Aneurysms of the Anterior Cerebral Artery
Aneurysms of the Anterior Communicating Artery
The anterior communicating artery (ACommA) is one of the most common locations associated with intracranial aneurysm development. Although some ACommA aneurysms can be treated successfully through an endovascular approach, many are best managed with open microsurgical clipping because of a broad neck or incorporation of one of the A2 segment origins into the aneurysm base. ACommA aneurysms are generally exposed through a standard pterional approach as described in Chapter 1. We add a bit more frontal exposure for these lesions than we do for other anterior circulation aneurysms to facilitate opening of the anterior interhemispheric fissure as needed.
With the bone flap completed, the dura is elevated from the floor of the anterior cranial fossa and from the sphenoid wing. The sphenoid wing is drilled down aggressively to the region of the anterior clinoid process, and any ridges in the subfrontal bone along the superior aspect of the orbit are flattened with the drill. The periorbita is often exposed as the subfrontal bone is drilled and should be protected during this phase. By creating a low, flat plane of approach, the surgeon can limit the need for brain retraction, facilitating exposure of the aneurysm while minimizing local trauma to the brain. Once the dura has been opened, the operating microscope is introduced.
At this point, we elevate the frontal lobe gently and open the arachnoid over the optic nerve and internal carotid artery (ICA) using sharp dissection. This releases cerebrospinal fluid (CSF) and typically produces excellent brain relaxation. Traditionally, the surgeon now opens the proximal Sylvian fissure to expose the carotid bifurcation, revealing the origin of the A1 segment, which can be followed to the aneurysm. In practice, one can elevate the frontal lobe at the level of the optic nerve to identify the mid A1 segment or the aneurysm itself without first exposing the A1 origin. In our experience, when the Sylvian fissure can be opened quickly and easily, this maneuver will untether the frontal lobe and minimize the need for subfrontal retraction. When the Sylvian fissure is scarred or adherent, a very limited opening of only the most proximal aspect of the fissure will suffice in most cases.
The exact maneuvers needed to expose the aneurysm, including the degree to which the Sylvian fissure should be opened, will depend on the specific anatomical configuration of the anterior communicating complex as well as the orientation and size of the aneurysm ( Fig. 2.1 ). When the aneurysm points inferiorly, one can simply elevate the frontal lobe, divide the arachnoid tethering the optic nerve to the frontal lobe, and the aneurysm will come into view ( Figs. 2.2, 2.3 ) . For other configurations, it is helpful to expose and open the anterior interhemispheric fissure to visualize the A2 vessels as they exit the aneurysm . By opening the interhemispheric fissure, the surgeon can avoid gyrus rectus resection in many cases ( Fig. 2.4 ). If needed, however, limited resection of the gyrus rectus can be important in exposing the key structures ( Figs. 2.5, 2.6 ) . In such cases, the pia is opened along an avascular area on the undersurface of the gyrus rectus, and subpial resection is then performed. Arterial branches crossing the gyrus rectus to irrigate the frontal lobe should be left intact. When the aneurysm points superiorly and posteriorly or when the ACommA rides high in the interhemispheric fissure, an orbitozygomatic (OZ) osteotomy or a more frontal exposure can be helpful.
Careful evaluation of the preoperative arteriogram should give the surgeon a good sense of the anticipated three-dimensional anatomy even before entering the operating room ( Fig. 2.7 ). It is important for the surgeon to understand the relationship of the aneurysm to the A2 vessels. As a rule, the surgeon should focus on identifying the ipsilateral and contralateral A1 vessels, the aneurysm neck, and the A2 vessels as early as possible during the surgery ( Fig. 2.8 ) . Once these maneuvers are completed, proximal control can be achieved readily, and dissection of the aneurysm neck can proceed safely and with confidence. Dissection should focus on the aneurysm neck at its origin from the communicating artery, clearing a path for the tips of the clip. It is rarely necessary to dissect the entire aneurysm dome, but it may be important to free up adherent perforators to allow for proper clip placement. All perforating arteries in the region of the ACommA should be left intact. The recurrent artery of Heubner, which most commonly arises from the proximal aspect of the A2 segment and then courses back over the A1 toward the anterior perforated substance, should be identified and preserved as well.
