ACA Bypass Strategy

Anterior cerebral artery bypasses are challenging for several reasons. First, most bypass surgeons perform traditional EC-IC bypasses such as the STA-MCA bypass for MCA aneurysms, atherosclerotic occlusive disease, and moyamoya disease. The STA is less useful for ACA indications because it is a lateral scalp artery that may not reach recipient arteries at the depths of the interhemispheric fissure, or may be too small in caliber if it does. Interpositional bypasses from the STA or cervical carotid artery that address this problem require extensive surgical exposures, are often long and circuitous, and may be mismatched in caliber. Therefore, ACA indications frequently call for IC-IC bypasses.

Second, the anatomy of the ACoA complex and the parallel course of its efferent arteries in the interhemispheric fissure facilitate many variations of IC-IC bypasses in the ACA territory. ACA bypasses can be communicating bypasses that bring left and right arteries together in the midline with side-to-side in-situ bypasses to allow crossing flow from one side to the other (e.g., L PcaA-R PcaA bypass). End-to-side reimplantation bypasses can also bring pericallosal and callosomarginal arteries (e.g., PcaA-CmaA reimplantation), or other similar pairs, together to allow flow from superior and inferior. ACA bypasses can be interpositional “jump” bypasses in the sagittal plane that carry flow from anterior to posterior (e.g., A2 ACA-RAG-A3 ACA bypass). The availability of donor flow from an MCA trunk or extracranial arteries such as the STA and ECA enable ACA interpositional bypass flow from lateral to medial, either coursing high over the convexity (“bonnet” bypass, STA-RAG-A3 ACA bypass) or low subfrontally (“basal” bypass, M2 MCA-RAG-A2 ACA bypass). The breadth of ACA bypass exceeds its indications.

Third, many of these bypasses require more than one simple, encompassing surgical corridor in which to perform a bypass. Unlike most lesions in other locations where proximal afferent and distal efferent arteries are accessed through one exposure for vascular control and anastomosis, many ACA lesions sit at the limit of transsylvian exposure without good access to the distal side of the pathology. For example, the pterional-transsylvian approach exposes the A1 segments and the ACoA aneurysm but provides only limited exposure of the proximal A2 segments. The bifrontal-interhemispheric approach exposes the A2 segments and the aneurysm but provides only limited exposure of the distal A1 segments. Therefore, more complicated exposures may be needed to perform an ACA bypass.

The ACA bypasses are indicated mainly for complex aneurysms. Ischemia in the ACA territory caused by atherosclerosis, moyamoya disease, or other steno-occlusive disease is rare and typically treated with an EC-IC bypass with or without an interposition graft, such as an STA-STA-ACA bypass (see Chapter 14). When cortical arteries along the paramedian convexity are large enough to serve as recipients, an interposition graft may not be necessary, but when they are diminutive or an interhemispheric recipient is needed, an interposition graft may be necessary. Therefore, this chapter focuses on ACA bypass strategy for aneurysms.

ACA Aneurysms

The ACoA aneurysms are the second most common aneurysms in my surgical experience, yet they require the fewest bypasses. Most ACoA and ACA aneurysms are amenable to conventional clipping or endovascular coiling, even when they are complex. Their anatomy—with two afferent arteries (bilateral A1 segments), four efferent arteries (bilateral A2 segments and recurrent arteries of Heubner), the ACoA and its perforators, the nearby branch arteries (orbitofrontal and frontopolar arteries bilaterally), and sometimes accessory A2 segments—is best preserved with direct aneurysm occlusion. The ACoA connection between the left and right ACA circulations cross-fills the distal territories and allows some latitude in deliberately sacrificing an afferent artery during aneurysm treatment. However, not every ACA aneurysm can be clipped or coiled or has the anatomy to tolerate A1 ACA sacrifice, and a select few will require revascularization after trapping.

The ACA aneurysms can be revascularized in a variety of ways and the management strategy is individualized according to patient-specific anatomy. The algorithm for the ACA bypass strategy is based on the location of the ACA aneurysm relative to the ACoA complex, classifying aneurysms into three groups: (1) pre-communicating (A1 segment), (2) communicating, and (3) post-communicating (A2 to A5 segments) (Fig. 22.1).

Pre-Communicating ACA Aneurysms

Pre-communicating aneurysms are not as likely as other ACA aneurysms to require a bypass because the contralateral A1 segment and ACoA provide collateral blood flow across the midline to the ipsilateral A2 segment. Bypass is only indicated in the absence of these elements of the circle of Willis. Therefore, strategy is dictated by the anatomy of the contralateral A1 ACA. Specifically, the presence of a symmetric contralateral A1 ACA allows most pre-communicating ACA aneurysms to be excluded simply with trapping and no bypass. This portion of the circle of Willis harbors some medial lenticulostriate arteries supplying the anterior perforated substance and anterior basal ganglia, but can be occluded safely in most patients. However, the absence of a contralateral A1 ACA and ipsilateral dominance places bilateral ACA territories at risk with aneurysm trapping. A dominant A1 ACA with a hypoplastic or atretic A1 ACA on the contralateral side requires a bypass to restore flow after trapping (Figs. 22.2 and 22.3).