Aneurysms of the Carotid Bifurcation
Aneurysms arising at the bifurcation of the internal carotid artery (ICA) are common and can be challenging because of the frequent small perforating vessels that can be adherent to, or hidden by, these lesions. Both the recurrent artery of Heubner as well as A1 and M1 perforators will often cross these aneurysms and can be densely stuck to the aneurysm’s neck or dome.
Proper exposure of these lesions requires visualization of the proximal A1 and M1 segments, and the amount of opening of the Sylvian fissure should be guided by the size and orientation of the aneurysm in these cases. As a rule, the proximal aspect of the fissure is opened sharply to reach the ICA bifurcation. This will enable visualization of the aneurysm neck. We have found it important to open the fissure further in most cases, exposing at least some length of the M1 segment ( Fig. 5.1 ). In general, we prefer to dissect these aneurysms completely out of their beds, reflecting them forward to see all perforators, and then applying a permanent clip to the neck ( Fig. 5.2 ). This may require a small amount of subpial resection of the deep frontal lobe, which has not carried any clinical consequence in our experience. This technique is illustrated in several videos in this collection ( Fig. 5.3 ). Once mobilized, these aneurysms can usually be clipped in straightforward fashion ( Fig. 5.4 ).
ICA bifurcation aneurysms can point anteriorly, superiorly, or posteriorly. Those directed either anteriorly or superiorly are more straightforward from a surgical perspective, as they are more readily exposed through a standard pterional craniotomy ( Fig. 5.5 ). Posteriorly directed lesions point away from the surgeon and can be more difficult to expose. The surgeon must carefully identify, dissect, and separate the perforators running along the ICA bifurcation to expose the aneurysm neck ( Fig. 5.6 ). These lesions can then be treated either by applying a clip over the top of the bifurcation (most common) or by working along the lateral aspect of the distal ICA above the anterior choroidal artery (AChA). In the latter instance, the surgeon must be careful not to injure the AChA. When clipping a carotid bifurcation aneurysm, it is important to avoiding placing the clip too low on the parent artery, potentially stenosing the A1 or M1 segments.
In the setting of a subarachnoid hemorrhage (SAH), establishing proximal control for an ICA bifurcation aneurysm requires access to the proximal supraclinoid ICA as well as the A1 segment. Assuming the presence of a patent anterior communicating artery (ACommA) as well as normal bilateral A1 segments, placing a temporary clip on the supraclinoid ICA may only minimally decrease the bleeding from an ICA bifurcation aneurysm that ruptures in the operating room. It should also be noted that when a carotid bifurcation aneurysm bleeds, it can occasionally result in a large hematoma involving the overlying frontal lobe. This hematoma can rupture directly into the ventricle with only minimal associated SAH. In these cases, it is important to recognize the true source of the bleeding as a ruptured ICA bifurcation aneurysm ( Table 5.1 ).