Anatomic Considerations

The region of the anterior communicating artery (ACoA) has an extremely variable normal anatomy ( ▶ Fig. 8.1). The complex relationships between the blood vessels, aneurysm, and other surrounding structures make ACoA aneurysms one of the most complex to treat surgically. The following factors should be thoroughly analyzed on high-quality computed tomography angiography with three-dimensional reconstructions and/or digital subtraction angiography for each patient:

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  The configuration of the aneurysm sac and the direction of projection.

  
  
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  The anatomy of the neck of the aneurysm.

  
  
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  The relationship of the aneurysm to the ipsilateral and contralateral pre- (A1) and postcommunicating (A2) segments of the anterior cerebral arteries (ACAs).

  
  
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  The side of the dominant A1 segment.

  
  
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  Anatomic variations.

The caliber of the A1 segment is asymmetric in 85% of patients with ACoA aneurysms, with 10% having a hypoplastic A1 segment. ³ This asymmetry almost invariably correlates with the aneurysm orientation as the base of the aneurysm typically emerges on the side of the larger A1 segment with the dome pointing toward the side of the hypoplastic segment. Thus, the aneurysm sac will possess a distinct orientation, which has important surgical implications and can be classified based on its relationship to the planum sphenoidale:

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  Inferior (toward the planum).

  
  
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  Anterior (toward the nose, parallel to the planum).

  
  
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  Superior (toward the vertex, perpendicular to the planum).

  
  
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  Posterior (toward the occiput, parallel to the planum).

  
  
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  Circumferential (ectasia of the ACoA).

Combinations of these projections are frequent, particularly in large aneurysms.

The ACoA may also demonstrate anomalous anatomy, with approximately 25% ranging from a network of multiple bridges to duplication or triplication of the artery. Furthermore, the A2 displays variability. In the majority (90%) of cases, there are two A2 arteries, but three A2 arteries can be found in 9% and a single (azygos) A2 artery can be seen in 1%. Another significant area of divergence in anatomy occurs with the recurrent artery of Heubner (medial striate artery), the largest and most important of the striate perforators in the ACA–ACoA complex. This artery originates from the A2 segment in 78% of cases, from the A1 segment in 14% of cases, and at the level of the ACoA in 8% of cases, curving sharply back on itself to course parallel to the A1 segment. ⁴ The size and position of this artery correspond to other components of the ACoA complex. For instance, in cases of small A1 arteries, the recurrent artery of Heubner may also be small or atretic with possible replacement by large medial perforating arteries that originate from the midportion of the A1 segment and travel more laterally than the typical recurrent artery of Heubner.

Another critical anatomical consideration is the relative parallel or perpendicular orientation of the ACoA to the planum sphenoidale, which influences the visualization of the contralateral A1 and A2 arteries and the neck of the aneurysm, as well as the aneurysm clip selection and final placement ( ▶ Fig. 8.2 and ▶ Fig. 8.3a,b).

8.2 Surgical Approaches

Once the aneurysm configuration and anatomic considerations have been thoroughly analyzed, the type and side of approach can be determined. ACoA aneurysms can be approached from an anterior (interhemispheric), anterolateral (subfrontal), lateral (pterional), extended skull base, or combination of these approaches. In our practice, the interhemispheric approach is not routinely used unless the aneurysm is superiorly projecting, located at least 15 mm above the anterior clinoid process, and the remaining anatomy is otherwise favorable. Advantages of the interhemispheric approach include less need for frontal lobe retraction and easier visualization of the ACoA complex and adjacent vessels. However, the main disadvantage is that the aneurysm sac is frequently encountered prior to dissection of the neck, particularly in anteriorly projecting and large aneurysms. In such cases, adequate aneurysm neck exposure and satisfactory proximal control can be difficult to obtain. Additionally, both frontal lobes are subject to potential injury, the operative field increases in depth, the olfactory tracts are at increased risk of injury, and entry into the frontal sinus elevates the risk of infection and cerebrospinal fluid leakage. Although the subfrontal approach provides the most direct course to the ACoA complex, the disadvantages are similar to those of the interhemispheric approach, primarily encountering the aneurysm dome prior to obtaining proximal control. The pterional approach with emphasis on a larger frontal exposure is the most frequently advocated approach for ACoA aneurysms and used in virtually all cases in our practice. We reserve the use of extended skull base approaches, such as supraorbital, transorbital, and orbitocranial zygomatic craniotomies, for specific complicated or giant aneurysms.

