasons for this is that ACom aneurysms have the greatest incidence of false-negative CT angiography reports ranging between 20% and 60% of patients with SAH as reported by Iwanaga et al. and Van Rooij et al. in their respective studies.[10,11] Van Rooij et al. also reported this could be due to equal flow rates in the bilateral A1 segments causing inadequate infiltration with contrast due to the almost equal opposing pressure gradients across the anterior communicating artery. These values were derived from both the above studies where clinical sorting of spontaneous and traumatic causes of SAH was not performed before determining the false-negative rate of computed tomography (CT) angiography. Hence, in patients with spontaneous SAH, the number of false-negative CT angiography could be significantly higher. In the diagnosis of an ACom aneurysm, variances of the artery have to be kept in mind. There are significant variances in the trunk of ACom, up to 60%, with the fenestration deformity being the most common.[12,13] It is important to keep the fenestration deformity in mind when CT angiography is used to evaluate the ACom region. This is because the CT angiography slice thickness affects the spatial resolution and the fenestration deformity might be misdiagnosed as an aneurysm.[12,13] Therefore, digital subtraction angiography should be used in the evaluation of ACom aneurysms in all doubtful cases. Most aneurysms remain asymptomatic until they rupture. Only a fifth of patients with aneurysms present with pre-rupture headaches and much less with cranial nerve palsies.[12,13] Due to the anatomic location of the anterior communicating artery, there is sufficient room for the growth of an ACom aneurysm and pre-rupture leak before presentation with a headache. Hence, almost all cases are only diagnosed post-rupture, unless incidentally diagnosed pre-operatively.
RISK OF RUPTURE OF ACOM ANEURYSMS AND FACTORS INFLUENCING SURGICAL MANAGEMENT
The unruptured cerebral aneurysms study of the Japanese cohort reported that the risk of rupture increased with the increasing size of the aneurysm, presence of daughter sac and location. The authors reported that the risk of rupture was more for the posterior and anterior communicating artery aneurysm than the middle cerebral artery aneurysm. Due to these varying results and unclear natural history, several treatment algorithms have been suggested for decision-making. All these algorithms assess the risk of rupture in unruptured aneurysms. Recent technological advances include the detection of altered pulsation during the cardiac cycle as a predictor of rupture in unruptured aneurysms in the elderly. Long-term follow-up results on the natural history of unruptured aneurysms found that the annual rupture risk was 1.1% and cumulative risk was 10.5% at 10 years; however, no index aneurysm ruptured after a gap of 25 years. They also found that cigarette smoking, size >7 mm, ACom location and patient age inversely affected the rupture risk of the unruptured aneurysm. Studies found that the risk of rupture of ACom aneurysm of size 4–7 mm is similar to the posterior circulation aneurysm and microaneurysms <3 mm rupture more frequently in a combination of hypertension and ACom location.[16,17] Therefore, it is suggested that there should be an active intervention in ACom aneurysms irrespective of size once they are diagnosed.
Projections of the ACom aneurysm have significance in the surgical approach and difficulty in clipping. Furthermore, the projection determines the risk of intraoperative rupture of the aneurysm. The different projection creates different haemodynamic stress on the aneurysm wall, growth of the aneurysm and rupture. Several studies have found that the risk of rupture is highest with the anterior projecting aneurysm than the posterior projection and also the Fisher grade and poor outcome with the anterior projecting aneurysm.[19,20,21,22]
Suzuki et al. reported the method to identify the projection of aneurysms on DSA and lateral CT angiogram images. In that report, the aneurysm dome was treated as a base, the parallel line of the sphenoid platform as the transverse axis and the longitudinal axis perpendicular to the transverse axis. With this, the projections of the ACom aneurysm identified were: anterosuperior, anteroinferior, posterosuperior and posteroinferior. The superior projection of the aneurysm is most commonly seen accounting for 19%–37%.[19,24,25]
A superiorly projecting ACom aneurysm is closely located in the depths of the cerebral hemispheres. It is challenging to separate the aneurysm and expose the contralateral A2 segment during surgery because the junction of the A1-A2 segments of the ACoA and the closed A2 segment can easily block the main body and the neck of the aneurysms. The gyrus rectus may need to be removed to expose the aneurysm; as a result, the procedure is more challenging and there are more post-operative complications.
In the posterior projecting aneurysm, there can be an injury to the recurrent artery of Heubner and contralateral A1, while the aneurysm dome is dissected and isolated.
CHOOSING THE SIDE OF APPROACH
While the size and extensions of the craniotomy are dependent on whether the aneurysm has presented with subarachnoid haemorrhage with or without intraparenchymal or intraventricular bleed or not, the side of the craniotomy is often independent of it. The side of the approach is determined largely by the aneurysm projection and the relative anatomy of the bilateral A2 vessels. For the selection of the operative side, the conventional idea is to use the non-dominant hemisphere side and the A1 dominant blood supply side, unless the dominant side has multiple aneurysms. Often the aneurysm dome is pushed opposite to the side of dominant A1 due to the downstream pressure of preferential blood flow. This, in turn, provides a better proximal control and better chances of exposing the neck before handling the fragile dome.