As with all aneurysms, it is important to select a clip with properly sized blades. Overly long blades can torque the aneurysm or the normal vessels and may inadvertently injure structures beyond the surgeon’s view. Blades that are too short will fail to obliterate the aneurysm neck fully. We often find bayoneted clips to be particularly useful for ACommA aneurysms, as they allow excellent visualization of the tips of the clip as they close the aneurysm neck. When the aneurysm points superiorly and is hidden by the ipsilateral A2 vessel, a fenestrated clip can be applied with the A2 encircled in the fenestration to obliterate the aneurysm. The use of intraoperative angiography, either formal digital subtraction angiography or indocyanine green (ICG) angiography, will limit unexpected complications related to inadvertent narrowing or occlusion of a critical neurovascular structure, particularly when dealing with larger aneurysms or those with atheroma and calcification.
In the setting of a subarachnoid hemorrhage (SAH), placement of a ventriculostomy at the start of the procedure may greatly facilitate exposure of the aneurysm without undue brain retraction. We generally use a ventricular drain if there is evidence of preoperative ventriculomegaly related to a recent hemorrhage, if the brain is “full” at the start of the procedure, and in most patients who are preoperatively Hunt/Hess Grade III or higher. Although some surgeons have recommended the use of a spinal drain for unruptured lesions, we have not found this necessary, as opening of the basal CSF cisterns will generally provide excellent brain relaxation.
We generally approach ACommA aneurysms from the side of the dominant A1, but if the complex is rotated such that exposure of the critical anatomy would be compromised from that side, we will often use a “contralateral” approach . Other mitigating factors may include the presence of additional aneurysms that can be clipped in the same setting if a particular side is chosen, a history of previous craniotomy that may make a “contralateral” approach appealing, or a large frontal hematoma from a recent hemorrhage. In this setting, it may be advisable to approach the aneurysm on the side of the hematoma to avoid injuring the preserved frontal lobe. Since the aneurysm dome will often point into the hematoma, the surgeon should always follow the basic principles already outlined, focusing attention on proximal control and working at the aneurysm neck rather than charging through the hematoma to reach the lesion. If a surgeon is working through the hematoma cavity and ruptures the aneurysm dome, it can be surprisingly disorienting and difficult to achieve proximal control without injuring critical neurovascular structures. Occasionally, a subfrontal, midline approach may be useful, although we have rarely found this necessary .
Large and giant aneurysms of the ACommA are challenging because they tend to obstruct the critical vascular anatomy and can interfere with proper establishment of proximal control. In these cases, a more aggressive opening of the Sylvian and anterior interhemispheric fissures can be combined with an orbitozygomatic or orbitocranial approach and judicious resection of the gyrus rectus to expose the critical surgical anatomy properly ( Fig. 2.9 ).
ACommA aneurysms are deep, midline lesions. Complications associated with their treatment generally relate to inadequate exposure of the critical anatomy, resulting in uncontrolled bleeding from a recently ruptured aneurysm or inadvertent occlusion of critical neurovascular structures. Occlusion of an anterior cerebral artery (ACA) can result in hemiparesis, particularly involving the lower extremity; temporary aphasia if the dominant supplementary motor cortex is involved; abulia and frontal lobe dysfunction; or hypothalamic/pituitary dysfunction. Severe impairment of short-term memory can be a devastating consequence of injury to the local perforating vessels and must be avoided.
The surgeon should thoroughly evaluate the preoperative imaging studies to exclude the possibility of a “third” A2 vessel, which is found in a small percentage of cases. If the surgeon is unaware of its presence preoperatively, inadvertent sacrifice of a third A2 may be missed even on intraoperative angiography as the surgeon identifies the “expected” two A2 vessels filling properly, never realizing that a third important efferent vessel has been compromised. This can result in a serious, yet completely avoidable, ischemic complication.
Because of the additional frontal exposure utilized for ACommA aneurysms, there is an increased chance of violating the frontal sinus when the bone flap is cut. In these instances, once the aneurysm has been treated, we close the dura carefully, strip the sinus mucosa, and then fill the sinus with fat harvested from the abdomen. A small amount of fat is left in the epidural space overlying the sinus opening. The patient is then maintained on an antibiotic with appropriate coverage of the sinus flora for a period of five days ( Table 2.1 ).