Aneurysms of the Middle Cerebral Artery
M1 Segment Aneurysms
Most M1 segment aneurysms are small lesions associated with the origin of the anterior temporal artery. Although some can be coiled, many incorporate the origin of the anterior temporal itself and are best treated surgically ( Figs. 6.1, 6.2 ). When the aneurysm is unruptured, we almost always open the lateral aspect of the Sylvian fissure, working from lateral to medial to follow the M2 branches back to the bifurcation and then back along the M1 trunk to the aneurysm. Over time, one develops a sense of where to begin the fissure dissection to come down directly on the aneurysm. In the setting of a ruptured M1 lesion, we will usually expose the supraclinoid internal carotid artery (ICA) proximally and then open the fissure from medial to lateral to achieve early proximal control. As a rule, we avoid sacrificing bridging Sylvian veins, but if the fissure cannot be opened adequately without sacrificing a tributary, it is usually well tolerated.
We have included several videos illustrating the clipping of M1 aneurysms . Some form of intraoperative angiography is helpful to be sure that the anterior temporal artery itself has not been compromised by the clip.
Rarely, an aneurysm will arise at the origin of a lenticulostriate artery. The fine perforator will often be adherent to the aneurysm dome, and we have found these lesions best treated with very fine clips applied parallel to the M1 trunk coming across the aneurysm neck and falling just short of the lenticulostriate origin ( Fig. 6.3 ). If necessary, a small amount of aneurysm can be left to preserve flow into the perforating branch and then wrapped with gauze or Gore-Tex.
Finally, we have encountered a subset of M1 aneurysms that are true fusiform lesions, which may represent dissections of the vessel ( Fig. 6.4 ) . In some cases, we have been able to primarily reconstruct these lesions with a clip that gathers the saccular component, effectively reconstructing the M1 trunk. In other cases, we have used circumferential wrapping with Gore-Tex or gauze held in place with clips. In the largest of such lesions, particularly when the entire circumference of the vessel is clearly involved, distal revascularization with either proximal or distal occlusion has been an effective strategy ( Fig. 6.5 ) ( Table 6.1 ).
MCA Bifurcation Aneurysms
For most aneurysm surgeons, the middle cerebral artery (MCA) bifurcation has become the most common location of aneurysms treated surgically. This probably relates to the wide necks associated with most MCA bifurcation aneurysms, which typically incorporate one or both of the M2 branches. This provides a serious challenge for endovascular therapy, whereas surgical clip application is well suited for these lesions. Currently in our practice, we clip the vast majority of MCA aneurysms.
In the author’s opinion, the emerging trend of using complex endovascular constructs such as “Y” stenting to treat these lesions is difficult to understand when a competent surgeon is available. These endovascular techniques carry significantly higher complication rates than those offered by an experienced neurovascular surgeon, and the durability of stent coil constructs in the setting of a wide-necked aneurysm cannot match that associated with proper neck clipping. Although the use of such constructs may have a role for aneurysms in locations where surgical results are less favorable (such as the basilar apex), it makes little sense to offer patients a procedure that carries a higher risk and has poorer results than open microsurgical clipping. Sadly, the growing enthusiasm for endovascular treatment performed outside the setting of a team approach that includes an experienced neurovascular surgeon has resulted in an increasing number of such cases being treated endovascularly, with an associated increase in the rates of early complications and complex, late recurrences.
For unruptured lesions, we generally use a limited opening of the lateral Sylvian fissure as described above ( Figs. 6.6, 6.7 ). The opening can be extended proximally or distally as needed to afford full exposure of the aneurysm or to reach additional aneurysms at the same setting ( Fig. 6.8 ). Many wide-necked lesions require multiple clips to reconstruct the MCA bifurcation properly. At times, the entire bifurcation is aneurysmal and the eccentric, saccular component can be clipped while the entire parent artery is wrapped with gauze or Gore-Tex that can be secured circumferentially around the vessel ( Fig. 6.9 ). In general, it is better to leave some “fullness” at the MCA bifurcation than to stenose the origins of one or both M2 branches, risking a serious ischemic injury.
We have included multiple MCA aneurysm examples in this series to highlight the variety of nuances and techniques that we have found useful in clipping these lesions . Younger surgeons should focus on becoming adept at opening the Sylvian fissure using sharp microsurgical dissection while avoiding pial transgression ( Fig. 6.10 ). This represents an excellent microsurgical exercise as well as the cornerstone of surgery for anterior circulation aneurysms and MCA aneurysms, in particular. For larger aneurysms, it may be helpful to free the dome fully from its bed, enabling precise clip reconstruction. In select cases, multiple clips can be used to reconstruct a complex aneurysm, or fenestrated clips may be useful, allowing the surgeon to leave a densely adherent branch attached to the aneurysm dome ( Figs. 6.11, 6.12 ). When the M1 segment is short, it may be easier to open the fissure from medial to lateral. In such cases, one should be careful to preserve every small vessel running near the aneurysm, as medial lenticulostriate vessels irrigating the internal capsule can be injured when treating a bifurcation aneurysm arising on a short M1 segment.
On occasion, middle cerebral aneurysms can present with a large temporal lobe hematoma, potentially resulting in a life-threatening cerebral herniation syndrome. As described in Chapter 4, we bring these patients to the operating room without a formal preoperative diagnostic angiogram, perform a generous craniotomy, open the dura, and then perform an intraoperative angiogram to map the vascular anatomy ( Fig. 6.13 ). The hematoma can then be evacuated, and the aneurysm can be treated, although we prefer to avoid working through the hematoma cavity to reach the aneurysm if at all possible. When treating smaller ruptured aneurysms of the MCA, we prefer to expose the M1 segment first to achieve proximal control and then dissect the aneurysm itself, often with the use of temporary clipping ( Fig. 6.14 ).
The MCA bifurcation is one of the most common locations to encounter a giant aneurysm. The management of giant aneurysms is detailed in Chapter 9. In these cases, bypass has become an important part of our treatment algorithm, with primary clip reconstruction or aneurysmorrhaphy representing alternative options ( Table 6.2 ).