Once an approach has been selected, the side of operation must be determined. It is our practice to virtually always approach an aneurysm from the side of the dominant A1, if one is present, as it allows greater visualization and easier dissection of the aneurysm neck without disturbance of the dome since the aneurysm sac typically projects away from the dominant A1. Additionally, earlier access to the dominant A1 can be obtained for proximal control. However, it should be emphasized that temporary clipping of only the dominant A1 without temporary clipping of the contralateral A1 virtually never results in complete vascular control. Conversely, approaching the ACoA aneurysm from the side of the nondominant A1 may be considered in the presence of other aneurysms of the anterior circulation that the surgeon feels compelled to treat concurrently, as well as in cases of large intraparenchymal hemorrhages or unilateral gyrus rectus hematomas requiring evacuation or to preserve the uninjured parenchyma. However, if the surgical factors favoring each side are otherwise equal, the aneurysm should be approached from the patient’s nondominant side.

8.3 Operative Procedure

8.3.1 Exposure

A pterional craniotomy is performed in the standard fashion with specific considerations made for ACoA aneurysms. Horizontal head rotation is no greater than 45 degrees in the contralateral direction, which is less than for other anterior circulation aneurysms. The frontal portion of the bone flap is larger than for other anterior circulation aneurysms and extends laterally to the midpupillary line, which is approximately 3 cm from the zygomatic orbitofrontal junction. Placement of frontal burr holes below the hairline is avoided for cosmesis. In order to minimize retraction on the frontal lobe, striving to be as low as possible on the anterior fossa floor is essential in this extended frontal approach. Although opening of the frontal sinus increases the risk of infection and cerebrospinal fluid leakage, an extensive view of the anterior fossa floor is crucial and takes precedence. The temporal portion of the craniotomy needs only to be large enough to expose the proximal aspect of the sylvian fissure.

8.3.2 Microdissection

After the dura is opened, the first step in microdissection is identification of the optic nerve, carotid artery, and ipsilateral A1 artery ( ▶ Fig. 8.2a). A frontal retractor is placed on the orbital surface of the frontal lobe immediately anterior to the sylvian fissure. Advancing the retractor blade beyond the olfactory nerve to the gyrus rectus and applying slight frontal lobe retraction will identify the ipsilateral optic nerve. The opticocarotid cistern is opened exposing the carotid artery. A retractor is then placed over the temporal lobe. Slight tension will expose the proximal sylvian fissure, allowing division of the most medial and inferior region. We routinely strive to preserve the temporal bridging veins during this dissection. Opening the sylvian fissure detaches the frontal and temporal lobes from each other, which greatly aids in frontal lobe retraction and exposes the proximal ipsilateral Al, facilitating early temporary clip application. In certain instances, the carotid bifurcation may be more distally located in the sylvian fissure and extensive dissection would be required. In such cases, however, there is usually significant looping of A1 inferiorly from the bifurcation and the more distal aspect of the ipsilateral A1 can be found where it crosses the optic nerve without opening the sylvian fissure. Likewise, in cases of subarachnoid hemorrhage (SAH) where the sylvian fissure is difficult to identify and dissect, attempted opening should be avoided.

With slight retraction of the frontal lobe and further arachnoid dissection, the ipsilateral A1 can be followed distally to where it crosses the optic nerve. At this stage of the dissection, the recurrent artery of Heubner is vulnerable to injury as it courses anterolateral to the A1 segment in 60% of cases. ³ Thus, it may be encountered before the A1 segment during the initial dissection and retraction of the frontal lobe. Moreover, the recurrent artery of Heubner should not be confused with the orbitofrontal artery, which is often the second major branch of the A2 segment and courses perpendicularly over the gyrus rectus and olfactory tract. The orbitofrontal artery may be important to identify because following it proximally can lead to the location of an obscured aneurysm neck. The gyrus rectus may be resected at this point. In our practice, resection is performed frequently, but only the minimum required to accomplish adequate exposure of the adjacent arteries and aneurysm. Gyrus rectus resection is accomplished by coagulating the pia of the gyrus rectus medial to the olfactory tract while protecting the orbitofrontal artery. The pia is then opened by sharp dissection and a small portion is carefully suctioned away while initially preserving the most medial pial layer to protect against damage of adjacent blood arteries and aneurysm. With a small retractor positioned in the resection bed, excellent visualization of the A1/A2 junction can be obtained ( ▶ Fig. 8.2b). Gyrus rectus resection may be unnecessary for inferiorly projecting aneurysms, ACoA–aneurysm complexes located close to the planum sphenoidale within the suprachiasmatic cistern, and in particularly relaxed brains.

Opening the interhemispheric fissure allows safer retraction of the ipsilateral frontal lobe. However, the anatomy of the aneurysm and the projection of the dome must be clearly understood prior to this dissection. Anteriorly and, especially, inferiorly directed aneurysms may be immediately beneath the arachnoid of the interhemispheric fissure and are prone to injury during this dissection. Conversely, extensive opening of the interhemispheric fissure is especially advantageous for superiorly projecting aneurysms located high within it.