This action plan is now being revisited, with many groups advocating the orientation of the A2 fork as the primary determinant of the side of approach for some aneurysms with special directions (superiorly projecting aneurysms), as the anteroposterior relationship of bilateral A2 could affect the exposure of the aneurysm neck, the difficulty of operation, the resection rate of the gyrus rectus and the post-operative complications. The effectiveness of surgical clipping may be impacted by the difficulties of surgically exposing the bilateral A1 and A2 and aneurysm neck on opposite sides. It is argued that a successful application of aneurysm clip along the line of sight, to completely occlude the neck, is better achieved when applied from the side where the fork is open. In addition, these groups also report that there is no compromise in the proximal control as bilateral A1s are equally well secured, if the head positioning is correct, and adequate cerebral relaxation has been achieved. When negotiating the clip blade across the neck from the side where the fork is open, there are lesser chances of compromising the flow across the remaining ACom vessel and the bilateral A2 as they are under direct vision. When approaching from the other side, the contralateral A2, as well as the ACom artery, shall always remain at least partially hidden. Furthermore, when approaching from the closed side of the A2 fork, the clip has to be brought in at an angle which requires more retraction of the brain and vessels, increasing the chances of parenchymal contusions and intraoperative dome rupture.
Finally, as a thumb rule could be that for the superiorly projecting aneurysms, the plane of the A2 is more important; whereas for inferiorly projecting aneurysms, it is the dominant A1 which is a better determinant for the side of the approach.
POSITIONING IN ACOM ANEURYSMS
As is with any other skull base approach, here also the positioning of the head is planned in a way so that gravity helps in minimising the use of a self-retaining retractor. Since the main corridors of approach are the Sylvian fissure and sub-frontal, the head is to be positioned so that the gravity should assist in opening up the Sylvian fissure and the frontal lobe should fall away from the anterior cranial fossa floor. In addition, the head should be elevated above the level of the heart to improve venous drainage. Effectively, the head-end of the table is elevated, and the neck is extended with a slight lateral tilt to the opposite side. This is done to make the Sylvian fissure perpendicular to the floor so that both the temporal and frontal lobes fall away from each other. While this strategy works well for all other dome projections, it does not work well with inferiorly projecting aneurysms, as they are often adherent to the dura of the planum region. The slightest traction on the dome can lead to intraoperative rupture. Hence, in these cases, the amount of extension is reduced, and the aneurysm is primarily approached trans-Sylvian rather than sub-frontally to achieve proximal and neck control. The next challenging projection of the anterior communicating artery aneurysm is the posterior–superior one. These aneurysms often have the perforators wrapped around their necks, which require a very careful and meticulous dissection of the neck.
Determining the degree of rotation of the head for each aneurysm projection has been reported extensively, and each projection has its own complexities and requirements.[25,28] Deriving a fixed rule of rotation for every projection may be confusing and difficult to follow. We find that with the current advances in neuroimaging and the availability of 3D rotating images on the go, the surgeon can fine-tune the degree of rotation on individual case-to-case basis.
DEALING WITH HIGH RIDING AND SUPERIORLY OR POSTERIORLY PROJECTING GIANT ACOM ANEURYSMS: THE ROLE OF INTERHEMISPHERIC APPROACH
While working through the pterional approach through the trans-sylvian and lateral supraorbital corridors, exposing a high riding or giant aneurysm projecting posteriorly or superiorly requires generous opening up of the interhemispheric fissure. This invariably requires resection of the gyrus rectus as splitting the fissure through pterional route become very challenging.[26,29] The Japanese groups extensively use the IHA with the meticulous splitting of the interhemispheric fissure. This leads to not only a good visualisation of both A1s and A2s but also hypothalamic perforators with minimal brain retraction and preservation of the gyrus rectus. However, one has to be careful about the risks involved with IHA including cerebrospinal fluid (CSF) rhinorrhoea (frontal sinus opening up) and anosmia (traction damage to the olfactory tract).
THE DILEMMA OF THE BLIND AREAS
While clipping the anterior communicating artery aneurysms, not all proximal and distal vessels are well visible all the time. The ipsilateral A2 is often hidden behind the gyrus rectus, while the contralateral one is behind a superiorly projecting dome. A limited subpial resection of the gyrus rectus medial to the olfactory tract often helps in exposing the vessels inside the interhemispheric fissure, thereby ensuring adequate vascular control before the neck dissection has begun.
PTERIONAL APPROACH: THE WORKHORSE
The pterional approach with its lateral supraorbital extension is the workhorse approach for ruptured anterior communicating artery aneurysms. The advantage of beginning the dissection from the trans-sylvian plane is that adequate CSF and haematoma can be evacuated to achieve a desired level of brain relaxation. In addition, a proximal control of parent vessels is superior to and without undue retraction as through the subfrontal corridor, thereby reducing injury to the olfactory tract. However, may often require resection of the gyrus rectus for aneurysms high up in the interhemispheric fissure.
Managing anterior communicating artery aneurysm is a challenging task and it requires an individualised approach and strategy. A thorough understanding of the anatomical relations and the important perforators is vital to achieve the desired outcome. The use of 3D rotation imaging is becoming indispensable in tailoring the most suitable approach in each case.
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