In order to obtain sufficient control of the aneurysm during temporary clipping, control over the contralateral A1 is advisable, even if this artery appears hypoplastic or atretic on vascular imaging. In nearly all cases, there will be a contralateral A1 artery that will cause bleeding of a prematurely opened aneurysm if only the ipsilateral A1 is controlled. Such bleeding may still occur even with temporary clipping of both A1s due to excellent collateral flow through the A2 arteries. Contralateral A1 ease of exposure depends on the configuration of the ACoA–aneurysm complex, specifically, the location where the contralateral A1 crosses the optic nerve. The location can vary, but is usually more medial than the surgeon might anticipate. Therefore, dissection should start medially and progress laterally in order to avoid overlooking a very medially located artery. Care must be taken to avoid confusing a large recurrent artery of Heubner for the contralateral A1. The relative orientation of the ACoA segment to the planum sphenoidale is another critical factor to evaluate. If the ACoA complex is parallel to the planum, both the ipsilateral and contralateral A1 and A2 may be visualized more easily. Conversely, if it is more perpendicular, the contralateral arteries will be obscured. During temporary clipping, the contralateral A1 clip should be placed first. A long, slightly curved clip may aid in clipping during this more technically difficult clip placement. Subsequently, the ipsilateral A1 can be clipped with a shorter clip in order to avoid obscuring the area of dissection.

The next step involves identification of both A2 arteries, if possible. Finding the ipsilateral A2 is essential and is usually accomplished by following the ipsilateral A1 distally and laterally, thereby exposing the region between the ipsilateral A2 and the aneurysm neck. At this point, a third, tapered retractor blade can be used to retract the frontal lobe. The second retractor is located more proximally and maintains exposure of the ipsilateral Al. Though finding the contralateral A2 is always preferable, this may be very easy to nearly impossible depending on the anatomy. In some instances, the contralateral A2 may not be identifiable until the aneurysm has been clipped. Finally, the numerous perforating arteries arising in this region are extremely important. They should be visualized and protected ( ▶ Fig. 8.1, Video 8.1).

8.3.3 Clip Application

ACoA aneurysm necks are seldom ready to be clipped upon initial exposure. The aneurysm neck should be defined accurately. Once defined, cleavage planes need to be created between the aneurysm neck, adjacent arteries, and perforating arteries. Premature clip placement without full understanding of the global anatomy is strongly discouraged as it will likely lead to premature aneurysm rupture, inadequate aneurysm occlusion, and violation of the perforating arteries. Wide-necked aneurysms can be reconfigured with bipolar coagulation at a low setting with ample irrigation, but the origin of perforating arteries must be carefully protected from damage. Temporary clipping may be helpful. Though no defined guidelines regarding duration and quality of temporary clipping exist, in ACoA aneurysms we believe it should be avoided or kept to the minimum required to obtain a safe and adequate dissection and clipping of the aneurysm neck due to the large number of critical perforators in this region. If the dissection is performed over a dominant A1 and the aneurysm anatomy is relatively straightforward, temporary clipping of the ipsilateral A1 can be helpful to relax the aneurysm complex for safer dissection. In addition, if the anatomy is obscure and there is a high likelihood of opening the aneurysm prematurely, temporary clipping is advised. When a lengthy and difficult dissection is expected, placing a contralateral A1 temporary clip is a safe initial maneuver because an ipsilateral A1 temporary clip can be placed quickly if urgently needed. When prolonged temporary clipping is anticipated or deterioration of evoked potentials, if monitored, occurs, thiopental loading to achieve burst suppression on the electroencephalogram should be considered. Furthermore, the patient should be maintained with a normal to slightly hypertensive blood pressure during temporary clipping.

Again, aneurysm anatomy should be carefully evaluated prior to clipping. ACoA aneurysms occur in a variety of projections and configurations. Yasargil classified aneurysms according to their surgical orientation, and when adapted to an anatomical classification in relationship to the planum, as used in this text, his findings were that 34% projected superiorly, 23% anteriorly, 13% inferiorly, and 14% posteriorly, while 16% were complex, multilobulated aneurysms pointing in multiple directions ( ▶ Fig. 8.4a–c). ⁴ Clipping strategies vary depending on the projection of the aneurysm.

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How to Repair the Intracranial Aneurysm: Clipping or Coiling Decision Making Distal Anterior Cerebral Artery Aneurysms: Anterior Interhemispheric Approach Middle Cerebral Artery Aneurysms Tentorial and Posterior Fossa Dural Arteriovenous Fistulas Superficial Temporal Artery to Middle Cerebral Artery Bypass Transsylvian Transclinoidal and Transcavernous Approach for Basilar Bifurcation Aneurysms

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